Effects of Organic Amendments on Soil Humic Substances Content and Physiological Properties of Water-Stressed Zea mays and Glycine max

Final Report for GS04-031

Project Type: Graduate Student
Funds awarded in 2004: $9,793.00
Projected End Date: 12/31/2005
Grant Recipient: Virginia Tech
Region: Southern
State: Virginia
Graduate Student:
Major Professor:
Greg Evanylo
Virginia Tech
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Project Information

Summary:

Field research determined that continuous application of various types and rates of compost elicited no greater plant physiological responses to environmental stress than than did annual applications of commercial fertilizer or uncomposted poultry litter. Compost rates designed to supply the nitrogen requirement for corn produced yields and seed quality as high as those with the fertilizer and litter. Benefits of compost on crop physiological responses are more easily observed in drought sensitive species grown in containers where the biologically active constituents are maintained in the root zone.

Tables, figures or graphs mentioned in this report are on file in the Southern SARE office.
Contact Sue Blum at 770-229-3350 or
sueblum@southernsare.org for a hard copy.

Introduction

Water stress is the most critical environmental factor limiting crop production in the Piedmont soil physiographic province, which extends from Maryland to Alabama (Southeast Regional Climate Center, 2003a). Short-term crop water stress is common during summer months due to high evapotranspiration rate. Although often temporary, moisture stress usually reduces yield potential of crops such as corn and soybean because adequate moisture is essential for optimal growth and development during the sensitive reproductive stage of summer crops.

Periodically, summer crops in the region sustain prolonged water deficits. The National Drought Mitigation Center issued ‘severe’ and ‘extreme’ drought indices for the Virginia Piedmont during the summers of 1999 and 2002 (National Drought Mitigation Center, 2003). Precipitation was 14% and 24% less in 1999 and 2002, respectively, than precipitation occurring in a normal year (Southeast Regional Climate Center, 2003b). Corn and soybean yields in 1999 and 2002 were 35% to 57% and 11% to 43% lower, respectively, than average yields produced in the region (National Agricultural Statistics Service, 2003).

During water stress, overall photosynthetic efficiency is compromised and excessive concentrations of reactive oxygen species are generated within the chloroplast. Reactive oxygen species are partially reduced forms of oxygen that are capable of unrestricted oxidation of cellular components including the thiol and iron-sulfur clusters of peptides in the DNA bases (Mano, 2002). Reactive oxygen species also cause lipid peroxidation of the chloroplast membrane (as measured by malondialdehyde concentration) (Yan et al., 1996; Hung and Kao, 1997) and eventually cause cell death. Reactive oxygen species are naturally generated due to the intrinsic inefficiencies of photosynthesis. Plants have evolved an antioxidant scavenging system to effectively remove excessive reactive oxygen species from the chloroplast and maintain their concentrations at steady-state levels (Asada, 1994). It has been well documented that an up-regulation of antioxidant activity during stress increases the stress tolerance of plants (Longo et al., 1993; Li et al., 1994; Jiang and Zhang, 2001; Du et al., 2005; Ge at al., 2005). Pastori and Trippi (1993) observed greater antioxidant activity in more drought resistant plants than in the less resistant ones. Some plants have evolved an alternative photosynthetic pathway to prevent the formation of reactive oxygen species such as the of C4 metabolism of corn (Mittler, 2002). Stepien and Klobus (2005) observed lower reactive oxygen species generation and greater antioxidant efficiencies of stressed corn than in wheat.

Increased antioxidant activity has also been documented to retard the natural process of leaf senescence (Lin et al., 1988; Pastori and Trippi, 1993). Prochazkova et al. (2001) observed greater antioxidant activity over a longer period in a later maturing corn cultivar than in a relatively early maturing one. The authors concluded that the earlier decrease in antioxidant activity in the faster maturing cultivar contributed to an earlier senescence.

There are three major antioxidants that quench free radicals. Superoxide dismutase (SOD) reduces superoxide to peroxide in the chloroplast (Asada, 1994) and is considered the first response antioxidant. Ascorbate peroxidase (APX) reduces peroxide to water in the chloroplast. Catalase (CAT), located in the peroxisome, also reduces peroxide to water. Researchers have observed that CAT activity in C4 species is predominately driven by the formation of reactive oxygen species in the chloroplast (Tolbert et al., 1969), though corn CAT activity has been documented to be unresponsive to changes in the intercellular redox status during stress (Alber and Scandalios, 1993). Catalase activity in C3 plant species appears to be driven by the oxidation of glycolate, a product of photosynthesis (Lyu-bimov and Zastrizhnaya, 1992).

The soil-based application of organic amendments to field grown crops may have an ameliorating effect on drought stressed crops. Sahs and Lesoing (1985) observed higher sweet corn yields in plots amended with beef feedlot manure than those that were inorganically fertilized during drought years. Heckman et al. (1987) found that field grown soybeans fertilized with sewage sludge had increased drought resistance and nitrogen fixation than the control treatment.

Improved drought tolerance of crops grown in organically amended soils has been linked to the maintenance of optimum leaf health. In five-week old water stressed maize seedlings, Xu (2000) recorded higher photosynthetic rates when the soils were organically amended. HuiLan et al. (1998) noted that the application of organic amendments increased water stress resistance of sweet corn leaves. In particular, stomatal and curticular conductances of the leaves were lower in these plants than in inorganically-fertilized plants. Researchers speculate that the hormone-like properties of humic substances may play a causal role in drought stress amelioration (Serdyuk et al., 1999; Kulikova et al., 2003; Chen et al., 2004; Quaggiotti et al., 2004; Zhang and Ervin, 2004).

Humic substances are the major constituents of stable organic matter. These materials are naturally occurring, ubiquitous organic compounds that contain relatively high molecular weights, are yellow-black in color, and are formed by secondary synthesis reactions between plant and animal remains and microbial metabolites (Stevenson, 1994b). Humic substances are operationally defined, based on solubility. Fulvic acids represent about 20% of humic substances (Epstein, 1997), are relatively low in molecular weight (1000-4000 g/mol), and soluble in both alkali and acidic solutions (Stevenson, 1994b). Humic acids represent roughly 80% of humic substances (Epstein, 1997), have relatively large molecular weights (12,000-300,000 g/mol), and are insoluble in acidic solutions (Stevenson, 1994b).

Humic substances have been shown to elicit ameliorative effects on drought stressed plants, but most experiments designed to elucidate such benefits were limited to foliar applications of humic substances in pot studies. Xudan (1986) foliarly applied fulvic acid to pot grown wheat plants prior to imposing a nine-day dry down period. These plants maintained greater stomatal conductances, contained greater chlorophyll contents and increased 32P uptake relative to the control. Yan and Schmidt (1993) applied a commercially available seaweed extract to pot grown drought stressed perennial ryegrass and observed increased cell membrane fluidity and permeability relative to the control treatment. Zhang and Schmidt (1999, 2000) foliarly applied a commercially available seaweed extract and humic acid solution to drought-stressed tall fescue, creeping bentgrass, and Kentucky bluegrass, and observed an increase in leaf water status and antioxidant activities relative to the control.

