A pecan-cotton alley cropping system was established in northwestern Florida in Spring 2001 to assess tree-crop competition for nitrogen (N) and its effect on mineralization rates and groundwater nitrate levels, and nitrogen use efficiency. Polyethylene root barriers were used to prevent belowground interaction between pecan and cotton in half the number of test plots, for the duration of the 17-month study (June 2001-October 2002).
The study first examined the effect of tree roots on nitrogen transformations in soil. It was observed that temporal variations in net ammonification, nitrification and mineralization were driven primarily by environmental factors (such as soil moisture content and soil temperature), and by initial ammonium and nitrate levels. In general, greater nitrification and mineralization rates were observed in the non-barrier treatment due to higher soil nitrogen. Cotton lint yield reductions were observed in the non-barrier treatment during both years compared to the barrier treatment, likely due to interspecific competition for water. In addition, source of N was found to have a significant effect on cotton yield, with inorganic fertilizer resulting in higher yields in the barrier treatment compared with organic poultry litter.
The study also examined the “safety net” hypothesis to determine whether tree roots were able to capture nitrate and ammonium leached below the crop root zone. In general, the presence of trees in the non-barrier treatment resulted in decreased soil solution nitrate concentrations and nitrate leaching rates.
Lastly, the results indicated that competition for fertilizer N was minimal because of differences in temporal patterns of pecan and cotton nitrogen demand, although NDF may have occurred in unstudied portions of pecan tree tissue. Nitrogen use efficiency of cotton in barrier treatment was shown to be higher, indicating a greater ability to utilize the available nitrogen.
Overall, this study reveals that the competitive presence of trees can be utilized to decrease soil nitrate concentrations and reduce nitrate leaching. This knowledge will help to improve our understanding of temperate alley cropping systems and to design systems that utilize the safety net process to maximize nitrogen use efficiency and minimize groundwater pollution.
Individuals and institutions in the world’s temperate regions are increasingly taking notice of the science and art of alley cropping. This is due in part to growing concerns over the long-term sustainability of intensive monocultural systems. In the temperate context, alley cropping involves the planting of timber, fruit or nut trees in single or multiple rows on agricultural lands, with crops or forages cultivated in the alleyways (Nair, 1993; Garrett and McGraw, 2000). Major purposes of this type of agroforestry system include production of tree or wood products along with crops or forage; improvement of crop or forage quality and quantity by enhancement of microclimatic conditions; improved utilization and recycling of soil nutrients for crop or forage use; control of subsurface water levels; and provision of favorable habitats for plant, insect or animal species beneficial to crops or forage (USDA, 1996; Garrett and McGraw, 2000). Important crops for alley cropping in the southern United States include cotton (Gossypium spp.), peanut (Arachis hypogaea), maize (Zea mays L.), soybean (Glycine max. L. (Merr.)), wheat (Triticum spp.) and oats (Avena spp.), combined with trees such as pines (Pinus spp.) and pecan (Carya illinoensis K. Koch).
As an association of plant communities, alley cropping is deliberately designed to optimize use of spatial, temporal and physical resources, by maximizing positive interactions (facilitation) and minimizing negative ones (competition) between trees and crops (Jose et al., 2000a). For example, trees in these systems are capable of improving soil nutrient status (Nair, 1993; Palm, 1995; Rowe et al., 1999), thereby improving overall system productivity. Trees are also capable of capturing and recycling lost soil nutrients and are thus a potential moderating factor in groundwater pollution caused by leaching of nitrates (Williams et al., 1997; Garrett and McGraw, 2000). In addition, trees on agricultural lands offer landowners the possibility of accruing carbon credits via the sequestration of stable carbon stock, an added incentive for adopting alley cropping (Dixon, 1995; Williams et al., 1997; Sampson, 2001; Nair and Nair, 2003). However, adoption of alley cropping and other agroforestry systems has been hampered by a lack of understanding of interspecific interactions involving system components and their impact on system productivity and sustainability. This is especially true for temperate agroforestry systems, where research efforts have gained momentum only in recent years.
