Role of soil particulate organic matter accrual in 15N fate to grain corn comparing long-term, first-time and no cover cropping

Abstract

Long-term cover cropping increases soil organic matter; however, the effect of cover crop duration on nitrogen (N) uptake by subsequent crops remains unclear. We hypothesized greater N uptake by grain corn with long-term cover cropping, due to both cover crop decomposition and increased mineralization of particulate and mineral-associated organic matter. Testing this hypothesis is critical for optimizing N management and enhancing the adoption of cover cropping. We introduced a novel experiment by growing cover crops for the first time on a subset of no cover crop plots to compare long-term, first-time, and no cover cropping. We applied ¹⁵N (12 kg N ha^–1) in microplots after cover crop emergence and traced it into cover crops, corn, and soil (0–90 cm depth). During cover crop season (late fall and the following spring), long-term and first-time cover cropping had about double total ¹⁵N recovery in cover crop biomass and soil, compared to no cover cropping where only 30% of ¹⁵N applied was accounted for, indicating that both long-term and first-time cover cropping are equally effective at reducing potential overwinter N losses. At corn harvest, long-term and first-time cover cropping had approximately ten times greater ¹⁵N recovered in soil than in corn biomass, indicating cover crops likely contributed more to soil organic N retention than soil inorganic N supply to subsequent crops. Moreover, piecewise structural equation modeling demonstrated that particulate organic matter, but not mineral-associated organic matter, was the main source of soil mineral N for grain corn in the sandy loam site. This finding implies that mineral N supply depends more on the N pool size of soil organic matter fraction than on N mineralization rate. Overall, our findings demonstrated that building-up soil organic matter under cover cropping is critical for enhancing N availability in agroecosystems.