Tracing crop residue derived nitrogen into subsequent crops and nitrous oxide emissions

Abstract

Research has demonstrated that including winter wheat with under-sown red clover into corn-soybean rotations has the potential to improve crop yields and N use efficiency. Yet, the mechanisms that explain these benefits are poorly understood. One possible explanation is that the crop rotation ‘diversification’ improves the soil N supply or that the soil N cycle ‘tightens’— thereby lowering potential N losses. To address this hypothesis, an isotope tracing experiment was setup i) to follow the fate of enriched 15N residues into subsequent soil and crop N pools; and ii) to measure N2O and CO2 emissions, and N residue decomposition dynamics. For my field experiment, natural abundance and enriched 15N urea were applied to 1 m2 micro-plots within a 37-yr long-term trial, where I had access to the ‘simple’ corn-corn-soybean-soybean (CCSS, SSCC) rotations; and ‘diverse’ corn--corn-soybean-wheat/red clover (CCSWrc, SWrcCC) rotations. These systems were maintained under conventional tillage or no-till. At harvest, a residue exchange operation was performed to transfer enriched 15N above-ground residues to 15N natural abundance micro-plots, and vise-versa, thus isolating enriched 15N above- and below-ground residue contributions. Subsequent crops were harvested and used to determine above- and below-ground previous year’s residue N contributions. For my soil incubation experiment in the lab, field soil cores were collected from the crop rotation and tillage treatments to establish 50 g soil microcosms that were amended with 15N-enriched corn stover or roots. Soil and gas samples were periodically collected to measure crop residue decomposition dynamics (via CO2 emissions and 15N mineralization) and 15N2O emissions. The field trial demonstrated that crop rotation had no impact on the overall crop residue N allocated to the subsequent crop systems. In contrast, no-till and below-ground residues increased corn residual N contributions to the subsequent crop, relative to conventional tillage and above-ground residues, respectively. Regardless, below-ground residual N pool contributed more N to subsequent crops than above-ground crop residue. The incubation results demonstrated higher residue-derived N mineralization, and greater overall N2O and CO2 emissions from ‘diverse’ vs. ‘simple’ rotations. Overall, my findings indicate that crop ‘diversification’ enhanced soil N stocks likely due to the additional N inputs (N fertilization or N fixation). Although ‘diversifying’ corn-soybean rotations with winter wheat and red clover may produce higher crop yields, it is necessary to adjust for nutrient credits or soil N surplus when applying N inputs year after year. Otherwise, N losses may be a side-effect and should be investigated at field scale.