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.