Soil Bacterial and Fungal Richness Forecast Patterns of Early Pine Litter Decomposition

Abstract

Discovering widespread microbial processes that drive unexpected variation in carbon cycling may improve modeling and management of soil carbon (Prescott, 2010; Wieder et al., 2015a, 2018). A first step is to identify community features linked to carbon cycle variation. We addressed this challenge using an epidemiological approach with 206 soil communities decomposing Ponderosa pine litter in 618 microcosms. Carbon flow from litter decomposition was measured over a 6-week incubation. Cumulative CO 2 from microbial respiration varied twofold among microcosms and dissolved organic carbon (DOC) from litter decomposition varied five-fold, demonstrating large functional variation despite constant environmental conditions where strong selection is expected. To investigate microbial features driving DOC concentration, two microbial community cohorts were delineated as "high" and "low" DOC. For each cohort, communities from the original soils and from the final microcosm communities after the 6-week incubation with litter were taxonomically profiled. A logistic model including total biomass, fungal richness, and bacterial richness measured in the original soils or in the final microcosm communities predicted the DOC cohort with 72 (P < 0.05) and 80 (P < 0.001) percent accuracy, respectively. The strongest predictors of the DOC cohort were biomass and either fungal richness (in the original soils) or bacterial richness (in the final microcosm communities). Successful forecasting of functional patterns after lengthy community succession in a new environment reveals strong historical contingencies. Forecasting future community function is a key advance beyond correlation of functional variance with end-state community features. The importance of taxon richness-the same feature linked to carbon fate in gut microbiome studies-underscores the need for increased understanding of biotic mechanisms that can shape richness in microbial communities independent of physicochemical conditions.

Microbial Communities Influence Soil Dissolved Organic Carbon Concentration by Altering Metabolite Composition

“…As described in Albright et al (2020) soil samples were collected from 206 locations throughout the southwestern United States between February and April 2015. Samples were typically collected at locations approximately 80 km apart, at least 15 m from roadways, and from the top 3 cm of the soil surface after removal of surface litter (if any).…”

“…Samples were stored at 6°C in the laboratory to avoid microbial lysis from freeze-thaw effects and were used within 6 weeks to inoculate microcosms. A map of the sampling locations is published in the Supplementary Material in Albright et al (2020) . Sampling included sites in Texas, Oklahoma, Kansas, Nebraska, Wyoming, Colorado, Utah, New Mexico, and Arizona with grassland-shrub and juniper woodland – grass being the predominant ecosystem types.…”

“…The microbial drivers of carbon fate are strongly influenced by environmental factors that vary widely by soil depth, location, and season that alter microbial composition and function ( Torsvik and Øvreås, 2002 ; Berg and Smalla, 2009 ; Fierer, 2017 ). Simplified microcosms that control for environmental factors provides a direct way to identify the impact of microbial metabolism in the soil ( Albright et al, 2020 ). Further, several experiments have been conducted to determine the rate and mechanisms by which added substrates are initially decomposed ( Bailey et al, 2006 ) but less understood is the way that compounds or compound classes within complex mixtures correlate with carbon storage ( Smith et al, 2007 ).…”

“…In the current study, we examine links between the community composition of microbes, and chemical composition and concentration of DOC. We extend the work of Albright et al (2020) ; they performed a common garden microcosm experiment where 206 soil microbial communities were inoculated on a common substrate of Ponderosa pine litter and sand in homogeneous laboratory microcosms. Cumulative DOC was measured after 44 days of decomposition and varied by 5-fold across all microcosms.…”

“…They can be used to inform models and new hypotheses that can then be tested in more complex systems. In this study, we characterized the chemical composition of a subset of samples collected from the high and low extremes of the DOC concentration distribution from the Albright et al (2020) study. We used Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and applied machine learning and indicator species approaches to the previously identified bacterial and fungal taxonomic dataset to reveal links between taxa and key compounds that may govern DOC concentration.…”