Interspecific competition for nitrogen can be an importanbt determinant of productivity since N is generally the most limiting soil nutrient in temperate alley cropping systems. Nitrogen is lost via various biogeochemical processes such as volatilization, denitrification or leaching. Nitrogen is also lost when crop biomass is removed from the field following harvest. In addition, plants of the same species and growth stage can compete heavily for nitrogen when zones of depletion in the soil overlap with neighboring plants. Moreover, in alley cropping systems, competitive forces can be even more intense, as most tree species have the bulk of their fine, feeder roots in the top 30 cm soil layer, thus placing them in a zone of competition with crop species for water and nutrients (Rao et al., 1993; Lehmann et al., 1998). Thus, tree-crop systems must be properly designed and managed in order to maximize fertilizer use efficiency and minimize deleterious effects of competition on crop yield.
The extent of competition between two species will depend on factors such as nutrient and water availability, root architecture, rooting depth and proximity to competing roots, and temporal nutrient demand (Jose et al., 2000a). In addition, the peak intensity of nutrient demand in trees and crops may differ by several months, as trees tend to exhibit highest nutrient demand in spring during leaf formation, and crops such as cotton would be at highest demand in mid-summer during boll formation.
Euqlly important to system productivity and sustaibaility is the fate of nitrogen fertilizer and its effect upon groundwater quality. On a national scale, over-application of N increases the production costs of farmers by millions of dollars each year (USDA, 1998a). Moreover, because nitrates are highly soluble, they are easily transported through the soil matrix (Aelion et al., 1997), where they may be carried away by runoff, or leached through the soil profile into the water table (USDA, 1998a; Nair et al., 1999). Such contamination can lead to pollution of drinking water wells, as well as create conditions for eutrophication and related ecological disruptions of rivers, lakes, estuaries and aquifers (Johnson and Raun, 1995; USDA, 1998a,b; Bonilla et al., 1999; Ng et al., 2000). From a human health standpoint, nitrate is of concern in drinking water because it can cause a respiratory deficiency known as methemoglobinemia (‘blue baby syndrome’) in infants under six months of age, and similar problems in older adults (Sawyer et al., 1994; Baker, 1998; Bonilla et al., 1999; Ng et al., 2000; Reddy and Lin, 2000).
In this regard, the effect of trees in alley cropping systems is of interest due to the mechanism of nutrient capture, in which deep roots of trees serve as a ‘safety net’ for capturing nitrates that leach below the root zone of crops (van Noordwijk et al., 1996; Rowe et al., 1999). At lower depths, tree roots can exploit subsoil nitrate and other nutrients beyond the rooting depths of crops. A portion of these nutrients that are absorbed by the trees are later returned to the soil surface through decomposition of fine roots and litterfall, representing a gain to the soil nutrient pool (Nair 1993; Jose et al., 2000b). This phenomenon is of importance because it serves as a possible mechanism for groundwater clean-up.
Pecan-based alley cropping systems offer potential for Southern landowners, given the large number of pecan orchards in the southeastern USA, and the possible environmental and financial benefits that may be accrued from such systems. However, competition for nitrogen, nitrogen mineralization and the movement of nitrogen in pecan-cotton alley cropping systems remain unstudied, albeit critical in affecting the productivity and sustainability of such systems. While nitrogen losses cannot be avoided completely, losses can be minimized through appropriate fertilizer and orchard management practices and by knowledge of how nitrogen moves in the soil-tree system (Herrera and Lindemann, 2001).Thus, more understanding is needed of the interactive dynamics of nitrogen in tree-crop systems, in order to maximize fertilizer use efficiency and optimize production from each component.
A three-year research project was conducted at the West Florida Research and Education Center Research Farm of University of Florida in Jay, FL to examine the competitive interactions involving nitrogen in a pecan-cotton alley cropping system. Pecan and cotton were chosen for the study because of their social and economic importance to producers in the Southeast. The study was undertaken with the following three objectives:
1.To quantify competition for nitrogen between pecan and cotton using 15N labeled fertilizer
2.To determine the effect of tree-crop competion on ammonfication, nitrification, and mineralization; and
3.To determine the degree to which nutrient uptake in trees affects groundwater ammoinum and nitrate levels in this system.
