Differential effects of redox conditions on the decomposition of litter and soil organic matter

Abstract

<jats:p>Abstract. Soil redox conditions exert substantial influence on biogeochemical processes in terrestrial ecosystems. Humid tropical forest soils are often characterized by fluctuating redox dynamics, yet how these dynamics affect patterns in soil versus litter decomposition and associated CO2 fluxes is not well understood. We used a 13C-labeled litter addition to explicitly follow the decomposition of litter-derived vs. native soil-derived organic matter in response to four different soil redox regimes – static oxic or anoxic, and two oscillating treatments – in soil from the Luquillo Experimental Forest, Puerto Rico. We coupled this incubation experiment with high-resolution mass spectrometry to characterize the preferential decomposition of specific classes of organic molecules. CO2 production from litter and soil organic matter (SOM) showed distinctly different responses to redox manipulation. The cumulative production of SOM-derived CO2 was positively correlated with the length of soil exposure to an oxic headspace (r = 0.89, n = 20), whereas cumulative 13C-litter-derived CO2 production was not linked to oxygen availability. The CO2 production rate from litter was highest under static anoxic conditions in the first half of the incubation period, and later dropped to the lowest among all redox treatments. In the consistently anoxic soils, we observed the depletion of more oxidized water-extractable organic matter (especially amino sugars, carbohydrates, and proteins) over time, suggesting that under anaerobic conditions, microbes preferentially used more oxidized litter-derived compounds during the early stages of decomposition. Results from kinetic modeling showed that more frequent anoxic exposure limited the decomposition of a slow-cycling C pool, but not a fast-cycling pool. Overall, our results demonstrate that substrate source – freshly added litter vs. native organic matter – plays an important role in the redox sensitivity of organic matter decomposition. In soil environments that regularly experience redox fluctuations, anaerobic heterotrophs can be surprisingly effective in degrading fresh plant litter. </jats:p>

Redox-Related Metabolic Dynamics Imprinted on Short-Chain Carboxylic Acids in Soil Water Extracts: A 13C-Exometabolomics Analysis

“…Understanding the dynamics of microbial metabolism in soils is of great interest due to its critical role in carbon cycling. , Organic matter in soils is composed of plant, microbial, and animal detritus at different degrees of decomposition by microorganisms. − In addition to producing carbon dioxide (CO 2 ), microbial organic matter decomposition involves metabolic intermediates or products, including short-chain carboxylic acids (SCCAs). − Previous soil studies have relied on measuring CO 2 effluxes as a proxy for microbial metabolism, , analyzing phospholipid fatty acids to deduce microbial phenotypes, − and stable isotope probing of DNA, RNA, and (meta)­proteomics to evaluate metabolic potentials of the microbial populations. ,− However, these measurements do not capture explicitly the metabolic activities, especially within the context of redox-influenced metabolism. ,− Metabolomics, which involves the profiling of metabolites directly involved in cellular metabolic reactions, is an emerging approach for obtaining metabolite-level information on soil microbial activities. − Intracellular metabolomics is widely used to elucidate metabolic pathways in pure cultures of microbial isolates. − However, metabolic functions in mixed cultures such as those found in soils tend to be different from those in microbial isolates. ,, Extracellular metabolomics (also termed exometabolomics) has been employed to probe metabolic consumption versus secretions in response to changes in environmental conditions or microbial community. ,, We explore the application of a 13 C-assisted exometabolomics approach to evaluate water-extracted SCCAs as redox-dependent metabolic proxies in soils.…”