Dynamics of mineral N availability in grassland ecosystems under increased[CO2]: hypotheses evaluated using the Hurley Pasture Model

The following arguments are outlined and then illustrated by the response o f the Hurley Pasture Model to [CO2] doubling in the climate of southern Bri tain. 1. The growth of N-limited vegetation is determined by the concentration of N in the soil mineral N pools and high turnover rates of these pools (i.e. , large input and output fluxes) contribute positively to growth. 2. The size and turnover rates of the soil mineral N pools are determined o verwhelmingly by N cycling into all forms of organic matter (plants, animal s, soil biomass and soil organic matter - 'immobilisation' in a broad sense ) and back again by mineralisation. Annual system N gains (by N-2 fixation and atmospheric deposition) and losses (by leaching, volatilisation, nitrif ication and denitrification) are small by comparison. 3. Elevated [CO2] enriches the organic matter in plants and soils with C, w hich leads directly to increased removal of N from the soil mineral N pools into plant biomass, soil biomass and soil organic matter (SOM). 'Immobilis ation' in the broad sense then exceeds mineralisation. This is a transient state and as long as it exists the soil mineral N pools are depleted, N gas eous and leaching losses are reduced and the ecosystem gains N. Thus, net i mmobilisation gradually increases the N status of the ecosystem. 4. At the same time, elevated [CO2] increases symbiotic and non-symbiotic N -2 fixation. Thus, more N is gained each year as well as less lost. Effecti vely, the extra C fixed in elevated [CO2] is used to capture and retain mor e N and so the N cycle tracks the C cycle. 5. However, the amount of extra N fixed and retained by the ecosystem each year will always be small (ca. 5-10 kg N ha(-1) yr(-1)) compared with amoun t of N in the immobilisation-mineralisation cycle (ca. 1000 kg N ha(-1) yr( -1)). Consequently, the ecosystem can take decades to centuries to gear up to a new equilibrium higher-N state. 6. The extent and timescale of the depletion of the mineral N pools in elev ated [CO2] depends on the N status of the system and the magnitude of the o verall system N gains and losses. Small changes in the large immobilisation -mineralisation cycle have large effects on the small mineral N pools. Cons equently, it is possible to obtain a variety of growth responses within 1-1 0 year experiments. Ironically, ecosystem models - artificial constructs - may be the best or only way of determining what is happening in the real wo rld.