Returning crop residues to the field after harvesting is a proven effective strategy for improving soil fertility, carbon sequestration, and crop productivity. However, the relationships between crop residue return modes, SOC and nutrient contents, and crop yields are still unclear. In this study, a field trial was conducted to investigate the effects of different maize straw return modes, i.e., straw mulching (SMU), straw deep ploughing (SDP), and control without straw return (CK), on soil organic carbon (SOC) and nutrient contents in soil layers of 0–40 cm in a Mollisol. The relationships between straw return modes, SOC and nutrient contents, and maize yield were evaluated. Compared with CK, SMU and SDP significantly increased SOC, total nitrogen (N), available N, total phosphorus (P), and available P contents in all soil layers. Relative to SMU, SOC, total N, available N, total P, and available P contents were significantly lower in soil layers of 0–10 cm, but they were significantly higher in soil layers of 20–40 cm in SDP. Redundancy analysis indicated that total N, available N, and SOC were major factors controlling maize yield. Structural equation modeling further showed that straw return modes indirectly affected maize yield by directly and preferentially affecting total N and available N contents. The results indicated that SMU and SDP were beneficial for increasing SOC and nutrient contents at the surface and subsurface soils, respectively. Optimizing a nitrogen management strategy is important to achieve high maize yield with straw return.

This study explored the effect of the combined application of chemical and organic fertilizers on phosphorus morphology and its conversion to an active state. A long-term field positioning experiment comprising five treatments was conducted in black soil. The results concluded that the soil total phosphorus (TP), available phosphorus (AP), inorganic phosphorus, and organic phosphorus contents of all treatments ranked as follows: 1.5M1NPK > M2NPK > M1NPK > NPK > CK. The long-term application of chemical and organic fertilizers increased the proportion of soil reactive phosphorus and moderately reactive phosphorus but decreased the proportion of mildly active phosphorus and residual phosphorus. A phosphorus-31 nuclear magnetic resonance (³¹P NMR) spectral analysis showed that the contents of orthophosphate, pyrophosphate, phosphoric acid diesters, and orthophosphate acid monoesters increased with the application of chemical and organic fertilizers, of which 1.5M1NPK usually resulted in the highest increases. In conclusion, the long-term application of chemical fertilizers could promote the conversion of soil phosphorus into active phosphorus and improve the effectiveness of soil phosphorus, and the long-term use of organic and chemical fertilizers was more effective than the use of chemical fertilizers only, with 1.5M1NPK providing the best effects.

Adopting an optimal cropping regime is crucial for sustainable soil use. However, how different cropping regimes impact phosphorus (P) availability and the underlying mechanism remain unclear. Here, a 10-year field experiment was performed to examine the influence of different cropping regimes, including maize–soybean rotation (MSR), continuous maize cropping (CMC), and farmland fallow (FALL), under unfertilized and fertilized conditions in Northeast China. The P forms were analyzed using chemical fractionation and solution phosphorus-31 nuclear magnetic resonance. Compared to FALL, total P and different forms of P contents were significantly lower under MSR and CMC systems. Moreover, the contents of total P and different forms of P were higher under MSR than those under CMC. Correlation analysis showed that there were significant and positive correlations between total P and different forms of P contents. Redundancy analysis revealed soil organic carbon (SOC) as the most significant factor influencing total P and different forms of P. Structural equation modeling demonstrated the direct positive impacts of SOC, total nitrogen, total phosphorus, and Olsen phosphorus on phosphatase activity, which exhibited direct positive influence on P availability. In summary, maize–soybean rotation is an effective cropping regime for promoting P accumulation and availability in this region.