Incorporating leguminous and non-leguminous cover crops into Fraser fir Christmas tree plantations appeared to improve soil biological and chemical properties as well as tree growth performance relative to a conventionally-managed fir production system. Groundcovers increased microbial biomass and soil available N after a 2-year study. Despite differences in external N fertilizer inputs and groundcover management, fir survival and growth in the novel and conventional systems were similar. However, allowing the cover crops, to grow continuously over the entire plot led to poor seedling performance likely because of a strong competition between cover crops and trees.
The production of plantation-grown Christmas trees is a significant agricultural industry in the United States (Koelling et al., 1992). An estimated 15,000 growers yearly harvest between 32 and 36 million trees contributing over $462 million to the nation economy (Nzokou et al. 2006; Chastagner and Benson 2000). The bulk of the production is centered in the northern half of the country and Michigan ranks third among the leading producing states. Many conifer species have long been grown as Christmas tree but during the last two decades, Fraser fir is increasing in popularity because of its superior post-harvest needle and moisture retention characteristics, and higher value compared to other species (Nzokou et al. 2006, Leuty 2005). However, Fraser fir is a very demanding species and requires close attention with respect to soil fertility, especially nitrogen needed to improve the tree growth and maintain the dark green color. In addition, weed control is also essential and rank among the top priorities in Christmas tree farming (Pollack et al. 1995). Conventional Fraser fir Christmas tree production requires heavy use of inorganic N fertilizer (100 to 150 lbs per acre every year) and herbicides (two glyphosate applications at 16 – 20oz per acre every year) in plantations. Such practices provide short term results but create an environment more prone to soil erosion, leaching, and development of weed resistance to the herbicides. In recent years, increased concerns have been raised over the environmental and health risks of the use of agrichemicals. Many studies have demonstrated that intensive management practices with excessive use of chemicals create a variety of economic, environmental and ecological problems (Huat and Bremner1982; Edwards, and Abivardi, 1998. Cox, 2002). Some of the potential problems are the increased contamination of groundwater systems. Excess Nitrogen is reportedly converted into nitrates know to be very mobile in soil, moving rapidly with deep percolation through the vadose zone to underlying groundwater (Smith et al. 1996a). For example, an investigation of the effects of nitrogen management in Fraser fir plantations conducted by Rothstein (2005) found that mineral N (NH4+ + NO3-) concentrations in water leaching below the rooting zone increased markedly with increasing N application rates. Additionally, a number of economic assessment studies on Christmas tree plantations reported that the high costs of inorganic fertilizers and herbicides reduce the profits for Christmas tree growers (Pollack et al. 1995; Nzokou et al. 2006). Despite the considerable environmental and economic costs associated with current N fertilizer and weed management practices in Christmas tree production, there is no published research work suggesting alternative management options for low input Christmas tree production systems. The choice of a cover crop with potential for fixing nitrogen biologically offers an attractive alternative for inclusion into the production system. This study was carried out to assess whether soil biology and fertility as well as Fraser fir survival and growth could be improved by incorporating leguminous and non-leguminous cover crops into Fraser fir (Abies fraseri) production system.
Most tree crop production systems are dependent on nitrogen availability and over-fertilization in such systems is common because most growers rely on synthetic fertilizers (Ingels and Miller, 1993). Cover crops offer a potentially valuable source of nitrogen, especially if the cover crop plants are nitrogen fixers such as white clover and other legumes (Shiferaw 1985; BSTID/NRC, 1994, Smith et al. 1996b). Healthy stands of white clover have been reported to produce 80 to 130 lb. nitrogen per acre, when killed the year after and is reported to be a first choice for “living mulch” systems planted between rows of vegetables or trees. They also offer an important way to control weed, conserve excess soil nitrate during winter and to supply nitrogen when needed by the trees in the spring and summer (Bowman et al., 1998). Cover crops in general have been used successfully in tree crop production to improve soil fertility, weed control (Schroth et al. 2000). Because of these and other benefits, growers should consider using cover crops rather than relying solely on synthetic nitrogen fertilizers for managing nitrogen. Several studies have have observed significant increases in aboveground biomass or volume production in mixtures of Eucalyptus spp. and a N2-fixing species compared to Eucalyptus monocultures. And this response is often attributed to increases in nutrient availability through N2-fixation or accelerated rates of nutrient cycling (Binkley et al. 1992; Kaye et al. 2000; Forrester et al. 2006; Jorge and Pekka 2005). The use of legume cover crops as nitrogen sources and as a mean to control weed in agricultural settings is quite well documented, however, applied research is still needed to evaluate this technology for Christmas tree cropping systems. For instance, the potential impact of the cover crop on the soil and tissue N in a production system including Fraser fir is not known. In addition the influence of the cover crop on the long term control of weed population should be investigated. Information on the overall N uptake patterns as well as the amount of supplemental fertilizer and herbicides growth needed to design cropping systems that reduce losses of applied fertilizers/ herbicides and optimize nutrients use.
The overall goal of this research was to develop a low input production system for Fraser fir in Christmas tree production with potential to meet the nutritional requirements of the trees, perform a more competitive biological weed control and improve the soil physical and biological conditions. The specific objectives of the research were to:
• Evaluate the impact of groundcover management practices on soil fertility, soil microbial properties and tree growth;
• Measure nitrogen flows between the cover crop, the soil and the tree to determine the best management practice and appropriate level of supplemental inorganic nitrogen for optimal growth;
• Determine the impact of ground cover management systems on weed populations
The study was conducted from May 2007 to October 2008 in a newly established Fraser fir plantation located at the Tree Research Center (TRC) (42.67ºN, 84.46ºW) on the campus of Michigan State University (MSU) in East Lansing, Michigan, U.S.A. Soil at this site is classified as a fine-loamy, mixed, active, mesic Aquic Glossudalf (USDA/NRCS-MAES 1992). The general soil chemical characteristics of the site are: 35% silt + clay, pH 5.6 and 13 cmol kg-1 CEC (Rothstein 2005). Soil total C and N, inorganic N and Mehlich III extractable nutrients (P, K, Ca and Mg), measured at the initiation of the plantation establishment are provided in Table 1. The site had been used for crop cultivation, primarily corn with occasional rotations of wheat and soybeans, for at least the past 30-40 years. Total monthly rainfall from May to September of 2007, 2008 and the past 12-yr average (1996-2008) are presented in Figure 1.
