Soil phosphorus dynamics during cropland reforestation

Phosphorus (P) commonly limits plant growth on strongly weathered soils in the humid subtropics. The reforestation of croplands alters soil P cycling, but the impact of reforestation on the chemical nature of soil P, mediated through climate and the alteration of soil physicochemical and microbial properties at regional scale, remain poorly understood. To address this, we quantified soil P fractions at 30 reforestation sites in Yunnan and conducted a meta-analysis including 797 records worldwide to analyze the responses of soil P fractions to reforestation. In Yunnan Province, reforestation of croplands decreased soil inorganic P (Pi), primarily in forms extracted in sodium bicarbonate (NaHCO3) and hot concentrated hydrochloric acid (cHCl-Pi), especially during conversion of maize to dawn redwood (Metasequoia glyptostroboides) forest or shrubland. However, organic P (Po) fractions were unchanged. After reforestation, total Pi, NaHCO3-Pi, cHCl-Pi, and total Po concentrations decreased from the central and southern to the northeast and northwest of Yunnan Province. Soil NaHCO3-Pi was positive correlations with soil moisture content, total nitrogen, organic carbon, and bacterial (16S rRNA) abundance. Exchangeable iron, manganese, and fungal abundance (ITS) were positively associated with cHCl-Pi, cHCl-Pi was negatively correlated with acid phosphatase activity. Total Po decreased with increasing time since reforestation. In contrast to Yunnan sites, our meta-analysis shows that reforestation reduces all soil Pi fractions (cHCl-Pi, 1 M HCl-Pi, NaOH-Pi, and NaHCO3-Pi), but does not alter other soil properties. Overall, these findings highlight that reforestation-driven decrease of soil Pi pools can inform targeted P fertilizer management in afforested agroecosystems, promoting sustainable use of soil P.