Optimized Phosphorus Inputs Enhances Maize Yield and Humus Stabilization in Albic Soils of Northeast China: Evidence from Three-Year Field Trial

Maize is a globally significant cereal crop, while Albic soils in Northeast China are characterized by low available phosphorus (P), poor humus (HS) quality, and constrained maize yield. The synergistic effects of P fertilization on maize yield and HS quality in these soils remain poorly understood. This three-year field experiment was conducted to determine the optimal P application rate for concurrently enhancing crop productivity and HS quality. Four P application rates were established: 0 kg P2O5 ha−1 (no P application, P0), 40 kg P2O5 ha−1 (low P application, LP), 80 kg P2O5 ha−1 (moderate P application, MP), and 120 kg P2O5 ha−1 (high P application, HP). Soil nutrients status, HS fractions, dissolved organic matter (DOM) fluorescence characteristics, and structural properties of humic acid (HA) were systematically analyzed following standard analytical procedures. Principal component analysis (PCA) and Pearson correlation analysis were integrated to facilitate comprehensive data interpretation. Results indicated that the MP treatment achieved the highest maize yield (12,257.1 kg ha−1) and soil organic matter (SOM, 14.8 g kg−1) content, with no further yield improvement observed under HP. The MP treatment significantly increased DOM carbon content (CDOM, 0.350 mg L−1) and its humification index (HIX, 6.80), promoting the transformation of labile DOM into stable HS. HA under MP treatment exhibited enhanced structural stability, as evidenced by a lower H/C ratio (1.72), a higher O/C ratio (0.880), and a reduced E4/E6 ratio, reflecting increased aromatic condensation and a greater abundance of oxygen-containing functional groups. Fourier transform infrared (FTIR) spectroscopy and differential thermal analysis (DTA) confirmed that MP improved the structural complexity and thermal stability of HA. In contrast, P0 and LP restricted nutrient availability and HS formation, whereas HP induced soil acidification (pH 5.68) and disrupted HS equilibrium. Principal component analysis (PCA) and correlation analysis revealed significant positive associations between the MP treatment and SOM, CDOM, and maize yield. This implied that moderate P input promoted stable soil organic carbon accumulation and nutrient availability, synergistically enhancing maize productivity—consistent with the study’s core goal of optimizing P management for concurrent yield and HS quality improvement in Albic soils. Accordingly, this study concluded that moderate P application (80 kg P2O5 ha−1) was optimal for Albic soils, synergistically enhancing both maize productivity and HS quality. These findings provided theoretical support for precise P management in sustainable agricultural systems within the Albic soil regions of Northeast China.