Research exploring possible ameliorative effects of
soil applied compost humic substances on drought stressed agronomic crops is lacking. Further investigation is required to discover whether organic matter fractions in compost may elicit plant physiological benefits under field conditions.

Project Objectives:

The objectives of this field study were to compare the effects of repeated applications of inorganic fertilizer, poultry litter, and two composts on
(1) soil physiochemical properties (i.e., nutrient concentrations, bulk density, water holding capacity, organic carbon, humic and fulvic acid carbon) and (2) physiological properties (i.e., chlorophyll content, leaf water potential, photochemical efficiency, leaf antioxidant activities, lipid peroxidation, leaf Delta T values, yield, seed quality) of corn and soybean grown in a Piedmont soil.

Cooperators

Click linked name(s) to expand
  • Chandra Bowden

Research

Materials and methods:

Experimental Design

Site Description

This research was conducted at the Northern Piedmont Agricultural Research and Extension Center in Orange, Virginia on a Fauquier silty clay loam (fine, mixed, mesic Ultic Hapludalf). Site elevation is approximately 180 m (590 ft) above sea level, 38° 2’ N latitude and 78° 1’ W longitude. Annual temperatures range from 8°C (47°F) to 19°C (67°F), and average annual precipitation is 118 cm (47 in).

Phase I of Study: 2000-2002

Seven treatments established in spring 2000 and implemented through 2002 were modified in spring 2003 and investigated through 2005. The original treatments were applied to 3.7 x 7.6m plots arranged in a completely randomized block design with four replications and included a control; annual yard waste compost at 20% agronomic N; annual yard waste compost at 20% agronomic N + supplemental ammonium nitrate fertilizer; biannual yard waste compost at 100% agronomic N rate; biannual yard waste compost at 100% agronomic N rate + ammonium nitrate fertilizer; annual poultry litter at agronomic nitrogen rate; and annual commercial fertilizer treatment. No amendments were ever applied to the control treatment (Table 3.1).

The first three years of this study examined changes in soil and water quality and yields in a vegetable production system (Evanylo et al., 2002; Sherony et al., 2002; Evanylo et al., 2003a; Evanylo et al., 2003b). Crops grown included pumpkin (Cucurbita pepo; V. Magic Latern) in 2000, sweet corn (Zea mays; V. Silver Queen) in 2001, and bell pepper (Capsicum annuum; V. Aristotle) in 2002. Cereal rye (Secale ceral L.) was planted in all plots in the autumn of each year as a winter cover crop and incorporated by disking prior to applying amendments each spring.

The nitrogen requirements for pumpkins, sweet corn, and bell peppers were estimated as 84, 168, and 140 kg/ha, respectively, based on Virginia Cooperative extension (VCE) vegetable production recommendations (Alexander et al., 2000). Limestone, phosphorus, and potassium requirements were determined by VCE soil test recommendations (Donohue and Heckendorn, 1994). Ammonium nitrate (33-0-0), triple superphosphate (0-46-0), and muriate of potash (0-0-60) were used to apply nitrogen, phosphorus, and potassium, respectively.

The original treatments were applied annually in mid-April and incorporated with a rototiller to a depth of 15 cm on the day of application. Crops were planted within a week of the application of the amendments. Vegetables were mulched with barley (Hordeum vulgare L.) straw in all years to control weeds.

Phase II of Study: 2002-2005

The treatments implemented in 2003 and 2004 included the continuation of three of the original treatments from years 2000-2002: annual commercial inorganic fertilizer applied according to soil test laboratory recommendations, annual poultry litter applied at calculated agronomic nitrogen rates, and the unamended control. The original annual low compost rate and biannual agronomic N compost rate treatments, with and without supplemental nitrogen fertilizer, were modified as such (also see Tables 3.1 and 3.2): Low rate yard waste-poultry litter compost treatments with and without supplemental nitrogen fertilizer selected randomly from two of the four replications remained at low yard waste-poultry litter compost application rates (30% agronomic N) in both 2003 and 2004. The remaining two replications from the original low yard waste-poultry litter compost application rate with and without supplemental nitrogen fertilizer were converted to low rate treatments of biosolids compost. Two of the four replications (randomly selected) of the biannual agronomic nitrogen rate of yard waste-poultry litter compost with and without supplemental nitrogen fertilizer were converted to annual agronomic N application rates of yard waste-poultry litter compost in 2003 and 2004. The remaining replications of these treatments were converted to agronomic N rates of biosolids compost in 2003 and 2004. These modifications enabled the new treatments to remain at the same relative application rates as in 2000 to 2002.

The new treatments were:
Inorganic fertilizer (FERT): per Virginia Tech Soil Test Lab recommendations
Agronomic yard waste compost (AYWC): 100% agronomic N compost rate
Low yard waste compost (LYWC): 30% agronomic N compost rate
Agronomic biosolids compost (ABSC): 100% agronomic N compost rate
Low biosolids compost (LBSC): 30% agronomic N compost rate
Poultry litter (PL): 100% agronomic N rate
Control (CTRL): no amendments or fertilizer added

Panorama Pay Dirt Compost (PYWC; Earlysville, VA) comprised of 1 part poultry litter to 2 parts yard waste (leaves only) was composted for 120 days using windrow technology. A biosolids compost obtained from the Rivanna Water and Sewer Authority (RBSC; Charlottesville, VA) was comprised of an anaerobically-digested biosolids dewatered with calcium hydroxide and composted with woodchips (1:2 ratio of biosolids and woodchips) for five consecutive days at 66°C via static pile technology. The material was cured for an additional 10 days after screening the compost through a 0.95 cm sieve to remove woodchips. The poultry litter used in the study was a commercial non-composted litter (PL) from Valley Pride (Harrisonburg, Virginia). All five organic amendments were analyzed at A&L Eastern Agricultural Laboratories, Inc. (Richmond, Virginia) for routine analysis (Table 3.3).

The crops included Zea mays (Pioneer 31G20) in 2003 and 2004 and Glycine max (Delta Pine 4933RR) in 2005. This report includes the data from the corn in 2004 and the soybean in 2005. Each winter, cereal rye (Secale cereal L.) was planted and incorporated by disking prior to applying amendments each spring. No organic amendments or inorganic fertilizer was applied in spring 2005 because the soybean crop required no nitrogen fertilizer.

Plant available N in the composts and poultry litter were estimated by adding 100% of the measured NO3-N and NH4-N and the fraction of organic N estimated to be mineralizable during the first season. The mineralization coefficients used were 0.1 for the composts and 0.6 for the poultry litter (Table 3.3) (Evanylo, 1994; DCR, 2002). The lime requirement was determined by the Adams-Evans single buffer method (Sims, 1996). Treatments were applied on June 11, 2004 and were immediately incorporated to a 15 cm depth with a rototiller. Total rates of organic by-products and accompanying carbon are listed in Table 3.4. Total macro- and micronutrients added are listed in Table 3.5. Corn (Pioneer 31G20) was planted June 11, 2004 and thinned to approximately 52,000 plants/ha three weeks after emergence. Lumax (2-chloro-4-ethylamni-6-isopropylamino-s-triazine) was applied at planting for weed control. Lorsban (O, O-diethyl-O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate) was applied in the row. Bradyrhizobium inoculated soybean was planted May 31, 2005.

