Analysis on the limiting hazardous chemical factors of acid sulfate paddy soils in Guangdong province

【Objectives】The growth of rice in acid sulfate paddy fields (ASPS) which were the typical low-mid yielding fields in south China was severely limited due to strong acid, thereby, and the rice yields were lower than the national average. In order to ameliorate soils and increase rice yields in ASPS, the limiting factors of acid sulfate paddy fields should be cleared. Soil chemical properties of ASPS with three levels of rice yield were studied to analyze limiting hazardous chemical factors of ASPS, and provide theoretical guidance on ASPS improvements. 【Methods】Soil samples were collected in three typical ASPS with three levels of rice yield (4500, 3000 and 1500 kg/hm2) according to preliminary survey and abandoned ASPS as the control treatment in Taishan city, Guangdong province on June 28, 2013. Eight replicate samples were collected on each sampling field. Soil organic matter, acidity, nutrient elements and micro-elements were measured, and the limiting hazardous chemical factors of ASPS were discussed by correlation analysis and principal components analysis. 【Results】The organic matter and alkaline hydrolyzable N of ASPS with different yield levels have no significant differences, which suggests that the organic matter and alkaline hydrolyzable N are not the limiting factors of ASPS. ASPS are very acid, their pH values are about 3.0-4.0, and their soluble acidity, exchangeable acidity and retained acidity are about 0.6-5.6、 2.7-6.3 and 1.3-14.1 cmol/kg. The soluble acidity, exchangeable acidity and retained acidity have a decreasing tendency as the yield level increasing, especially greater declines of the soluble acidity and retained acidity are observed. As rice yield level decreasing, available P and K decrease significantly, while soluble S, exchangeable S, exchangeable Mn, exchangeable Al increase significantly, and exchangeable Ca, Zn, Cu are not significantly different, which indicates the P deficiency, K deficiency, more SO4, Al toxicity and Mn toxicity are the limiting hazardous chemical factors of ASPS. The correlation analysis indicates that available P and K correlates significantly and negatively with various forms acidity and sulfate, while exchangeable Ca, Mn, Cu, Zn and Al correlate significantly and positively with various forms acidity and sulfate, which suggests that the limiting hazardous chemical factors of ASPS are affected by acidity and sulfate. The principal components analysis (PCA) shows that soluble S, exchangeable S, exchangeable Al, exchangeable acidity, soluble acidity, retained acidity, pH value, available P and K constitute a relatively homogeneous group, which summarize 64.99% of the total variability of soil chemical properties of ASPS with different rice yields, they are the critical limiting hazardous chemical factors of ASPS. The results of PCA also suggests that soluble S, exchangeable S, exchangeable Al, exchangeable acidity, soluble acidity, retained acidity are the variables which have negative effects on rice yield in ASPS, while pH value, available P and K are the variables which have positive effects on rice yield in ASPS. 【Conclusions】The SO4 stress, Al toxicity, Mn toxicity, acid toxicity, P deficiency and K deficiency are the critical chemical factors that limiting yield of rice in ASPS. Acid and sulfate formed in ASPS development are native and fundamental obstacle factors of ASPS in Guangdong province. The Al toxicity, Mn toxicity, P deficiency, and K deficiency are secondary obstacle factors. Thus, ASPS improvement should focus on slowing down pyrite oxidation and promoting jarosite hydrolysis, and decreasing acid and sulfate contents at the plough layer.