Effect of several organic acids on phosphate adsorption by variable charge soils of central China

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Abstract

The effect of several organic acids on phosphate adsorption by acidic soils in subtropical zone of central China was studied. Results showed: (1) citrate and oxalate remarkably reduced the amount of phosphate adsorption, but tartrate, benzoate and acetate had only a very slight influence on phosphate adsorption; (2) the ability of citrate in reducing phosphate adsorption was greater than that of oxalate, moreover, the reduction percentage was dependent on the concentration of organic ligands in the solution; (3) the effect of organic acids on phosphate adsorption was related to the pH value of organic acid solution. The minimum reduction in adsorption of phosphate was present at a specific pH value of organic acid solution which ranged from 2 to 10; (4) a minimum reduction of phosphate adsorption occurred when phosphate was added to the soils before organic acid, whereas a maximum occurred when organic acid was introduced before the addition of phosphate. Meanwhile, the treatment for the mixture of two organic acids resulted in more reduction in phosphate adsorption than each of the organic acids and less than the total of them. Based on these observations, we suggested that the competition between phosphate and organic acids relied on their relative affinity to soil mineral surface at different conditions.

Introduction

Phosphorus (P) is an essential nutrient element for plant growth. The misapplication of phosphate fertilizer usually causes eutrophication of water body, unbalanced plant nutrition and low P utilization efficiency. How to properly apply phosphate fertilizers and increase the efficiency of soil P are common interests of soil and fertilizer researchers and environmental protection workers.

Soil organic acids are derived from the decomposition of plant and animal residues, microbial metabolism, canopy drip and rhizosphere activity Huang and Violante, 1986, Mo, 1986, Stevenson, 1967. The rhizosphere is a favorable habitat for acid-producing bacteria, and therefore the amounts of low-molecular-weight organic acids in the immediate vicinity of the plant roots are much higher than in the bulk soil. The nonvolatile aliphatic acids, such as oxalic, citric, tartaric acids are relatively abundant in soil Mo, 1986, Stevenson, 1967. The supply of P to plant is strongly influenced in the rhizosphere by the presence of organic ligands Deb and Datta, 1967, Earl et al., 1979, Evans, 1985, Han and Jordan, 1995, Lopez-Hernandez et al., 1986, Nagarajah et al., 1970, Violante and Gianfreda, 1995, Violante et al., 1991. Although the competitive adsorption of P and organic ligands by synthetic clay minerals and oxides have been studied Kafkafi et al., 1988, Sibanda and Young, 1986, Violante and Gianfreda, 1993, Violante and Gianfreda, 1995, Violante et al., 1991, there is limited information on the fate of P in the presence of organic acids in natural soil (Yuan, 1980). Therefore, the exact mechanism among soil inorganic colloids, organic acids and P has not been well-established.

For this reason, we investigated the effect of various organic acids on P adsorption by acid soils, so as to provide the scientific base for revealing the mechanism of P adsorption on acidic soils in the presence of organic acids, and to increase the efficiency of P and diminish its environmental pollution.

Section snippets

Materials and methods

The soil samples used included five acidic soils (four Ultisols and one Inceptisol), which were derived from Q2 and granite, and located in Hubei and Hunan Provinces, China. They possessed lower contents of organic matter and higher amounts of free iron and aluminum oxides (Table 1).

The effect of organic acid kinds on phosphate adsorption by acidic soil

When 2 or 4 mmol/l organic acids were added to Wenquan brown red soil together with P, the P adsorption quantity reduced from 28.39 mmol/kg to 12.21–26.6 and 9.90–25.55 mmol/kg, and the reduction reached 6.3–57.0% and 10.0–65.1%, respectively (Fig. 1). When citrate was introduced, the reduction was the most among five kinds of organic acids examined. However, when benzoate and acetate were introduced, the reduction in adsorption was less obvious. The effect of oxalate on P adsorption was more

Conclusion

The present results showed that the effect of organic acids was relied on the constituents of soil solid phase, organic acid concentration, pH value and the way of organic acid addition. The adsorption of P and organic acid depended on the relative affinity in the surface group at different conditions. It seemed that the soil surface for adsorbing P and organic acid should be similar, although the surface site might have different energy states. In order to increase the utilization efficiency

Acknowledgements

We thank the National Natural Science Foundation of China for financial support.

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