The research was divided in two separate trials, with trial#1 addressing objective 1 and 3 and trial#2 addressing objective 2. Each trial was set up as a randomized complete block design, with seven (7) treatments replicated 3 times.
Trial#1: The experiment was established in a randomized complete block design with three replications in a flat field measuring 32.4 m × 50.4 m. Blocks and experimental plots were 10.8 m × 50.4 m and 7.2 m × 10.8 m, respectively. Each rectangular plot contained a total of 35 trees (5 × 7 trees). Trees in border rows were used as buffers and not included in measurements, therefore restricting data collection to the remaining 15 interior trees in each plot. Each block had two plots per cover crop, managed either with bands (B) or with no bands (NB) (Figs 2-a and 2-b). The NB treatment consisted of growing each cover crop continuously over the entire plot. In contrast, in the B treatment, the assigned cover crop was intercropped between the tree rows while maintaining, through glyphosate application, a clear band of 0.61 m wide, centered on the tree row. Control plots were managed conventionally (CONV); they contained no cover crop and weeds were completely controlled with glyphosate (active ingredient concentration =1.1 kg ha?1) (Fig 2-c). Thus, the treatments were as follows: conventional system (CONV), Dutch white clover with bands (DWCB), Dutch white clover with no bands (DWCNB), alfalfa with bands (ALFB), alfalfa with no bands (ALFNB), perennial ryegrass with bands (PRGB) and perennial ryegrass with no bands (PRGNB). No N fertilizer was applied to any plot in 2007 as it may injure first-year seedling roots (Koelling 2002). However, during the second growing season, 16 g N per tree (50 kg N ha-1) as ammonium sulfate [(NH4)2SO4] was applied one time on May 13, 2008 to the control plots.
Trial#2: Here, we tested the combined effects of two leguminous (i.e. N-fixing) cover crops and the addition of nitrogen fertilizer (a slow release ammonium sulfate fertilizer) on soil nitrogen status, nutrient leaching and subsequent Fraser Fir growth performance. The two leguminous cover crops selected for this trial were Dutch white clover and alfalfa. As in trial 1, no nitrogen fertilizer was applied during the first growing year (2007) of the trial following nitrogen fertilization recommendation for one year-old Christmas tree plantation. However, in 2008, we added 14, 28 and 42 kg N ha-1 yr.-1 of a slow release ammonium sulfate fertilizer to some plots as treatments. These rates correspond to 25, 50 and 75% of the amount recommended for 2-year-old trees (54 kg N ha-1 yr.-1). Plots with no cover crop and which received 100% of the recommended rate were used as control plots. The groundcover treatments were as follows:
1. Alfalfa plus 14 kg N ha-1 yr.-1 (ALF, N25);
2. Alfalfa plus 28 kg N ha-1 yr.-1 (ALF, N50);
3. Alfalfa plus 42 kg N ha-1 yr.-1 (ALF, N75);
4. Dutch white clover plus 14 kg N ha-1 yr.-1 (DWC, N25);
5. Dutch white clover plus 28 kg N ha-1 yr.-1 (DWC, , N50);
6. Dutch white clover plus 42 kg N ha-1 yr.-1 (DWC, N75);
7. Conventional treatment (bare ground + 54 kg N ha-1 yr.-1)
Planted seedlings were initially raised in styroblocks (65.5 cm3) for one year before transferring them to a commercial Nursery (Peterson’s Riverview) for two additional growing seasons (plug + 2). The seedlings, at this stage were graded 25.4/40.6 cm in size, and lifted bare root. Then, they were machine-planted (Whitfield planter) at the TRC into a chisel-plowed and dragged field soil on May 8, 2007 at a spacing of 1.8 m × 1.8 m. Seeds of common Dutch white clover, alfalfa (SS 100 brand) and perennial rye (VNS) were purchased from Michigan State Seeds (Grand Ledge, Michigan, U.S.A.) and hand-seeded on May 22, 2007. The seeding rates used were 28 kg ha?1 for clover and alfalfa, and 16 kg ha?1 for ryegrass. Once the cover crops were fully established, mechanical mowing was performed at 3 cm above the ground every three to four weeks in 2007 (July 2, July 26, August 21 and September 18) and 2008 (May 27, June 24, July 17, August 14, and September 11) to control cover crop growth, minimize the competition with the trees, and add green manure to the surface soil. Glyphosate was sprayed twice during each growing season (2007 and 2008) to control weeds in the CONV plots (Trials#1 and 2) and within the tree rows in the B plots (trials #1 and 2). The whole field was protected with an electric fence to prevent deer browsing.
Soil microbial biomass C (SMB-C) and biomass N (SMB-N) were assessed from soil samples by the chloroform fumigation-extraction method described by Brookes et al. (1985) and Beck et al. (1997). Soil solutions obtained from the fumigated and non-fumigated samples were analyzed for total dissolved C and N by oxidative combustion-infrared analysis and oxidative combustion-chemiluminescence, respectively (Shimadzu models TOC-VCPN analyzer and TNM-1 unit, Kyoto, Japan). SMB-C and SMB-N were calculated as the difference between C and N in the fumigated and non-fumigated samples using 0.45 as a correction factor for SMB-C and 0.54 as a correction factor for SMB-N (Brookes et al. 1985; Beck et al. 1997).
