Relay strip intercropping enhances soybean rhizodeposition and soil carbon sequestration through light-induced compensatory root growth

In a relay strip intercropping system, the later-sown crop experiences shading during the co-growth stage but gains a period of high light availability (‘light recovery’) after the harvest of the early-sown crop. Whether this light recovery can compensate for the initial reduction in carbon (C) input to the soil remains unknown. Using ¹³CO₂ pulse-labeling at the seventh trifoliolate stage (V7: co-growth) and beginning pod stage (R3: solo-growth), we traced the photosynthetic C flow in soybean (Glycine max L.) within a soybean-maize relay strip intercropping system. We found that light recovery at the R3 stage, significantly enhanced photosynthetically active leaf area (16.7 %), light interception (24.2 %), and photosynthetic C assimilation (16.7 %) in intercropped soybeans. This triggered compensatory root growth, increasing belowground C allocation by 13.8 %, and slowing microbial biomass C turnover by 38.6 %, indicating more efficient C stabilization. Consequently, intercropped soybeans contributed 1.8 times more C to the soil (354.4 vs. 192.9 kg C ha⁻¹) than monocropped soybeans, with 80 % of this input derived from the light recovery phase. Long-term (10-year) intercropping increased soil macroaggregates (> 2 mm) by 25.5 % and SOC content by 15.0 %. Our results demonstrate that light recovery induces compensatory root growth that not only offsets early shading effects but also significantly enhances rhizodeposition and SOC sequestration. This study establishes a mechanistic link between light capture, photosynthesis, and belowground C allocation in an intercropping system, highlighting its potential as a sustainable strategy for soil C sequestration.