Soil phosphorus availability is enhanced by nitrogen and litter addition during the growing season

Litter input plays an important role in maintaining soil phosphorus (P) availability in terrestrial ecosystems. Moreover, nitrogen (N) deposition can alter litter accumulation and decomposition and, consequently, soil P availability. However, the combined effects of N and litter addition on soil P availability remain unclear. We examined how N addition (0 and 10 g N m−2 yr−1) and litter manipulation (initial litter, litter removal and litter addition) affected the seasonal dynamics of soil P availability in an 11-year grassland experiment. During the mid-growing season, N addition significantly increased soil available P but decreased soil total P, while litter addition increased soil total P. At the end of the growing season, litter addition increased soil available P, while N addition increased soil total P. Structural equation modelling (SEM) revealed that N addition increased soil available P by increasing aboveground biomass and decreasing soil pH during the mid-growing season. At the end of the growing season, litter addition increased soil available P by enhancing microbial biomass P. Our findings suggested that N and litter addition increased soil P availability from the middle to the end of the growing season. Moreover, the positive effects of litter on soil available and total P under N addition at the end of the growing season indicated that grassland management that reduce litter accumulation, such as grazing and mowing, may decrease the soil P supply under increased N deposition.