Speciation,Microbial Enzymatic Conversion And Activation Of Legacy Phosphorus In Paddy Soils

Excessive application of swine manure greater than nutritional P requirements of plants can lead to most of the P stored in soils. This legacy P has become the focus of the current "agro-ecological stability and security". The legacy P transformation is strongly related to microbial aticity. Therefore, accounting for legacy P fractions and related microbial activities under swine manure application are important to reduce P inputs and minimize P transfer from land to aquatic environments. The field experiment treatments included three swine manure organic amendment rates (26 [Low],39 [Middle], and 52 [High] kg P ha-1) and two controls (no-fertilizer and 26 kg P ha-1 superphosphate). Soil samples were taken for two rice seasons to uncover the responses of soil P fractions and phosphomonoesterase activities in a middle-term swine manure applied rice field. Meanwhile, the concentration of TP and DP in paddy water under different fertilization were determined to analyse P loss potential. To promote the release of legacy phosphorus in the paddy soils, the locally typical paddy soils were selected to further activate legacy phosphorus. The main results were shown as follows:(1) Rice yield was significantly increased with the increase of manure P rates from 0 to 39 kg P ha-1. However, when manure P rates increased from 39 to 52 kg P ha-1, swine manure amendment had no significant effect on grain yields. Most of P fractions showed the highest content at rice seedling stage for all treatments. On seasonal average, the relative sizes of labile (H2O-P, NaHCO3-Pi and -Po), moderately labile (NaOH-P; and -Po) and stable (HCl-P and Residual-P) P pools were in 1:3.1: 3.6 and 1:2.3:3.7 ratios in 2011 and 2012, respectively. According to P fractions (except for Residual-P) treatments could be ranked as M52>M39>M26>P26>Mo over the whole rice growth stages, but the difference between between P26 and M26 treatments were not significant, which further indicated that changes in P fractions depended on the amount of applied P irrespectively of P sources in short- and medium-term soil management systems. Moreover, repeated swine manure did not affect most of P fractions (except for NaOH-Po) under the plow layer due to the mature plow pans. Results from this study suggested that heavy application of swine manure over 7 years promotes NaOH-Po movement to lower portions of the soil profile.(2) Analysis of enzymatic properties indicated that acid phosphomonoesterase activity was the predominant phosphomonoesterase in the tested soil. The soil acid and alkaline phosphomonoesterase activity activities showed a sharp increase in the rice tillering stage in all treatments. The M39 and M52 treatments had relatively high initial velocity (Vo), maximal velocity (Vmax) and activation grade (lgNa) but low Michaelis constant (Km), temperature coefficient (Q10), activation energy (Ea) and activation enthalpy (△H). These results implied that the M39 and M52 treatments could easily stimulate the enzyme-catalyzed reactions than other P rate treatments.(3) TP and DP concentration appeared a peak within the first day after fertilization in surface water, whereas the peak values appeared within the first 7 days after fertilization in seepage water for all treatments (except for Mo). During the whole rice growth season, the TP concentration in surface water and seepage water have exceeded 0.02 mg L-1 which level can result in eutrophication. Particulate phosphorus was the main form of phosphorus in surface water, whereas dissolved phosphorus was the main form of phosphorus in the seepage water. Both TP and DP concentration in surface water with respect to time (t, days) could be fitted well by logarithmic function (y=a+b×lnt), while TP and DP concentration in the 50 cm seepage water with respect to time could be fitted well by exponential function (y=a×exp(b×t)). TP and DP concentrations significantly higher in M52 treatment than other treatments in surface water and seepage water, therefore, it was not suitable to continuously apply 52 kg P ha-1 in paddy soils for a long time. Based on these results, we suggested that the swine manure application should be kept at about 39 kg P ha-1 to decrease potential environmental risks and to improve the soil enzymatic properties and rice yield.(4) Each dosage ascorbic acid (0.5%,1.0%,2.0%) had significantly improved Olsen-P content in all paddy soils, and each dosage citric acid significantly improved Olsen-P content in Husong, Xiaofen and Huangban paddy soils, while oxalic acid increased Olsen-P content in Qingzi paddy soil. Each dosage citric and ascorbic acid had significantly positive effect on acid phosphomonoesterase in Husong and Huangban paddy soils, but each dosage reduced glutathione had significantly positive effect on acid and alkaline phosphomonoesterase in Xiaofen and Qingzi paddy soils. 1% citric and ascorbic acid application improved NaHCO3-Pi and NaOH-P; contents but decreased NaHCO3-Po and NaOH-Po contents in Husong paddy soil.1% citric and ascorbic acid application improved H2O-P, NaHCO3-Pi, NaHCO3-Po, NaOH-Pi and HCl-P contens but decreased NaOH-Po contens in Xiaofen paddy soil.1% citric and ascorbic acid application significantly improved NaHCO3-Pi, NaOH-P; and HCl-P contents but decreased H2O-P, NaHCO3-Po, NaOH-Po in Huangban paddy soil.1% oxalic acid application activated NaHCO3-Pi, NaOH-Pi and HCl-P but reduced H2O-P, NaHCO3-Po, NaOH-Po contents in Qingzi paddy soil.