Effects of soil and water conservation tillage on soil water and nitrogen supply in sloping rotation farmland

To systematically investigate the effects of various soil and water conservation tillage techniques on soil water storage (SWS) and nitrogen use efficiency (NUE) in sloping black soil farmland under a typical maize-soybean rotation system, a two-year consecutive field experiment was conducted in a representative black soil region, where soil erosion and nutrient loss have become critical constraints to agricultural sustainability. Seven tillage treatments were scientifically designed to compare single and combined conservation practices, as follows: (1) Single tillage group: contour tillage (TP), ridge tillage with furrow diking (RF), and subsoiling tillage (SF); (2) Combined tillage group: contour tillage integrated with subsoiling (TP-S), ridge tillage with furrow diking combined with subsoiling (RF-S), and contour tillage coupled with ridge tillage with furrow diking (TP-R); (3) Control group: conventional tillage (CK), which adhered to local traditional farming procedures without any additional soil and water conservation measures. This study comprehensively analyzed the impacts of these tillage techniques on key agro-ecological and production indicators, including farmland SWS at 0–100 cm soil depth, nitrogen (N) absorption capacity, N translocation and utilization efficiency, crop growth-related traits (e.g., plant height, dry matter accumulation, leaf area index), and final grain yield under the maize-soybean rotation regime. The results obtained from two years of continuous field monitoring and statistical analyses demonstrated that: (1) All soil and water conservation tillage techniques significantly increased SWS compared with the conventional tillage (CK). Notably, the TP-R treatment achieved the highest SWS for both maize and soybean during their entire growing seasons, which was 12.40% and 13.78% higher than that under CK, respectively. This indicated that the combination of contour tillage and ridge tillage with furrow diking exerted a synergistic effect on enhancing soil water retention, which is crucial for alleviating drought stress in sloping black soil. (2) These tillage practices effectively improved the N absorption and utilization efficiency of both maize and soybean. During the maize growing season, the TP-S treatment exhibited superior performance in multiple N-related indicators: compared with CK, it increased the N translocation efficiency of stems and leaves by 34.81% and 53.79%, respectively, while enhancing N absorption efficiency by 60.00% and partial factor productivity of nitrogen fertilizer (PFPN) by 26.44%. Furthermore, the N harvest index (NHI) of TP-S was 3.97%～12.21% higher than that of other treatment groups, and the N grain contribution rate of TP-R was 54.52% greater than that of CK. In soybean fields, the RF-S treatment increased the N translocation rate of stems and leaves by 42.44% and 15.12% compared with CK. For the TP-S treatment in soybean, significant improvements were observed in petiole N translocation rate (20.08%), N grain contribution rate (56.99%), N absorption efficiency (82.93%), and NHI (10.14%) relative to CK, with its PFPN being 9.49% higher than the average value across all seven treatments. (3) Compared with conventional tillage (CK), all soil and water conservation tillage treatments promoted crop growth and yield formation, with the TP-S treatment consistently showing the best performance in all measured indicators, including plant height, aboveground biomass, and final grain yield. In conclusion, soil and water conservation tillage techniques, particularly the combined tillage practices, represent a promising and sustainable agricultural approach that can effectively enhance soil water content, improve NUE, and increase crop yield in sloping black soil under maize-soybean rotation. Among the evaluated treatments, contour tillage combined with subsoiling (TP-S) yielded the most significant comprehensive beneficial effects, thereby holding considerable potential for agricultural extension and application in similar sloping black soil regions. This finding is of great significance for promoting the sustainable utilization of black soil resources, mitigating soil degradation, and ensuring regional food security.