Paddy Cultivation Significantly Alters Phosphorus Sorption Characteristics and Loss Risk in a Calcareous Paddy Soil Chronosequence

Abstract

Core Ideas Phosphorus absorbents in calcareous paddy soils increased with rice cultivation age. Calcium primarily determined P availability in calcareous paddy soils without P inputs. Added P was preferentially retained by Fe or Al oxides. The first few years were the main high‐risk periods for P loss. To reveal the dynamics of phosphate behavior in a calcareous paddy soil chronosequence (after 0, 3, 8, 30, and 70 yr of rice [Oryza sativa L.] cropping), P adsorption–desorption isotherms, chemical fractions, and loss risks were investigated. The contents of P adsorbents (i.e., Fe and Al oxides and Ca salts) in the paddy soils increased with cultivation age, mainly through accelerated primary mineral destruction and weathering in an Alfisol, along with a small portion from irrigation waters. The Langmuir equation gave increasing P adsorption capacities (Qmad) of 520.67, 545.67, 662.78, 860.79, and 834.45 mg kg‐1 for 0, 3, 8, 30, and 70 yr. The Qmad was closely correlated with free Fe and Al oxides, amorphous Al oxides, and HCl‐replaceable Ca contents. The fraction of Ca–P was 52.04 to 63.64% without P inputs, indicating that Ca salts primarily determined P availability. The preferential forms of P retained by paddy soils were in the following order: Ca–P > Al–P > Fe–P > loosely sorbed P. The contents of Olsen‐P in 3‐ and 8‐yr paddy soils were 42.1 and 28.1 mg kg‐1, respectively, much higher than for the two older paddy soils and the upland soil (11.6–24.8 mg kg‐1). The highest degree of P saturation and equilibrium P concentration were in the two younger paddy soils, indicating the increasing potential environmental risk of P loss in paddy soils in the first few years. Hence, more attention should be given to P loss in newly reclaimed paddy soils.