Plant community change mediates the response of foliar delta15N to CO2 enrichment in mesic grasslands

Authors: Polley, Herbert; Derner, Justin; JACKSON, ROBERT - Duke University; GILL, RICHARD - Brigham Young University; PROCTER, ANDREW - Duke University; Fay, Philip

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2015
Publication Date: 5/26/2015
Publication URL: http://handle.nal.usda.gov/10113/60969
Citation: Polley, H.W., Derner, J.D., Jackson, R.B., Gill, R.A., Procter, A.C., Fay, P.A. 2015. Plant community change mediates the response of foliar delta15N to CO2 enrichment in mesic grasslands. Oecologia. 178:591-601.

Interpretive Summary: The concentration of carbon dioxide (CO2) gas in air is increasing. CO2 gas, like soil nitrogen (N), is an essential resource of plants. Rising CO2 thus may increase plant growth provided that N remains available. In unfertilized grasslands and other ecosystems, N becomes available to plants when soil micro-organisms (microbes) decompose dead roots and other N containing plant material that has been deposited in or on soil. We used subtle differences in microbial preferences for the two stable forms (isotopes) of N, known as 14N and 15N, to determine CO2 effects on microbial release of N for plant growth. The ratio of the heavier 15N to lighter 14N isotope in plants is expected to increase if plant acquire N released during microbial decomposition of very old plant material in soil. We measured the two N forms in leaves of grassland plant sampled from a pasture and native tallgrass prairie in central Texas, the latter of which was grown on each of three soil types. Pasture and prairie were grown for four years at CO2 levels ranging from lows of the pre-industrial period to elevated levels predicted by mid-century before sampling. High CO2 increased growth of both pasture and prairie species. Increased growth was accompanied by an increase in the ratio of the heavier to lighter N isotope in prairie species grown on one of three soil types only. CO2 did not affect the N isotope ratio of prairie plants on other soils or of pasture species. Change in the N isotope ratio provided evidence that microbial release of N during the decomposition of old plant material in soil lessened the N limitation to increased plant growth at high CO2. CO2 thus may enhance plant growth more or for a longer period than would have been predicted had N not been ‘transferred’ from soil reserves to plants.

Technical 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 delta15N by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, 15N-enriched organic matter. We predicted that foliar delta15N 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 delta15N. Increased CO2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils. However, CO2 increased leaf delta15N 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 delta15N 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 delta15N values than the dominant grass (Bothriochloa ischaemum) in pasture. Results highlight the role of changes in community composition in CO2 effects on grassland delta15N values.