The impact of soil microorganisms on the global budget of delta18O in atmospheric CO2.

Improved global estimates of terrestrial photosynthesis and respiration are critical for predicting the rate of change in atmospheric CO(2). The oxygen isotopic composition of atmospheric CO(2) can be used to estimate these fluxes because oxygen isotopic exchange between CO(2) and water creates distinct isotopic flux signatures. The enzyme carbonic anhydrase (CA) is known to accelerate this exchange in leaves, but the possibility of CA activity in soils is commonly neglected. Here, we report widespread accelerated soil CO(2) hydration. Exchange was 10-300 times faster than the uncatalyzed rate, consistent with typical population sizes for CA-containing soil microorganisms. Including accelerated soil hydration in global model simulations modifies contributions from soil and foliage to the global CO(18)O budget and eliminates persistent discrepancies existing between model and atmospheric observations. This enhanced soil hydration also increases the differences between the isotopic signatures of photosynthesis and respiration, particularly in the tropics, increasing the precision of CO(2) gross fluxes obtained by using the delta(18)O of atmospheric CO(2) by 50%.

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

In semi-arid regions, where plants using both C(3) and C(4) photosynthetic pathways are common, the stable C isotope ratio (delta(13)C) of ecosystem respiration (delta(13)C(R)) is strongly variable seasonally and inter-annually. Improved understanding of physiological and environmental controls over these variations will improve C cycle models that rely on the isotopic composition of atmospheric CO(2). We hypothesized that timing of precipitation events and antecedent moisture interact with activity of C(3) and C(4) grasses to determine net ecosystem CO(2) exchange (NEE) and delta(13)C(R). Field measurements included CO(2) and delta(13)C fluxes from the whole ecosystem and from patches of different plant communities, biomass and delta(13)C of plants and soils over the 2000 and 2001 growing seasons. NEE shifted from C source to sink in response to rainfall events, but this shift occurred after a time lag of up to 2 weeks if a dry period preceded the rainfall. The seasonal average of delta(13)C(R) was higher in 2000 (-16 per thousand) than 2001 (20 per thousand), probably due to drier conditions during the 2000 growing season (79.7 mm of precipitation from April up to and including July) than in 2001 (189 mm). During moist conditions, delta(13)C averaged -22 per thousand from C(3) patches, -16 per thousand from C(4) patches, and -19 per thousand from mixed C(3) and C(4) patches. However, during dry conditions the apparent spatial differences were not obvious, suggesting reduced autotrophic activity in C(4) grasses with shallow rooting depth, soon after the onset of dry conditions. Air and soil temperatures were negatively correlated with delta(13)C(R); vapor pressure deficit was a poor predictor of delta(13)C(R), in contrast to more mesic ecosystems. Responses of respiration components to precipitation pulses were explained by differences in soil moisture thresholds between C(3) and C(4) species. Stable isotopic composition of respiration in semi-arid ecosystems is more temporally and spatially variable than in mesic ecosystems owing to dynamic aspects of pulse precipitation episodes and biological drivers.