- Agronomic: cotton
- Crop Production: agroforestry
- Education and Training: demonstration
- Production Systems: holistic management
- Soil Management: nutrient mineralization, organic matter, soil quality/health
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.