Agricultural phosphorus flow and its environmental impacts in China.

The transfer of nutrients from agricultural land to waters attracts the attention of policy makers as well as scientists as it plays an increasingly significant role in affecting the water environment. It is therefore essential to first understand the nutrient flow in agricultural systems and then correspondingly formulate a series of cost-effective policies and best management practices (BMPs). On the basis of an emission inventory analysis (EIA) and a nutrient full balance (NFB) calculation, this paper presents a partial substance flow analysis (SFA) method, as well as an Agricultural Phosphorus Flow Analysis (AgiPhosFA) model, to describe the phosphorus (P) flow in the agricultural systems in China and assess the impact of human activities on waters driven by agriculture and rural life. It is estimated that average P input and output were 28.9 kg ha(-1) a(-1) and 14.2 kg ha(-1) a(-1) respectively in China in 2004, while the total P utilization efficiency (Plant uptake P/P input) in agriculture was 45.7% leading to an average P surplus of 14.7 kg ha(-1) a(-1). Excessive P application through mineral fertilizer in the arable farming system has led to the accumulation of soil P and constituted a risk to the recipient water quality, whereas the grassland grazing system is confronted with a severe P deficit problem which has resulted in widespread grass degradation. Therefore it may be an efficient way to mitigate the problems simultaneously by regulating and balancing the P flows between the two systems. Uncertainties of the method and model are also discussed in terms of model conceptualization, data and parameters, and spatial and temporal variability.