Optimizing Nitrogen Application in a <scp> <i>Pennisetum hydridum</i> </scp> – <scp> <i>Sesbania cannabina</i> </scp> Intercropping System to Enhance Soil Carbon Sequestration and Soil Health in Agricultural Landscapes

ABSTRACT Land degradation is currently one of the major threats to global food security and ecosystem stability, often leading to diminished soil organic carbon (SOC) and reduced agricultural productivity. Restoring soil health through sustainable practices is, therefore, paramount, with a critical knowledge gap regarding the synergistic mechanisms between the high‐biomass grass Pennisetum hydridum and the nitrogen‐fixing legume Sesbania cannabina , and how nitrogen management optimizes their combined potential to enhance soil carbon sequestration. Soil carbon sequestration is critical for mitigating climate change and improving soil health in agricultural systems. While well‐studied in forests and grasslands, the interactive effects of planting patterns and nitrogen (N) fertilization on carbon dynamics in cultivated farmland require further exploration. This study investigates the influence of N application levels (0, 100, and 200 kg N ha −1 ) and cropping systems on SOC sequestration and its mechanisms. Through integrated pot and field experiments, changes in SOC components, including particulate organic carbon (POC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC) were assessed. Our findings revealed that cultivated soils significantly enhanced all measured carbon fractions compared to abandoned land, with an increase of 16.01% in soil organic matter, 12.94% in total nitrogen, 46.07% in POC, 31.78% in ROC, and 248.59% in MBC. Higher carbon storage capacity was noted under monocropped Pennisetum hydridum , whereas the leguminous Sesbania cannabina (MSc) contributed significantly to nitrogen fixation and labile carbon pools. The intercropping system (IPc‐Sc) synergized these benefits, boosting carbon sequestration by enhancing organic matter input and stabilizing soil structure more effectively than monocropping. Furthermore, N fertilization significantly altered soil enzyme activities, indicating a shift in microbial‐mediated carbon cycling. However, excessive N application (200 kg N ha −1 ) risked accelerating SOC decomposition. This study demonstrates that intercropping P. hydridum with S. cannabina under optimized N fertilization (100 kg N ha −1 ) effectively enhances soil carbon storage, boosts agroecosystem productivity, and promotes sustainable land management.