Characteristics and controls of inorganic and organic phosphorus transformation during long-term paddy soil evolution

Information on phosphorus (P) transformation during long-term paddy soil evolution is important for maintaining rice yield and nutrient management. This study was aimed to reveal the characteristics and controls of inorganic and organic P transformation during paddy soil evolution over a millennial time scale using both sequential chemical extraction and solution 31P nuclear magnetic resonance spectroscopy. Results show that the rate and trajectory of P evolution in the topsoil differ markedly from that of the illuvial horizon. The amount of apatite P, non-occluded P, occluded P and total P in the topsoils increased rapidly during the early stage of paddy soil evolution (< 50 yrs) due to anthropogenic P additions, but declined in the older paddy soils due to the leaching loss of P sorbents (CaCO3 and Fe/Al oxides); thus, the added P cannot be efficiently retained by the older paddy soils. Total organic P including phosphate monoester in the topsoils reached maximum in the 300-yr paddy soil and declined thereafter. However, the phosphate diester pool (including DNA) and its proportion in the topsoil increased continuously across the paddy soil chronosequence, suggesting that long-term paddy cultivation promoted labile organic P accumulation. In the deeper soil horizons with less human impacts, the temporal trends in inorganic and organic P across the paddy soil chronosequence can be fully predicted by previous established P transformation models during natural pedogenesis, although the rates of P changes differ among different sites. Our study demonstrates that long-term paddy cultivation alters the rate and trajectory of inorganic and organic P transformation in the topsoils over a millennial time scale, and highlights the importance of labile organic P activation for nutrient supply in the older paddy soils.