Objectives From the dual perspectives of microbial nutrient resource limitation and ecosystem multifunctionality in farmland black soil, this study systematically explores the regulatory effects of different tillage practices under long-term straw returning, aiming to provide theoretical support and practical basis for the efficient utilization of straw resources and scientific optimization of tillage schemes in the black soil region of Northeast China.

Methods This study relied on a long-term positioned field experiment platform of black soil established in Harbin, Heilongjiang Province in 2011. Five treatments were set up: conventional tillage (T1), deep plowing with straw returning (T2), subsoiling with straw returning (T3), no-tillage with straw mulching (T4), and no-tillage with straw stubble retention (T5). Key indicators including soil physicochemical properties and the activities of extracellular enzymes related to carbon, nitrogen and phosphorus transformation were determined. Subsequently, the soil extracellular enzyme stoichiometry characteristics and ecosystem multifunctionality index (EMF) were calculated, and the main driving factors for the changes in soil microbial nutrient resource limitation and ecosystem multifunctionality were finally clarified.

Results Compared with conventional tillage T1, both subsoiling with straw returning T3 and no-tillage with straw mulching T4 significantly increased the contents of surface soil organic carbon (SOC), total N (TN), and the activities of β-glucosidase (BG), β-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP) and acid phosphatase (ACP). The SOC, TN and ACP activity under T4 were significantly higher than those under T3. Compared with T1, all straw returning treatments significantly increased soil C:N ratio and C:P ratio, while significantly decreased the enzymatic activity C:N ratio and increased the enzymatic activity N:P ratio. Enzyme stoichiometry analysis showed that soil microorganisms under all treatments were co-limited by carbon and P. Compared with T1, the vector length and vector angle of soil enzyme stoichiometry in T2, T3 and T4 treatments were significantly reduced, and the reduction amplitudes in T2 and T3 were significantly greater than that in T4. The overall performance of soil ecosystem multifunctionality under different treatments was T3≈T4 > T2 > T1≈T5. Combined with Pearson correlation and random forest model analysis, β-N-acetylglucosaminidase (NAG) was the core factor regulating microbial carbon and phosphorus limitation, while soil total nitrogen (TN) was the dominant factor with the highest contribution to ecosystem multifunctionality.

Conclusions The activity of soil nitrogen-acquiring enzymes and its stoichiometric balance relationship with carbon- and phosphorus-acquiring enzymes are the key factors determining the pattern of microbial nutrient resource limitation in black soil, while soil total nitrogen content is the core contributor supporting soil ecosystem multifunctionality. Long-term straw returning combined with different tillage practices can significantly improve the surface soil organic matter, total nitrogen level and extracellular enzyme activity, effectively alleviate the carbon-phosphorus nutrient resource limitation of microorganisms, and further enhance soil ecosystem multifunctionality. In general, subsoiling with straw returning and no-tillage with straw mulching have higher ecosystem multifunctionality than deep plowing with straw returning, and subsoiling with straw returning shows a better alleviation effect on microbial carbon and phosphorus nutrient limitation than no-tillage with straw mulching.