Reforestation alleviates the impacts of forest-to-cropland conversion on the phosphorus cycle: a four-decade study

Land-use change, including deforestation and reforestation, has driven significant alterations in soil properties, phosphorus (P) dynamics, and microbial communities. Reforestation might alleviate the disturbance of forest-to-cropland conversion. This study investigates the effects of converting natural forests (NF) to croplands (CP) and subsequent reforestation into cultivated forests (CF) on soil P fractions, and 18 P-cycling functional genes. The results showed that deforestation was associated with reductions in soil total nitrogen (TN), total organic carbon (TOC), pH, and microbial biomass phosphorus (MBP), along with a 37.5 % increase in available phosphorus (AP) pools. Reforestation decreased total P to levels comparable to those in NF. Additionally, CF exhibited lower concentrations of moderately labile (NaOH-Po and -Pi) and stable P pools (concentrated HCl-Pi and -Po), which may reflect enhanced biological P utilization or a gradual depletion of recalcitrant P reserves due to ongoing ecosystem recovery. Pi content was consistently 2–3 times higher than Po, with NF maintaining this ratio through greater Po conservation and stable P pools. Land-use change disrupted this balance, as reflected by shifts in P-cycling gene abundance. Deforestation was associated with higher abundances of genes related to inorganic P solubilization and organic P mineralization, whereas reforestation was associated with reductions in these genes to levels comparable to those observed in NF. For instance, PhoB and phoU showed higher abundances in CP with higher P availability. Ugp and phnCDE responsible for P uptake and organic P mineralization, which correlated with organic P content (NaOH-Po and concentrated HCl-Po). Utilizing a 40-year land-use chronosequence, this study uniquely captures the legacy effects on soil P cycling and microbial function by integrating metagenomic sequencing with phosphorus fractionation to offer a holistic view of nutrient cycling under land-use changes. The study highlights the importance of land management practices in maintaining P cycling and microbial community functions, with implications for potential improvements in reforestation strategies and nutrient management. • Deforestation increased P pools but decreased C stocks compared to nature forest • Reforestation restored P-cycling microbial functions closer to natural forest level • Inorganic P dominated over organic P, serving as the main bioavailable P source • Deforestation shifted P-cycling genes and reforestation promoted partial recovery