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Responses of net ecosystem CO2 exchange in managed grassland to long‐term CO2 enrichment, N fertilization and plant species

The effects of elevated pCO2 on net ecosystem CO2 exchange were investigated in managed Lolium perenne (perennial ryegrass) and Trifolium repens (white clover) monocultures that had been exposed continuously to elevated pCO2 (60 Pa) for nine growing seasons using Free Air CO2 Enrichment (FACE) technology. Two levels of nitrogen (N) fertilization were applied. Midday net ecosystem CO2 exchange (mNEE) and night-time ecosystem respiration (NER) were measured in three growing seasons using an open-flow chamber system. The annual net ecosystem carbon (C) input resulting from the net CO2 fluxes was estimated for one growing season. In both monocultures and at both levels of N supply, elevated pCO2 stimulated mNEE by up to 32%, the exact amount depending on intercepted PAR. The response of mNEE to elevated pCO2 was larger than that of harvestable biomass. Elevated pCO2 increased NER by up to 39% in both species at both levels of N supply. NER, which was affected by mNEE of the preceding day, was higher in T. repens than in L. perenne. High N increased NER compared to low N supply. According to treatment, the annual net ecosystem C input ranged between 210 and 631 g C m - -2 year -1 and was not significantly affected by the level of pCO2. Low N supply led to a higher net C input than high N supply. We demonstrated that at the ecosystem level, there was a long-term stimulation in the net C assimilation during daytime by elevated pCO2. However, because NER was also stimulated, net ecosystem C input was not significantly increased at elevated pCO2. The annual net ecosystem C input was primarily affected by the amount of N supplied.