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