Soil Sampling and Monitoring

Ten soil samples were collected at a depth of 0-15 cm on June 4, 2004 and August 26, 2005 for fulvic and humic acid extractions. Three samples for bulk density determination were collected from the CTRL, LYWC, and AYWC treatment plots to a depth of 15 cm on June 30, 2004 (Grossman and Reinsch, 2002). Three soil cores were collected in 5-cm increments to a depth of 15cm on June 23, 2005 for bulk density (Grossman and Reinsch, 2002) and plant available water determination (Dane and Hopmans, 2002). In 2004, soil moisture readings were taken every 7 to 9 days using a hand-held TDR (Hydrosense; Campbell Scientific, Edmonton, Alberta) beginning July 14 (seven days after tasseling). In 2005, soil moisture readings were taken every 7 days using the Trace System I TDR (Soil Moisture Corporation, Santa Barbara, CA) beginning August 12 (early podfill). Ten soil samples collected on September 20, 2004 and August 26, 2005 were used to determine C and N content (VarioMax CNS macro-elemental analyzer; Nelson and Sommers, 1996; Bremmer, 1996) and for routine soil test analyses by the Virginia Tech Soil Testing Lab (pH, Mehlich 1-P, K, Ca, Mg, Cu, Fe, Zn; Donohue and Friedericks, 1984).

Humic Substances Extraction

Humic and fulvic acid contents of the organic amendments and the amended soils were determined using a modified International Humic Substances Society Method (Zhang, 2004). One gram of organic amendment or 20 g of air dried soil were passed through a 2 mm sieve was mixed with 200 ml of 0.1 N NaOH in a 250 ml centrifuge bottle, flushed with N2 to prevent oxidation of organic matter, and shaken for 16 hours. The suspension was allowed to settle for 12 hours and then centrifuged at 6,000 g for 20 minutes. The supernatant was acidified to pH 1 with 6 ml of 6 N HCl, allowed to settle for 12 hours and then centrifuged at 6,000 g for 20 minutes. The supernatant, representative of the fulvic acid fraction, was decanted into a 50-ml disposable plastic vial and stored at 4°C. The organic carbon content was determined with a carbon auto analyzer by the analytical services laboratory of North Carolina State University (Nelson and Sommers, 1996). The humic acid was redissolved in 50 ml of 0.1 N KOH and then mixed with 0.745 g KCl to give a potassium concentration of 0.3 N. Oxygen was removed by flushing with N2. The suspension was centrifuged at 6,000 g for 20 minutes. The supernatant was acidified to pH 1 with 2.01 ml of 6 N HCl and allowed to settle for 12 hours. After centrifugation at 6,000 g for 20 minutes, the supernatant was decanted and the precipitate was oven-dried in an aluminum tin at 55°C for 24 hours. Organic carbon analysis was determined using a CNS macro-elemental analyzer by the analytical services laboratory of North Carolina State University (Nelson and Sommers, 1996).

In-field Plant Measurements

Five corn ear leaves were sampled on July 14, 2004 (silking), dried at 65°C, ground in a Wiley Mill to pass a 0.85 mm sieve for TKN (EPA 351.2) and TKP (EPA 365.4) analyses (USEPA, 1979). Ten soybean leaflets from the second oldest, fifth node leaflet were sampled August 18, 2005 for N (VarioMax CNS macro-elemental analyzer; Bremmer, 1996) and P (Inductively Coupled Plasma Spectroscopy; Soltanpour et al., 1996) contents. Corn leaf sampling and measurements for physiological variables were performed every 7 to 9 days, beginning seven days after silking and culminating at the R5 stage (56 days after silking). Soybean leaf sampling was performed every 7 days, beginning at podfill and culminating at R6 stage (35 days after podfill). Measurements were made on three plants in each treatment to form a composite sample. Delta T measurements were only taken on the 2005 soybean crop. Delta T quantifies the difference between canopy and air temperatures and is a non-destructive indicator of leaf stress. Leaves with a higher Delta T value are warmer than the surrounding air and have less active transpiration. Leaves with a lower Delta T value have surface leaf temperatures closer to that of the ambient air and are relatively more stressed than the warmer leaves. Chlorophyll content and PSII photochemical efficiency measurements were determined from a corn ear leaf blade or soybean second oldest, fifth node leaflet using a chlorophyll meter (Minolta SPAD 502 Meter; Spectrum Technologies, Inc., Plainfield, Illinois) and a dual wavelength fluorometer (OS-50; Opti-Sciences, Inc., Tyngsboro, Massachusetts), respectively. Midday leaf water potentials were measured from the same blade or leaflet using a pressure chamber (PMS-600; PMS Instruments Co., Corvallis, Oregon.). One half of the remaining corn blade and the two remaining soybean leaflets were freeze-dried and used for antioxidant and MDA analysis using standard procedures as described in Zhang et al., (2003).

Corn ears were hand harvested at black layer formation (September 14, 2004) from one 3m section of the center rows. Yields were adjusted to a 155 g/kg moisture content basis. Soybean was harvested from the two center rows at full maturity using a combine on October 20, 2005. Yield was adjusted to 130 g/kg moisture content basis. The remaining corn and soybean plants were removed from the plots both years.

Leaf Laboratory Analyses

Chlorophyll content: Quantification of chlorophyll content was determined using the dimethyl sulphoxide (DMSO) chlorophyll extraction method of Hiscox & Isrealstam (1979) with minor modifications. Glass centrifuge bottles containing 7 ml of DMSO were preheated to 65°C in a water bath. Five discs (total area, 1.41 cm2) punched from the previously frozen (-80°C) composite sample were incubated in vials for 50 minutes and were subsequently topped to 10 ml with additional DMSO. Two milliliters of the extract were transferred to a disposable polyestrene cuvette having a transmittance of 340-800 nanometers (VWR, 5801 7-847; West Chester, Pennsylvania). A spectrophotometer (Biomate 3 series; Rochester, New York) calibrated at 645 and 663 nm using pure DMSO was used to determine total chlorophyll concentration using Arnon’s (1949) equation: 0.0202A645 + 0.00802A663 = mg chlorophyll/ml solution. The chlorophyll concentration was then converted to leaf chlorophyll content by the equation: (mg chlorophyll/ml solution)*(10ml solution/1.41 cm2 total leaf area) = mg chlorophyll/cm2, and a regression equation between SPAD readings and leaf chlorophyll content was established.

Crude protein extraction: Leaf tissue (0.25 g fresh weight) previously stored at -80°C was macerated with mortar and pestle in liquid nitrogen, and mixed with 3 ml of 0.05 M sodium phosphate buffer, pH 7, containing 1% polyvinylpyrrolidone and 0.2 mM EDTA. Two milliliters of the homogenate were centrifuged at 4°C for 20 minutes at 15,000 g. The supernatant was stored at 4°C until further analysis.

Total leaf protein assay: Leaf protein analysis was conducted using the crude protein extract and the method of Bradford (1976) with bovine serum albumin as a standard.

