Seasonal variations in the carbon isotope composition of soil-respired CO2 and the dominance of root/rhizsophere respiration in desert soils (Invited)
Abstract
Quantifying the sources of CO2 produced in soils is important for closing ecosystem scale carbon (C) budgets and predicting the response of soil C pools to global change. Sourcing soil-respired CO2 is also important for accurately using paleosol carbonates as paleoenvironmental indicators. Here we present ten records of seasonal change in C isotope compositions of soil-respired CO2 (δ13Cr) and examine their implications for soil respiration. Measured concentrations and δ13C values of soil CO2 below 30 cm were used to calculate all δ13Cr values reported here. Distinct seasonal cycles occur in all records and the lowest/highest δ13Cr values occur during the winter/summer in 9 of the 10 records. The magnitude of seasonal δ13Cr fluctuations varies inversely with mean annual precipitation (MAP), increasing from 3‰ at 500 mm to 8‰ at 200 mm. Values for two Vertisols in subhumid climates plot off the trend, perhaps in part because winter ponding induces a closed system resulting in calculated winter δ13Cr values that are lower than actual and therefore overestimated seasonal δ13Cr amplitudes. The large seasonal variation in desert soil δ13Cr values has been attributed to seasonal variation in the magnitude of photosynthetic discrimination expressed in soil-respired CO2. Seasonal changes in C3 versus C4 productivity do not explain the observations as some of the largest δ13Cr variations occur in nearly monospecific C3 shrublands (creosotebush). A number of other explanations involving heterotrophic respiration, including soil temperature- and moisture- induced changes in respiration depth and substrate, are also rejected based on observed soil temperatures and average depths of respiration, which frequently exceed 50 cm in the driest soils studied. The observed decrease of seasonal amplitude with increasing precipitation is consistent with a stomatal control on desert soil δ13Cr values and may be caused by 1) MAP-driven increase in the component of heterotrophic respiration, which likely has less variable δ13C values and therefore dampens fluctuations in δ13Cr caused by rhizosphere respiration and/or 2) smaller seasonal changes in photosynthetic discrimination as MAP increases. Measured δ13Cr values closely approximate (within 0.2 ‰) the δ13C values of shallow A horizon soil organic matter (SOM) in two C3-domianted soils at the time of year that soil carbonate accumulates, supporting the use of SOM as a proxy for δ13Cr despite the large seasonal variability and autotrophic control of the latter. We suggest that soil carbonate records root/rhizosphere respiration and that proper correction (subtraction of perhaps 1-2‰) to measured δ13C values of paleosol B horizon SOM are required to accurately determine the δ13Cr values needed to reconstruct paleoatmospheric CO2 using paleosols. Furthermore, the δ13Cr values reported here provide no evidence for an inorganic C source to soil respiration and suggest that much of the CO2 flux from desert soils consists of rapidly (perhaps with days) cycled C.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B22A..02B
- Keywords:
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- 0428 BIOGEOSCIENCES Carbon cycling;
- 0486 BIOGEOSCIENCES Soils/pedology;
- 1041 GEOCHEMISTRY Stable isotope geochemistry