Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

Plant community change mediates the response of foliar δ15N to CO2 enrichment in mesic grasslands.

Abstract

Rising atmospheric CO2 concentration may change the isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO2 may increase leaf δ15N by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, 15N-enriched organic matter. We predicted that CO2 would increase aboveground productivity (ANPP; g biomass m-2) and foliar δ15N values of two grassland communities in Texas, USA: (1) a pasture dominated by a C4 exotic grass, and (2) assemblages of tallgrass prairie species, the latter grown on clay, sandy loam, and silty clay soils. Grasslands were exposed in separate experiments to a pre-industrial to elevated CO2 gradient for 4 years. CO2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils, but increased leaf δ15N only for prairie plants on a silty clay. δ15N increased linearly as mineral-associated soil C declined on the silty clay. Mineral-associated C declined as ANPP increased. Structural equation modeling indicted that CO2 increased ANPP partly by favoring a tallgrass (Sorghastrum nutans) over a mid-grass species (Bouteloua curtipendula). CO2 may have increased foliar δ15N on the silty clay by reducing fractionation during N uptake and assimilation. However, we interpret the soil-specific, δ15N-CO2 response as resulting from increased ANPP that stimulated mineralization from recalcitrant organic matter. By contrast, CO2 favored a forb species (Solanum dimidiatum) with higher δ15N than the dominant grass (Bothriochloa ischaemum) in pasture. CO2 enrichment changed grassland δ15N by shifting species relative abundances.