Soil organic carbon dynamics shift by incorporating wheat straw in paddy soil in China

The objective of this study was to explore the effects of unfertilized control (CK), mineral NPK fertilizer (NPK), NPK plus medium-rate wheat straw (MSNPK), and NPK plus high-rate wheat straw (HSNPK) on soil organic carbon (SOC) fractions and C-cycle enzymes at distinct depths (0-5, 5-10, 10-20, 20-30, and 30-50 cm) in paddy soil. The SOC content at 0-50 cm depth ranged from 8.50 to 21.15 g kg-1 , following the trend HSNPK > MSNPK > NPK > CK. Correspondingly, the content of water-soluble organic carbon (WSOC), microbial biomass carbon (MBC), particulate organic carbon (POC), and easily oxidizable carbon (EOC) ranged from 0.08 to 0.27 g kg-1 , 0.11 to 0.53 g kg-1 , 1.48 to 8.29 g kg-1 , and 3.25 to 7.33 g kg-1 , respectively, and HSNPK exhibited the highest values for these parameters among all treatments and soil depths, with significant differences observed compared to NPK and CK (p < 0.05). The cellulase activity of HSNPK was significantly (p < 0.05) higher by 6.12%-13.30% compared to the CK at 0-30 cm depth; while the activity of invertase and β-glucosidase of HSNPK were significantly (p < 0.05) higher by 34.09%-433.43% and 26.61%-130.50%, respectively, compared to the CK at 0-50 cm depth. Enzyme activities were significantly (p < 0.05) correlated with SOC fractions and the dominant factors driving changes in enzyme activities were WSOC, POC, and EOC. HSNPK was associated with the highest SOC fractions and enzyme activities, indicating that it was the most favorable management practice for promoting soil quality in rice paddy fields.