Spatial and temporal variation of cropland pH and the driving factors in Jiangsu over the past 40 years

Objectives The spatial and temporal variation characteristics of cropland soil pH were surveyed at county scale, and the driven factors were investigated for prevention of future cropland acidification.

Methods The data were collected from the Second National Soil Survey (1980), the National Soil Formula Fertilization Survey (2006−2012), and the Survey of Soil Agrochemical Traits in Jiangsu Province (2018). The four decades were divided into three periods, according to the data acquisition time. The whole province was composed of 13 regions, according to the administrative divisions. The main driving factors were assessed using stepwise regression analysis.

Results The mean pH cropland in Jiangsu Province in 2018 was 7.02 (5.61−8.31). Soil pH was high in Nantong (7.80) and Yancheng (7.52), and low in Wuxi (6.05) and Changzhou (6.20). In the past 40 years, the cropland pH in Jiangsu Province decreased by 0.57 pH units on average, with quick decrease period and gentle stable period. The rapid annual decrease rate was 0.016 pH unit during 1980−2006, and the gentle decrease rate was 0.008 and 0.007 pH units, respectively, during 2006−2012 and 2012−2018. During 1980−2006, the proportion of sample sites in level I (pH>7.5) decreased from 60.6% to 38.0%, and the proportion of sample sites in level III (pH in 5.5−6.5) increased from 8.5% to 28.2% in whole Jiangsu Province. During 2006−2018, the decreasing trend of cropland soil pH was effectively suppressed. The variation of cropland pH was closely related with the initial soil organic carbon density, crop straw incorporation, and chemical fertilizer application rate, and the contribution of the three factors to soil pH change were 48.7%, 26.9%, and 24.4% for the forty years.

Conclusions The overall spatial distribution of soil pH in Jiangsu was characterized by “acidity in the south and alkalinity in the north”, which was mainly driven by soil properties, soil types and land use practices. Over the forty years, soil pH did not decrease evenly. The pH decrease was quick from 1980 to 2006 due to the rapid increase of chemical fertilizer application rate, and gentle from 2006 to 2018 as straw returning to field increased.