Plant Community Change Mediates the Response of Foliar δ15 Nitrogen 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. Isotope fractionation theory and limited experimental evidence indicate that 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 foliar δ¹⁵N values would increase as a positive function of the CO₂ effect on aboveground productivity (ANPP) of two grassland communities, a pasture dominated by a C₄ exotic grass and assemblages of native tallgrass prairie species, the latter grown on each of three soils, a clay, sandy loam, and silty clay. Both grasslands are located in Texas, USA and were exposed to a pre-industrial to elevated CO₂ gradient for four years. CO₂ enrichment did not consistently increase both ANPP and δ¹⁵N. Increased CO₂ stimulated ANPP of pasture and of prairie assemblages on each of the three soils. However, CO₂ increased leaf δ¹⁵N only for prairie plants grown on a silty clay soil. CO₂ enrichment led to a shift in dominance from a mid-grass (Bouteloua curtipendula) to a tallgrass prairie species (Sorghastrum nutans) that contributed to increased leaf δ¹⁵N on the silty clay soil by increasing ANPP and apparently stimulating mineralization of recalcitrant organic matter. By contrast, CO₂ enrichment favored a forb species (Solanum dimidiatum) with higher δ¹⁵N values than the dominant grass (Bothriochloa ischaemum) in pasture. Results highlight the role of changes in community composition in CO₂ effects on grassland δ¹⁵N values.