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.

Soil phosphorus (P) deficiency is an important factor limiting plant growth in the semi-arid Loess Plateau region in China. The topsoils in this area undergo repeated drying–rewetting (DRW) cycles, which can influence soil P availability, a process that may become more pronounced due to climate change. However, little is known about how soil P availability responds to DRW cycles under different land-use types. To investigate this issue, we conducted three 120-day soil culture experiments to investigate the direction and magnitude of soil available P and the responses of its influencing factors to repeated DRW cycles and their frequency and intensity under three typical land-use types (cropland, grassland, and shrubland) in Gansu Province, North-western China. The results showed that the available P concentration of cropland, grassland, and shrubland soils after repeated DRW cycles significantly decreased by 8.9%, 11.5%, and 14.2%, respectively, compared with a constant humidity control. With increasing intensity of the DRW cycles, the available P concentration of grassland and shrubland soils significantly increased by 14.3% and 15.5%, respectively, while in cropland soil P significantly decreased by 10.4%. Compared with low-frequency DRW cycles, high-frequency DRW cycles significantly reduced the available P concentration by 6.4% in grassland soil and increased it by 9.8% in shrubland soil but had no significant effect in cropland soil. Overall, the responses of soil P availability to repeated DRW cycles vary among different land-use types, and the magnitude of the soil P availability response to repeated DRW cycles depended strongly on soil microorganism biomass, phosphatase activity, and the initial soil properties, being more pronounced in grassland and shrubland soils than in cropland soils. It is therefore essential to consider land-use type when studying the effects of DRW on soil P cycling in semi-arid regions, especially in the context of climate change.