Distinct impact of land use and soil development processes on coupled biogeochemical cycling of C, N and P in a temperate hillslope-flood plain system

Understanding the biogeochemical cycling of phosphorus (P), particularly organic P (OP) in soils, under varying land use and soil development processes is essential for optimizing P usage under P fertilizer crisis. However, the complexity of OP impedes the mechanistic understanding. Therefore, by using well-documented organic carbon (OC) and total nitrogen (TN) cycling, we studied their stoichiometric correlation with P in soil fractions to indicate soil organic matter (SOM) and P turnover under two land uses (Cropland VS. Grassland) in Germany. Our results showed that grassland soils on the hillslope have higher OC and TN stocks than cropland soils. Total P (TP) stocks were unaffected by land use. However, grassland topsoil exhibited higher OP stocks and OP/TP proportions than cropland, with a constant IP stock throughout the soil profile, as this was determined by soil development processes in the subsoil. This proves that the flood plain soils are decoupled from hillslope soils due to different soil development processes. The stoichiometric assessment revealed a higher enrichment of OP in fine fractions of grassland soils, indicating stronger resistance to P loss by soil degradation. Mechanistic insights from OC:OP ratio of fine fractions indicate two potential OP cycling pathways: a ratio similar to microbial biomass C:P ratio suggesting a greater OP stabilization within microbial biomass/necromass; whereas a narrower ratio indicating more OP associated directly with mineral surfaces. This study illuminates the complex interplay between land use and soil development processes on OC, TN and P cycling, emphasizing the potential of stoichiometric assessment in soil fractions to understand OP biogeochemical cycling.