Superoxide dismutase (SOD) activity: In 2004, 1.38 ml of 50 mM sodium phosphate buffer, pH 7.8, 15 µl of 10mM EDTA, 60 µl of 0.325 M methionine, 15 µl of 6.3 mM NBT, 5 µl of the crude protein extract, were mixed in a disposable polyestrene cuvette. Fifteen microliters of 130 µM riboflavin was added, and the cuvettes were secured on a rotating cutoff under one circular fluorescent bulb (irradiance = 60 mol•m-2•s-1) for 30 minutes at 25°C to initiate reaction. The absorbance was read 560 nm, with one non-irradiated mixture serving as a blank (Zhang, 2003).
One unit of activity =
{1000µl*leaf protein (mg)}/{amount of extract*[(Abs(sample) -1)/ Abs(light blank)]}
In 2005, SOD activity was determined using a microplate reader (Ospys MR; Thermo Labsystems, Chantilly, VA). The reaction solution included 50 mM Pipes buffer, pH 7.5, 0.4 mM o-dianisidine, 0.5 mM DTPA, and 26 μM riboflavin. In each well, 20 μl of enzyme extract was added to125 μl of reaction solution. The absorbance was read at 25°C after a 30 minute reaction period under one circular fluorescent bulb (irradiance = 60 mol•m-2•s-1). Superoxide dismutase activity was determined from a standard curve and expressed as unit of activity/mg protein (Zhang, 2005).

Ascorbate peroxidase (APX) activity: The reaction mixture contained 1.405 ml of 50 mM of phosphate buffer, pH 7, 15 µl of 10 mM EDTA, 15 µl of 50mM ascorbic acid, and 50 µl of enzyme extract in a polyestrene cuvette. Fifteen microliters of 10 mM H2O2 was added just prior to measuring the absorbance at 290 nm at 0 and 1 minute for corn and 0 and two minutes for soybean (Zhang, 2003).
One unit of activity = change in absorbance/(0.01)/minute/ mg total leaf protein

Catalase (CAT) activity: The reaction mixture contained 1.424 ml of 50 mM phosphate buffer, pH 7, and 26 µl of 3% H2O2 in a polyestrene cuvette. Fifty microliters of crude protein extract was added just prior to measuring the absorbance at 240 nm at 0 and two minutes (Zhang, 2003).
1 unit activity = change in absorbance/(0.1)/minute/mg total leaf protein.

Malondialdehyde (MDA) concentration: Lipid peroxidation of chlorplasts was determined using the method of Jiang and Zhang (2001). The concentration of MDA, a product of lipid peroxidation, was determined using the molar extinction coefficient of 155 mM-1cm-1.
Seed biochemical measurements

Seed quality (i.e., protein, oil, starch, density, fiber (soybean only)), and soybean 100 g seed weight were determined by the Iowa State Grain Quality Laboratory using nuclrea resonance spectroscopy techniques.

Data Analysis

Analysis of variance and mean separation data were performed using the complete randomized block design PROC GLM procedure for mid-season leaf N & P contents, yield, and seed quality parameters (SAS Institute, 2002). Analysis of variance and mean separation data were performed using the PROC MIXED repeated measures procedure for end of season soil parameters and leaf physiological measurements. The least significant difference procedure (LSD) with a probability level of 0.05 was used to determine significant differences between treatment means.

Research results and discussion:

Climatological parameters at the Northern Piedmont Agriculture and Research Extension Center

In 2004, monthly and total precipitation was greater than the 30 year average (Table 3.6). Leaf sampling began July 14, 2004 and ended August 27, 2004. The corn did not exhibit signs of water stress during the sampling season. The exceptionally high precipitation in July 2004 was due to the regular occurrence of afternoon thundershowers and Hurricane Isabel.

In July 2005, monthly precipitation was much greater than the 30 year average (Table 3.6). Leaf sampling began August 12, 2005 and ended September 16, 2005. Despite the relatively low precipitation values during these months, they were no biological indicators of water stress. It is possible the deep rooting system of the soybean provided sufficient water for the developing crop.

Monthly air temperatures in 2004 and 2005 were similar to the 30 year average (Table 3.6). Air temperatures on the 2004 and 2005 sampling dates were also near average values (Table 3.7). On August 26, 2005, the air temperature was relatively low at 20.51°C and the sky was overcast.

Soil Fertility

Nitrogen (N)

All treatments were applied on an N basis. End of season soil data indicate that the AYWC treatment had the greatest soil N content followed by the ABSC>LYWC>LBSC=PL= FERT=CTRL treatments (Table 3.8). Both Panorama yard waste compost treatments (i.e., AYWC and LYWC) had greater soil N values than the ABSC, PL, and LBSC treatments, respectively. The percent N fertilizer value of Rivanna biosolids compost (30%) and poultry litter (49%) was greater than the percent nitrogen fertilizer value of Panorama yard waste compost (10%) as ascertained in a related greenhouse N mineralization study. The greater N content in the soils amended with the Panorama yard waste compost were likely due to the higher application rates of the yard waste compost than the biosolids compost and the poultry litter and a higher than estimated nitrogen fertilizer value of the material.

Phosphorus (P)

All organically amended treatments applied at the agronomic nitrogen rate (i.e. AYWC, ABSC, PL) contained greater soil P contents than the FERT treatment (Table 3.8). The 2005 soil P concentrations increased in all treatments following the soybean crop (Table 3.9). The ratio of calculated plant available N to total P of the organically amendments were between 0.38 and 2.10 (Table 3.3). Gilbertson et al. (1979) have calculated a mean N:P uptake ratio of 5.9 for corn. Thus, the N-based application rates of the organic amendments resulted in P applications that exceeded crop needs.

Several researchers have observed elevated soil P concentrations when organic amendments were applied on an N basis (Eghball and Gilley, 1999; Sharply and Moyer, 2000; Eghball, 2002). The Virginia Tech Soil Testing Lab reported that soil P concentrations for the organically amended treatments were sufficient for plant growth and that applications of additional P fertilizer would not improve future crop yields. High soil P concentrations are not detrimental to plant growth, but may cause significant environmental damage if transported from the field site via erosion. An extensive study on the potential movement of P at this site was conducted by Spargo (2004).

Potassium (K)

Soil K concentrations were greatest in the AYWC treatment (Table 3.8). The PL and LYWC treatments had greater soil K concentrations than the ABSC and LBSC treatments. The relatively large soil K content of the PL treatment despite its low application rate (Table 3.4) was due to the exceptionally high K levels of the poultry litter amendment (Table 3.3). It is common practice to add mineral supplements to animal feed in confined animal farming operations (Blezinger, 2001; Kegley, 2001). The amount of nutrient supplied often exceeds the assimilatory capacity of the animal, and excess minerals are excreted through waste. The low soil K contents of the ABSC and LBSC treatments were due to the relatively low K concentration of the amendment (Table 3.3). The LBSC, CTRL, and FERT treatments had the lowest soil K contents. The lower K concentration in the FERT compared to the CTRL treatment may be due to increased root exploration and nutrient uptake in the adequately fertilized soil. Soil K increased in 2005 (Table 3.9).

