Phosphorus adsorption, availability, and potential loss characteristics in an Ultisol-derived paddy soil chronosequence, using a stirred-flow chamber study

The adsorption characteristics, availability, and a loss risk of phosphorus (P) in paddy soil chronosequences had not been well established. In the present study, P adsorption characteristics of paddy soil chronosequence with 0, 7, 22, or 80 years of paddy cultivation history (PS0, PS7, PS22, and PS80, respectively) were identified by P adsorption isotherm, envelope, and stirred-flow chamber (SFC) kinetics, along with the assessment of P availability and loss risks. Results indicated that P adsorption and desorption followed the same order of PS0 > PS7 > PS22 > PS80. Pearson's correlation analysis revealed P adsorption capacity was significantly and positively correlated with the concentrations of both free iron (Fe) and aluminum (Al) (hydro)oxides (Fe_d and Al_d, respectively), suggesting that the major P sorbents were Fe_d and Al_d. Findings from SFC adsorption kinetics suggested that the kinetics of both P adsorption and desorption were biphasic, being fast in the first 100 min (75.11%–87.57% of P adsorbed and 79.00%–88.65% of P released), followed by a slow phase for the remaining 180 min. Bray-P increased with the duration of rice cultivation and reached a maximum of 77.90 mg kg⁻¹ at PS80, along with the highest P saturation degree and equilibrium phosphorus concentrations of 7.56% and 2.33 mg L⁻¹, respectively. Hence, with an increasing duration of paddy cultivation, there was a less pronounced P sink function and a lower P availability, but a higher P loss risk, indicating that P availability in the younger and loss risk in the older Ultisol-derived paddy soils deserve greater attention.