The functional diversity of the soil microbial community was measured on soil samples collected from 0-15 cm depth using the BiologTM Eco Plate system (Biolog Inc., Hayward, CA, USA). The BiologTM approach is based on relative changes in C source utilization from biolog micro-plates, each containing 31 of the most useful C sources for community analysis of mixed cultures. Substrate-utilization patterns of the soil microbial population were determined by a procedure adapted from Garland and Mills (1991). Ten grams of field-moist soil were shaken with 90 ml of a sterilized saline solution (0.85% NaCl, w/v) for 60 min and then brought to 103 final dilutions. A 150 µL aliquot was inoculated into each micro-plate well. The plates were kept at a constant temperature (25 °C) in the dark. The absorbance of the content of each well at 595 nm was measured at 0, 24, 96, 120, 168, and 240 h using an automated plate reader (Dynatech, MR-7000, Dynatech Laboratories – USA). Readings of the plates at day 0 and readings generated from the control wells were subtracted from subsequent readings to eliminate background color generated by the substrates and the bacterial suspension.
Microbial activity in each micro-plate, expressed as average well color development (AWCD), was calculated for each sample at each time point by dividing the sum of the optical density data by 31 (number of substrates). We used an optical density (OD) of 0.25 as the threshold for a positive response (Garland 1997) to calculate richness (R), or the total number of oxidized C substrates, a Shannon-Weaver index (H’) of metabolic diversity and evenness of response (E).
The Shannon-Weaver diversity index was calculated as follows:
• H’ = -?pi ln(pi) Eq.(1)
where pi is the ratio of the activity on each substrate (ODi) to the sum of activities on all substrates (?ODi). Its value usually ranges from 0.4 to 4 and expresses a greater metabolic diversity when it is close to 4 and a lower metabolic diversity when close to 0.4 (Frontier and Pichod-Viale 1988).
Substrate evenness (E), which is a measure of the uniformity of activities across all substrates, was calculated as follows:
• E = H’/log(R) Eq.(2)
where H’ is the Shannon-Weaver diversity index and R is the number of different substrates used by the community (counting all positive OD readings). E values range between 0 and 1 with lower uniformity of activity when the values are close to 0 and a greater uniformity when the values get close to 1.
Micro-plate readings measured at 0, 24, 96, 120, 168, and 240 h of incubation were used to calculate AWCD and R. However, plate readings at 120 h were used to calculate R, H’ and E for statistical analysis, since it was the shortest incubation time that allowed the best resolution among treatments.
Seedling initial height and root collar diameter, recorded at 1 cm from the ground, were measured two weeks after planting. In addition, in mid-October of 2007 and 2008, we measured tree survival as well as each seedling’s height and root collar diameter. Stem volume of each tree was calculated assuming the stem to be cone. Stem volume relative growth rate (RGR) of individual trees was calculated using the formula by Hunt (1982):
RGR = (lnV2-lnV1)/(t2-t1) (eq.1)
where V2 and V1 are tree stem volume at times t2 and t1.
The maximum photochemical efficiency of photo-system (PS) II (Fv/Fm), of current year needles (one needle per tree) was measured using a Handy PEA Chlorophyll Fluorometer (Hansatech Instruments, England). Midday branch water potential (?w) of new branches (one branch per tree) collected from the upper third of the crown was also measured with a Pressure Bomb (PMS Instruments Company, Oregon, USA). Both Fv/Fm and ?w were measured on three trees per plots, three times a year in 2007 (May 29, July 18 and September 27) and 2008 (May 14, July 17 and September 23). Fv/Fm and ?w data were averaged per plot and date before performing statistical analysis.
Needle samples were collected at the end of each growing season (mid-October) for determination of foliar nutrient concentration. During this time of the year, roots are dormant, trees have produced most of their annual woody growth, and nutrient levels particularly in the trees are stable (Hart et al. 2004). Soil samples were taken at the end of each growing seasons (mid-October of 2007 and 2008) for determination of soil available N and N mineralization rates. Soil samples collected in 2008 were used for the determination of P concentration. Fifteen randomly selected soil samples per plot were collected with 5.2-cm diameter PVC tubes and composited into one sample. In the banded cover crop plots, nine cores of soil were collected within the cover crop zone and six cores within the bands (bare ground zones), proportional to the area of the banded and cover crop zones. Soil samples were collected from 0-15, 15-30, and 30-45 cm depths.
From each field-moist soil sample, about 10 g of soil was used for determination of gravimetric moisture content by weighing the soil before and after oven drying at 105°C for 24 h. A second subsample was used for inorganic N determination while a third subsample was used for potential N mineralization determination after 4 weeks of incubation at 25oC and maintaining the samples at 50% of field capacity by periodically adding distilled water. Nitrate and ammonium (NH4+-N) concentrations of the non-incubated and incubated soil samples were extracted with 2 M KCL (5:1 extractant to soil ratio) and extracts were analyzed spectrophotometrically (Spectrophotometric plate reader, Model ELx Bio Tek Instruments, Inc. Winooski, Vermont, U.S.A.) for NO3–N and NH4+-N following the procedures described by Doane and Horwath (2003) and Sinsabaugh et al. (2000), respectively. Soil macronutrients (e.g. P, K, Ca and Mg) were extracted with Mehlich III extractant (Mehlich 1984) from soil samples collected in 2008, and analyzed using an inductively coupled plasma atomic emission spectrometry (ICP-AES; PerkinElmer, Model Optima 2100DV, Shelton, CT, U.S.A.).
Before each mowing event in 2008, the aboveground biomass of the cover crops was measured by using four quadrats of 0.3 m x 0.3 m, randomly placed between the tree rows in each plot. Vegetation was cut at 3 cm above the ground to mimic a mechanical mowing. Then, quadrat clippings from each individual plot were put in a paper bag and taken to the laboratory where cover crop fresh material was separated from weeds.
Needles samples were collected by pinching five to eight needles of new or current season growth from different locations on the upper one third of the tree crown. This procedure was repeated for three to six trees within each plot and all needles samples bagged per plot.
All plant tissue samples were gently washed with a non-ionic detergent and rinsed five times in distilled water before they were oven-dried at 65ºC for 48 hours. Dried weight of cover crop clippings was used for the determination of herbage yield at each mowing event. Then, each plant tissue sample was ground in a ball-mill and a sub-sample of 500 mg was acid-digested (4.5 mL of concentrated 70% nitric acid) for elemental analysis (ICP-AES). Additional subsamples of about 2.5 mg were used for total C and N determination by combustion using an elemental analyzer (Model ECS 4010, COSTECH Analytical, Valencia, California, U.S.A.).
