Response of Soil Organic Carbon and Its Components to Mixed Sowing of Green Manure

Mixed sowing of green manure in winter is a unique farming mode in southern China, which has the potential to replace or partially replace nitrogen fertilizer. To investigate how mixed sowing of green manure combined with nitrogen reduction regulates soil organic carbon and its fractions, this study was conducted in the 3/4 Chinese milk vetch × 1/4 rapeseed farming mode, without nitrogen fertilizer (CK), 100% N fertilizer (150 kg ha−1, N1MR), 20% N fertilizer reduction (120 kg ha−1, N2MR), 40% N fertilizer reduction (90 kg ha−1, N3MR), and 60% N fertilizer reduction (60 kg ha−1, N4MR). The main results were the N2MR treatment could guarantee stable and increased rice yield. The N1MR and N2MR treatments were more conducive to the accumulation of TOC. The N4MR and N2MR treatments were more conducive to the increase and accumulation of AOC and DOC. The effective spikes were positively correlated with TOC and ROC. The grain number per panicle was positively correlated with DOC. The seed-setting rate was positively correlated with ROC. Overall, mixed sowing of Chinese milk vetch and rapeseed combined with 20% nitrogen reduction ensures a stable yield and increase in rice. Nitrogen reduction by 60% and 20% is beneficial to the increase and accumulation of TOC, AOC, and DOC in soil.