Solubility of phosphorus in subtropical forest soils as influenced by low-molecular organic acids and key soil properties

Low phosphorus (P) solubility in soil limits primary productivity in terrestrial ecosystems worldwide. However, our understanding about drivers of soil P solubility is still incomplete. This study examined the levels of water soluble inorganic P (Pi) and organic P (Po) in soils under mature subtropical forests in China, and how they can be influenced by three low-molecular organic acids (LOAs, i.e. citric, malic and oxalic acids) and key soil properties (e.g. organic C concentration). Water soluble Pi (mean 0.05 mg kg− 1) and Po (0.32 mg kg− 1) in the soils studied were generally low, compared to in many other areas of the world. While all three LOAs significantly solubilized P from the soils studied as a whole, the solubilized amount varied greatly among soils and also varied largely with the type of LOA and P forms. Solubilized Po by the three LOAs were > 2.5 times of the solubilized Pi. Soluble total P (Pi + Po) increased in the order of oxalic acid < malic acid < citric acid and was positively related to the soluble aluminum and iron. Rock fragment (4-50 mm) content explained 69%, 51%, and 31%, respectively, of the variations in water, oxalic acid, and malic acid soluble Pi. Efficiencies of the three LOAs in solubilizing Pi, but not Po, generally increased with soil organic C, total N, total P, and microbial biomass C concentrations. Our results suggest that interplay among P forms, LOAs, and soil properties control soil P solubility.