Safety-net role of tree roots: evidence from a pecan ( Carya illinoensis K. Koch)–cotton ( Gossypium hirsutum L.) alley cropping system in the southern United States

Abstract

Excessive application of nitrogen (N) fertilizer in commercial agriculture and forestry can result in leaching of nitrate (NO 3-N) into surface and subsurface drainage water. Temperate alley cropping is considered to be a potentially useful land use practice for mitigating this problem because deep roots of trees could serve as a “safety net” for capturing the N that is leached below the root zone of crops. This hypothesis was tested in a pecan ( Carya illinoensis K. Koch)–cotton ( Gossypium hirsutum L.) alley cropping system in northwestern Florida from June 2001 to August 2002. A belowground polyethylene root barrier was used to isolate tree roots from cotton alleys in half the number of test plots in order to provide two treatments—barrier and non-barrier. Soil water nutrient concentrations were quantified 1–2 times monthly from lysimeter water samples collected from two depths (0.3 and 0.9 m) at specific distances (1.5, 4.2 and 8.4 m from tree) in alley rows. Using this data, nutrient leaching rates were calculated using water drainage estimates derived from the LEACHMN soil modeling program. Overall, ammonium concentrations in soil solution were found to be close to the minimum limit of detection, a result similar to other lysimetric studies. Nitrate concentrations varied by depth but not by treatment in both growing seasons, with 0.9 m levels being significantly lower than 0.3 m levels. However, N leaching rates were found to be lower in the non-barrier treatment compared to barrier treatment, and also lower at 0.9 m depths in both treatments compared to 0.3 m depths. Cumulative amounts of nitrate leached during the study period were 63.83 and 13.05 kg ha −1 for 0.3 and 0.9 m depths, respectively, in non-barrier treatment, compared to 121.94 and 45.56 kg ha −1 for respective depths in barrier treatment, representing a significant reduction in total NO 3-N leached at both depths. Hence, it is likely that tree roots were able to capture N in the non-barrier treatment, resulting in lower rates of leaching below the root zone. Further, tree water uptake, in addition to cotton water uptake in the non-barrier treatment, may have decreased water drainage in comparison to the barrier treatment, thereby influencing leaching rates. It appears that tree roots play a significant role in alleviating groundwater nitrate leaching through their safety-net role in temperate alley cropping systems.