Calcium (Ca) and Magnesium (Mg)

Lime was added differentially to treatment plots in spring 2004 to limit the confounding effect of soil pH on P availability. The ABSC treatment, nevertheless, had the highest pH and Ca contents in (Table 3.8). The biosolids were dewatered using Ca(OH)2, which increased CCE (Table 3.3). All other treatments had similar pH values. The Ca contents of the organically amended treatments were greater than the FERT and CTRL treatments likely due to its complexation within the organic matter matrix.

All organically amended treatments had greater soil Mg contents than the FERT treatment in 2004 (Table 3.8). The AYWC treatment had the greatest soil Mg concentrations. The poultry litter amendment contained the highest Mg concentration (5.80 mg/kg, Table 3.3), which resulted in greater soil Mg contents than the ABSC treatment despite the relatively low application rate (Table 3.4). The FERT treatment had a slightly lower Mg concentration than the CTRL. This may be due to increased root growth and nutrient uptake of crops planted in the adequately fertilized soil.
There were no changes in soil magnesium contents from 2004 to 2005 (Table 3.9).

Micronutrients (Cu, Fe, Mn, Zn)

End of season soil copper contents were greatest in the PL treatment followed by the CTRL and FERT treatments (Table 3.8). The relatively high copper content of the poultry litter amendment (Table 3.3) is due to its use as a supplement and animal feed (Blezinger, 2001; Kegley, 2001). The LYWC and LBSC treatments added more copper to soils than the PL treatment (Table 3.5), yet extractable copper was lower. The AYWC and ABSC contributed the greatest amount of copper to the soil (Table 3.5), yet extractable concentrations were the lowest among all treatments (Table 3.8). A similar phenomenon occurred with soil iron content where increases in organic carbon decreased the amount of extractable iron (Tables 3.4, 3.5, 3.8).

This inverse relationship between the total amount of nutrient applied and the quantity of the nutrient extracted its opposite of what was observed with zinc and manganese concentrations where extractability increased with increasing application rates (Tables 3.5, 3.8). Copper and iron form relatively stronger complexes with organic ligands (Havlin et al., 1999), and are tightly bound to the organic material due to the humification process of composting (Epstein, 1997). This greatly decreases the extractability of these minerals. Although the PL amendment contributes organic matter to the soil, it is in a non-composted form. The addition of copper with this material is therefore highly extractable.

Soil Organic Carbon (C)

Soil organic C contents were similar in spring 2000 prior to the implementation of this study (Table 3.10). The application rates of all organic amendments were on an N basis, and there were large differences in the total amount of carbon applied in each treatment (Table 3.4). The AYWC treatment contained the greatest organic carbon content among the organically amended treatments, while PL had the lowest and was similar to the CTRL and FERT treatments (Table 3.10). Entry et al. (1997) also observed that a poultry litter amended Typic Hapludult in Alabama contained a soil carbon content significantly lower than soils amendments of the organic amendments and was similar to the fertilized control. Paul and Beauchamp (1989) observed that CO2 evolution in a poultry litter amended silt loam was over 10 times greater than in the same soil amended with several different composts during a seven-day incubation period. In addition to supplying low rates of C, the poultry litter had not gone through as extensive a humification process as had the composted residuals; thus, the poultry litter C was readily available for microbial use and did not improve soil organic matter content. The organic carbon contents of the ABSC, LYWC, and LBSC were greater than the FERT and CTRL treatments.

Fulvic and Humic Acid

Organic Amendments

Rivanna biosolids compost had the lowest NaOH extractable fulvic acid-carbon content (Table 3.11). Panorama yard waste compost fulvic acid-carbon content was greater and poultry litter had the greatest fulvic acid-carbon content. According to the tenants of the polyphenol theory, fulvic acid is relatively less humified than humic acid (Stevenson, 1994). The poultry litter was the only non-composted amendment. The greater quantity of extractable fulvic acid-carbon from this residual may indicate that much of the carbon was weakly humified.

The relative rankings of extractable humic acid-carbon from the amendments are PYWC>>PL>RBSC (Table 3.11). The lignin theory contends that humic acid is comprised of humified lignin molecules. NaOH is often used to extract lignin from wood fibers in paper processing plants. The greater quantity of humic acid-carbon in the Panorama yard waste compost and poultry litter is likely not due to the sole extraction of humified carbon, but the extraction of lignified materials from the woody debris of the compost and poultry bedding as well. Although the composting process of Rivanana biosolids uses wood chips as a bulking agent, these materials are sieved from the organic fraction after five days of composting. The result is a lower content of a humified materials in the compost.

Total NaOH extractable carbon was greatest in the poultry litter, due to the very high extractable fulvic acid-carbon content. Panorama yard waste compost had a greater total NaOH extractable carbon content than the Rivanna biosolids compost.
Amended soil

The AYWC treatment contained the greatest fulvic acid-content (Table 3.12). The ABSC and LYWC treatments had moderately lower fulvic acid-carbon contents. The PL and LBSC treatments had the lowest fulvic-acid content among the organically amended treatments. These rankings are likely due to the total amount of amendment applied in 2004 (Table 3.4).

The humic acid-carbon content of the AYWC treatment was greatest among all treatments followed by the LBSC treatment (Tables 3.12). All other treatments had similar humic acid-carbon contents. The extraction efficiency of humic acid-carbon from soil can vary between 30-50% (Chao Shang, personal communication) as humic acid is often covalently bound with cations, especially iron and aluminum (Donisa et al., 2003). The relative ranking of humic acid-carbon: AYWC>>ABSC>LYWC=LBSC=FERT=PL=CTRL. It appears that the humic acid-carbon content follows the ranking of the amendment application rates. The non-composted PL treatment has the lowest humic acid-carbon content among all treatments. The carbon applied via poultry litter is easily mineralized by microbes. Paul and Beauchamp (1989) observed that increases in microbial activity decreases soil organic matter content. The relatively lower humic acid-carbon content in this treatment indicates that pelletized poultry litter does not contribute to the humified carbon content.
The fulvic acid-carbon content is considerably larger than the humic acid-carbon content in nearly all of the treatments (Table 3.12). This was rather surprising as the addition of organic amendments to soils has been documented to increase humic acid-carbon more than of fulvic acid-carbon (Nardi et al., 2004; Zinati et al., 2001; Schnitzer and Kodama, 1992). An increase in humic acid-carbon content has been considered an indicator of soil organic matter humification and stability (Ji-ping et al., 2002). Wei and Xiao (1996) observed greater fulvic acid-carbon contents in soils when the predominant clay mineral was kaolinite, while higher humic acid-carbon contents were observed in soils where montmorillonite was the predominant clay. The large presence of 1:1 clays at this study site may favor the formation of fulvic acid-carbon.

Soil Bulk Density, Soil Water Holding Capacity, and Moisture Potential

In 2004, AYWC had a lower bulk density from 0 to 15 cm than the LWYC and CTRL treatments (Table 3.13). No differences in soil water holding capacity occurred at the 0 to 15 cm depth in the disturbed (ground, sieved, and replacked) soil samples. There were no differences in Ψsoil throughout the sampling season (Table 3.13). Soil moisture potential was not determined on Day 7.

