Changes in the Size of the Active Microbial Pool Explain Short-Term Soil Respiratory Responses to Temperature and Moisture.

Alejandro Salazar-Villegas,Evgenia Blagodatskaya,Jeffrey S Dukes

doi:10.3389/fmicb.2016.00524

Abstract

Heterotrophic respiration contributes a substantial fraction of the carbon flux from soil to atmosphere, and responds strongly to environmental conditions. However, the mechanisms through which short-term changes in environmental conditions affect microbial respiration still remain unclear. Microorganisms cope with adverse environmental conditions by transitioning into and out of dormancy, a state in which they minimize rates of metabolism and respiration. These transitions are poorly characterized in soil and are generally omitted from decomposition models. Most current approaches to model microbial control over soil CO2 production relate responses to total microbial biomass (TMB) and do not differentiate between microorganisms in active and dormant physiological states. Indeed, few data for active microbial biomass (AMB) exist with which to compare model output. Here, we tested the hypothesis that differences in soil microbial respiration rates across various environmental conditions are more closely related to differences in AMB (e.g., due to activation of dormant microorganisms) than in TMB. We measured basal respiration (SBR) of soil incubated for a week at two temperatures (24 and 33°C) and two moisture levels (10 and 20% soil dry weight [SDW]), and then determined TMB, AMB, microbial specific growth rate, and the lag time before microbial growth (tlag) using the Substrate-Induced Growth Response (SIGR) method. As expected, SBR was more strongly correlated with AMB than with TMB. This relationship indicated that each g active biomass C contributed ~0.04 g CO2-C h−1 of SBR. TMB responded very little to short-term changes in temperature and soil moisture and did not explain differences in SBR among the treatments. Maximum specific growth rate did not respond to environmental conditions, suggesting that the dominant microbial populations remained similar. However, warmer temperatures and increased soil moisture both reduced tlag, indicating that favorable abiotic conditions activated soil microorganisms. We conclude that soil respiratory responses to short-term changes in environmental conditions are better explained by changes in AMB than in TMB. These results suggest that decomposition models that explicitly represent microbial carbon pools should take into account the active microbial pool, and researchers should be cautious in comparing modeled microbial pool sizes with measurements of TMB.

Highlights

Microbial respiration responds rapidly to changing environmental conditions, strongly influencing soil carbon cycling, and its feedbacks to climate change (Allison et al, 2010; Frey et al, 2013; Wieder et al, 2013; Sulman et al, 2014)
Since active microbes overwhelmingly drive soil carbon processes, we investigated whether active microbial biomass (AMB) is a more accurate predictor of soil CO2 fluxes than total microbial biomass (TMB)
To meet assumptions regarding the normality of residuals and homogeneity of variances we log10-transformed AMB for statistical analyses, but for clarity we present the untransformed data in the text and figures

Summary

Introduction

Microbial respiration responds rapidly to changing environmental conditions, strongly influencing soil carbon cycling, and its feedbacks to climate change (Allison et al, 2010; Frey et al, 2013; Wieder et al, 2013; Sulman et al, 2014). More than 80–90% of soil microorganisms are in a dormant or inactive physiological state in which they have minimal respiratory activity (Anderson and Domsch, 1985; Lennon and Jones, 2011) These active and dormant fractions of soil microbial biomass can change in response to environmental and nutritional conditions (Van de Werf and Verstraete, 1987) but typically are not considered when analyzing microbial control over soil CO2 production. If they have represented microbes at all, decomposition models have most commonly represented microbial biomass as a single pool (Zhang et al, 2014; Wieder et al, 2015) without differentiating between its active and dormant fractions. Since active microbes overwhelmingly drive soil carbon processes, we investigated whether active microbial biomass (AMB) is a more accurate predictor of soil CO2 fluxes than total microbial biomass (TMB)

Methods

Results

Conclusion