Deteriorated Water Quality of Agricultural Catchments in South China by Net Anthropogenic Phosphorus Inputs

Improper anthropogenic phosphorus (P) management is considered a major cause of water quality deterioration, however the relationship between anthropogenic P input and catchment water quality is rarely quantified in China. The study area encompassed eight small catchments with areas ranging from 58.6 to 13,442.4 ha in the subtropical region of South China. On-site observations of P concentrations, stream fluxes, and social investigation of P input were conducted over a 3-year period. The regional variations of net anthropogenic phosphorus inputs (NAPI) and responses of riverine P export were quantitatively analyzed. Results showed that the total NAPI of catchments varied from 11.04 to 40.52 kg P ha(-1) year(-1), where cropland systems (NAPI(c)) were the largest P sources, accounting for 47.7-67.7% in total. Meanwhile, net food and feed P input varied from 3.87 to 30.73 kg P ha(-1) year(-1), accounting for 35.0-75.8% in total, followed by fertilizer and non-food P input with 4.65-10.48 and 0.63-2.89 kg P ha(-1) year(-1), respectively. Riverine P export and the soil total P and Olsen P contents in croplands were all positively related to NAPI (p < 0.05). A simple empirical model was simulated to predict the annual riverine total P fluxes using NAPIc with greater accuracy than with using NAPI or NAPI for residential land (NAPI(r)). Gray relational analysis suggested that livestock density was the most important influencing factor for NAPI. It is concluded from these results that, although the livestock accounted for the largest part of the NAPI, the cropland contributed the greatest to catchment riverine P export. This probably due to recycling of animal manure for plant cropping systems. Therefore, maintaining a reasonable scale of livestock production, and reducing the internal cycle of manure or replacing part of the chemical fertilizer should be a major approach in reducing NAPI and corresponding riverine P export in the study area.