In 2005, AYWC had the lowest bulk density (Table 3.14). The differences in bulk density decreased with depth among the treatments. Soils were sampled at 0-5 cm increments to a depth of 15 cm to preserve soil structure. The AYWC treatment had a greater soil water holding capacity than the CTRL at the 0-5 cm depth (Table 3.14). No differences in soil water holding capacity occurred at the 5-10 cm depth, but the LYWC treatment retained the most soil water holding capacity at the 10 to 15 cm depth. Cumulative soil water holding capacity in the top 15 cm was greater in the LYWC but was similar to the AYWC treatment. The CTRL treatment had the lowest soil water holding capacity. The Ψsoil was lowest on Day 7 and increased through Day 28. Soil moisture potential was not determined on Day 35. The decreased bulk densities and increased soil water holding capacities of the organically amended treatments relative to the CTRL is likely due to increased aggregation of soil particles by the addition of organic matter (Khaleel et al., 1981; Grandy et al., 2002; Elsharawy et al., 2003).

Corn (2004)

Midseason Leaf Nutrient Content

Leaf N and P concentrations in ear leaves sampled at silking (July 14, 2004) were greatest in the agronomic N treatments (i.e., FERT, AYWC, ABSC, PL) (Table 3.15). The low nitrogen treatments (i.e. LYWC, LBSC) had N and P concentrations lower than the agronomic N treatments but higher than the CTRL.

Leaf Water Potential (Ψleaf)

The Ψleaf determines the pressure inside the leaf that is exerted to extract water from the roots. Leaf water potential measurements were not taken Day 7, 42, 51 due to instrument availability. There were no biologically significant differences in Ψleaf (Table 3.16). The Ψleaf decreased with time throughout the sampling season possibly due to the senescence process of the leaves (Table 3.17).
Total leaf protein

The agronomic N treatments had the greatest total leaf protein contents (Table 3.16). The low N treatments had relatively lower values but were greater than the CTRL. Total protein contents remained relatively constant the first 29 days of sampling (Table 3.17). There was an increase in leaf protein on Day 42; after which, leaf protein contents began to decline as the leaves senesced.
Leaf chlorophyll

Chlorophyll contents were greatest in the agronomic N treatments (Table 3.16). The low nitrogen treatments had greater chlorophyll values than the CTRL treatment. Chlorophyll contents increased on Day 22 and remained stable through Day 35. There was a decrease in leaf chlorophyll contents on Days 42 and 51, likely due to the senescence of the leaves. Prochackova et al. (2004) also observed an increase in maize leaf chlorophyll content during early reproductive growth followed by a decline in chlorophyll contents.

Photochemical Efficiency (Fv/Fm)

Photochemical efficiency of PSII was determined using a flurometer. No Fv/Fm measurements were taken on Day 7 due to instrument malfunction. Most agronomic N treatments had greater Fv/Fm values than the low N treatments (Table 3.16). The PL treatment had a similar Fv/Fm value to the low N treatments. The low N treatments had greater Fv/Fm values than the CTRL treatment (Table 3.16). Khamis et al. (1990) observed greater Fv/Fm values in N replete than N deficient corn seedlings. Fv/Fm readings increased with time and were highest at Day 29 (Table 3.17). Treatment differences in Fv/Fm, although significant, are less than expected, given the relatively large variation of leaf chlorophyll contents (Table 3.16). Fv/Fm readings were taken after the ear leaves were covered with an aluminum foil sleeve for at least 15 minutes. The sleeves had to be partially lifted during measurement. It is likely that the differences in Fv/Fm are less definitive due to stray light that reached the fluorometer sensor.
Superoxide Dismutase (SOD)

Superoxide dismutase is the first response antioxidant reducing newly formed superoxide to peroxide. It is adjacent to PS I and has a very high sensitivity for its substrate (Asada, 1994). Superoxide dismutase is generally considered the most important antioxidant in the chloroplast (Perl-Treves and Perl, 2002). SOD activity was lowest in the agronomic nitrogen treatments (Table 3.16). The low nitrogen and CTRL treatments had relatively higher SOD activities. Tewari et al. (2004) observed greater SOD activity in nitrogen starved maize. The lower leaf nitrogen and chlorophyll contents of the low nitrogen and CTRL treatments likely created an environment that generated more reactive oxygen species. At Day 22 (R3-milk stage), there was an increase in SOD activity (Table 3.17). Kernels at this stage of development are undergoing rapid cellular expansion due to the accumulation of starch. The increase in SOD activity was simultaneous to an increase in chlorophyll content (Table 3.16). Reactive oxygen species are naturally generated due to the intrinsic inefficiencies of PSI. It appears, the greater photosynthetic activity not only increased the amount of carbohydrate available to transport from the developing leaves, but also increased superoxide formation. Prochazkova et al. (2001) also observed increased SOD activity in maize leaves up to 25 days after tasseling. Superoxide dismutase activity decreased from Day 29 through 51, likely contributing to leaf senescence.

Ascorbate Peroxidase (APX)

The CTRL treatment had the greatest APX activity and was similar to the LYWC and LBSC treatments (Table 3.16). All agronomic N treatments had lower APX activities. Ascorbate peroxidase activity is regulated by the formation of peroxide from SOD activity. The N sufficient treatments had greater photosynthetic efficiencies and generated less ROS than the low N and CTRL treatments. Ascorbate peroxidase activity increased from Day 7 to Day 29 (Table 3.17). Prochazkova et al. (2001) also observed an increase in maize leaf APX activity during early reproductive growth. Increased APX activity during this time is likely due to the increased formation of peroxide from elevated SOD activity. APX activity decreased on Day 35, and then increased to values similar to Day 29 on Days 42 and 51 (Table 3.17). The increase in APX activity during this period occurred after SOD activity declined (Table 3.17). The much greater SOD activity relative to APX activity indicates that APX can never reduce all peroxide produced within the chloroplast, but maintain steady-state reactive oxygen species levels instead. It appears the down regulation of APX activity is related to intercellular peroxide concentrations and not absolute SOD activity. Increased APX activity during the latter part of the season contrasts with the results of Prochazkova et al. (2001) who observed decreased APX activity with age. Pastori and Trippi (1993) observed no change in maize leaf APX activity during senescence. Leaf APX activity varies among cultivars and is related to general stress tolerance. The greater activity observed in the cultivar used in this study may indicate that it is relatively more resistant to oxidative stress than the cultivars used in the other studies. The increased APX activity could not delay oxidation indefinitely as lipid peroxidation increased over time (Table 3.17).

