Abstract
Forest soil ecosystems are associated with large pools and fluxes of carbon (C) and nitrogen (N), which could be strongly affected by variation in rainfall events under current climate change. Understanding how dry and wet cycle events might influence the metabolic state of indigenous soil microbes is crucial for predicting forest soil responses to environmental change. We used 454 pyrosequencing and quantitative PCR to address how present (DNA-based) and potentially active (RNA-based) soil bacterial communities might response to the changes in water availability across three different forest types located in two continents (Africa and Asia) under controlled drying and rewetting cycles. Sequencing of rRNA gene and transcript indicated that Proteobacteria, Actinobacteria, and Acidobacteria were the most responsive phyla to changes in water availability. We defined the ratio of rRNA transcript to rRNA gene abundance as a key indicator of potential microbial activity and we found that this ratio was increased following soil dry-down process whereas it decreased after soil rewetting. Following rewetting Crenarchaeota-like 16S rRNA gene transcript increased in some forest soils and this was linked to increases in soil nitrate levels suggesting greater nitrification rates under higher soil water availability. Changes in the relative abundance of (1) different microbial phyla and classes, and (2) 16S and amoA genes were found to be site- and taxa-specific and might have been driven by different life-strategies. Overall, we found that, after rewetting, the structure of the present and potentially active bacterial community structure as well as the abundance of bacterial (16S), archaeal (16S) and ammonia oxidizers (amoA), all returned to pre-dry-down levels. This suggests that microbial taxa have the ability to recover from desiccation, a critical response, which will contribute to maintaining microbial biodiversity in harsh ecosystems under environmental perturbations, such as significant changes in water availability.
Highlights
Climate change effects on seasonal precipitation regimes and on rates of soil evapotranspiration are predicted to strongly influence soil water availability across forest ecosystems worldwide through its effects on both seasonal precipitation regimes and soil evapotranspiration (Dale et al, 2001; Westerling et al, 2006; Malhi et al, 2008)
We found that soil organic C and N content did not change during the dry-down and wet-up treatments across the three forest sites (Figures 1B,C)
Overall our findings demonstrate that the microbial community of three different forest soils shows high desiccation-tolerance and high resilience to changes in soil water availability
Summary
Climate change effects on seasonal precipitation regimes and on rates of soil evapotranspiration are predicted to strongly influence soil water availability across forest ecosystems worldwide through its effects on both seasonal precipitation regimes and soil evapotranspiration (Dale et al, 2001; Westerling et al, 2006; Malhi et al, 2008). In particular large increases in water availability during intense rainfall events occurring at the end of prolonged dry seasons might greatly enhance soil CO2 efflux pulses Such increase in soil respiration is primarily due to enhanced microbial mineralization of carbon (C) and nitrogen (N) organic substrates and is referred to as “Birch Effect” (Birch, 1958; Borken and Matzner, 2009; Inglima et al, 2009). This effect well explains large CO2 pulses, which result from rewetting events after dry summer periods and accounts for a significant proportion of annual C budgets in forest ecosystems (Xu and Qi, 2001). Despite soil microbial communities may play a crucial role in mediating the “Birch Effect” it is not clear how soil microbial abundance and community structure will change under drying and rewetting cycles in forest ecosystems
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