Evolution of soil fertility and the main driving factors in Yangzhou city during the past 30 years

Objectives  Study of the temporal and spatial evolution of fertilities of farmlands and the driving factors is of great value for guiding agricultural production and farmland conservation in the main grain producing areas of the middle and lower reaches of the Yangtze River.

Methods  Yangzhou is one of the main grain producing areas in the middle and lower reaches of the Yangtze River. The soil pH and the contents of total N, available P, available K and organic matter in 1984 and 2014 were selected as the evaluation indexes of soil comprehensive fertility. The soil comprehensive fertility index was calculated by Nemero index method. The spatial-temporal variation characteristics and distribution pattern of each fertility index and comprehensive fertility index were determined by GIS and geostatistics. The main differences of soil fertility were explored by regression analysis control factor.

Results  1) During 30 years (1984–2014), in the main grain producing areas of the middle and lower reaches of the Yangtze River basin, the contents of organic matter, total N and available P in farmland increased by 6.01 g/kg, 0.33 g/kg and 21.21 mg/kg, respectively; the contents of available K decreased by 9 mg/kg, and the pH values decreased by 0.77. The contents of soil organic matter, total N, available P and available K showed moderate variability, and the pH showed weak variability. 2) In the past 30 years, the soil fertility index of farmland increased by 0.31, which was stable between years, showing medium variability and medium spatial autocorrelation, and the level of soil fertility increased significantly. The soil fertility of farmlands in 1984 mainly fell in grade III, accounting for 73.27% of the total farmland area, and that in 2014 was mainly in grade II, accounting for 76.20%. 3) Soil parent material, soil type, soil subclass, soil texture and land use pattern had a significant effect on the spatial evolution of soil fertility (P < 0.01). Land use patterns could independently explain 53.9% of the spatial differences in soil fertility evolution in 1984, and explain 58.0% in 2014.

Conclusions  The fertility of farmlands in Yangzhou City, the main grain producing areas of the middle and lower reaches of the Yangtze River basin, are generally enhanced, and the fertility grade of farmlands has increased from mainly grade III in 1984 to grade II in 2014. The main driving factors of temporal evolution of soil fertility during the past 30 years are the land use pattern.