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

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 (Q(m)(ad)) of 520.67, 545.67, 662.78, 860.79, and 834.45 mg kg(-1) for 0, 3, 8, 30, and 70 yr. The Q m ad 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.