Belowground Response to Drought in a Tropical Forest Soil. II. Change in Microbial Function Impacts Carbon Composition.

Eoin L Brodie,Nicholas J Bouskill,Hsiao Chien Lim,Zaw Ye,Tana E Wood,Trent R Northen,Benjamin Gilbert,Ben P Bowen,Whendee L Silver,Peter S Nico,Richard Baran,Zhao Hao,Hoi-Ying Holman

doi:10.3389/fmicb.2016.00323

Abstract

Climate model projections for tropical regions show clear perturbation of precipitation patterns leading to increased frequency and severity of drought in some regions. Previous work has shown declining soil moisture to be a strong driver of changes in microbial trait distribution, however, the feedback of any shift in functional potential on ecosystem properties related to carbon cycling are poorly understood. Here we show that drought-induced changes in microbial functional diversity and activity shape, and are in turn shaped by, the composition of dissolved and soil-associated carbon. We also demonstrate that a shift in microbial functional traits that favor the production of hygroscopic compounds alter the eﬄux of carbon dioxide following soil rewetting. Under drought the composition of the dissolved organic carbon pool changed in a manner consistent with a microbial metabolic response. We hypothesize that this microbial ecophysiological response to changing soil moisture elevates the intracellular carbon demand stimulating extracellular enzyme production, that prompts the observed decline in more complex carbon compounds (e.g., cellulose and lignin). Furthermore, a metabolic response to drought appeared to condition (biologically and physically) the soil, notably through the production of polysaccharides, particularly in experimental plots that had been pre-exposed to a short-term drought. This hysteretic response, in addition to an observed drought-related decline in phosphorus concentration, may have been responsible for a comparatively modest CO2 eﬄux following wet-up in drought plots relative to control plots.

Highlights

Tropical forests have a disproportionate capacity to affect Earth’s climate relative to their areal extent
We address three specific questions: (1) do changes in the functional potential of perturbed microbial communities feed back onto the composition of soil carbon? (2) Do pre-excluded soils exhibit shifts in carbon composition at the molecular scale that signify a mechanism for resilience to prolonged drought? (3) Do changes in soil carbon composition relate to the magnitude of carbon lost under simulated conditions of wet-up from precipitation relative to soils not exposed to drought?
Using LC-MS/MS mass spectrometry, Fourier Transform Infrared Spectroscopy (FTIR) and Excitation Emission Matrix fluorescence spectroscopy (EEM) we assessed the effects of drought on the composition of the Dissolved Organic Carbon (DOC) pool and mineral associated compounds

Summary

Introduction

Tropical forests have a disproportionate capacity to affect Earth’s climate relative to their areal extent. Within the clay rich mineral soils common to tropical regions, soil moisture plays a deterministic role in soil oxygen and redox dynamics (Yu et al, 2006; Liptzin et al, 2010; Silver et al, 2013) This is significant because an increase in soil oxygen as moisture declines can trigger the precipitation of soluble ferrous iron to ferric iron complexes, and the simultaneous coprecipitation of organic matter and phosphorus (Kleber et al, 2015), forming a strong geochemical C and nutrient sink (Silver et al, 1999; Baldock and Skjemstad, 2000; Lalonde et al, 2013). These changes in the physical environment feed back on the phylogenetic and functional composition of microbial communities (Bouskill et al, 2013), and their activity (Manzoni et al, 2012; Bouskill et al, submitted), and the rate of soil biogeochemical processes (Manzoni and Katul, 2014), and consequent trace gas fluxes (Wood and Silver, 2012)

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Discussion

Conclusion