Regulation of soil phosphorus availability and composition during forest succession in subtropics

Although phosphorus (P) is a limiting nutrient for plant growth in subtropical forests, the effects of forest succession on soil P dynamics, which in turn influences P availability, are unclear. The objective was to access the impacts of forest succession on P fractions of different availability (Hedley sequential fractionation) in highly weathered subtropical soils. We compared the P dynamics and availability under the chronosequence of forest succession from four stages: i) Cunninghamia lanceolata plantation, ii) through mixed broadleaf-conifer, iii) deciduous broadleaved, and finally iv) evergreen broadleaved forest. The soil P was dominated by stable P fractions (69–76%) in all successional stages. Forest succession increased total P content from plantation (199 mg kg−1) to evergreen broadleaved forest (253 mg kg−1), whereas P reached the peak in deciduous broadleaved forest and then remains stable due to balance between input with litter and litter decomposition and tree uptake. Stable P (non-available P) increased for 31–39% with forest succession because soil acidification led to more Fe and Al (oxyhydr)oxides strongly bounding P. Moderately labile P (moderately available P) contents under deciduous and evergreen broadleaved forests were higher than under plantation and mixed forest due to organic matter accumulation. However, labile (easily available) P content was reduced 35–50% by succession because of P removal by plant uptake. Available P content reached the peak under deciduous broadleaved forest (64–81 mg kg−1) and decreased again, indicating that forest ecosystem transit from P acquiring to P recycling system (litter input and plant uptake of mineralized P). Fine root biomass was the primary driver that controlled total, moderately labile and stable P contents during forest succession. Available P increased with soil organic carbon (SOC), suggesting that organic matter is crucial to maintain P availability. The C/P ratio of litter was the primary driver decreasing available P because litter decomposition released P is the main source determining P availability in soil. The increase of moderately labile P following forest succession played a crucial role for accumulation of available P. These results suggest that forest succession increases soil P availability until deciduous broadleaved forest. Therefore, strong measures to facilitate succession to the deciduous broadleaved forest stage should be a key approach to increase long-term soil P availability in subtropics.