Diversified cropping systems with limited carbon accrual but increased nitrogen supply

Diversified cropping systems offer a chance to mitigate environmental impacts of conventional agriculture, but effects on soil organic carbon (SOC) sequestration and nitrogen (N) dynamics remain debated. We integrated a 20-year field experiment and laboratory measurements with three stable-isotope-enabled mechanistic models to examine SOC stocks and decomposition in a conventional corn–soybean system and two more diversified systems including small grains, legumes and manure inputs, in addition to corn and soybean. Contrary to the prevalent hypothesis that diversified systems increase SOC, we found no differences in 0.3 m topsoil or 1 m profile SOC and N stocks. Diversified systems markedly increased N mineralization rates and decomposition of older SOC from previous corn inputs. Models revealed that increased C decomposition with residence times of months to years counteracted higher C inputs but increased N supply. Our findings highlight a critical trade-off between C storage and N supply in these diversified systems, demonstrating that key climate benefits may arise from decreased N fertilizer use, not SOC sequestration.