Abstract
Variation in microbial use of soil carbon compounds is a major driver of biogeochemical processes and microbial community composition. Available carbon substrates in soil include both low molecular weight-dissolved organic carbon (LMW-DOC) and volatile organic compounds (VOCs). To compare the effects of LMW-DOC and VOCs on soil chemistry and microbial communities under different moisture regimes, we performed a microcosm experiment with five levels of soil water content (ranging from 25 to 70% water-holding capacity) and five levels of carbon amendment: a no carbon control, two dissolved compounds (glucose and oxalate), and two volatile compounds (methanol and α-pinene). Microbial activity was measured throughout as soil respiration; at the end of the experiment, we measured extractable soil organic carbon and total extractable nitrogen and characterized prokaryotic communities using amplicon sequencing. All C amendments increased microbial activity, and all except oxalate decreased total extractable nitrogen. Likewise, individual phyla responded to specific C amendments-e.g., Proteobacteria increased under addition of glucose, and both VOCs. Further, we observed an interaction between moisture and C amendment, where both VOC treatments had higher microbial activity than LMW-DOC treatments and controls at low moisture. Across moisture and C treatments, we identified that Chloroflexi, Nitrospirae, Proteobacteria, and Verrucomicrobia were strong predictors of microbial activity, while Actinobacteria, Bacteroidetes, and Thaumarcheota strongly predicted soil extractable nitrogen. These results indicate that the type of labile C source available to soil prokaryotes can influence both microbial diversity and ecosystem function and that VOCs may drive microbial functions and composition under low moisture conditions.
Highlights
The response of soil ecosystems to climate change depends on microorganisms’ access to a variety of soil carbon (C) substrates that shape their growth and activity
There was a significant interaction between C amendment and soil moisture (χ216 = 67.98; P < 0.001; Figure 1B) which was primarily due to α-pinene marginally increasing activity above both low molecular weight-dissolved organic carbon (LMW-DOC)
While α-pinene - and to some extent methanol consistently interacts with soil moisture to affect soil functional and compositional variables, our other predictions related to the moisture dependence of C source effects were largely not supported
Summary
The response of soil ecosystems to climate change depends on microorganisms’ access to a variety of soil carbon (C) substrates that shape their growth and activity. Soil moisture content is an important regulator of these C substrates (Averill et al, 2016, Hawkes et al, 2017), as many C compounds are limited by slow diffusion through soil water (Kuzyakov and Blagodatskaya, 2015). These compounds include low molecular weight-dissolved organic carbon (LMW-DOC) e.g., carbohydrates, amino acids, and organic acids. This study examines soil microbial responses to LMW-DOCs and VOCs, and the extent to which they are dependent on soil water content
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