Separation of Root Respiration from Total Soil Respiration Using Carbon‐13 Labeling during Free‐Air Carbon Dioxide Enrichment (FACE)

Soil respiration constitutes a major component of the global carbon cycle and is likely to be altered by climatic change. However, there is an incomplete understanding of the extent to which various processes contribute to total soil respiration, especially the contributions of root and rhizosphere respiration. Here, using a stable carbon isotope tracer, we separate the relative contributions of root and soil heterotrophic respiration to total soil respiration in situ. The Free‐Air Carbon dioxide Enrichment (FACE) facility in the Duke University Forest (NC) fumigates plots of an undisturbed loblolly pine ( Pinus taeda L.) forest with CO 2 that is strongly depleted in 13 C. This labeled CO 2 is found in the soil pore space through live root and mycorrhizal respiration and soil heterotroph respiration of labile root exudates. By measuring the depletion of 13 CO 2 in the soil system, we found that the rhizosphere contribution to soil CO 2 reflected the distribution of fine roots in the soil and that late in the growing season roots contributed 55% of total soil respiration at the surface. This estimate may represent an upper limit on the contribution of roots to soil respiration because high atmospheric CO 2 often increases in root density and/or root activity in the soil.