Plant community change mediates the response of foliar δ15N to CO2 enrichment in mesic grasslands
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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.
KeywordsIsotope Plant productivity Soil carbon Soil type Tallgrass prairie
Chris Kolodziejczyk operated CO2 chambers. Field and laboratory assistance from Anne Gibson, Katherine Jones, Chris Kolodziejczyk, Alicia Naranjo, Kyle Tiner, and numerous students was invaluable. Joseph Craine and Kevin Mueller provided insightful suggestions that improved the manuscript. R.J. acknowledges support from the U.S. Department of Energy (Program in Ecosystem Research #ER64242). Mention of trade names or commercial products does not imply endorsement by the US Department of Agriculture. USDA is an equal opportunity provider and employer.
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