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Plastic film mulch and phosphorus (P) fertilization usually increase alfalfa (Medicago sativa L.) forage yield in semi-arid regions by improving soil moisture and P availability. However, this does not always occur. The present study explored the P level for highest forage yield under film mulch and the mechanism underlying yield decrease at high P levels. A six-year study was conducted with randomized blocks of split-plot design with and without plastic film mulch as main plots: M0, no film mulch with flat planting; and M1, film mulching with ridge-furrow field regimes. Four P rates were applied in sub-plots: P0, P1, P2 and P3 with 0, 9.73, 19.3 and 28.9 kg P ha(-1), respectively. Film mulch greatly increased forage yield, resulting in 14% higher yield in M1P0 than in M0P3. Forage yield under mulching decreased for 19.3 and 28.9 kg P ha(-1). The P rate for maximum yield was 16.1 kg P ha(-1). Soil total nitrogen (N) and inorganic-N decreased at 19.3 and 28.9 kg P ha(-1 )with mulching compared with 9.73 kg P ha(-1), and the latter was about 90% lower than that of alfalfa lands in semi-humid region. Soil microbial biomass carbon (C) and C/N ratio in microbial biomass in mulched P2 and P3 subplots were significantly lower than for P1. With mulching, soil organic C mineralization was significantly less for P3 than for P1 and P2. Shoot N/P ratio decreased with increasing P with mulching. Thus shoot N uptake for P2 and P3 mulched plots was associated with low soil N availability. It is concluded that the highest forage yield was obtained with mulching, P-saving and low risk of N loss on the semiarid Loess Plateau.