Influence of fertilization and atmospheric CO₂ enrichment on ecosystem CO₂ and H₂O exchanges in single- and multiple-species grassland microcosms

This paper reports on measurements of net CO₂ and H₂O exchange from single- and multiple-species microcosms composed of California annual grassland species grown at either ambient or elevated (ambient + 36 Pa) CO₂. Microcosms consisted of grassland species grown in PVC tubes (~ 0.95 m deep x 0.2 m diameter) containing ~45 kg of either serpentine or sandstone derived soil or parent material in open-top enclosures under ambient meteorological conditions. Half of the microcosms were left unfertilized (low nutrient) while the other half received an intermediate level of a slow-release (N,P,K) fertilizer (high nutrient). Gas exchange was performed by sealing individual microcosms within a transparent chamber (on clear sunny days) and coupling this to an open gas-exchange system. In fertilized single-species microcosms, elevated CO₂ consistently enhanced net 'ecosystem' CO₂ exchange (NCE) on a ground area basis in both early and late spring. Among unfertilized single-species microcosms, no significant trends or differences were observed in NCE between those grown at ambient versus elevated CO₂. The NCE in sandstone and serpentine multiple-species microcosms was monitored seasonally over a majority of the 1993-1994 growing season. Rates were largely unaffected by growth CO₂ or fertilization until after mid-February, 1994. Water-use efficiency (WUE = NCE/evapotranspiration (ET)) was generally enhanced by elevated CO₂, but this was primarily a result of enhancements in NCE as opposed to decreases in ET. Enhancements in NCE by elevated CO₂ in fertilized single-species microcosms at the growth-CO₂ concentration were partially explained by higher above-ground biomass in elevated CO₂ microcosms. However, ecosystem-level 'acclimation' occurred such that microcosms grown at elevated CO₂ consistently had lower NCE than ambient CO₂ treatments at a single measurement CO₂ concentration (ambient or elevated). The reduction in apparent ecosystem-level photosynthetic capacity in elevated CO₂ microcosms was accompanied by decreases in foliar Rubisco activity, such that NCE measured at ambient CO₂ was highly correlated. (r = 0.98) with foliar Rubisco activity across the three single-species microcosms in which it was measured.