Soil respiration is one of the largest components of the global C cycle, but also one of the hardest to model. A key aspect of this difficulty is that we do not have a good mechanistic understanding of the tens of gigatonnes per annum of respiration arising from cycles of soil drying followed by rewetting. The overarching aim of this project is to improve our understanding of how soil responds to drying and rewetting.
In all soils drying reduces microbial activity (including respiration), while rewetting increases microbial activity and increases rates of respiration 500-1000%. Following re-wetting rates of soil respiration can remain faster than in well-watered controls for up to six weeks. Understanding processes associated with soil drying-wetting cycles has proven difficult because they reflect inter-relationships among microbial population dynamics, microbial physiology, substrate dynamics and diffusion. It had been suspected for some time that the massive pulse of respiration when soil is re-wet reflected microbes burning off a pool of organic C that accumulated during the drying phase, though quantitative evidence was sorely lacking. The breakthrough came when my lab showed for the first time that the explosive respiration pulse was partially fuelled by two sources of organic C that accumulated during the drying phase (Warren, 2014, 2016). • A part of the respiration pulse was fuelled by depolymerisation products, viz. protein amino acids and mono-saccharides, that had accumulated in the extracellular fraction of drying soil -- presumably because the drought-induced reduction in microbial uptake was larger than the decrease in enzyme activity (Stark and Firestone, 1995). • In some soils the respiration pulse was partially fuelled by organic osmolytes that accumulated in the microbial biomass of drying soil (Warren, 2014c, 2016). Quantitative analysis revealed that the depolymerisation products and osmolytes in dry soil accounted for maybe half of the CO2 pulse after re-wetting (Warren, 2014, 2016), which leads to the possibility that a proportion of the pulse of soil respiration was fuelled by old C that existed before the drying phase and became accessible after re-wetting. The source of the C that is respired is important because if the respiration pulse is fuelled by “old” C then cycles of drying-rewetting will reduce the total C stock of soil, whereas total C stocks would be unaffected f the source of C is new/microbial C (osmolytes & depolymerization products). Hence the aim of this project is to determine the relative amounts of new versus old C that are respired when dry soil is re-wet. This project will make use of advanced isotope labeling and mass spectrometry techniques and would suit candidates with an interest in biogeochemistry and analytical methods. Warren, C.R., 2014. Response of osmolytes in soil to drying and rewetting. Soil Biology & Biochemistry 70, 22-32. Warren, C.R., 2016. Do microbial osmolytes or extracellular depolymerisation products accumulate as soil dries? Soil Biology & Biochemistry 98, 54-63.
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The opportunity ID for this research opportunity is 2854