Soil phosphorus availability and rice phosphorus uptake in paddy fields under various agronomic practices

Agronomic practices affect soil phosphorus (P) availability, P uptake by plants, and subsequently the efficiency of P use. A field experiment was carried out to investigate the effects of various agronomic practices (straw incorporation, paddy water management, nitrogen (N) fertilizer dose, manure application, and biochar addition) on soil P availability (e.g., soil total P (STP), soil available P (SAP), soil microbial biomass P (SMBP), and rice P uptake as well as P use efficiency (PUE)) over four cropping seasons in a rice-rice cropping system, in subtropical central China. Compared to the non-straw treatment (control, using full dose of chemical N fertilizer), straw incorporation increased SAP and SMBP by 9.3%-18.5% and 15.5%-35.4%, respectively; substituting half the chemical N fertilizer dose with pig manure and the biochar application increased STP, SAP, and SMBP by 10.5%-48.3%, 30.2%-236.0%, and 19.8%-72.4%, respectively, mainly owing to increased soil P and organic carbon inputs; adding a half dose of N and no N input (reduced N treatments) increased STP and SAP by 2.6%-7.5% and 19.8%-33.7%, respectively, due to decreased soil P outputs. Thus, soil P availability was greatly affected by soil P input and use. The continuous flooding water regime without straw addition significantly decreased SMBP by 11.4% compared to corresponding treatments under a mid-season drainage water regime. Total P uptake by rice grains and straws at the harvest stage increased under straw incorporation and under pig manure application, but decreased under the reduced N treatments and under biochar application at a rate of 48 t ha^-1, compared to the control. Rice P uptake was significantly positively correlated with rice biomass, and both were positively correlated with N fertilizer application rates, SAP, SMBP, and STP. Phosphorus use efficiency generally increased under straw incorporation but decreased under the reduced N treatments and under the manure application (with excessive P input), compared to the control. These results showed that straw incorporation can be used to increase soil P availability and PUE while decreasing the use of chemical P fertilizers. When substituting chemical fertilizers with pig manure, excess P inputs should be avoided in order to reduce P accumulation in the soil as well as the environmental risks from non-point source pollution.