Variations in microbial isotopic fractionation during soil organic matter decomposition

Abstract

International audiencehe soil microbial biomass (SMB) is known to participate in key soil processes such as the decomposition of soil organic matter (SOM). However, its contribution to the isotopic composition of the SOM is not clear yet. Shifts in the 13C and 15N natural abundances of the SMB and SOM fractions (mineralised, water soluble and non-extractable) were investigated by incubating an unamended arable soil for 6 months. Microbial communities were also studied using Fatty Acid Methyl Ester specific isotope analysis. The SMB was significantly 13C and 15N-enriched relative to other fractions throughout the incubation. However, significant isotopic variations with time were also observed due to the rapid consumption of relatively 13C-enriched water soluble compounds. The increase in the difference in SMB and water soluble 15N compositions as the water soluble C/N ratio decreased, indicated a shift from N assimilation to N dissimilation during the incubation. These changes also induced modifications of the microbial community structure. Once the system reached a steady-state (after 1 month), the isotopic trends appeared to corroborate those obtained in long term experiments in the field in that there was a constant microbial isotopic fractionation leading to a 13C and 15N enrichment of the SOM over the long-term. This work also suggests that caution must be exercised when interpreting short term incubation studies since perturbations associated with experimental set-up can have an important effect on C and N dynamics, microbial fractionation of 13C and 15N and microbial community structure

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: a review

“…3). However, Lerch et al (2011) found fractionation to vary over time when calculated in this manner, and while changes in substrate could account for this pattern it is also likely that the active microbial community is changing. Fast changes in microbial composition have been documented after addition of labile substrate (Cleveland et al, 2007), rapidly changing environmental conditions (Gordon et al, 2008), and other environmental stresses (Schimel et al, 2007).…”

“…For example, the use of positional labeling of carbon in glucose has given metabolic insight into carbon pathways in mycorrizhae (Scandellari et al, 2009). Studies that assess the isotopic composition of soil microbial biomass usually treat soil microbes as a single C pool without differentiating between metabolically active and dormant microorganisms (Santrûcková et al, 2000;Lerch et al, 2011). Soil microbial biomass is composed of both active and dormant microorganisms, yet, CO 2 respired from microorganisms derives solely from those that are metabolically active (Stenstrom et al, 2001;Kuzyakov, 2008, 2009;Millard et al, 2010;Werth and Kuzyakov, 2010).…”

“…For example, Kramer et al (2010) showed that the source of carbon in the biomarkers present in their incubation studies did not originate from litter or SOM, leaving only roots as the primary source. Using fatty acid methyl ester isotopic composition, Lerch et al (2011) found a switch in the active microbial community from Gram-negative bacteria initially, which consumed the easily degradable and water-soluble substrates, to Grampositive bacteria and fungi later. Based on their isotopic measurements, Lerch et al (2011) also suggested that there is a potential lag between changes in the bacteria actually consuming carbon belowground, and the community structure as a whole.…”

From land to water, an overview of organic matter research in France––proceedings of the Sainte-Maxime 2009 symposium organized by the French network on organic matter research

“…Indeed, numerous observations showed that plant compounds are slightly depleted in 13 C and 15 N compared to bulk SOM. Lerch et al (2011) tested the assumption according to which fractionation accompanying microbial metabolism may be at the origin of this enrichment of SOM (Balesdent and Mariotti 1996;Bird et al 2003;Dijkstra et al 2006). They quantified 13 C and 15 N fractionation factors between soil and microbial biomass and confirmed that microbial isotopic fractionation leads to the enrichment of SOM in heavy isotopes.…”

Adaptations to sea level change and transitions to agriculture at Khao Toh Chong rockshelter, Peninsular Thailand

“…The depletion in carbon isotope values may be due to 482 several factors, including changes in the ratio of C 3 and C 4 plants on the landscape, changes in 483 the growing conditions of plants (such as canopy structure, and water or nutrient stress), changes 484 in the ratios of isotopically distinct organic fractions in the sediment organic matter, and changes 485 in organic inputs from microorganisms in soils (Tieszen 1991 cooler/dryer conditions indicated by a decrease in magnetic susceptibility occurring at KTC at 504 the same time. However, the trend in δ 13 C values at KTC is relatively low magnitude, and 505 isotopic fractionation and microbial activity cannot be fully dismissed as contributing factors 506 (Lerch et al 2011;Schweizer et al 1999;Tieszen 1991;Wynn 2007). Carbon isotope values of 507 leaf wax n-alkanes may help to overcome these ambiguities because these are more diagnostic 508 than those from bulk sediments, which contain materials of both terrestrial and aquatic origin.…”