In each treatment from trial#2, suction lysimeters were installed on June 30, 2008 to collect water leaching below the tree rooting zone (1.20 m below the ground). Leachate samples were collected on a weekly basis in each plot. After quantifying the volume of leached water from each lysimeter, subsamples were filtered through Whatman # 2 filter paper and analyzed for total C and total N (Shimadzu Corp., Kyoto, Japan), base cations such as K+ and Ca2+ (AA ) and mineral nitrogen (NO3- and NH4+) (Micro plate reader)
All soil and plant variables were analyzed as a randomized complete block design in SAS (SAS Institute, 2002 Cary, NC). Normality of the residuals and homogeneity of variances assumptions were checked using normal probability plots. Data were found to be normally distributed and homogeneity of variances was not violated. Seedling survival data were subjected to a generalized linear model with binomial distribution and the logit link function to assess the association of the degree of competition-management (0=bare ground control, 1=banding and 2=no banding) with fate (0=dead; 1=live,) of a seedling. With the exception of tree growth data, all other data were first averaged per plot for each sampling date and analyzed using repeated measures ANOVA. The statistical model for tree growth data included one random factor (block) and two (2) fixed factors: groundcover treatments and year, with year considered as repeated measurements. For Fv/Fm, ?w and needle nutrients, the statistical model included one random factor (block) and the three fixed factors: groundcover treatments, sampling date and year with date and year as repeated measurements. For cover crop and soil Mehlich III extractable nutrient data which were assessed only during the second growing season (2008), year was not included as a fixed factor or repeated measurements in the model. As for data on the concentration of soil available nutrients (NO3–N and NH4+-N,) and N mineralization, we used block as a random factor while groundcover treatments and depths were fixed factors, with depth considered the repeated measurements. N leaching data were analyzed using repeated measures ANOVA with sampling data as repeated measures. Variance/ covariance structures were assessed in PROC MIXED repeated measures analysis and the model with the lowest criteria (AIC, BIC) values (type=CSH or heterogeneous compound symmetry) was use for the parameter estimate calculations. Then, the conclusions about factor effects, mean separations and contrasts were all done using the model with this selected variance-covariance structure. Differences were accepted as significant at the 0.05 level.
- Table 1 Initial soil chemical characteristics (means ± se; n=3) measured in May 2007 at three soil depths of the experimental site, Michigan State University Tree Research Center, East Lansing, MI, U.S.A.
- Figure 1 (A) Total monthly precipitation and during the 2007 and 2008 growing seasons compared with the 12-year monthly average at MSU’s Horticultural Farm, East Lansing, MI.
- Figure 2 Banding (A), no banding (B) and conventional (C) groundcover treatments
Groundcover treatments significantly affected soil microbial biomass (SMB-C and SMB-N) at the 0-15 cm depth (P<0.001). However, no statistical difference was observed among groundcover treatments for both SBM-C and SMB-N at the 15-30 cm and 30-45 cm depths (Figs. 3-a&b). At the 0-15cm depth, ALFB and ALFNB averaged 49% and 35% higher SMB-C compared to CONV controls (Fig. 3-a). Similarly, SMB-N in ALFB and ALFNB plots was 80% and 53% higher than in CONV treatments (Fig. 3-b). SMB-C and SMB-N were also significantly higher in all Dutch white clover and perennial rye grass plots (banded and non-banded) compared to the CONV plots. SMB-C was 57% and 49% higher, and SMB-N was 80% and 76% higher in DWCB and DWCNB than in CONV. For PRGB treatments, SMB-C was 15% and SMB-N was 33% higher than in CONV while PRGNB yielded 19% and 40% SMB-C and SMB-N, respectively, higher than in CONV.
The results indicated that creating bands did not significantly affect SMB-C or SMB-N in both legume and non-legume cover crop treatments. SMB-C and SMB-N in plots managed with bands averaged 558.8 mg C kg-1 dry soil and 83.2 mg N kg-1 dry soil while plots without bands averaged 535.8 mg C kg-1 dry soil and 79.3 mg N kg-1 dry soil, respectively. Soil microbial biomass C:N ratio is often used to describe the structure and state of the microbial biomass (Moore et al., 2000). Bacteria and fungi generally comprise 90% of the total soil microbial biomass (Six et al. 2006) and the substrate quality is known to have a major influence on fungal: bacterial ratios, with low quality substrates (high C:N ratio) favoring fungi and high quality substrates (low C: N ratio) favoring bacteria (Bossuyt et al. 2001). High soil microbial biomass C:N generally indicates higher fungi with respect to bacteria populations in the soil (Jenkinson, 1976; Moore et al., 2000). This was not statistically significant (P>0.05) among groundcover management practices at any soil depth (Fig. 3-c). Across all treatments, SMB-C and SMB-N decreased with soil depth for all cover crop treatments (Fig 3-a &-b) , and legume cover crops treatments generally yielded the highest microbial biomass C compared to grass cover crop and the conventional treatments. However, soil microbial biomass C: N ratio significantly (P<0.05) increased with soil depth (Fig. 3-c), ranging from 6.5-7.1; 7.1-8.8 and 11.1-13.7 at the 0-15, 15-30 and 30-45 cm soil depths, respectively. This trend is the opposite of that observed for soil total C:N and could indicate a shift in community composition, perhaps from a bacterial-dominated community on the top soil layer to a fungal-dominated community in deeper soil layers. Similar results indicating no changes in total organic C after one or two years suggested that small gradual changes in soil organic matter are difficult to monitor because of the high background carbon level and the natural variability of soils (Sparling 1999; Mendes et al. 1999). Other research documenting long term experiments have reported that cover cropping increased soil organic C levels (Dinesh et al. 2004; Sanchez et al. 2007; Hoagland et al. 2008). However, the time frame necessary to observe significant changes in soil carbon is not clearly defined and most cropping systems should be continuously monitored to detect gradual changes.
