Do microbial osmolytes or extracellular depolymerisation products accumulate as soil dries?

When dry soil is re-wet there is a pulse of C mineralization. It is likely that the pulse of mineralization is fuelled by soluble C that accumulates as soil is drying. When soil is drying soluble C could accumulate as products of exo-enzyme mediated depolymerisation (i.e. protein amino acids and small carbohydrates) in the extracellular fraction of the soil (i.e. adsorbed and in free solution). Alternatively there could be an accumulation of osmolytes within the microbial biomass. To test whether extracellular depolymerisation products and/or microbial osmolytes accumulate as soil dries, soil from a Themeda triandra grassland was dehydrated and then depolymerisation products and osmolytes were quantified by capillary electrophoresis-mass spectrometry and gas chromatography–mass spectrometry. A secondary aim of this experiment was to determine if the response of soil to drying is the same when soil is dehydrated rapidly in the laboratory as when an intact soil containing plants is slowly dehydrated by withholding water from large (200 L) mesocosms. The responses of soil to dehydration differed between lab incubations and mesocosms, despite involving the same soil being dehydrated to the same final water content. When soil was dehydrated slowly in 200-L mesocosms the accumulation of osmolytes was more quantitatively significant than accumulation of depolymerisation products, whereas when soil was dehydrated more rapidly in lab incubations there was negligible accumulation of osmolytes but large accumulation of depolymerisation products. This study has highlighted that when soil dries the accumulation of osmolytes within microbial biomass and depolymerisation products within the extracellular fraction of soil are both quantitatively important and likely underpin the flush of soil CO2 efflux when dried soil is re-wet.