Divergent linkages of soil phosphorus fractions to edaphic properties following afforestation in the riparian zone of the upper Yangtze river, China

Soil phosphorus (P) is an essential nutrient element for plant growth but it is also one of the elements of agricultural-dominated watershed pollution. While the vegetation in the riparian zone usually plays an important role in regulating P pollutants. However, how afforestation affects soil P dynamics and fractions in the riparian zone remains largely unclear. Here, we investigated soil P fractions, and associated drivers including edaphic properties, microbial attributes, and soil enzyme activities under conversion from cropland to different afforested lands in order to better understand the dynamics of soil P fractions in the riparian zone of the upper Yangtze River. We found that afforestation significantly decreased the concentrations of available phosphorus, microbial biomass P, and labile P fractions, but the moderately labile P and Stable P did not significantly differ among afforestation types. Particularly, the lowest concentration of labile P was observed in Morus alba (M.a.) forests followed by the Salix babylonica (S.b.) forests, whereas croplands generally exhibited an inverse trend with a higher labile P concentration compared to woodlands, especially in croplands nearby Morus alba forests. Generally, P fractions were negatively associated with soil pH and C:N ratio, while positively related to microbial attributes, N:P ratio, and alkaline phosphatase activities. The labile P and moderately labile P fractions were predominantly regulated by biotic factors (i.e., microbial biomass P, microbial biomass N, leucine amino peptidase), whereas the stable P was strongly related to abiotic factors (i.e., total C concentration, pH, C:N ratio). These findings indicate afforestation is conducive to intercept more labile P, resulting in reduced P leaching to rivers. Collectively, our results not only offer direct experimental insight into predicting the effects of afforestation on soil P fractions but also have important implications for agricultural pollution management and reforestation strategies in the riparian zone.