Appropriateness of nutrient budgets for environmental risk assessment: a case study of outdoor pig production

Outdoor pig production has expanded rapidly in recent years due to economic and market pressures. Recent research has shown that a high degree of heterogeneity in soil nutrient distribution can occur as a result of this agricultural system. This study examined the consequences of dietary manipulation on total excretion and spatial distribution of N and P in soils under outdoor sow production. Small areas of land chosen for excretion received excessive amounts of both N and P. Values equivalent to up to 2861 kg ha(-1) of soluble N were observed in preferred areas, up to 90% of this was in organic forms. Within 15 months, applied nutrients were sufficient to saturate the soil profile in preferred areas with respect to P and produce areas that represented a significant environmental risk. Lowering the amount of N and P in the diet has potential to reduce environmental impact but cannot eliminate spatial variability. The use of simple input output nutrient budgeting approaches may underestimate losses of N and P as they cannot account for mineralisation rates of organic nutrients excreted or for P saturation in areas preferentially used for excretion. (C) 2003 Elsevier B.V. All rights reserved.