Hypotheses that explain the pulse of C and N mineralisation in dry soil that has been rewet
Cycles of drying and rewetting are common features of most terrestrial ecosystems. One globally important consequence of dry-wet cycles is their effect on ecosystem cycles of C and N. In general water deficits reduce microbial activity of soils while rewetting increases microbial activity and leads to a pulse of C and N mineralization. In many ecosystems these pulses of mineralisation can account for a substantial fraction of annual fluxes. The mechanisms underpinning these responses to drying & rewetting remain elusive. One theory is that in drying soil there is a massive accumulation of osmolytes within microbes. Rewetting leads to lysis of microbes and excretion of osmolytes, with the massive pool of released osmolytes serving as substrates for those microbes (and plants) that survived water stress – and thereby underpinning the flush of N and C mineralisation. However, there has been little experimental evidence for a substantive role of osmolytes in responses to dry-wet cycles. At least part of the reason for this may be the difficulty of comprehensively profiling osmolytes in soil. The aims of this study were to develop and refine methods for quantifying microbial osmolytes in soil, and then re-visit the theory that osmolytes underpin the flush of mineralisation in re-wet soils.
No single analytical platform could obtain broad coverage of known N and C-containing organic osmolytes, but broad coverage could be obtained by using a combination of capillary electrophoresis-mass spectrometry and gas chromatography-mass spectrometry.
Experiments with large (>200 L) mesocosms of soil from a sub-tropical grassland tested whether osmolytes underpin the flush of soil CO2 efflux following rewetting. Compared with controls, drought-stressed mesocosms contained >10-fold larger amounts of known microbial osmolytes: ectoine, hydroxyectoine, betaine, proline-betaine, trigonelline, proline, trehalose, arabitol. The pool of osmolytes accounted for 3.6% of CHCl3 labile TOC in control mesocosms and 17% of CHCl3 labile TOC in drought-stressed mesocosms. The microbial pulse of CO2 efflux was of the same order of magnitude as the difference in microbial osmolytes between drought-stressed and control mesocosms. Taken together these observations were consistent with the theory that osmolytes (partially) underpin the flush of mineralisation in re-wet soil.
In stark contrast, studies on other soils provided equally convincing evidence that osmolytes do not accumulate in dry soil. Ongoing experiments are trying to answer the question of why some soils do accumulate osmolytes, whereas others do not.