Plant community change mediates the response of foliar _15N to CO2 enrichment in mesic grasslands

Authors: Polley HW, JD Derner, RB Jackson, RA Gill, AC Procter, PA Fay

Rising atmospheric CO2 concentration may changethe isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO2 may increase leaf _15N 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 _15N 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 _15N only for prairie plants on a silty clay. _15N 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 _15N on the silty clay by reducing fractionation during N uptake and assimilation. However, we

interpret the soil-specific, _15N-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 _15N than the dominant grass (Bothriochloa ischaemum) in pasture. CO2 enrichment changed grassland _15N by shifting species relative abundances.