Contrasting Phosphorus and Potassium Responses to Tillage and Straw Return in a Wheat–Maize Rotation: Associations with Soil Microbial Communities

Abstract Background and Aims: Straw return strengthens nutrient recycling in cereal rotations, yet its effects on phosphorus (P) and potassium (K) acquisition under contrasting tillage remain unclear. Method : A two-year split-plot experiment in northern China compared rotary tillage (RT) and moldboard plowing (MP) under no, half, full, and double straw return in a winter wheat–summer maize rotation. Soil and crop measurements covered four harvests; topsoil microbial communities were assessed after both maize harvests. Results : Full and double return increased grain yield and aboveground P accumulation in both crops, with higher maize yield under MP. Wheat K accumulation increased with straw return, whereas maize K accumulation showed no significant overall tillage or straw return effect. Estimated from additional uptake, apparent straw P recovery was higher under MP, while apparent K recovery increased with return rate. Maize P accumulation increased between years as K accumulation declined. MP and higher straw inputs generally increased topsoil organic matter, available P (AP), and available K (AK). Between maize harvests, AP increased while AK declined, indicating contrasting trajectories. AP correlated positively with maize P accumulation in both years (R² = 0.61–0.62), and AK with K accumulation in both crops. Microbial richness increased between years; combined analysis detected modest treatment differentiation for fungi but no significant differentiation for bacteria. Nutrient-associated taxa varied between years, providing candidates for functional investigation. Conclusion : Findings support separate P and K monitoring when integrating straw return with tillage; recommending double over full return requires evidence of additional agronomic and economic benefits.