Complex crop rotations improve organic nitrogen cycling

Nitrogen (N) availability in agroecosystems is often poorly coupled to plant N uptake, leading to inefficient fertilizer use and environmental losses. Building soil organic N pools and enhancing internal recycling of N with crop rotations while reducing synthetic inputs may help improve N use efficiency. The organic N pool could be a valuable source of N that could help farmers reduce reliance on large inorganic N inputs if controls on its availability were better understood. While we know that the breakdown of high-molecular weight organic N compounds is the rate-limiting step to accessing bioavailable N from organic sources in natural ecosystems, there has been little work in agroecosystems to identify how management influences this inflection point in the N cycle. To provide insight into how growers can manage the organic N pool to reduce fertilizer input, we examined gross protein depolymerization rates within an agricultural context. Specifically, we investigated 1) how crop rotations affect organic N pools and alter the rate of organic N cycling, and 2) whether inorganic N fertilization enhances, has no effect, or suppresses soil N cycling responses to crop rotation. To test this, we measured gross rates of protein depolymerization, amino acid consumption, mineralization, and ammonium consumption using 15N isotope pool dilution assays on soils collected from a long-term crop complexity experiment near Mead, NE, USA. Treatments sampled included both 0 kg and 180 kg N ha−1 fertilization levels in continuous corn, corn-soybean, and corn-soybean-sorghum-oat/clover rotations. We found that higher cropping complexity coupled with zero fertilization increased gross depolymerization and amino acid consumption rates by 193% and 93%, respectively, relative to fertilized, monocrop plots. Gross mineralization was 2.7 and 3.9x higher in complex rotations than corn-soybean and continuous corn rotations, respectively, while ammonium consumption was 4x higher in fertilized plots than unfertilized plots across all cropping regimes. We show that within our study system internal N cycling is stimulated by cropping system complexity; however, N fertilization suppresses some of the benefits of temporal crop diversification. Balancing reduced mineral fertilizer application rates with increased cropping complexity has the potential to promote internal N cycling while minimizing N losses in agroecosystems.