Spatio-temporal variability and environmental risk assessment on soil available phosphorus in tobacco-planting fields of paddy-upland rotation

To investigate the spatio-temporal variability of soil available phosphorus and provide a scientific basis for phosphorus fertilization and the control of nonpoint phosphorus pollution in typical paddy-upland rotation areas, the spatio-temporal variability and environmental risk of soil available phosphorus in Liuyang City were assessed with spatial autocorrelation, semi-variance function and fractal dimension methods by taking the available phosphorus content in fields of Liuyang in 2000 and 2014 as objects. The results showed that soil available phosphorus content increased by 23.07 mg/kg (up to 119.53%) from 2000 to 2014. Moran's I index of soil available phosphorus decreased, while fractal dimension and nugget effect increased, it thus indicated that the spatial structure of soil available phosphorus weakened and the stochastic variability enhanced over this period. The areas with "extremely higher" and "very higher" soil available phosphorus content increased by 46.95% and 29.68%, respectively; while those with "high" and "appropriate" soil available phosphorus content decreased by 23.27% and 53.36% , respectively. In 2014, the probability of available phosphorus content exceeding risk critical value in Liuyang increased sharply. "Very high risk" region (probability>80%), "high risk" region (probability 60%-80%) and "moderate risk" region (probability 40%-60%) accounted for 1.27%, 16.52% and 31.95% of the tested areas, respectively. The northeast areas around Dahu Town and the Midwest areas around Chunkou Town in Liuyang were the key control regions for agricultural nonpoint phosphorus pollution.