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Environmental Benefits of
Agroforestry Systems, Nutrients
Rationale/Objectives It has been hypothesized that agroforestry systems can minimize nutrient losses from the soil because of enhanced nutrient uptake by tree and crop roots from varying soil depths, compared to more localized and shallow rooting depths of sole crop stands. Thus, agroforestry practices may reduce runoff and leaching of nutrients from farmland, and thereby improve environmental quality. Our objectives were: i) to evaluate nitrogen and phosphorus dynamics in the soil at pecan + cotton intercropping experimental plots at Jay, FL and compare nutrient movement (from both organic and inorganic sources) to that in an adjacent control plot, and ii) to evaluate nutrient transport from a slash pine silvopastoral system at Ona, FL and compare the transport to that from an adjacent treeless pasture and a native silvopasture (native flatwood vegetation). Work Done At Jay: This study was conducted in collaboration with Dr. Shibu Jose at the West Florida Research and Education Center farm of the University of Florida, located near Jay in northwestern Florida. The soil at Jay is classified as an Ultisol – a Red Bay sandy loam (fine-loamy, kaolinitic, thermic Rhodic Kandiudult). For the study, a total of 90 soil samples were collected from 12 experimental plots and 3 control areas in a pecan (Carya illinoensis)-cotton (Gossypium sp.) alley-cropping system in early 2002, following initial cotton harvest in late 2001. For each plot, three 1-m deep soil samples were composited at each depth (0-5, 5-15, 15-30, 30-50, 50-75, and >75 cm) within a soil profile. Total carbon and nitrogen, soluble reactive phosphorus (SRP, 0.01 M CaCl2 solution), Mehlich 1-extractable P, Ca, Mg, Fe, and Al, pH, as well as NH4-N and NO3-N were determined for each sample collected. After this initial soil collection, poultry manure was applied in 2002 to 6 plots and inorganic fertilizer was applied to 6 of the other plots at the same rate of nitrogen application. For both fertilizer sources, three out of the 6 plots were trenched (i.e., the root zones of the pecan trees and the cotton plants were separated), and three were untrenched, to show effects of interspecific competition. Three additional sole-tree plots served as a control area (i.e., no treatment). Cotton was planted at all 12 plots and was harvested in November 2002. Soil samples were again collected in early 2003 from each of the above plots by depth (total 90 samples) with the same chemical parameters for determination as before. Comparison is being made of the soil parameters before and after the treatments, by depth. Phosphorus and NH4-N and NO3-N transport from the manure and fertilizer plots will be evaluated and compared to concentrations in the control plots. The entire experiment was repeated for the 2003 cotton-growing season. Follow-up soil samples of the same plots were collected in March 2004, and analyses (similar to the initial and second soil samplings) are currently being conducted. At Ona: This study was conducted in collaboration with Dr. Rob Kalmbacher at the Range Cattle Research and Education Center of the University of Florida, located near Ona in central Florida. The soil at Ona is classified as a Spodosol – Ona fine sand (sandy, siliceous, hyperthermic Aeric Alaquod). Pasture species present are bahiagrass (Paspalum notatum) with the legumes Desmodium heterocarpon and Vigna parkeri. The silvopasture system under study is planted to slash pine (Pinus elliottii) at 500 trees ha-1. The open pasture has been fertilized annually with 56 kg N ha-1 for the past 15 years. The silvopasture has received 5 kg P ha-1 for the past 5 years, and the open pasture 4 kg P ha-1 once (2003) in the past 20 years. Grazing density for pasture is 1 cow/calf pair ha-1 for the past 20 years, and for range is 1 cow/calf pair 7 ha-1 for the past 60 years. For the current study, which began in late 2002, ten soil profiles each were sampled at successive depth (0-5, 5-15, 15-30, 30-50, 50-75, and 75-100 cm) in the alleys of the silvopastoral site (thinned), and another 10 profiles between the tree rows. Soil profiles (10 profiles each) were also collected