Document: REB-3-66-27

The effects of global change on belowground carbon storage: Under elevated CO₂, increased N-deposition does not lead to increased C storage in a California grassland.

SHAW, M.R.* ¹, M.S.TORN ², C.B.FIELD ¹, M.RILLIG ³ and H.A.MOONEY ⁴

Carnegie Institution of Washington, Stanford, CA 94305, USA ¹
Lawrence Berkeley Laboratory, Berkeley, CA 94707 ²
The University of Montana, Missoula, MT 59812Missoula, MT 59812 ³
Stanford University, Stanford, CA 94305 ⁴

Abstract:
At the Jasper Ridge Integrated Global Change Experiment (JRI-GCE), we evaluated the effects of global change on belowground C inputs, soil respiration, and the contribution of newly-fixed C to soil C efflux. The field experiment implemented a full factorial combination of 2 levels of CO₂ (360 and 700 ppm), temperature (ambient and ambient + 2C), nitrogen deposition (ambient and +7 g (N) m^-2), and precipitation (ambient and ambient + 50%). Because C fixed in the elevated CO₂ plots had a distinct isotopic signature, we used these plots to distinguish between sources of C to soil respiration signature. We used 5-point Keeling plots to determine the isotopic signature of soil respiration and to partition the sources of respiration between C fixed during the first growing season of the treatment application ("new C") versus that derived from decomposition of soil organic matter produced in years prior. The isotopic signature of respiration from the control plots was -27.9 ppmil, matching the ¹³C of plant inputs (-28.2 ppmil). The signature of elevated CO₂ plots varied from -34 to -40 ppmil. At the beginning of the 2nd growing season, we evaluated treatment effects on soil respiration and the proportional contribution of new C to soil respiration as well as on the absolute rate of soil respiration. These measurements were made within a week of the first rain so they reflect only microbial decomposition without any root respiration. Total respiration was higher under all N deposition treatments but did change with any of the other treatments. In addition, isotopic values indicated that a greater proportion of the respiration was derived from new C under N deposition: New C made up 65-75% of soil respiration in the nitrogen treatments and about 55% in the CO₂, CO₂ + water and CO₂ + warming treatments. At peak belowground biomass of the first growing season of treatment application, root density was higher in all N-deposition treatments. Many studies have hypothesized that C sequestration may be enhanced when a doubling of atmospheric CO₂ is accompanied by increased N deposition. In this study, however, the increased belowground plant inputs were accompanied by a subsequent increase in C flux from the soil under N-deposition. Under a 2 fold increase in atmospheric CO₂, there may be little new storage of C despite N stimulation of plant inputs.

Keywords: global change, C storage, elevated CO₂ , N deposition, partitioning soil respiration, isotopic signature, belowground biomass, Keeling plot

This abstract is being presented at: 1:15 PM in session:
Oral Session #52: Carbon Storage in Ecosystems.