Phosphorus spatial distribution and pollution risk assessment in agricultural soil around the Danjiangkou reservoir, China.

The phosphorus (P) content in agricultural soils in catchments of lakes or rivers is an important issue because P is both an essential nutrient for high plant yields and a major contributor to eutrophication of water bodies. Thus, we have assessed the spatial distribution of soil P, impacts of environmental factors on its spatial variation, and associated pollution risks, in farmland around the Danjiangkou Reservoir, China. A total of 217 sites were sampled, including 121, 44 and 52 in dry cropland, paddy land and orchards, respectively. Total phosphorus (TP) contents in the sampled soils ranged from 0.230 to 1.893 g kg-1 and Olsen-P (a measure of available P) from 0.38 to 176 mg kg-1. Soil TP and Olsen-P had moderate and high variability, with coefficients of variation of 40% and 115%, respectively. Geostatistical analysis showed that both TP and Olsen-P had positive nugget effects, but TP had stronger spatial autocorrelations than Olsen-P (nugget-to-sill ratios: 22 and 50%, respectively). Total P was significantly influenced by temperature, elevation, aspect, soil pH, organic matter and precipitation, while Olsen-P was controlled by aspect and land use. The TP in soil was very similar in dry cropland and orchards (0.64 ± 0.19 and 0.61 ± 0.23 g kg-1, respectively), but substantially lower in paddy land (0.15 ± 0.25 g kg-1). Risks for P loss appeared to be very high, high, moderate and low in approximately 10, 40, 15 and 30% of the farmland in the Danjiangkou reservoir area, respectively.