Study area and configuration:
This study was conducted at the West Florida Research and Education Center Farm of the University of Florida, located near Jay in northwestern Florida, USA (30°89’ N Lat., 87°13’ W Long.). The climate is temperate with mild winters and hot, humid summers. The soil at this site is classified as a Red Bay sandy loam, which is a fine-loamy, siliceous, thermic Rhodic Paleudult, and the average water table depth at the site is 1.8 m. For this study, a pecan-cotton alley cropping system was initiated in Spring 2001 from an existing orchard of pecan trees that had been planted at a uniform spacing of 18.28 m in 1954 and that had remained under grass cover until the current study. Ten plots were established within the orchard and arranged into five blocks using a randomized complete block design. Each plot, which consisted of two rows of trees oriented in an E/W direction, was 27.43 m long and 18.28 m wide, with a practical cultivatable width of 16.24 m, and was separated from its adjacent plot by a border area of the same dimensions. To assess tree root competition for nitrogen fertilizer, each block was randomly divided into a ‘barrier’ plot and a ‘no barrier’ plot. ‘Barrier’ plots were subjected to a root pruning treatment in the spring of 2001 in which a trenching machine was used to dig a 1.2 m deep trench along both sides of the plot at a distance of 1 m from the trees. A double layer of 6-mil polyethylene sheeting was then used to line the ditch, after which the trench was backfilled. ‘No barrier’ plots did not receive this root pruning treatment. The ‘barrier’ plots thus served as the no-competition treatment, while the ‘no barrier’ plots served as the tree-crop competition treatment.
For this study, cotton (DP 458 B/RR) was planted in rows 0.96 m apart, at 16 rows per alley, in a N/S orientation, on 16 May 2001, following disking of the alleys. Conventional insecticide and herbicide were applied during the growing season as recommended. In each plot, one microplot (2.60 m x 0.76 m), containing 8-10 plants, was established on the first, fourth and eighth rows of cotton, respectively (going west to east in each plot). To quantify nutrient competition, 15N enriched fertilizer ((NH4)2SO4, 5% atom enrichment) was uniformly hand-applied to microplots at a rate of 89.6 kg N ha-1, on 19 June 2001, at the same time, rate and formulation as the regular fertilizer application (Timmons and Cruse, 1990). Each microplot was arranged so that one of the pecan trees in the tree row was in the center and could serve as the target tree for 15N sampling (Figure 1).
Six plants (aboveground portions) from each microplot were sampled for 15N content in leaf, stem and boll components. Cotton leaf samples were collected prior to leaf senescence, on 8 November 2001. The same plants were harvested at physiological maturity on 4 December, and separated into stem and boll components. In addition, foliar samples from each associated tree were collected on 10 October 2001. For this purpose, the tree canopy was divided into an upper and lower half, and leaves were collected via shotgun harvest method from all four cardinal directions in both the halves, to provide one composite sample per tree. Following collection, all plant tissue samples were air dried at 65°C for 72 hours. In preparation for combustion analysis, all green plant tissue samples (cotton leaves and stems, and tree leaves) were ground with a model 4 Wiley Mill (Arthur H. Thomas Company, Philadelphia, PA) to pass through a 1 mm screen, and then re-ground using a burr coffee grinder. All grinders were thoroughly cleaned between samples to prevent cross-contamination of the 15N plant material. Cotton lint was de-seeded and manually shredded in preparation for analysis. Soil cores, measuring 120 cm in length and 5 cm in diameter, were collected in pairs at random points within each microplot in January 2002, using a tractor-mounted hydraulic corer and polyethylene collection tubes. The cores were divided into 30 cm increments to a depth of 120 cm, composited for each microplot depth, air dried, and a subsample was fine-ground with a mortar and pestle. For determination of total N and 15N concentrations, subsamples of the ground plant material and soil samples were analyzed by the University of Florida Geological Sciences Department (Gainesville, FL) using a Finnigan-MAT DELTAplus isotope ratio mass spectrometer with a ConFlo III interface attached to a Costech ECS 4010 elemental analyzer (Schepers et al., 1989). Percent nitrogen derived from fertilizer, percent utilization of fertilizer nitrogen, and percent nitrogen recovery in soil, were calculated from the enrichment data to determine the degree of interspecific competition for nitrogen.
Monthly ammonification, nitrification, and N mineralization were determined from July 2001 to October 2002 using the in-situ buried bag technique (Eno 1960) at specific distances (0, 1.5, 4.2 and 8.4 m from tree) in tree rows and alleys (Figure 1). For each of 10 (later 12) plots, soil was collected from 7 sites per plot: at tree base, on the first, fourth and eighth rows of cotton, and at commensurate distances in the tree row. For each sampling site, soil samples were collected in pairs in the top 10 cm of soil using a hand spade, after which half were removed to the lab for processing and half were placed into 0.05 mm-thick Fisherbrand polyethylene zipper-seal bags (12.7 cm x 20.32 cm) and returned to the soil profile for month-long incubation. Coarse roots and large organic debris were removed by hand to avoid extraneous N immobilization during incubation. Following incubation, the in-situ samples were collected and transported to the laboratory for processing. All soil samples were kept at 4ºC until processing.
