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Title: RHIZODEPOSITION STIMULATED BY ELEVATED CO2 IN A SEMI-ARID GRASSLAND

Author
item PENDELL, E - U WYOMING
item Mosier, Arvin
item Morgan, Jack

Submitted to: New Phytologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/5/2004
Publication Date: 6/2/2004
Citation: Pendell, E., Mosier, A.R., Morgan, J.A. 2004. Rhizodeposition stimulated by elevated co2 in a semi-arid grassland. New Phytologist. 162: 447-458.

Interpretive Summary: By the end of this century the concentration of atmospheric carbon dioxide (CO2) is expected to more than double. This continued increase in CO2 concentration is likely to have profound effects most ecosystems including the grasslands of the Great Plains of the USA. Elevated CO2 stimulates carbon cycling in many ecosystems, but how much additional carbon may be stored in long lived pools such as soils or wood has been difficult to assess. Rhizodeposition was roughly doubled in elevated compared to ambient CO2 chambers, over the last 4 years of the experiment. Net carbon sequestration will depend on how decomposition rates are altered by elevated CO2. The net carbon sequestered will be a main factor which regulates the future productivity of this grassland. Approximately 40% of the Great Plains is used for livestock production. Since the impact of increasing CO2 concentrations had not been investigated in the short grass prairie an open top chamber study was initiated in 1997 at a shortgrass steppe site in northeastern Colorado, USA. One of the questions to be resolved was the effect of increasing CO2 the quantity and quality of forage produced. This study suggests that the negative effects of elevated CO2 on forage quality are likely to be greater than the positive effects on quality, because quality drops to critically low levels that can inhibit utilization of the quantity that is available.

Technical Abstract: Elevated CO2 stimulates carbon cycling in many ecosystems, but how much additional carbon may be stored in long lived pools such as soils or wood has been difficult to assess. Utilizing large open-top-chambers and the unique 13C signal from the CO2 used, the deposition of plant carbon into the soil under elevated CO2 was determined. The main findings from these studies are: Rhizodeposition, or the addition of C from roots to soil C pools, is expected to increase if net primary production is stimulated and some excess C is allocated belowground. We investigated the effects of 5 years of elevated CO2 on belowground C dynamics in a native, C3-C4 grassland ecosystem in Colorado, USA. Cylinder harvests following each growing season and monolith excavation at the end of the experiment provided data on root biomass, root C:N ratios, and root and soil ÿ13C values. We applied an isotopic mixing model to quantify new soil C inputs on elevated and ambient CO2 treatments. Root biomass increased by 23% and root C:N ratios increased by 26% after 5 years of elevated CO2. Species-specific differences were found in root residence times, which ranged from 6 to 8 years. Rhizodeposition was roughly doubled in elevated compared to ambient CO2 chambers, at 83±16 vs. 35±9 g C m-2 y-1 over the last 4 years of the experiment (t-test, P=0.006). Net C sequestration will depend on how decomposition rates are altered by elevated CO2.