N-POOR ECOSYSTEMS MAY RESPOND MORE TO ELEVATED [CO2] THAN N-RICH ONESIN THE LONG-TERM - A MODEL ANALYSIS OF GRASSLAND

The Hurley Pasture Model was used to examine the short and long-term r esponses of grazed grasslands in the British uplands to a step increas e from 350 to 700 mu mol mol(-1) CO2 concentration ([CO2]) with inputs of 5 or 100 kg N ha(-1) y(-1). In N-rich grassland, [CO2] doubling qu ickly increased net primary productivity (NPP), total carbon (C-sys) a nd plant biomass by about 30%. By contrast, the N-poor grassland under went a prolonged 'transient', when there was little response, but even tually NPP, C-sys and plant biomass more than doubled. The 'transient' was due to N immobilization and severe depletion of the soil mineral N pool. The large long-term response was due to slow N accumulation, a s a result of decreased leaching, decreased gaseous N losses and incre ased N-2-fixation, which amplified the CO2 response much more in the N -poor than in the N-rich grassland. It was concluded that (i) ecosyste ms use extra carbon fixed at high [CO2] to acquire and retain nutrient s, supporting the contention of Gifford et al. (1996), (ii) in the lon g term, and perhaps on the real timescale of increasing [CO2], the res ponse (in NPP, C-sys and plant biomass) of nutrient-poor ecosystems ma y be proportionately greater than that of nutrient-rich ones, (iii) sh ort-term experiments on nutrient-poor ecosystems may observe only the transient responses, (iv) the speed of ecosystem responses may be limi ted by the rate of nutrient accumulation rather than by internal rate constants, and (v) ecosystem models must represent processes affecting nutrient acquisition and retention to be able to simulate likely real -world CO2 responses.