Composition of low molecular weight organic substances affect total soil microbial activity independently of community composition.
Authors: Dufour Louis J.P. , Herrmann Anke. M. , Leloup Julie , Przybylski Cédric , Abbadie Luc , Nunan Naoise 
Published: March 2020

&lt;p&gt;It has recently been suggested that microbial-derived material is an important constituent of soil organic matter, and accumulation of organic matter in soil is related to microbial activity and the composition of the microbial communities present. However, microbial activity is generally intimately related to the properties of the carbon substrate (e.g. molecular diversity, energy content) available for decomposition. It is important, therefore, to understand in more detail what the drivers of microbial activity and their associated metabolic pathways are: the composition of the microbial communities or the properties of the available organic substrate.&lt;/p&gt;&lt;p&gt;Water extractable organic matter from 6 different grassland and forest soils were added cross-wise to samples of each of the soils. The total heat dissipation was measured by isothermal calorimetry during a 23 h incubation period. Heat dissipation is a measure of total metabolic activity in the soil. The total CO&lt;sub&gt;2&lt;/sub&gt; emission was also determined, by gas chromatography-mass spectrometry. The elemental composition of low molecular weight organic substances (LMWOS) was obtained by Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry, which allowed us to calculate the Nominal Oxidation State of Carbon (NOSC) from each exact molecular mass detected. The NOSC is related to the energy content of the molecules. The microbial composition was determined by 16S rRNA gene sequencing. This experimental design allowed us to evaluate potential links between microbial community composition and their metabolic pathways when different LMWOS are undergoing decomposition.&lt;/p&gt;&lt;p&gt;First, we found that the total heat dissipated and CO&lt;sub&gt;2&lt;/sub&gt; emitted were associated with differences in the composition of LMWOS, independent of microbial community composition in the soil. We observed that the median of the total CO&lt;sub&gt;2&lt;/sub&gt; emission was positively correlated with the weighted sum of the NOSC of each detected LMWOS. These results emphasise that a supply in available substrate with lower energy density (i.e. higher NOSC) may result in an increase in decarboxylation processes. Furthermore, total heat dissipated but not CO&lt;sub&gt;2&lt;/sub&gt; was positively correlated to the molecular richness of LMWOS. This indicates that substrate with a higher molecular richness include also other metabolic pathways where CO&lt;sub&gt;2&lt;/sub&gt; is not a decomposition end-product. Finally, we observed three different dynamics of heat dissipation during the 23 h incubation period. These dynamics were related to the microbial communities, but were independent of the LMWOS composition. The dynamic of heat dissipation were summarised by a Q&lt;sub&gt;t50&lt;/sub&gt; index, i.e. the time required to release half of the heat dissipated in 23 h. The Q&lt;sub&gt;t50&lt;/sub&gt; index was significantly correlated to the relative abundance of different bacterial taxa. In conclusion, whilst the short-term dynamics of substrate use appear to be associated with microbial community composition, the overall activity is more closely related to the composition of the available substrate.&lt;/p&gt;