from an un-thinned area in the silvopasture, and from an adjacent grazed pasture area and a nearby native flatwoods vegetation site. All 426 samples corresponding to 70 soil profiles (GPS locations available) were subjected to the same chemical analyses as for the Jay samples. A second set of soil samples were collected (approximately 500) from the silvopasture and the regular pasture in January 2004. These samples are currently being processed for SRP, NH4-N and NO3-N. Results will also be used for computer projection of nutrient concentrations and Bh-horizon by depth using 3-D spatial programming software. Results and Conclusions At Jay: First-year results showed that NO3-N concentration in the surface soils of the control plots (pecan tree plots with no fertilizer applications) were lower compared to both the fertilized nontrenched plots (F-NT) and the fertilized trenched plots (F-T), representing alley cropping and sole crop stands, respectively. For Year 2, NO3-N was highest in the surface soils of F-T. Trenching seemed to increase the presence of NO3-N in the lower soil profiles, suggesting that more NO3-N was taken up by the agroforestry system where there were interactions between the rooting systems of the pecan and cotton cropping systems. The NO3-N concentrations in the soil profiles of F-T seem to have moved much more through the soil profile to the 1-meter depth sampled compared to both the F-NT plots and the control plots. Although absolute values for NO3-N are low throughout the soil profile compared to concentrations found in other agricultural systems, we recognize that this is due to a limited fertilizer application (two years of study) and project the consequences on water quality due to repeated applications of fertilizer in an agricultural farm to be problematic. NH4-N concentrations were higher for Year 2 than Year 1 at all soil depths and treatments. Some increases in Mehlich 1-P (soil test P) and water soluble P were noted after the second soil sampling, particularly in the surface soils, both for the manure-treated and inorganic fertilizer trials. Manure P sources seem to support higher phosphorus concentrations than the inorganic fertilized soils. However, these trends can only be confirmed after analysis of the third set of soil samples are completed, and the results statistically analyzed. At Ona: Both NH4-N and NO3-N were higher in all surface soils, and decreased down the soil profile. Soluble reactive phosphorus (SRP) concentrations in the surface soils varied in the order: treeless pasture > silvopasture > native silvopasture, although the treeless pasture has received only a single inorganic P application in 20 years. Soluble reactive P concentrations also varied down a soil profile in the order: pasture > silvopasture > native flatwood vegetation. This finding suggests that the silvopastoral system is more efficient in removing P from the soil profile than a regular pasture and therefore might contribute significantly in reducing nonpoint source pollution. The deeper tree roots in the silvopastoral site could have removed more P from the lower depths. The native flatwoods had the lowest SRP concentrations, since the site had never been fertilized. The spodic horizon with its high P retention capacity was at an average depth of 30 cm for all the three sites, and would explain the general reduced SRP concentrations beyond this depth. However, because of the high water table there is the likelihood of lateral P loss before reaching the spodic horizon at these sites. The above two experiments on highly different soil types, involving both fertilizer and animal manure applications, can be taken as indicative of the likely benefits of agroforestry practices in reducing nutrient leaching through the soil profile. Work planned for 2004 – end of contract period Analyses are currently being conducted on all Jay soils collected after the third cotton harvest. Comparison will be made of the soil parameters before and after the fertilizer treatments, by depth. Phosphorus and NH4-N and NO3-N movement from the manure and fertilizer plots will be evaluated and compared to concentrations in the control plots. The flatwood soil samples from Ona (first set) have been