The fact that we observed lower soil total N in the conventional soils compared to all of the other treatments suggested that there must have been additions of N to the soil under groundcovers, lower losses of N under groundcovers or some combination of the two processes. Input from N-rich litter generated by cover crops can increase soil N levels. In addition, maintenance of a vegetative groundcover, or “living mulch”, can reduce nutrient loss by acting as a “catch crop,” immobilizing and retaining available soil N, and returning it to the soil via residue decomposition. For the leguminous species, the addition of N to the soil could result from atmospheric N2 fixation. Indeed, previous studies documented that when a leguminous cover crops was used as an intercrop, additional N could be added to the soil through fixation of atmospheric N2 (Marsh et al. 1996; Sanchez et al. 2003; Stork and Jerie 2003). For the non-leguminous species (ryegrass), the significant increase in soil total N might result from an upward redistribution of N from depth, or trapping of N that would otherwise be lost through leaching. Previous research documented that non leguminous crops could successfully uptake significant amount of residual soil N between cash crops (Meisinger et al. 1991). Such processes almost certainly contributed to the overall increase in total soil N content in the cover crop treatments in this study.
The color response in a given well is related to the number of microorganisms (functional diversity) which are able to use the substrate within the well as a sole carbon source and are therefore used to assess microbial community structure in a given ecosystem (Garland and Mills 1991). Average well color development (AWCD) recorded as optical density (OD) and the number of well responses expressed as the catabolic diversity from all groundcover management treatments followed the same pattern (sigmoidal curve) throughout the incubation period (Fig.4-a). However, the rate of increase varied with different treatments. Both the AWCD and the catabolic diversity of communities from control plots were lower than the cover crop managed plots. These results suggest that microbial communities from the cover crop plots had higher substrate utilization rate than the control plots. Perennial rye grass plots treated with bands showed significantly higher overall AWCD values throughout the incubation period compared to perennial rye grass plots with unbanded treatments. However, the AWCD recorded from the two legume cover crop treatments, irrespective of the management type, were not statistically different.
The number of well responses (catabolic richness) followed the same pattern as AWCD throughout incubation (Fig.4-b). In all the soil samples from the different groundcover treatments, only a few wells showed no color response after 96 h of incubation. Microbial community richness was significantly lower in the conventional plots than in all cover crop treatments (P<0.01). Significant differences among treatments (P<0.01) were found in catabolic richness, Shannon diversity and evenness (Fig. 5-a, b & c). All cover crop treatments, both with bands and no bands, had significantly higher (P<0.001) microbial species catabolic richness levels than the conventional treatments (Fig. 5-a). However, there were no significant differences in microbial catabolic richness among cover crop treatments (P>0.05).
Plots managed with both legume cover crops, either with or without bands, had significantly higher Shannon-Weaver index means than the control. Similarly, the H’ value was significantly higher in the banded non-legume cover plots than the control plots. Conversely, no statistical difference was found (P>0.05) between plots managed with perennial rye grass without banding and the conventional plots. There was a statistically significant difference in H’ between PRGB and PRGNB, suggesting that rye grass with banding management could develop a more diverse microbial catabolic diversity than rye without banding and control treatments, while for both legume cover crops, there was no significant difference in Shannon-Weaver index level between banding and no banding treatments.
In general, microbial species catabolic response was significantly different among the various groundcover management practices (P=0.01). Microbial species evenness index was the highest in DWCB and PRGB plots. The plots managed with DWCB and PRGB showed significant differences in microbial species evenness index compared to all other groundcover treatments, including conventional.
In order to determine the extent of differentiation between the conventional and the cover crop treatments with regard to carbon source utilization, the OD data from the various treatments were subjected to multivariate analysis (principal component and similarity distance cluster analyses). The trends observed on soil microbial biomass and CLPP data were supported by results from the multivariate analysis. Contrasting patterns were apparent between the cover crop treatments and the conventional plots as a result of the different groundcover management practices (Fig 6-a&-b).
The separation of groundcover treatments in PC space can be related to differences in carbon source utilization by examining the correlation of the original variables to the PCs. The principal component analysis showed that the first principal component had high coordinate values (Eigen value) of 6.30 which explained 90.0% of the total variance in the data. The second principal component had variance 0.08 and accounted for 7.9% of the data variability. Together, the first two components of this PCA accounted for 97.9% of the variation in the data, with good discrimination (P<0.05). The plots managed with the two legume cover crops with banding exhibited dissimilar patterns of substrate utilization.
The importance of cover crops for nutrient cycling in cover crop-tree based cropping systems is dependent upon how many, and how fast, the nutrients are recycled. The magnitude of nutrient cycling is known to be a function of biomass production of any given system, the nutrient contents and the biomass decomposition rate. It was therefore important to evaluate the aboveground biomass and the amount of certain nutrients which were being returned to the system from each mowing operation.
Results of average aboveground dry matter production for each cover crop species and weed population (all weed species combined) are presented in Figure 7 and Table 2. Overall, white clover had the highest annual dry matter production followed by alfalfa and rye grass. For perennial rye grass, the average biomass harvested during the first four clippings (early to mid-season) were higher than the biomass produced from mid to late season. White clover however had better yields in early and late season compared to the mid season harvests. Indeed, the growing season of 2008 was characterized by an extended drought in mid season (July –August) and this may have negatively affected white clover yieds during that period. Alfalfa biomass yields were less affected by the drought which occurred during the middle of the growing season.
The highest weed biomass was recorded in alfalfa plots, yielding 2.48 t ha-1 during the growing season of 2008. There was a trend toward an increase in weed population biomass from early to late season in alfalfa plots. Weed biomass yields, recorded in perennial rye grass and alfalfa plots were similar (1.52 t ha-1) and revealed to be significantly lower than those recorded in alfalfa fields. It can therefore be concluded that white clover and perennial rye grass were more effective at suppessing weed population than afalfa.
