No-Tillage Combined with Appropriate Amount of Straw Returning Increased Soil Biochemical Properties

(1) Background: Few studies have focused on the interaction of tillage and straw returning on soil carbon and nitrogen. Therefore, this study was conducted for investigating the effects of tillage and straw returning on soil biochemical properties under a rice–wheat double cropping system; (2) Methods: Six treatments were set up to determine soil biochemical properties, including no-tillage with all straw returning (NTS), wheat plow tillage and rice no-tillage with half straw returning (RT1), wheat no-tillage and rice plow tillage with half straw returning (RT2), plow tillage with all straw returning (CTS), less tillage with half straw returning (MTS), and plow tillage with no straw returning (CT); (3) Results: Straw returning increased soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN), but had no significant effects on total nitrogen (TN) and soil organic carbon (SOC). In the treatments of straw returning, the contents of SMBC, SMBN, TN, and SOC under no-tillage were increased in the 0–7 cm soil layer. Tillage and straw returning had no significant effects in the 7–14 cm and 14–21 cm soil layers. In addition, SMBC/SMBN for all the treatments was maintained within a reasonable range, and microbial quotient (SMQ) and SMBN/TN in the no-tillage treatment had a significant improvement; (4) Conclusions: The results showed that no-tillage with an appropriate amount of straw returning improved the soil biochemical properties and maintained the nitrogen mineralization capacity in the 0–7 cm soil layer for this region.