Film Mulching with Low Phosphorus Application Improves Soil Organic Carbon and Its Decomposability in a Semiarid Agroecosystem

Soil organic carbon (SOC) storage and decomposability are crucial for soil quality. Film mulching and phosphorus (P) application are important agricultural practices on the semiarid Loess Plateau. This study analyzed the combined effects of film mulching and P application on SOC, its fractions, and mineralization kinetics under alfalfa (Medicago sativa L.). The six-year field experiment incorporated randomized blocks of split-plot design with two mulching treatments (no film mulching with flat planting and film mulching with ridges and furrows) as main plots and four P levels (P0: 0 kg ha⁻¹, P1: 9.73 kg ha⁻¹, P2: 19.3 kg ha⁻¹, P3: 28.9 kg ha⁻¹) as subplots. Mulching increased SOC content, SOC fractions (light and heavy fraction organic C, microbial biomass C, and dissolved organic C), and mineralization. After six years, mulching increased SOC content by 2.18, 2.60, 2.37, and 0.17 g kg⁻¹ at P0, P1, P2, and P3, relative to no mulching. With increasing P levels, SOC fractions and mineralization increased under no mulching but increased initially and then decreased under mulching. P1 with mulching displayed the highest SOC utilization efficiency and stability. Kinetic models divided SOC into an active and a slow SOC pool, with the latter showing the lowest decomposability and highest stability in P1 with mulching. Overall, film mulching with a low P level, especially 11.9 kg ha⁻¹ P fertilizer, promoted SOC storage under alfalfa on the semiarid Loess Plateau due to the high SOC content with high C utilization efficiency and stability and low decomposability.