From each soil sample, a 20 g subsample was separated for ammonium and nitrate extraction, and an 18-22 g subsample was collected for water content determination. Soil water content was determined gravimetrically by drying a subsample for 24 hours at 105ºC. For N extraction, 20 g of soil was mixed with 50 ml of 1 M KCl solution in a 120 ml sample vial, shaken for 1 hr using a Lab-Line Orbit Shaker (Barnstead International, Dubuque, Iowa), allowed to equilibrate for 24 h, and 20 ml of extractant was gravity filtered and then frozen in 20 ml scintillation vials (Keeney and Nelson 1982).
The samples were analyzed for NH4-N and NO3-N content by the Analytical Research Laboratory of the University of Florida (Gainesville, Florida) using an Alpkem Flow Solution IV semi-automated spectrophotometer according to EPA methods 350.1 (for NH4-N) and 353.2 (for NO3-N). Values were expressed in mg kg-1 of dry soil.
Assuming no N losses to plant uptake, leaching or volatilization, monthly net ammonification, nitrification and mineralization rates were calculated (Hart et al. 1994, Reynolds et al. 2000)
Soil solution (free soil water that is not in equilibrium with the soil matrix) (Weston and Attiwill, 1996) was sampled 1-2 times monthly over a 15-month period from ceramic cup lysimeters installed in pairs at depths of 0.3 and 0.9 m at specific distances of 1.5, 4.2 and 8.4 m from a reference tree in each plot (Figure 1). Lysimeters were fitted with a highly porous (~45% porosity) ceramic cup (Soil Moisture Equipment Corp., Santa Barbara, CA) that allowed for collection of soil solution 24-48 hr after application of a vacuum (30-50 kPa) (Talsma et al., 1979). Samples were collected in 20 ml scintillation vials and kept frozen until analysis. Samples were analyzed for NH4-N and NO3-N concentrations by the Analytical Research Laboratory of the University of Florida (Gainesville, FL) using spectrophotometric analysis. Data for each month were averaged across rows to produce one observation per plot at each depth since initial analysis revealed no row effect (van Miegroet et al., 1994). In addition, a Hydrosense (Decagon Devices, Pullman, WA) water content reflectometry soil moisture probe was used on a monthly basis to determine volumetric water content within a 12 cm surface layer.
Our results indicate that cotton plants are subject to competition for nitrogen and perhaps water. Competition for nitrogen was alleviated to a great extent by the application of fertilizer nitrogen. Further, nitrogen uptake and allocation patterns in both pecan and cotton were influenced largely by temporal difference in N demand and the abundance of mineralized nitrogen in soil. We observed increases in nitrogen content of cotton in the presence of root barrier, although the barrier had no significant effect on pecan leaf nitrogen concentration or canopy nitrogen content. NDF was lower for cotton in ‘barrier’ plants, indicating that cotton in this treatment was taking up a higher percentage of its nitrogen from nitrogen already present in the soil. However, NDF in pecan was minimal, indicating an early and substantial uptake of N prior to the cotton season and fertilizer application. Total UFN was higher in ‘barrier’ cotton plants, indicating a greater ability to utilize the available fertilizer efficiently. In soil, depth was the primary factor influencing nitrogen recovery, although a slight trend was observed at lower depths in the ‘no barrier’ treatment, where N levels were somewhat lower than levels in ‘barrier’ treatment. Apparently, fertilizer nitrogen was taken up by tree roots from these deeper horizons.
Overall, while direct competition for nitrogen fertilizer appears to have been minimal, the alley cropping system in this study exhibits potential for nutrient capture and increased fertilizer use efficiency, given the ability of pecan trees to intercept nitrogen fertilizer and to provide litterfall to the cropping zone.
