Soil acidification characteristics in southern China??s croplands under long-term fertilization

【Objectives】The objective was to illustrate characteristics of soil acidification in southern China??s croplands under long-term fertilization, which will help to reduce acidification rate in these areas by rational fertilization.【Methods】Soil acidification characteristics of typical croplands were evaluated using a 10-year monitoring data in 32 sites located in Hunan province, including paddy soils and red soils under the long-term fertilization.【Results】There is no obvious soil acidification in paddy soils, generally. Significant soil acidification is observed in the paddy-upland rotation with the acidification rate of 0.071 unit/a which is 10 times of that in the rice-rice succession cropping system. Soil acidification is mainly ascribed to the increase of chemical N fertilizer amounts in the paddy-upland rotation. Similarly, obvious soil acidification is also observed in red soils. Further, the acidification tendency with fertilization year consists of two different stages, stage Ⅰ, rapid decline in rate of soil acidification after the fertilization for 1-6 years, and Stage Ⅱ, slow decline or relatively stable in rate of soil acidification after the fertilization for 7-10 years. Additionally, more significant soil acidification (average rate of 0.075 unit/a) is observed in red soils with initial higher pH (pH 6) than that with lower pH (pH 4-5), and there is a positive correlation between the soil acidification rate and application amounts of chemical nitrogen fertilizers. 【Conclusions】 In southern China's croplands the long-term chemical nitrogen fertilization accelerates soil acidification in paddy soils under the paddy-upland rotation and red soils with higher initial pH (pH 6). Therefore, controlling and reducing the application amounts of chemical nitrogen fertilizers is an effective measure for preventing further acidification in paddy-upland rotation soils and red soils.