Redistribution of Soil Phosphorus Fractions Alleviates Phosphorus Limitations Following Afforestation

Afforestation plays a crucial role in mitigating climate change, yet the transfer of soil phosphorus (P) to aboveground biomass may lead to soil P limitation. However, the influences of afforestation on soil P fractions and availability remain unclear. Here, we investigated soil P fractions at 144 paired sampling sites by comparing afforested lands with adjacent non-afforested lands in subtropical China. Afforestation increased the proportions of available P, labile P, and moderately-labile P, while decreasing inorganic P concentrations, with no significant effect on soil total P. These shifts were strongly coupled with the increase in organic P and the decline in stable P. Notably, afforestation with broad-leaved species (i.e., Quercus variabilis) increased soil organic P concentration, while afforestation with coniferous species (i.e., Pinus massoniana and Platycladus orientalis) reduced soil inorganic P concentration. Afforestation weakened the effects of soil microclimates on labile P fractions (e.g., decreased with moisture) in non-afforested soil. In contrast, plant nutrients (e.g., leaf P concentration) exhibited positive effects on inorganic P fractions in afforested soils. Soil iron oxides (Fe_o) were the primary cause of P variations in both afforested and non-afforested soils; particularly, lower Fe_o and soil pH under afforested lands likely promoted the desorption of more non-stable P fractions compared to the non-afforested soils. Overall, our findings suggest that afforestation may alleviate soil P limitation by enhancing the concentration of labile and moderately-labile P fractions, while soil inorganic P fractions are crucial for regulating soil P dynamics, thereby providing insights for optimizing afforestation practices.