Plant Community Change Mediates the Response of Foliar δ15 Nitrogen to CO2 Enrichment in Mesic Grasslands

Wednesday, 17 December 2014
Wayne Polley1,2, Justin D Derner3, Robert B Jackson4, Richard A Gill5, Andrew Procter6 and Philip A Fay2, (1)USDA ARS, USDA ARS, Pendleton, OR, United States, (2)USDA-ARS, Temple, TX, United States, (3)USDA ARS, Cheyenne, WY, United States, (4)Duke University, Durham, NC, United States, (5)Brigham Young University, Provo, UT, United States, (6)US Environmental Protection Agency, Research Triangle Park, NC, United States
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.