Abstract
Rising global temperatures may increase the rates of soil organic matter decomposition by heterotrophic microorganisms, potentially accelerating climate change further by releasing additional carbon dioxide (CO2) to the atmosphere. However, the possibility that microbial community responses to prolonged warming may modify the temperature sensitivity of soil respiration creates large uncertainty in the strength of this positive feedback. Both compensatory responses (decreasing temperature sensitivity of soil respiration in the long-term) and enhancing responses (increasing temperature sensitivity) have been reported, but the mechanisms underlying these responses are poorly understood. In this study, microbial biomass, community structure and the activities of dehydrogenase and β-glucosidase enzymes were determined for 18 soils that had previously demonstrated either no response or varying magnitude of enhancing or compensatory responses of temperature sensitivity of heterotrophic microbial respiration to prolonged cooling. The soil cooling approach, in contrast to warming experiments, discriminates between microbial community responses and the consequences of substrate depletion, by minimising changes in substrate availability. The initial microbial community composition, determined by molecular analysis of soils showing contrasting respiration responses to cooling, provided evidence that the magnitude of enhancing responses was partly related to microbial community composition. There was also evidence that higher relative abundance of saprophytic Basidiomycota may explain the compensatory response observed in one soil, but neither microbial biomass nor enzymatic capacity were significantly affected by cooling. Our findings emphasise the key importance of soil microbial community responses for feedbacks to global change, but also highlight important areas where our understanding remains limited.
Highlights
Soils contain a massive stock of terrestrial carbon (C) [1, 2], estimated at approximately 2500 Pg C, much of which is considered vulnerable to climatic warming [3, 4]
Canonical Variates Analysis (CVA) did not separate clearly the initial microbial community structures of soils associated with different respiration responses (S1 Fig), but there was a significant correlation between the initial microbial community structure and RRMT (Fig 1a)
operational taxonomic units (OTU) associated with Actinobacteria, Acidobacteria, Chloroflexi, Planctomycetes and Verrucomicrobia phyla had the highest relative abundance (Fig 2) and Actinobacteria comprised mainly Actinomycetes
Summary
Soils contain a massive stock of terrestrial carbon (C) [1, 2], estimated at approximately 2500 Pg C, much of which is considered vulnerable to climatic warming [3, 4]. In long-term warming studies the initial stimulation of soil respiration often declines over time [8, 15, 16] This can be explained partly by loss of the most readily-decomposable SOC pool, but there have been suggestions that microbial communities respond to warming in such a way as to compensate for the increase in soil temperature, promoting a gradual reduction in respiration rates in warmed soils [17,18,19,20]. Mechanisms behind such responses could include physiological responses of individual microbial phylotypes, genetic changes within species (adaptation) and ecological responses associated with change in community composition
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