Nitrogen dynamics in a pecan (Carya illinoensis K. Koch)-cotton (Gossypium hirsutum L.) alley cropping system in the southern United States

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.