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

John H. M. Thornley and Melvin G. R. Cannell
Plant and Soil
Vol. 224, No. 1 (2000), pp. 153-170
Published by: Springer
Stable URL: http://www.jstor.org/stable/42950826
Page Count: 18
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Dynamics of mineral N availability in grassland ecosystems under increased [CO₂]: hypotheses evaluated using the Hurley Pasture Model
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Abstract

The following arguments are outlined and then illustrated by the response of the Hurley Pasture Model to [CO₂] doubling in the climate of southern Britain. 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 overwhelmingly by N cycling into all forms of organic matter (plants, animals, soil biomass and soil organic matter - 'immobilisation' in a broad sense) and back again by mineralisation. Annual system N gains (by N₂ fixation and atmospheric deposition) and losses (by leaching, volatilisation, nitrification and denitrification) are small by comparison. 3. Elevated [CO₂] enriches the organic matter in plants and soils with C, which leads directly to increased removal of N from the soil mineral N pools into plant biomass, soil biomass and soil organic matter (SOM). 'Immobilisation' 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 gaseous and leaching losses are reduced and the ecosystem gains N. Thus, net immobilisation gradually increases the N status of the ecosystem. 4. At the same time, elevated [CO₂] increases symbiotic and non-symbiotic N₂ fixation. Thus, more N is gained each year as well as less lost. Effectively, the extra C fixed in elevated [CO₂] is used to capture and retain more 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 amount 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 elevated [CO₂] depends on the N status of the system and the magnitude of the overall system N gains and losses. Small changes in the large immobilisationmineralisation cycle have large effects on the small mineral N pools. Consequently, it is possible to obtain a variety of growth responses within 1-10 year experiments. Ironically, ecosystem models — artificial constructs — may be the best or only way of determining what is happening in the real world.

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