Influence of fertilization and atmospheric CO 2 enrichment on ecosystem CO 2 and H 2O exchanges in single- and multiple-species grassland microcosms

Abstract

This paper reports on measurements of net CO 2 and H 2O exchange from single- and multiple-species microcosms composed of California annual grassland species grown at either ambient or elevated (ambient+36 Pa) CO 2. Microcosms consisted of grassland species grown in PVC tubes (∼0.95 m deep×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 CO 2 consistently enhanced net `ecosystem' CO 2 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 CO 2. 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 CO 2 or fertilization until after mid-February, 1994. Water-use efficiency (WUE=NCE/evapotranspiration (ET)) was generally enhanced by elevated CO 2, but this was primarily a result of enhancements in NCE as opposed to decreases in ET. Enhancements in NCE by elevated CO 2 in fertilized single-species microcosms at the growth-CO 2 concentration were partially explained by higher above-ground biomass in elevated CO 2 microcosms. However, ecosystem-level `acclimation' occurred such that microcosms grown at elevated CO 2 consistently had lower NCE than ambient CO 2 treatments at a single measurement CO 2 concentration (ambient or elevated). The reduction in apparent ecosystem-level photosynthetic capacity in elevated CO 2 microcosms was accompanied by decreases in foliar Rubisco activity, such that NCE measured at ambient CO 2 was highly correlated ( r=0.98) with foliar Rubisco activity across the three single-species microcosms in which it was measured.