analyzed for the soil chemical parameters. The second set of samples will be analyzed only for P, NH4-N and NO3-N. These analyses will not only provide information on soil nutrient movement under silvopastoral systems, adjacent treeless pastures, and unfertilized tree-only areas, but will provide useful background information for future experimental trials. Movement of nutrients in these flatwood soils (Spodosols) is likely via both leaching and runoff, and different from P movement in the Red Bay sandy loam at Jay (predominantly via runoff). Two presentations, one on the Jay trials and the other on the Ona experiments will be presented at the 1st World Congress of Agroforestry in Orlando, in June/July 2004. These are "Tree-Crop Competition for Nutrients in a Pecan-Cotton Agroforestry System in the Southeastern USA" (Allen, Nair, Jose and Graetz), and "Phosphorus and Nitrogen Dynamics in Silvopasture, Open Pasture and Rangeland in South-central Florida, USA" (Nair, Allen, Kalmbacher, Graetz, Ellis, and Ezenwa). In addition, information generated from the Ona experiments will be presented at a Silvopasture workshop featured as part of the Congress activities. Publications/Outputs Allen, S.C., S. Jose, P.K.R. Nair, B.J. Brecke, V.D. Nair, D.A. Graetz, and C.L. Ramsey. 2004. Nitrogen mineralization under a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system in the southern United States. Biology and Fertility of Soil (in review). Allen, S.C., V.D. Nair, S. Jose, and D.A. Graetz. 2004. Tree-crop competition for nutrients in a pecan-cotton agroforestry system. 1st World Congress of Agroforestry, June 27-July 2, 2004, Orlando, FL. Book of Abstracts (in press). Nair, P.K R., and V.D. Nair. 2003. Carbon storage in North American agroforestry systems. Chapter 20. pp. 333-346. In J. Kimble, L. Heath, R. Birdsey, and R. Lal (eds.), The Potential of U.S. Forest Soils to Sequester Carbon and Mitigate the Greenhouse Effect. CRC Press, Boca Raton, FL. Nair, P.K.R., M.E. Bannister, V.D. Nair, J.R.R. Alavalapati, E.A. Ellis, S. Jose, and A.J. Long. 2004. Silvopasture in southeastern United States: More than just a new name for an old practice. p. 52 In Book of Abstracts (eds.) M.R. Mosquera-Losada, J. McAdam, and A. Rigueiro-Rodriguez. Silvopastoralism and Sustainable Management International Congress, Lugo, Spain. Crop Production Department, High Politechnic School Universidad Santiago de Compostela, Unicopia, Lugo. Nair, P.K.R., M.E. Bannister, V.D. Nair, J.R.R. Alavalapati, E.A. Ellis, S. Jose, and A.J. Long. 2004. Silvopasture in southeastern United States: More than just a new name for an old practice. Silvopastoralism and Sustainable Management International Congress, Lugo, Spain, April 18-24, 2004. CAB International (in press). Nair, V.D., and R.S. Kalmbacher. 2004. Silvopasture as an approach to reducing nutrient loading of surface water from farms. p. 141 In Book of Abstracts (eds.) M.R. Mosquera-Losada, J. McAdam, and A. Rigueiro-Rodriguez. Silvopastoralism and Sustainable Management International Congress, Lugo, Spain. Crop Production Department, High Politechnic School Universidad Santiago de Compostela, Unicopia, Lugo. Nair, V.D., and R.S. Kalmbacher. 2004. Silvopasture as an approach to reducing nutrient loading of surface water from farms. Silvopastoralism and Sustainable Management International Congress, Lugo, Spain, April 18-24, 2004. CAB International (in press). Nair, V.D., and D.A. Graetz. 2004. Agroforestry as an approach to minimizing nutrient loss from heavily fertilized soils: The Florida experience. Agroforestry Systems 61-62 (in press). Nair, V.D., R.S. Kalmbacher, S.C. Allen, D.A. Graetz, E.A. Ellis, and I. Ezenwa. 2004. Phosphorus and nitrogen dynamics in silvopasture, open pasture and rangeland in south-central Florida. 1st World Congress of Agroforestry, June 27-July 2, 2004, Orlando, FL. Book of Abstracts (in press). Invited presentation: (Environmental Sciences Staff Meeting, St. Johns River Water Management District, Palatka, Florida). V.D. Nair. Cattle, Soils and Water Quality: Spodosols of the Lake Okeechobee Basin. March 11, 2004. St. Johns River Water Management District, 4049 Reid Street. Palatka, FL 32178-1429.
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