Competition for ¹⁵N labeled nitrogen in a loblolly pine-cotton alley cropping system in the southeastern United States

The ecological sustainability of agroforestry systems is dependent on minimizing competitive interactions between system components. However, our understanding of resource competition and resource use efficiency in agroforestry systems such as alley cropping is limited. The objective of this study was to quantify the extent of competition for nitrogen (N) between loblolly pine (Pinus taeda L.) and cotton (Gossypium hirsutum K. Koch.) and its effect on fertilizer-use efficiency and N movement, and the role of trees in capturing the N that is leached below the root zone of cotton. Two pine-cotton alley cropping systems, established in 1999, with narrow (8 m alley width) and wide (16 m width), were studied. Belowground competition between pines and cotton was eliminated through the installation of a belowground polyethylene root barrier in half the number of plots to provide two treatments - barrier and non-barrier. Percentage of N derived from fertilizer (%NDF) and fertilizer-use efficiency (UFN) were determined using ¹⁵N-enriched ammonium sulfate (5% atom enrichment) applied at 89.6 kg N ha^-1. The barrier treatment in both the narrow and wide alley resulted in higher total cotton biomass (36% and 14%, respectively) compared to the non-barrier treatment. Mean %NDF of cotton was significantly lower in barrier treatment in both systems, representing 14% and 55%, respectively, for the narrow and wide alleys, compared to the non-barrier treatment. For %UFN, this trend was reversed, with plants in barrier treatment having a higher %UFN. Root trenching did not affect loblolly pine foliar N concentration, NDF and UFN, but it affected total leaf N content. In soil, N recovery at 90-120 cm depth was lower in non-barrier treatment, indicating tree root uptake of fertilizer N. It is likely that tree roots were able to capture N in non-barrier treatment, resulting in lower rates of leaching below the root zone. The alley cropping systems in this study demonstrates potential for efficient N cycling, given the apparent ability of loblolly pine to intercept and uptake fertilizer from deeper soil layers and return to surface soil via litterfall.