Long‐Term Consequences of Crop Residue Retention: Impact of Higher Soil Organic Carbon on N ₂ O Emissions

Long‐term management of crop residues strongly influences soil organic carbon (SOC) dynamics, what may also influence nitrous oxide (N 2 O) emissions, two key components determining the impact of agricultural systems on climate change. We investigated N 2 O fluxes in a long‐term experiment after four decades of continuous residue retention or residue export. Gas flux measurements were collected over 28 months, encompassing both crop‐growing and bare‐soil periods. Cumulative N 2 O emissions between crop residue management strategies were compared against long‐term SOC stock differences. Soil microbial biomass and extractable C and N were also investigated. Residue retention showed lower total N 2 O fluxes and higher SOC stocks and microbial biomass in the top 25 cm of soil. Our results suggest that an increased microbial immobilization of N and enhanced microbial processing of soil organic matter accumulated over the years reduced the availability of C and N for denitrification. This was supported by the enhanced levels of microbial biomass and slightly higher porosity under crop residue retention, being the main link to the lower N 2 O emissions, despite higher SOC. When expressed in CO 2 equivalents, the yearly difference in N 2 O emissions between the residue treatments accounts for only 1.8% (≈373 kg CO 2 ‐eq ha −1 a −1 ) of the long‐term climate mitigation benefit from higher SOC stocks (≈21 Mg CO 2 ‐eq ha −1 ) after 40 years of continuous management. Our findings indicate that long‐term residue retention in this site contributes to climate change mitigation by synergically increasing long‐term SOC storage and reducing N 2 O emissions. In this sense, managing and monitoring both SOC and N 2 O emissions are a crucial basis for continuous climate change mitigation in croplands. Further mitigation studies looking for this aim should also evaluate additional strategies for more ambitious management of N input, especially when increases in SOC may diminish after approaching a new steady state.