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Title: Wetting and drying cycles drive variations in the stable carbon isotope ratio of respired carbon dioxide in semi-arid grassland

Author
item SHIM, J - COLORADO STATE UNIVERSITY
item PENDALL, E - UNIVERSITY OF WYOMING
item Morgan, Jack
item OJIMA, D - COLORADO STATE UNIVERSITY

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2009
Publication Date: 5/1/2009
Citation: Shim, J.H., Pendall, E., Morgan, J.A., Ojima, D.S. 2009. Wetting and drying cycles drive variations in the stable carbon isotope ratio of respired carbon dioxide in semi-arid grassland. Oecologia. 160:321-333.

Interpretive Summary: The problem of anthropogenically released greenhouse gases into Earth’s atmosphere and consequences for global climate change has spurred research into understanding sources and sinks for these gases in hopes of developing management strategies which can reduce or in some cases even eliminate net losses to the atmosphere. Carbon dioxide (CO2) is one of the most important of those greenhouse gases, and agriculture plays an important role in the land-atmosphere exchange of this gas since CO2 is assimilated in plants through photosynthesis, and as is emitted back into the atmosphere through soil microbial, plant and animal respiration. This research uses non-radioactive isotopes of carbon (C) to investigate how native plants of the shortgrass prairie with differing types of photosynthetic metabolism affect the net fluxes of CO2 between this region along the western edge of the Great Plains and Earth’s atmosphere. We found that not only do different native grass species differ in their contribution of CO2 to the atmosphere depending on their photosynthetic metabolism, but that natural variation in annual precipitation that results in wet and dry years can have a significant impact on how carbon is released from grassland soils and emitted to the atmosphere. Respiration from soil microorganisms in a wet year was composed primarily of C that had recently been assimilated by the plants in photosynthesis, whereas respiration in dry years came either directly from plant roots or older soil C. These results will help model how weather and species affect the exchange of C between native Great Plains grasslands and the atmosphere.

Technical Abstract: We investigated pulse precipitation events and net CO2 exchange in the shortgrass steppe to explain seasonal and interannual variability of delta 13C of ecosystem respiration (delta 13CR). We hypothesized that timing of pulse precipitation events and antecedent moisture conditions interact with activity of C3 and C4 grasses to determine net CO2 exchange and delta 13CR. Field measurements including near- surface atmospheric CO2 flasks, plant and soil sampling, chamber-scale gas samplings, and micrometeorological measurements were conducted over the two growing seasons of 2000 and 2001. Net ecosystem exchange (NEE) shifted from positive (C source) to negative (C sink) in response to rainfall events, but this shift occurred after a time lag of up to two weeks if a dry period preceded the rainfall. On average, 2000 was drier during the early growing season (79.7 mm of precipitation from April through July) than 2001 (189 mm), and the seasonal average of delta 13C was higher in 2000 (-16.2 ‰) than in 2001 (-19.9 ‰). The delta 13C of ecosystem respiration ranged over 5.2 ‰ and 7.1 ‰ over the 2000 and 2001 growing seasons, respectively. The seasonal variations in delta 13CR were used to evaluate how NEE responses to pulses of rainfall might be associated with transferring the delta 13C signal from recently assimilated (labile) carbon to ecosystem respiration. Both vegetation and NEE responses to the local precipitation regime explained much of the variations of delta 13C. The delta 13C of soil respiration from C3 plots averaged -21.7 ‰, -16 ‰ from C4 plots, and -19 ‰ from mixed C3 and C4 plots during moist conditions, indicating respiration of labile substrates. However, labile substrates were not utilized in microbial and root respiration under drought conditions. Air and soil temperatures were negatively correlated with delta 13CR; vapor pressure deficit was a poor predictor of delta 13CR, in contrast to more mesic ecosystems.