O'connor: Nitrogen Balances in Grasslands Nitrogen Balances in Natural Grasslands and Extensively-managed Grassland Systems

An approach is outlined for synthesising current understanding of nitrogen dynamics in natural grasslands and extensively managed grassland systems. The increasing complexity of models is illustrated from recent literature, first conceptualising and eventually process-simulating the dynamics of nitrogen, especially in the soil sub-systems of grasslands. Some comparisons are made between New Zealand and North American grasslands in the magnitude of some N pools and fluxes and some of the principal features of soil biological studies are noted. Natural environmental variations in N balances are reviewed, especially with regard to climatic influences on organic N pools, mineral N pools and nitrification. The role of nitrogen fixation is related to ecological succession and is presented as sometimes a facultative response of natural systems to natural and some cultural perturbations. Attention is redirected at symbiotic nitrogen fixers in New Zealand tussock grasslands. The effects of management factors on N balances are reviewed. The special susceptibility of New Zealand tall tussock grasslands to large N losses from fire is indicated. Grazing is especially examined for its influence on mineral N pools and losses as well as for its N redistribution effects. A summary assessment of the understanding of nitrogen regimes of New Zealand tussock grasslands is made in terms of higher plant uptake and transfers, litter accumulation and decomposition, and soil biology and biochemistry. Special attention is directed to the possible effects of pastorally-induced changes in these regimes. The review ends with an indication of the likely significance of model-guided, co-ordinated research into the dynamics of nitrogen and other biogeochemical transformations, as of phosphorus and sulphur, in both natural grasslands and the culturally developed grasslands and forests which may now replace many of them.