Reduced nitrogen losses after conversion of row crop agriculture to alley cropping with mixed fruit and nut trees

Abstract

Agriculture across the temperate zone is dominated by a maize-soybean rotation (MSR) characterized by a “leaky” nitrogen (N) cycle. MSR N losses have considerable negative impacts on water quality via N leaching and climate change via soil emissions of nitrous oxide (N2O), a potent greenhouse gas. Alley cropping (AC) focused on food- or fodder-producing tree crops has the potential to substantially reduce environmental N losses while maintaining agricultural productivity. To compare the N cycling of MSR and AC, this study (1) summarized literature values of N pools and fluxes in both systems, (2) directly measured N leaching and N2O emissions in a side-by-side trial of MSR and an establishing AC over four years, and (3) used AC yield projections to estimate the trajectory of yield-scaled N losses as AC grows to productive maturity. Ample literature data on MSR permitted the construction of a robust working N budget, while a paucity of existing data on N cycling in AC revealed gaps and high uncertainty in our existing knowledge. In the field trial, AC quickly reduced both N leaching and N2O emissions compared to MSR. Nitrate leaching at 50 cm depth in MSR ranged from 21.6 to 88.5 kg N ha−1 yr−1, whereas leaching was reduced by 82–91% in AC. Cumulative annual net N2O fluxes in MSR ranged from 0.4 to 2.0 kg N ha−1, but AC reduced annual fluxes by 25–83%. Overall, conversion of MSR to AC reduced unintended N losses over four years by 83% from 240 to 41 kg N ha−1. Even when accounting for the low yield in AC during the establishment years studied here, yield-scaled N leaching in AC and MSR were not significantly different. In contrast, yield-scaled N2O fluxes were an average of 4.8 times higher in AC across years and were only estimated to reach a comparable range to MSR after reaching productive maturity. Our results demonstrate rapid tightening of the N cycle and a competitive trajectory of yield-scaled N losses as row crop agriculture is converted to AC.