Soil carbon and nutrient variations in an arable-ley rotation with organic pig production

Soil carbon, nitrogen and phosphorus concentrations were determined and the soil structure evaluated for four annual courses in a rotation (Year 1, fattening, non-nose-ringed pigs at a stocking density of one pig produced per 150–200 m2 depending on seasonal conditions; Years 2 and 3, spring wheat; Year 4, reseeded grass-clover) for organically accredited pig production on a shallow calcareous soil in southwest England. Soil nitrogen and phosphorus concentrations and organic decomposition rates were highest when pigs were present, consistent with recent inputs of feed and nutrient redistributed in dung and urine. Soil nitrogen and phosphorus concentrations and organic decomposition rates declined over the subsequent courses. The overall production system was sustained by inputs of pig feed one year in four, with nutrients being carried over to subsequent years. The pigs caused significant physical damage leading to soil disaggregation, but the soil structure recovered over the following 2 years. The high concentrations of inorganic nitrogen, particularly nitrate at high concentration ‘hot-spots’ around feeders and housings, and the poor soil structure caused by the pigs represented a significant risk of loss of nutrients and soil. At this site, vertical movement of disaggregated soil and nutrients through cracks and fissures in the underlying limestone may have been an additional route of loss alongside denitrification, volatilisation, erosion and run-off.