Temporal variations in net ammonification, nitrification and mineralization were driven primarily by environmental factors (e.g., soil moisture content and soil temperature), and by initial ammonium and nitrate levels. However, these and other factors appear to have exerted a combined influence on N transformations over the study period. Competitive interactions for resources such as water and nitrogen resulted in a decreased ability for nitrogen uptake in plants in the non-barrier treatment compared to those in the barrier treatment. This, in turn, may have resulted in a higher build-up of soil N in the non-barrier treatment. Effects of the pre-trial fallow period appear to have diminished by the second growing season.
the results of our study indicate that the competitive presence of trees can be utilized to decrease soil ammonium and nitrate concentrations and reduce N leaching in alley cropping systems. The “barrier” treatment had the potential to leach 23.79 kg N ha-1 yr-1 down below 0.9 m depth. The “no barrier” treatment exhibited a significantly lower potential for leaching with only 8.21 kg N ha-1 yr-1 below 0.9 m. For both treatments NO3-N accounted for 99.9% of the total inorganic nitrogen. These findings will improve our understanding of nitrogen dynamics in temperate alley cropping systems, which in turn, will help in designing systems that can utilize the ‘safety net’ role to maximize fertilizer use efficiency while minimizing groundwater nutrient-pollution.
Educational & Outreach Activities
1.Jose S., Allen S.*, and Nair P.K.R. 2003. Ecological interactions: Lessons from temperate alley cropping systems. In Agroforestry. Batish S. and Singh H.P. (eds). Haworth Press, New York. In press (invited)
1.Allen, S.*, Jose, S., Nair, P.K.R., and Brecke, B.J. Competition for nitrogen in a pecan-cotton alley cropping system in the southern United States. Plant and Soil (in press)
2.Jose, S., Allen, S.*, and Nair, P.K.R. Tree-crop interactions: Lessons from temperate alley cropping systems. Journal of Crop Production (in press)
3.Allen, S.*, Jose, S. Nair, P.K.R., Brecke, B.J., Ramsey C.L. 2004. Experimental evidence for the safety-net hypothesis. Forest Ecology and Management (in press)
Allen, S. 2003. Nitrogen dynamics in a pecan-cotton alley cropping system. Ph.D. dissertation, University of Florida, Gainesville, FL.
1.Jose, S. 2003. Tree-crop interactions in temperate alley cropping: Ecological principles and evaluation techniques. Ecology Seminar Series, School of Environmental Science and Management, Southern Cross University, October 3, Lismore, NSW, Australia (invited).
2.Allen, S.C.*, Jose, S., Nair, P.K.R., Nair, V.D., Graetz, D. 2003. Competition for nitrogen in a temperate alley cropping system with pecan and cotton. Eight North American Agroforestry Conference, June 22-25, Corvallis, Oregon.
3.Allen, S.*, Jose, S., Nair, P.K.R., Nair, V.D., Graetz, D., and Ramsey, C.L. 2003. Nitrogen mineralization in a temperate alley cropping system in the southern United States. ASA-CSSA-SSSA Annual Meeting, November 2-6, Denver, CO.
4.Allen, S.*, Jose, S., Nair, P.K.R., Brecke, B.J, and Ramsey, C.L. 2003. Experimental evidence for the safety-net hypothesis from a temperate alley cropping system. ASA-CSSA-SSSA Annual Meeting, November 2-6, Denver, CO.
1. Workman S., Allen, S.C.*, Jose, S. 2003. Alley cropping combinations for the southeastern U.S. Florida Cooperative Extension Service Fact Sheet, FOR 106. 6p. UF/IFAS EDIS Database, http://edis.ifas.ufl.edu/BODY_FR142
Outcome from the agroforestry project will be used to provide a profile of the interaction effects of trees and crops on the status of nitrogen and groundwater nitrate levels in temperate alley cropping systems. Findings from crop yields, soil moisture measurements, and water, soil and plant tissue analyses, will shed light on crop and tree root activity, and their effects on site environmental conditions. This knowledge will help to improve our basic understanding of temperate alley cropping systems, so that better systems can be created, with tighter nutrient cycling and reduced groundwater pollution.
Ultimately, it is hoped that this research will encourage farmers and landowners to adopt agroforestry practices. The researchers believe that such systems can help to diversify and strengthen the family farm, by providing alternate forms of income at various times of the year, while utilizing land that would otherwise remain unused. We also hope that landowners would be encouraged to plant longleaf pines on their property, given the possibility of incorporating this species with other more immediate cash crops. The prospect of reducing nitrate levels in groundwater is also an exciting possibility for these types of systems, which is a vision that we hope landowners are able to catch.
Areas needing additional study
Further reserach is needed to
1. Incorporate information from above and belowground interaction studies into a process level agroforestry model so that production dynamics can be predicted for a suite of species and management practices
2. Investigate the “safety-net” role of tree roots in younger agroforestry systems.