Organic/Inorganic phosphorus forms and transformation in long-term paddy cultivation in the Pearl River Delta, China

Abstract Soil P availability in subtropical regions is often limited due to depletion as well as immobilization by abundant Fe minerals. Identification and quantification of different soil phosphorus (P) fractions is important for improving agricultural productivity and developing sustainable management practices in these agricultural soils under long-term cultivation due to limited P efficiency. However, few studies have been conducted to investigate P composition and transformation in these soils. This study was conducted to characterize P fractions as affected by different paddy cultivation ages (200, 400 and 900 years) among soils of the Pearl River Delta Plain in China. A sequential chemical fractionation scheme was employed as well as enzyme assay and 31P nuclear magnetic resonance spectroscopy (31P NMR) to quantify various P fractions and species. Results showed Total P (TP) and available P (AP) increased with cultivation age, ranging from 0.60–2.12 g kg− 1 and 3.67-224.17 mg kg− 1, respectively. Easily-labile, moderately-labile and non-labile P had similar relationship in topsoils (Ap1, Ap2 horizon) with TP and AP, and inversed relationships in the subsoil (Br3, G horizons). Analysis with 31P NMR spectroscopy revealed that inorganic P including orthophosphate (Ortho-P) and pyrophosphate (Pyro-P) increased with cultivation age, while organic species monoester phosphate (Mono-P) and diester phosphate (Diester-P) decreased. Moreover, acid phosphatase (AcP) and neutral phosphatase (NeP) activities increased with cultivation age and positively correlated with P fractions (p < 0.05). NeP, AcP, exchangeable Ca and sand content are the main factors of affecting soil P composition, however, non-labile P (Dil.HCl-Pi) and Pyro-P made a significant contribution to P availability. Therefore, long-term paddy cultivation, influenced by these soil parameters including NeP, AcP, exchangeable Ca and sand, accelerated the transformation of soil organic to inorganic P.