Effects And Mechanisms Of Organic Acids On Soil Phosphorus Release From Typical Soils In The Three Gorges Reservoir Area

The environmental issues have aroused widespread concerns at home and abroad since the operation of Three Gorge Reservoir (TGR) in China, one of the main concerns is the potential eutrophication of its water body. Phosphorus (P) is a major limited factor for eutrophication. The release of phosphorus from soils in the water-level fluctuation zone of TGR may greatly impact the P loads in the water body and thus its potential for eutrophication, due to the fact that the soils are under periodical submerge and exposure. The activation and leaching of soil P as influenced by low molecular weight organic acids has been a focus by many researchers due to its significance on water quality, while few studies deal with those from soils in the water-level fluctuation zone in TGR, where vegetation roots can secrete low molecular weight organic acids, which is bound to result in a significant impact on the subsequent release of phosphorus during flooding. Surrounding the reservoir area, rainfall run off, industrial and living waste water may also contain simple organic acid and chelating agent,etc, through the influence with soil phosphorus forms and the migration activity, resulting in a significant impact on water phosphorus load. For this purpose, two types of typical soil in these areas, including purple soil and grey-brown purple soil were selected to investigate the effect of leaching solution acidity and three kinds of low molecular weight organic acid and chelating agent EDTA on P leaching from soil columns, through batch-type soil column leaching experiment in lab. Analyzed the morphological transformation characteristics of soil inorganic phosphorus under the influence of pH and simple organic acids(oxalic acid,citrate acid and tartaric acid) and EDTA, to explore the effect of different organic acids on soil inorganic phosphorus forms transformation, and inorganic phosphorus relationship with effective phosphorus. The main results obtained are as follows:(1) Ca10-P and O-P were the main form of inorganic phosphorus in both types of bottom soil. Ca10-P took percentage of 48.93% and 28.20% in the total inorganic phosphoru of purple soil and grey-brown purple soil, respectively. Reactive phosphorus (Al-P, Fe-P, Ca8-P, Ca2-P) of the total accounted for 13.12% and 36.91% of the total inorganic phosphorus, respectively. Adding 50mg/kg of exogenous inorganic phosphorus with the process of aging, through the conversion balance, various forms of soil phosphorus levels were absolute increased, and Al-P increased the most, increased by 149% and 71%, respectively. The Al-P increment account for exogenous inorganic phosphorus is 26.36% and 33.27%, respectively. The ratio of active phosphorus amount of inorganic phosphorus also increased significantly. Reactive phosphorus took percentages of 19.53% and 43.51%, respectively. Hence, the exogenous phosphorus could keep higher activity in the soil. The exogenous phosphorus balance in the soil after the transformation, Ca10-P and O-P were also the main form of inorganic phosphorus of the absolute content in both types of soil, Ca10-P took percentages of 42.39% and 25.95% in the total inorganic phosphoru,respectively.(2) Different leaching solution acidity experiment showed that leaching solution acidity had a certain influence on the dissolution of phosphorus, either acidic or slightly alkaline solution could promote phosphorus leaching. Treatments with pH 5,8 and 9 could all promot the phosphorus release of two types of soil. For the purple soil, leaching amount of DTP and MRP increased by 43%～62% and 45%～70%, respectively, compared with the control (pH7.13, DTP-0.197mg, MRP-0.155mg) while for the grey-brown purple soil, leaching amount of DTP and MRP increased by 12%～26% and 6%～11%, respectively, compared with the control(DTP-0.344mg, MRP-0.31 Omg). There was a difference in the phosphorus leaching ability of the two types of soil, the phosphorus of purple soil was more likely to pour out, compared with the control. For purple soil and grey-brown purple soil, pH 5,8,9 treatment all dispalyed a strong effect of activation on O-P and Ca10-P. For both of the two type of soils, the treatment of pH9 increased the amount most. Under the pH5,8,9 leaching treatment, the leaching liquid and soil pH of the two types of soil were maintained around 8 and 8.65, respectively. Compared with controls leaching liquid and soil pH, there was no obvious change observed. This implied that despite the influence of inorganic acid to the soil pH is still within the scope of the buffer, while the leaching of soil phosphorus release had been significantly affected. Morphological analysis showed that the leaching solution could promote O-P and Ca10-P activation in the soil, either acidic or slightly alkaline solution.