Soil acidification degree difference and impact factors of subtropical cropland

Soil pH is an important indicator of cropland soil fertility and quality. Decline in soil pH is a vital problem in soils of intensive agricultural systems in China, which heavily obstructed crop growth and improvement of ecological environments. Thus accurate knowledge on the degree of acidification and its causes of regional cropland soils is crucial for the enhancement of the quality of cropland soils and sustainable development of agriculture. In this study, data on topsoil attributes collected from 36 777 sampling sites in the second soil survey in 1982, 236 445 sampling sites of soil fertility investigation for fertilization in 2008, average annual temperature and precipitation for 1980-2008 in Fujian Province were used to establish a 1:250 000 map of relevant cropland soil attributes database in ArcGIS software. This database, containing a total of 34 593 patches, was used to study the extent of acidification, and combined with the Grey slope correlation analysis model to explore the causes of cropland soils acidification in the province. The aim of the study was to lay the scientific basis necessary for understanding soil acidification regulation. The results showed that the area of acidification during the studied period accounted for 67.60% of the total area of croplands in the province. Also the areas with strong, moderate and low acidifications accounted for 0.83%, 18.26% and 48.52% of total croplands in the province, respectively. Analysis on prefectural city scale showed that strongly acidified croplands were mainly distributed across Longyan City and Quanzhou City, accounting respectively for 86.88% and 8.39% of total strongly acidified croplands in the province. Moderately acidified farmlands were mainly distributed across Nanping City, Longyan City and Quanzhou City, accounting respectively for 29.88%, 18.10% and 16.94% of total moderately acidified cropland in the province. The weakly acidified croplands were widely distributed across Fujian Province. On the whole, strongly and moderately acidified farmlands were mainly distributed across the north, southwest and southeast littoral regions of Fujian Province. In terms of soil type, gley paddy soils, percogenic paddy soils and acid purplish soils had significantly acidized in 1980 to 2008, with acidified areas accounting respectively for 82.87%, 72.37% and 69.20% of total gley paddy soils, percogenic paddy soils and acid purplish soils in the province. However, the acidification degrees of percogenic and hydromorphic paddy soils were relatively severe, with the total area of strong, moderate and low acidifications accounting respectively for 98.94%, 84.51% and 87.36% of total acidified cropland area in the province. In terms of land use type, paddy and irrigated fields were significantly acidified, accounting for a high proportion of the study area. Acidified areas accounted respectively for 70.35% and 60.78% of total paddy and irrigated fields in the study area. Grey slope correlation analysis suggested that increasing severity of acid rain, high temperature, high precipitation and high fertilizer input were the main factors contributing to the acidification of croplands in Fujian Province. An effective control approach of cropland acidification in Fujian Province was by strict control the emissions of industrial acidic exhaust gases containing sulfur. It was also necessary to optimize fertilizer management programs by increasing organic fertilizers application and reducing chemical nitrogen and phosphate fertilizers application.