Short-term changes in soil nutrients and vegetation biomass and nutrient content following the introduction of extensive management in upland sown swards in Scotland, UK.

摘要

Agri-environmental policy changes promote more extensive grazing management but the temporal responses of soil nutrients and vegetation biomass and quality to reductions in grazing intensity are still unresolved. We measured soil nutrients and the biomass and nutrient content of vegetation over 5 years following the introduction of extensive management treatments at three sites in Scotland, UK. Five unfertilised treatments, representing different levels of extensive management, were established on existing sown perennial ryegrass/white clover swards. One treatment was ungrazed (UN) and the others had sward surface height treatments of 4 or 8 cm during two grazing season treatments, summer and autumn, within each year in a factorial combination (4/4, 4/8, 8/8, 8/4 cm). A further treatment, representative of current more intensive systems, received an annual total of 140 kg N ha-1 plus maintenance P and K and was grazed by sheep to maintain a sward surface height of 4 cm (4F). When compared with more intensive management, there was little effect of 5 years of extensive management on soil nutrients at any of the three sites. The extensive treatments created swards with different above-ground biomass during the season, thus changing the balance between the litter and excretal routes for the recycling of plant nutrients. When expressed relative to treatment 4F, the proportion of live dry mass and N, P and K contents in sown species was on average higher in swards maintained at 4 cm in summer than at 8 cm, and there was evidence of a decline over time in the latter treatments. The proportion of unsown species in live mass increased over time in the unfertilised treatments, and the increase was most rapid in the ungrazed treatment. Nutrient contents of the vegetation in unfertilised swards were lower than those in fertilised swards, and changes over 5 years in the different management treatments differed between sites. However nutrient contents remained above levels that could adversely affect sheep performance throughout this period. Agri-environment schemes that promote extensive grazing management will change vegetation biomass and nutrients but are unlikely to reduce soil nutrients in the short term.