Catalase (CAT)

There were no biologically significant differences in CAT activity (Table 3.16). Catalase is located in the peroxisome in large quantities and its activity is stimulated only by millimolar concentrations of peroxide (Asada, 1994). It doesn’t require reducing equivalents like APX to reduce peroxide. Subsequently, it may be relatively less sensitive to the redox status of the cell and its activity may not be affected by stress (Arora et al., 2002; Mittler, 2002). Alber and Scandalios (1993) observed that a maize mutant deficient in two CAT isozymes had adequate growth under atmospheric conditions and resembled the wild type in phenotype. Catalase activity decreased with time likely contributing to the senescence of leaves.
Malondialdehyde (MDA)

MDA concentration quantifies the lipid peroxidation of chloroplast membranes (Heath and Packer, 1967). The CTRL treatment had the greatest MDA content (Table 3.16). The two low N treatments had greater MDA content than the agronomic N treatments. The relative rankings of MDA content were consistent with the other parameters observed to this study. The chlorophyll and Fv/Fm measurements were lowest in the CTRL and low nitrogen treatments, and the antioxidant activities of these treatments were greater than in the agronomic N treatments.
At Day 7, MDA contents were relatively high compared to later sampling dates (Table 3.17). Dhindsa et al. (1981) observed that younger tobacco leaves had greater MDA contents than fully expanded leaves. They concluded that increased lipid peroxidation of younger leaves may have a role in the mechanism involving premature leaf abscission. MDA was not determined on leaves collected Day 22 as these were destroyed. MDA contents increased in all treatments at Day 42. Leaf senescence is associated with increased lipid peroxidation, and decreased antioxidant activities, total leaf protein, and chlorophyll contents (Table 3.17) as the plant comes closer to maturity (Dhindsa et al., 1981; Pastori and Trippi, 1993 Hung and Kao, 1996; Yan et al., 1996; Jiang and Huang, 2001; Prochazkova et al., 2001). The greater MDA contents of the CTRL and low nitrogen treatments are likely due to the cumulative effects of elevated reactive oxygen species throughout the sampling and not simply general senescence.

Yield and Seed Composition

The corn was harvested September 14, 2004, 126 days after planting. Yields closely followed the midseason leaf nitrogen contents with the agronomic nitrogen treatments having yields at 10.70 Mg/ha or greater (Table 3.18). The low nitrogen treatments had yields between 7.90-8.95 Mg/ha. The CTRL treatment had the lowest yield at 5.40 Mg/ha. The exceptionally high yields of the agronomic nitrogen treatments and the acceptable yields of the low nitrogen treatments demonstrated the favorable weather conditions of the summer 2004 growing season.

The ABSC treatment had the greatest kernel protein content followed by the FERT and AYWC treatments (Table 3.18). These treatments had greater protein contents than the PL treatment. The LYWC and LBSC treatments had greater protein contents than the CTRL. Kernel oil contents were greatest in the ABSC, LYWC, FERT, and AYWC treatments (Table 3.18). The authors have no explanation why the LYWC had such a high oil content. The PL, CTRL, and LBSC treatments had relatively lower oil contents, and the LBSC, CTRL, and LYWC treatments had the greatest kernel starch components (Table 3.18). The AYWC, ABSC, FERT, LYWC and PL treatments had the greatest kernel densities, and the CTRL treatment had the lowest kernel density (Table 3.18).

Differences in leaf health, corn yields, and seed composition were strongly associated with midseason leaf nitrogen contents and not humified carbon content (Table 3.19). The agronomic nitrogen treatments had greater chlorophyll, total leaf protein contents, Fv/Fm values, lower SOD activities and MDA contents, and outperformed the low nitrogen and CTRL treatments in yield, seed protein and oil contents, and density.

Soybean (2005)

Midseason Leaf Nutrient Content

There were no differences in midseason (August 18, 2005) leaf N and P concentrations among the treatments (Table 3.20).

Leaf Water Potential (Ψleaf)

There were no biologically significant differences in Ψleaf among treatments (Table 3.21). The relatively low Ψleaf measurements at Day 7 were due to overcast skies during sampling (August 26, 2005) (Table 3.22).

Total Leaf Protein

There were no differences in total leaf protein contents among treatments (Table 3.21). There was an increase in leaf protein content at Day 7. Total leaf protein declined from Day 14 to Day 35 (Table 3.22).

Leaf Chlorophyll

There were no differences in leaf chlorophyll contents among the treatments (Table 3.21). Chlorophyll contents increased in all treatments on Day 7 to Day 15 and remained constant until Day 28 (Table 3.22). Chlorophyll contents decreased on Day 35.

Photochemical Efficiency (Fv/Fm)

Photochemical efficiency measurements were only taken twice due to instrument problems. There were no treatment differences in Fv/Fm (Table 3.21) and Fv/Fm did not change over time (Table 3.22).

Delta T

Delta T measures the difference between canopy and air temperatures and is a non-destructive indicator of leaf stress. Delta T values were positive at all sampling dates except Day 21 when the air temperature was relatively warm (27.47°C) (Tables 3.7, 3.22). The same air temperature occurred at Day 0 as at Day 21. It appears that the metabolic efficiencies of the relatively younger leaves at Day 0 were less able to efficiently transplanting and decreased canopy temperatures at the higher air temperatures. Malondiadehyde concentrations were also higher on this date (Table 3.22). The low Delta T values at Day 7 were due to overcast skies at the time of sampling (August 26, 2005). Delta T increases the time indicating that as leaves senesce, transpiration decreases and the canopy temperatures became increasingly higher than ambient air temperature. The CTRL treatment had the lowest Delta T values and was similar to most other treatments except the ABSC, AYWC, and LYWC treatments which had higher Delta T values. The warmer canopies of these treatments may indicate that these leaves were more senescent than the FERT and CTRL treatments.

Superoxide Dismutase (SOD)

There were no treatment differences in SOD activity (Table 3.21). The lowest SOD activities occurred at Day 7 when the skies were overcast during sampling (August 26, 2005). The low SOD activities indicates that relatively few reactive oxygen species were generated on this date. Superoxide dismutase activity increased from Day 14 to Day 28 indicating an increase in the formation of active oxygen species (Table 3.22). Superoxide dismutase activity declined in all treatments had Day 35, likely contributnig to the senescence of leaves.
Ascorbate Peroxidase (APX)

There were no treatment differences in APX activity (Table 3.21). Ascorbate peroxidase activity increase on Day 7 (Table 3.22). Perhaps indicating that APX activity may be stimulated by the formation of peroxide from increased glycolate oxidation within the peroxisome (see following section). Ascorbate peroxidase activity also increased on Day 35. It is likely that the down regulation of APX activity is not directly related to SOD activity, but intercellular peroxide concentrations.

Catalase (CAT)

There were no treatment differences in CAT activity (Table 3.21). Tolbert et al. (1969) observed that the oxidation of glycolate, a product of photosynthesis, accounts for most of the peroxide scavenged by CAT in C3 species. The greater CAT activity on Day 7 was likely due to the effect of overcast skies on photosynthesis during sampling (Table 3.22). The diffused light conditions increased the photosynthate to reactive oxygen species ratio and increased the glycolate content within the peroxisome. The rate of photosynthesis decreases during rapid seed fill (Day 14 to Day 28) as photosynthate is primarily used for seed development and not root or nodule growth (Burton, 1997). This may explain the lower CAT activity observed during these sampling dates. Catalase activity was lowest at Day 35 as the leaves were senescent (Table 3.22).

Malondialdehyde (MDA)

There were no treatment differences in MDA content (Table 3.21). At Day 0, MDA contents were relatively high compared to later sampling dates. Dhindsa et al. (1981) observed that younger tobacco leaves had greater MDA contents than fully expanded leaves. They concluded that increased lipid peroxidation in younger leaves may have a role in the mechanism involving premature leaf abscission. Malondialdehyde content decreased at Day 7 and 14 and then increased on Days 21 through 35 (Table 3.22). Increased MDA contents at Day 35 contrast the decreasing SOD and CAT activities on that date (Tables 3.22). Superoxide dismutase and CAT are suspected to control the oxidative process of lipid peroxidation (Pastori and Trippi, 1993; Prochazkova, 2001; Mittler, 2002). Several researchers have observed the inverse relationship between antioxidant activities and lipid peroxidation (Dhindsa et al., 1981; Xu and Zou, 1993; Jiang and Huang, 2001).