Our measurements of soil mineral N and N mineralization in 2007 and 2008 provided an indication of the fate of N from the cover crop green manures (Table 3). By the end of the first growing season (2007), cover crop treatments significantly increased soil NO3–N concentration relative to CONV in the two top soil layers). The increase of soil NO3–N levels was in the range of 1.3 – 1.6% and 1.5 – 2.2% at the 0-15 and 15-30 cm soil depths, respectively. Similarly nitrification rate and net N mineralization increased substantially in all cover crop treatments relative to CONV at the two top soil depths. In 2008, however, treatment differences were apparent at all three soil depths for soil NO3–N, nitrification rate and net N mineralization. In 2007, cover cropping also significantly increased soil available N levels relative to the CONV treatment at the top soil layer, whereas treatments differences were not significant at the two deeper soil layers. In 2008, cover crop treatments significantly enhanced soil available N by 1.8-2.6%, 1.8-3.4% and 1.6-2.2% at 0-15, 15-30 and 30-45 cm soil depths, respectively.
Although more extensive research would be needed to follow the dynamic of the cover crop biomass N in the plantation soil, a single sampling of soil during a growing season may provide an indication of treatment differences. High available N and N mineralization rates were generally obtained in the B and NB plots. However, this did not necessarily lead to enhanced tree foliar N nutrition in these plots relative to the control. It is possible that we may have found a stronger relationship between soil nutrient status and foliar nutrition if the samples were taken at a different time (e.g. middle) of the growing season. The reason for the lack of response of seedling foliar chemistry to groundcover treatments could also be that water stress might have reduced the ability of the trees to effectively absorb plant available nutrients from the soil solution. As indicated by the weather data, July and September of 2007 were drier than the 12-year average. Similarly, May and August of 2008 received 70 and 75%, respectively, less rainfall than the 12-year average. The expected low soil moisture contents during these periods may have reduced the effective diffusion of elements in the soil solution.
In addition, plant water stress may in turn, have reduced the mass flow and plant uptake of elements. Similar to moisture availability, root volume and nutrient uptake and utilization are linked. Prior to planting, seedlings were root pruned to 20 cm following standard planting practices. This resulted in substantial root loss, especially the absorbing roots, and may also limit seedling nutrient uptake. As reported by Knapp and Smith (1982), root penetration to greater depths is an important factor determining the survival of conifer seedlings. Although we did not investigate seedling root development in this study, it is also likely that the root system was still shallow during the first year of the plantation establishment. The newly transplanted seedlings might have suffered from a combination of high temperature, drought, root damage and shallow rooting that contributed to poor nutrition and high mortality of seedlings (Dalton and Messina 1994), especially in the NB plots. However, most of the seedlings that survived during the first growing season also survived during the second year. Second year seedlings likely had ample time to restore their root system to the pre-transplant size, making them less susceptible to environmental stresses.
The total amount of leached water, collected from the suction lysimeters (about 1.20 m underground) is shown in Figure 8. In general, there was a slightly higher in conventional plots compared to cover crop plots although differences in July and September were not statistically significant. Slightly higher water in conventional plot can be caused by two factors. First, it could be that cover crops has improved the water holding capacity of the soil, thus reducing the total amount of water moving below the rooting zone. It is also likely that the presence of cover crop increased crop water use in the system therefore, reducing the total volume of water available for leaching. Leaching of water through the soil profile can occur whenever precipitation exceeds potential evapo-transpiration. The quantity and frequency of leaching is therefore determined by the amount, type and seasonal distribution of precipitation. Vegetation is known to influence leaching losses by decreasing the amount of water and nutrients available to move through the soil profile (Sharpley, 1992). August received less rains (data not shown) than July and September and the difference in the total amount of leached water was statistically significant between alfalfa plots compared to Dutch white clover and the conventional.
The dissolved carbon concentration in water leaching from the rooting-zone was not statistically different among all treatments, although total nitrogen concentrations differed with groundcover management practices (Table 4). Conventional plots had significantly higher levels of leached nitrogen than plots where cover crops were grown.
The concentrations of N leached in the conventional plots were more than twice those recorded where cover crops were grown. For both alfalfa and Dutch white clover, there was no trend of increased N concentration in leachate with increased nitrogen input. For the N25 input level, Ca2+ concentrations in leachates of the alfalfa and Dutch white clover plots were significantly lower than the plots which received higher N input levels (N50 and N75) as well as the conventional treatment. Although the highest Ca2+ concentrations in leachates of the alfalfa and Dutch white clover increased with increasing N input level, no statistically noticeable differences were recorded between N50 and N75. There was no significant difference between Ca2+ in leachate of the conventional treatment and both cover crop species plots amended with N50 and N75 input levels. Potassium concentrations in the conventional plots were twofold higher than those with either cover crop, regardless of N input levels.
Although N and K concentrations in leachate were not statistically different between the two cover crops across N fertilization levels, values for alfalfa were slightly lower. Compared to Dutch white clover, alfalfa has a deeper-rooting system than white clover, which may make this cover crop more efficient in recycling nutrients. Indeed, Sharpley et al. (1992) demonstrated that including cover crops in cropping systems or crop rotation increased sustainability with regard to nitrogen recovery when compared to conventional systems. Alfalfa can develop roots to depths greater than 5.5 m has and has been shown to utilize nitrate from any depth where soil solution is extracted by its roots. Mathers et al. (1975) reported that alfalfa removed nitrate from the soil profile at depth of 1.8 m during the first year of establishment and to the depth of 3.6 m during the second and third year of stand development.
In Bolivia, Barber at Navarro (1994) found a significant increase in K in soil from 14 different cover crops plots. This was thought to be an effect of K translocation from the subsoil (Lal et al. 1993) which was also reported by other authors (e.g. Eckert, 1991).
For leached Ca2+, there were no significant differences in concentrations among treatments, possibly due to the fact that Michigan soils, especially the soils in the study area, are known to be calcareous and the availability of this nutrient may far exceed the crop needs. This may also explain the greater concentrations of Ca2+ in leachate samples compared to the other nutrients.