(3) The simple organic acids (oxalic acid,citrate acid and tartaric acid) in leaching solution could significantly promote the P leaching in grey-brown purple soil, while not in purple soil, and the promoting capacity of citrate was the best, followed by tartaric acid and oxalic acid. Furthermore, for the pure soil, when the organic acid concentration in the range of 5～100mmol/L, DTP and MRP increased by 8%～176% and 1%～137%, respectively,compared with the control treatment.For grey-brown purple soil, as comcentration of organic acid increased, a higher enhancement was observed, especially when the organic acid concentration higher than 10mmol/L or higher.When the organic acid concentration was 100mmol/L, the leaching amount of DTP and MRP increased by 4.40 and 2.94 times, respectively, compared with the control treatment (0.344mg and 0.310mg).While with the treatment of citric acid, the leaching amount of DTP and MRP increased by 15.32 and 11.76 times, respectively, and in the case of tartaric acid, the amount increased by 10.17and 9.58 times, respectively, and the difference reached statistical highly significant (P<0.01) levels. When the concentrations of oxalic acid, citric acid and tartaric acid were higher than 50mmol/L, grey-brown purple soil pH dropped from around 8 to lower than 4, accordingly the soil phosphorus leaching release also increased significantly. For the purple soil, if the organic acid concentration ranged from 5 to 100 mmol/L, the soil pH still remained between 6.84 and 8.71, accordingly the amount of soil phosphorus leaching increased to a lower extent. This indicated that the acidolysis effect of organic acid was an important mechanism of activation for grey-brown purple. Another important mechanism is the complexation.(4) During the leaching process, the existence of organic acid had a significant effect on the transformation of soil inorganic phosphorus. In the purple soil, the amount of oxalic acid added were significant negative correlated with Fe-P and Ca8-P, the amount of citric acid added were significant negative correlated with O-P, and the amount of tartaric acid added were significant negative correlated with Fe-P and Ca10-P (r=<-0.764, P<0.01, N=12).In grey-brown purple soil, the amount of oxalic acid, citric acid and tartaric acid added were significant negative correlated with Fe-P and Ca10-P (r(Fe-P)=-0.938,-0.865,-0.935, r(Ca10-P)=-0.933,-0.896,-0.916, P<0.01, N=12), and the activating effect of organic acids on the Fe-P and Ca10-P was remarkable. The ability of organic acids on two soils inorganic phosphorus activation ability is higher in grey-brown purple soil than in purple soil. For grey-brown purple soil, with oxalic acid and tartaric acid treatment, Olsen-P was very significant negative correlated with Fe-P and Ca10-P(r<-0.795,P<O.01,N==12),while with citric acid treatment, no correlation law between grey-brown purple soil Olsen-P and various forms of inorganic phosphorus was observed.(5) EDTA had a significant influence on phosphorus dissolution, and its promoting effect was associated with EDTA concentration. When EDTA concentration was 5mmol/L or lower, with the increase of, the leaching amount of TP,DTP and MRP increased significantly as the concentration of EDTA increased, when EDTA concentration was higher than 0.5mmol/L, the leaching amount of TP increased gradually with the increase of EDTA concentration, and the leaching amount DTP and MRP increased slightly. When EDTA concentration was 0.5mmol/L, the leaching amount of TP, DTP, MRP in the purple soil increased by 102.35%,56.70%,56.03%, respectively, for grey-brown purple soil, the leaching amount of DTP increased by 41.95%,43.17%,26.81%. When EDTA concentration was 5mmol/L, the leaching amount of TP, DTP, MRP increased by 174.81%,86.45%,78.18% in the purple soild and 92.59%,55.44%,44.97% in grey-brown purple soil, compared with the control. EDTA was mainly to promote the release of insoluble phosphorus in the soil by Al, Fe and Ca and other metal ion complexation reactions. Correlation analysis showed that EDTA had a significantly negative correlation with Ca10-P in the purple soil (r=-0.781, P<0.01, N=12), while not with other forms of phosphorus. For purple soil, while did not with other forms of phosphorus. For grey-brown purple soil, EDTA had a significantly negative correlation with Al-P, Fe-P and Ca2-P (r<-0.746, P<0.01, N=12). EDTA’s promoting effect of P release on grey-brown purple was stronger than purple soil. In addition, under EDTA treatment, the relationship of effective phosphorus and various forms of inorganic phosphorus was not obvious.