Long-term paddy cultivation significantly alters topsoil phosphorus transformation and degrades phosphorus sorption capacity

• Altered rate and trajectory of phosphorus transformation during paddy soil evolution.
• Evident declines in phosphorus sorption capacity after long-term paddy cultivation.
• Loss of P-binding materials is responsible for the decline of P-sorption capacity.
• Higher risk of phosphorus loss in older paddy soils is expected.

Information on the dynamics of phosphorus (P) fraction and sorption–desorption characteristics of paddy soils with long cultivation history is essential to improve our understanding of P transformation and transport in agro-ecosystems and critical for sustainable and environment-friendly soil management. We investigated changes in P fraction and sorption–desorption characteristics in topsoils from two contrasting paddy chronosequences developed on calcareous marine sediments at Cixi and acidic quaternary red clays at Jinxian, respectively, in subtropical China. Both chronosequences showed similar patterns and pedogenetic trends of their topsoil P status. Total P (PT) and various P fractions (calcium-associated P (PCa), organic P (Po), non-occluded and occluded P) accumulated to a maximum after 50-yrs and 150-yrs of cultivation, respectively, at Cixi and Jinxian due to P addition, consistent with the prior agronomic field studies on decadal time scales. However, rapid decrease of PT occurred in the older paddy soils at both sites, despite the continuous P additions, which we attributed to the decline of P sorption capacity since P sorbents (CaCO3, Fe- and Al-oxides, and clay) rapidly decreased. Compared with Walker and Syers’ model of gradual P depletion and decreasing bioavailabity in natural ecosystems, our results show that long-term paddy cultivation alters both the rate and trajectory of topsoil P transformations during agro-ecosystem development. The ability of the topsoil to sorb added P decreased with paddy cultivation history in both chronosequences, due to the decline in the maximum P sorption capacity (Smax); whereas the P release potential increased as a result of paddy cultivation at both sites, as indicated by the higher degree of P saturation (DPS) and greater P desorbability in the paddy soils than the uncultivated soils. Our study suggests that after long-term cultivation paddy soils are getting degraded by lowering their P holding capacity so may enhance the P release potential and lead to increasing environmental risk through P loss.