By the end of 2008 growing season, tree survival ranged from 13 to 68%. PRGNB and DWCNB had the lowest tree survival of all groundcover treatments. In both 2007 and 2008, no significant differences were found between the CONV and the B plots. However, in both years, seedling survival was significantly (P=0.021 in 2007 and P<0.01 in 2008) higher in the B plots relative to the NB treatments. After two growing seasons, no survival differences were found among species. In general, most seedling mortality occurred during the first growing season. It is noteworthy that on average, 91% of the seedlings that survived the first year also survived the second year in the CONV and B treatments. In the NB plots, however, second-year survival was 70%, with the lowest survival (47%) recorded in Dutch white clover plots. In 2007, some dead seedlings were sent for analysis at MSU’s Center for Integrated Plant Systems and the result indicated that the mortality was not related to any disease problems. There was a strong (P<0.001) association between cover crop management practices and tree survival. Seedlings in the B plots tend to have a higher survival than subjects in the NB treatment as indicated the negative coefficient of -0.78 and odds ratio of 0.46 indicate from the logistic regression analysis.
Stem volume relative growth rate for the first growing season was not significantly (P=0.931) affected by groundcover treatments.
However, stem volume RGR for the second growing season and the combined two growing seasons differed significantly (both P<0.001) among groundcover treatments. Seedlings grown in NB plots had lower stem volume RGR than their counterparts in the CONV and B treatments. However, stem volume RGR of seedlings grown in all B plots was similar to that of their counterparts in the CONV plots. By the end of the second growing season, relative diameter growth of seedlings in the ALFNB, DWCNB and PRGNB plots was reduced by 31, 38 and 37%, respectively, relative to their counterparts in the CONV plots. Continued root and shoot growth during the establishment period depends on a number of factors such as the availability of nutrients and moisture, a well-established root system taking-up water and nutrients as well as healthy leaves producing high levels of carbohydrates during the growing season (Lambers et al. 1998). Stem volume RGR was not affected (P=0.931) by groundcover treatment during the first growing season probably because root systems were still in the recovery phase, during which more carbohydrates are allocated for root growth than shoot growth. During the second growing season, however, stem volume RGR in all NB treatments was significantly (P<0.001) reduced relative to their counterparts in the B and CONV plots. The highest seedling mortality recorded in most NB plots during the first year, coupled with the fact that, irrespective of groundcover treatments, most trees that survived during the first year also survived the during the second year, suggest that the implantation of the cover crops during the second growing season (or later) of the plantation establishment could be more advantageous to the trees than implanting in the first year.
The presumed reduction of belowground competition between Fraser fir seedlings and cover crops by banding was expected to influence stem ?w (Fig 8). The banding treatment substantially raised ?w of Fraser fir seedlings relative to the NB treatments, suggesting that seedlings in B plots had less water stress than their counterparts in the NB plots. Despite differences in cover crop management practices, seedlings in the B plots displayed similar ?w to that of their counterparts on the CONV systems. It is noteworthy that seedlings in NB plots had their ?w consistently below the threshold of -2.0 MPa, which is considered severe water stress and critical for stomatal opening and photosynthesis in seedlings and saplings of a variety of conifers (Tseng et al. 1988). In 2008, mean ?w was -1.1 MPa for seedlings in B and CONV treatments and -1.5 MPa for their counterparts in the NB plots. This suggests that seedlings in NB plots suffered more severe water stress than their counterparts in the B and CONV plots. The lower seedling ?w values of seedlings recorded during the first growing season relative the second growing season is in agreement with Muyi and Smith (1991) who also reported increasing ?w value with increasing age in subalpine fir (A. lasiocarpa) seedlings. Differences in ?w of seedlings between the two years might have resulted from differences in root development and distribution in the soils. Two-year old planted seedlings were likely to have more extensive and deeper root system than one-year old seedlings which played a fundamental role in alleviating water stress experienced in the second growing season.
The capacity to perform photosynthesis, given appropriate environmental conditions, is reflected by Fv/Fm. Environmental conditions such as extreme temperature, drought or nutrient deficiency can cause photo-oxidation or photo-inhibition, changing the efficiency of non-photochemical quenching and decreasing Fv/Fm (Westin et al. 1995; Maxwell and Johnson 2000). In this manner, groundcover management may have created specific environmental conditions that stimulated or inhibited photosynthetic capacity. Similar to seedling survival and ?w, we observed a significant response of Fv/Fm to groundcover treatment in 2007 (Fig 8). Seedlings from NB treatments had their needles Fv/Fm below the 0.8 threshold considered characteristic of healthy foliage (Lambers et al. 1998) whereas mean Fv/Fm values for seedlings in the B and CONV were greater or equal to this threshold. Cover crop management through banding, however, led to enhanced Fv/Fm level comparable to the CONV system during the first growing season. Moreover, we found a positive relationship between ?w and Fv/Fm in both 2007 (r=0.78, P<0.001) and 2008 (r=0.51, P<0.05), suggesting that water stress was responsible for the decline in seedling Fv/Fm on the NB plots. Water stress, in turn, can reduce the ability of tree foliage to produce energy (carbohydrates), diminish growth, and leave the tree susceptible to many other environmental stresses. Together, these problems, might have contributed to transplant stress that led to reduced seedling survival and growth in the NB plots.
- Figure 3 Soil microbial biomass carbon (a), microbial biomass nitrogen (b) and microbial biomass C:N ratio (c) as influenced by groundcover management. * Treatments with the same letter at each soil depth are not statistically different at P>0.05. NS: treatment means are not significant
- Figure 4 (a) Average well color development (AWCD) and (b) average catabolic diversity obtained from BIOLOGTM ecoplate incubation of all groundcover treatments in a Fraser fir plantation
- Figure 6 (a) PCA and (b) Cluster analysis performed on BIOLOGTM of soil extracts from different groundcover management treatments.
- Figure 7 Average of dry matter of weed and perennial rye grass (a) , alfalfa (b) and Dutch white clover (c).Values are treatment means ± 1 SE.
