Interplay of grazing intensity and topography on soil phosphorus dynamics in semiarid grasslands

The bioavailability of soil phosphorus (P) fractions is essential for sustaining primary productivity in grassland ecosystems, and it is influenced by grazing intensity and topographical features. However, the specific effects of grazing intensity on soil P fractions across varying topographies remain largely unexplored. Here, we utilized a 14-year grazing manipulation experiment in a semi-arid grassland, which included two topographical settings (flat vs. slope) and seven levels of grazing intensity (0 to 9.0 sheep ha⁻¹), to investigate how these factors impact the storage and transformation of soil P fractions. Soil P fractions were categorized into labile P (Ca₂-P), moderately labile P (Ca₈-P, Al-P, Fe-P), and stable P (O-P, Ca₁₀-P). Our results demonstrate that high grazing intensities lead to the accumulation of labile and moderately labile P fractions on flat areas due to runoff deposition from adjacent slopes, while these fractions are depleted on slopes themselves. Grazing exerts a direct influence on topsoil by reducing vegetation and litter cover, which accelerate the conversion of labile P fractions into more stable forms due to increased exposure and diminished protection. The mechanisms by which grazing impacts the distribution and dynamics of soil P fractions differ according topographical features. In flat areas, biochemical processes driven by soil organic carbon and microbial communities are predominant, resulting an initial decrease and followed by an increase in the proportion of less stable P with rising grazing intensity. Conversely, on slopes, environmental factors, particularly soil pH, play a more dominant role, leading to an initial increase followed by a decrease in the proportion of less stable P as grazing intensity escalates. These findings highlight the complex interplay between grazing intensity and topography in shaping soil P dynamics. Understanding these interactions is crucial for developing effective grazing management strategies and promoting sustainable grassland productivity in semi-arid environments.