Plant community change mediates the response of foliar [delta].sup.15N to CO.sub.2 enrichment in mesic grasslands

• Published in: Oecologia 2015-06, Vol.178 (2), p.591
• Main Authors: Polley, H. Wayne, Derner, Justin D, Jackson, Robert B, Gill, Richard A, Procter, Andrew C, Fay, Philip A
• Format: Article
• Language: English
• Subjects: Clay soils; Grasslands
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/s00442-015-3221-x

Rising atmospheric CO.sub.2 concentration may change the isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO.sub.2 may increase leaf [delta].sup.15N by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, .sup.15N-enriched organic matter. We predicted that CO.sub.2 would increase aboveground productivity (ANPP; g biomass m.sup.-2) and foliar [delta].sup.15N values of two grassland communities in Texas, USA: (1) a pasture dominated by a C.sub.4 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 CO.sub.2 gradient for 4 years. CO.sub.2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils, but increased leaf [delta].sup.15N only for prairie plants on a silty clay. [delta].sup.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 CO.sub.2 increased ANPP partly by favoring a tallgrass (Sorghastrum nutans) over a mid-grass species (Bouteloua curtipendula). CO.sub.2 may have increased foliar [delta].sup.15N on the silty clay by reducing fractionation during N uptake and assimilation. However, we interpret the soil-specific, [delta].sup.15N-CO.sub.2 response as resulting from increased ANPP that stimulated mineralization from recalcitrant organic matter. By contrast, CO.sub.2 favored a forb species (Solanum dimidiatum) with higher [delta].sup.15N than the dominant grass (Bothriochloa ischaemum) in pasture. CO.sub.2 enrichment changed grassland [delta].sup.15N by shifting species relative abundances.