- Table 3 Average soil available nitrate, available N, nitrification and net N mineralization at three depths, from samples collected at the end of the growing season of 2007 and 2008 with different cover crops in a Fraser fir Christmas tree plantation in Michigan, U.S.A. Treatments are: Conventionally managed (CONV), Dutch white clover with banding (DWCB), Dutch white clover with no banding (DWCNB), alfalfa with banding (ALFB), alfalfa with no banding (ALFNB), perennial ryegrass with banding (PRGB), and perennial ryegrass with no banding (PRGNB). *Mean values followed with the same letters are not statistically significant. ns = not significant.
- Figure 5 (a) Species richness (S), (b) Shannon index of diversity (H’) and (c) Evenness index (E) as influence by different groundcover management. * Treatments with the same letter at each soil depth are not statistically different.
- Figure 8 Water leaching collected in each cover crop treatments during the summer of 2008. * Treatments with the same letter at each soil depth are not statistically different
- Table 4 Dissolved C, total N and base cations leached as affected by various groundcover management practices and different rate of N fertilizer addition in alternative groundcover management plots in a Fraser fir plantation.
- Figure 9 Average maximum photochemical efficiency of PSII (Fv/Fm) measured in 2007 and 2008, average midday branch water potential (?w) measured in 2007 and 2008 and needles N concentration measured in 2007 and 2008, with different groundcover management in a Fraser fir Christmas tree plantation in Michigan, U.S.A. Values are means (±se). Mean values followed with the same letters are not statistically different; ns = not significant.
- Table 2 Aboveground biomass and N, P and K accumulation of a cover crop-Fraser fir intercropping system in Michigan; values in one column followed by the same letter are not significantly different at P<0.05* . Treatments with the same letter at each soil depth are not statistically different.
Educational & Outreach Activities
- This study was a demand driven research, specifically designed to address the needs of Christmas tree growers in Michigan and the entire United States, with emphasis on lowering the costs of tree production. Cover crops were used as an alternative to commercial N fertilizer in the system. The recognition of the relevance of this project to the end users (Christmas tree growers) was demonstrated by the funding commitment that many institutions including Michigan Christmas Tree Association, NCR-SARE and MSU’s project GREEEN provided toward the project. In return, the key findings of the project have been shared with growers at various forums. For example, in the summer of 2008, the research team made a presentation at the Michigan Christmas Tree Growers Association meeting (MCTA) where a number of farmers expressed an interest to take up the study findings in the production of commercial Christmas trees.
Publications in referee journal
Nikièma P, Nzokou P, Rothstein D (2011) Effects of groundcover management on soil properties, tree physiology, foliar chemistry and growth in a newly established Fraser Fir (Abies fraseri [Pursh] Poir) Plantation in Michigan, United States. New Forests. DOI 10.1007/s11056-011-9274-8
Nikièma P, Nzokou P, Rothstein D, NGouajio M (201x)- Effects of Groundcover Management Practices on Soil Microbial Biomass and Community Catabolic Diversity in a Fraser Fir (Abies fraseri [Pursh] Poir)-Plantation. Biology and fertility of Soils (in Review)
Conference papers and posters
Nikièma P., Rothstein D. (2009) Initial effects of pastureland conversion to hybrid poplar and willow bioenergy systems on N leaching and greenhouse gas emissions from soil- Poster presented at the Great Lake Bioenergy Research Center (GLBRC) retreat at the W.K. Kellogg Biological Station, Michigan State University-February 10-12, 2010
Nikièma P., Nzokou P. (2009) Groundcover Management affects Soil Fertility, Tree Physiology and Foliar Chemistry in a Fraser fir-Cover Crop cropping System. Book of Abstracts, 2nd World Congress of Agroforestry, Agroforestry-The Future of Global Land Use; Nairobi: World Agroforestry Centre. p342.
Nikièma P., Nzokou P. (2009) Microbial properties of soil as affected by intercropping Fraser fir (Abies fraseri) and cover crops; Book of Abstracts, 2nd World Congress of Agroforestry, Agroforestry-The Future of Global Land Use. Nairobi: World Agroforestry Centre: 71-72.
Nikièma P. Nzokou P., Rothstein R. Gouajio M. (2009) Effects of Groundcover Management Practices in a Fraser fir (Abies Fraseri) – Cover Crop Intercropping System on Soil Microbial Biomass and Community Catabolic Diversity: Conference Proceedings, the 11th North American Agroforestry Conference, Agroforestry Comes of Age: Putting Science into Practice. Colombia. Association for Temperate Agroforestry: 385-395
This research has direct benefits both in terms of reducing production costs for farmers while at the same time, contributing to a more sustainable production system interaction that is able to meet the nutritional requirements of the trees while minimizing environmental problems associated with the current farming practices. Farmers have been enthusiastic to take up this production method, not least because it helps them to reduce costs while also introducing a more competitive biological weed and pest controls which enables them to reduce costs even further. I was invited to make an oral presentation of some of these findings at the 11th North American Agroforestry Conference in Missouri and growers, researchers and extension specialists from other states of the U.S. who attended the conference were extremely interested in adopting this approach.
Areas needing additional study
Although incorporating cover crops into Christmas tree plantations appeared to be a promising approach, an economic evaluation of the costs and benefits should be conducted to determine its profitability before recommending such a practice to growers. The study should focus on determining the costs and efforts required to purchase and sow the cover crop seed, periodically mow the cover crop, and maintain the bands. These costs should be compared to the costs and labor involved in the conventional system such as complete weed control and inorganic N fertilization. The potential contribution of the cover crops to soil carbon sequestration over the entire plantation rotation period should also be investigated, so that growers who adopt this approach could be adequately rewarded for the carbon sequestration services they provide. Another possible option would be to simply band the plantation and allow local vegetation to fill in between the rows and serve as a “wild cover crop.” This would remove the cost of purchasing and establishing a cover crop. While it may not provide as much nitrogen as a leguminous cover crop, it should approximate the advantages of perennial ryegrass.