Nitrogen cycling in upland pastures of the U. K

Abstract

Substantial areas of upland pastures occur in central Wales, the Lake District and Pennines, and over much of Scotland. Plant growth and N availability are limited by soil acidity, low temperatures and a short growing season. Grass heaths and heather moorland dominate the vegetation; grazing prevents the re-establishment of forest. The land is used mainly for hill sheep farming and game shooting; it is also an important source of water for urban and industrial use. Comprehensive data on N cycling are sparse. Upland pastures have accumulated large amounts of soil organic N, 6-25 t ha -1 but only very small amounts, 20-60 kg N ha -1 are mineralised each year. Nitrogen in precipitation and dry deposition provides a total input of about 14 kg ha -1 organic and inorganic N, only part of which may be directly available for plant use. The annual input of mineral N in rainfall has increased by about 10 kg N ha -1 at Rotham sted this century; if this increase has occurred in upland pastures it could have significant effects. Biological fixation may add 5-10 kg N ha -1 annually; no fertilizers or manures are applied. Winter feed is unlikely to exceed 1 kg N ha -1 . Periodic burning of accumulated vegetation is a principal output, averaging 4-6 kg ha -1 annually. Streams may discharge 3-6 kg mineral N ha -1 annually and also organic N detached from streambanks. Output as livestock and wool is proportional to grazing intensity; at 0.5 ewes per hectare the annual output would be 0.4 kg N ha -1 . There are large differences in all items of the nitrogen cycle within a catchment, associated with inherent variations in soils, topography and climate. Consequently the N balance may be positive, negative or in equilibrium for different areas within a catchment and the internal variation may vitiate any evaluation of regional N balances.