Yield and seed composition

The ABSC and AYWC, followed by PL, LYWC, and LBSC, had the greatest yields (Table 3.23). The CTRL and FERT treatments had the lowest yields. Seed protein contents occurred in the same order as yield (Table 3.23).

Seed oil, fiber, and carbohydrate contents occurred in reverse order from yield and seed protein, with the FERT and CTRL treatments, followed by PL, LBSC, and LYWC, having the largest quantities. The ABSC and AYWC treatments had the lowest non-protein contents (Table 3.23). The weight of 100 seeds was greatest in the ABSC and AYWC treatments (Table 3.23). The LYWC, LBSC, and PL 100 gram seed weight was relatively lower but greater than the FERT and CTRL treatments.

The differences in yield and seed composition were not substantial. It is possible, however, to speculate that the greater protein content and heavier seeds in the organically-amended than the FERT and CTRL treatments may have been due to relatively faster maturation rates of the developing seeds. The lower chlorophyll contents SOD and CAT activities, and higher Delta T values (Table 3.21) of these treatments indicate that these treatments were more senescent than the CTRL and FERT treatments. It is possible the residual soil N in these treatments may have indirectly hastened maturity through stimulating root growth, and eventually enhancing nodulation numbers prior to nitrogen fixation (Al-Kahal et al., 2001). The slightly greater midseason leaf N contents (Table 3.27) may have increased the amount of N remobilized from the leaf to the seed and caused podfill to be completed faster in these treatments.

Comparison of Corn and Soybean Antioxidant Activities

On a protein basis, corn had relatively higher antioxidant activities than soybean (Tables 3.16, 3.21). Stepien and Klobus (2005) also observed greater antioxidant activities in corn as compared against wheat. The greater leaf protein content of soybean, however, caused total antioxidant activities to be higher than in corn. This is likely because the C3 metabolism of soybean is less efficient than corn at prevention the formation of reactive oxygen species. The function of CAT was very different between the two crops. In corn, CAT appears insensitive to the intercellular redox concentration. This may be due to the evolutionary characteristics of C4 metabolism that reduces reactive oxygen species formation. In soybean, CAT activity appears to be driven by the oxidation of glycolate and not the formation of reactive oxygen species within the chloroplasts. The greater MDA contents of soybean indicate that the corn antioxidant scavenging mechanism was more efficient at quenching free radicals. A similar result was obtained by Stepien and Klobus (2005) in comparing corn MDA contents against wheat.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

Publications

Bowden, C.L. 2006. Effects of Organic Soil Amendments on Soil Physiochemical and Crop Physiological Properties of Field Grown Corn (Zea mays L.) and Soybean (Glycine Max L.). M.S. Thesis, Virginia Tech, Blacksburg, VA.

Bowden, C. and G. Evanylo .2004. Effect of Organic Amendments on Soil Humic Substances Content and Physiological Properties of Water-Stressed Zea mays and Glycine max. ASA, CSSA, SSSA Annual Meeting, Seattle, WA. Oct. 31-Nov. 5.

Spargo, J., G.K. Evanylo, and C.L. Bowden. 2004. P availability from composted biosolids and poultry litter. ASA, CSSA, SSSA Annual Meeting, Seattle, WA. Oct. 31-Nov. 5.

Bowden, C.L., G.K. Evanylo, and B. Sukkariyah. 2005. Effects of compost on soil humic substances and crop physiological variables. In Mid Atlantic Composting and Compost Use Conference. Beltsville, MD. Sep. 21-23.

Bowden, C.L., G.K. Evanylo, and J.T. Spargo. 2005. Long term compost applications on quality attributes of a Piedmont soil. ASA, CSSA, SSSA Annual Meetings. Salt Lake City, UT. Nov. 6-10.

Outreach

High school students of southwestern Virginia participating in a summer enrichment program learned about the value of soil organic matter and received hands-on training in the measuring of leaf physiological properties of the soybean crop at the Northern Piedmont Agricutlural Research and Education Center research site in Orange, VA.

High school participants in the Virginia Governor’s school were presented a poster on this research and developed hypotheses concerning the beneficial impacts of organic matter on soil physical properties.

Project Outcomes

Project outcomes:

The application of organic amendments improved soil fertility and increased total organic and humified carbon contents relative to the inorganically fertilized and control treatments.

Bulk density was lower in the treatments that received composts, but the differences diminished with depth in 2005. Differences in soil moisture holding capacity were observed in 2005 only. The LYWC and AYWC treatments had slightly greater soil moisture holding capacities than the CTRL, likely due to the increased soil content of carbon.

Improvements in soil fertility, total carbon, and humified carbon were carried over into 2005, when no amendments were applied. The value of amending fine-textured soils with compost seems to be more important in improving soil fertility than moisture retention properties (viz. water holding capacity).

Neither the corn nor soybean experienced water stress during sampling season as measured by our methods. Midseason leaf N concentrations and not soil humified carbon were highly correlated with improvements in corn yield and seed composition. There were minimal differences in soybean yield and seed composition as all treatments were inoculated with Rhizobium. Treatment differences in corn antioxidant activity were based on plant available nitrogen and were observed throughout the sampling season. Soybean antioxidant activities changed with time but not in response to treatments. On a protein basis, corn had a relatively higher antioxidant activity than soybean. The greater leaf protein content of soybean, however, caused total antioxidant activities to be higher than in corn. The function of CAT was very different between the two crops. In corn, CAT appears insensitive to the intercellular redox concentration. This may be due to the evolutionary characteristics of C4 metabolism that reduces reactive oxygen species formation. In soybean, CAT activity appears to be driven by the oxidation of glycolate and not the formation of reactive oxygen species within the chloroplasts. The greater MDA contents of soybean indicate that corn was more efficient at quenching free radicals.

Overall, we did not observe non-nutritional benefits of soil-applied organic amendments on crop leaf physiology, yield, and seed composition. It is likely that these benefits of organic amendments on crop physiology are more important for container-grown drought sensitive plants, whose root systems are maintained in closer contact with potentially physiologically-altering humic substances.

Farmer Adoption

Farmers producing organic and high value crops have been increasingly using compost as an amendment for improving soil chemical, physical, and biological properties. The lack of positive plant physiological responses and associated yield quantity and quality attributes to compost additions under environmentally stressful situations will not likely increase or decrease the use of compost by organic farmers. Traditional farmers may be less apt to apply low rates of compost to their soils than if we had discovered beneficial non-nutritive effects of the composts.

Recommendations:

Areas needing additional study

Our research was unable to detect non-nutrient, biological benefits and few soil physical improvements from repeated and varying rates of compost applied to fine-textured soils. Such benefits may occur from long term compost applications to coarse-textured soils that have little structure, low water-holding capacity, and are more prone than fine-textured soils to summer drought conditions.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.