Conservation Tillage Coupled with Controlled-Release Nitrogen Enhances Soil Aggregate Stability, Nutrient Retention and Soybean Yield in the Loess Plateau

Soil erosion, structural degradation, and inefficient nitrogen (N) use jointly constrain dryland agriculture on the Loess Plateau—a globally recognized ecologically fragile region. A common concern is that soil conservation practices may reduce short-term crop productivity. This study evaluated whether conservation tillage based on no-tillage and straw mulching, particularly when coupled with controlled-release N, could enhance soil aggregate stability, apparent nutrient availability, soybean productivity, and N-use efficiency. A two-year field experiment was conducted in 2022–2023 at the Shenmu Erosion and Environment Research Station, Shaanxi Province, China. Five treatments were compared: conventional tillage without N application (CTCK), conventional tillage with normal N application (CTU), conservation tillage without N application (SMCK), conservation tillage with normal N application (SMU), and conservation tillage with controlled-release N (SMS). SMS consistently increased the proportion of >0.25 mm water-stable macroaggregates, mean weight diameter, and geometric mean diameter. It also increased soil organic carbon and its particulate and mineral-associated fractions in the 0–30 cm profile, improved total N, mineral N, total P, and available P, and produced the highest soybean yield. Across two years, SMS yielded 1960.57 kg ha−1, 162.26% higher than CTCK and 66.66% higher than CTU. The treatment also achieved the highest agronomic efficiency despite reduced N input. These findings indicate that integrating conservation tillage with controlled-release N can enhance soil structural stability, apparent nutrient availability, and soybean productivity. It confirms that soil conservation and high crop output are mutually reinforcing rather than conflicting. Given the ecological sensitivity of the Loess Plateau, this integrated strategy offers a promising pathway toward more resilient dryland farming systems.