Abstract
High microbial carbon (MBC) demand, a proxy for energy demand (cost), during soil microbial response to stressors such as drought are a major gap in understanding global biogeochemical cycling of carbon (C) and nitrogen (N). The dynamics of two dominant microbial pools (amino acids; AA and exopolymeric substances; EPS) in soils exposed to drying and C and N amendment to mimic both low and high nutrient soil habitats were examined. It was hypothesized that dynamics of EPS and AA (osmolytes) would be greater when soil drying was preceded by a pulse of bioavailable C and N. Drying reduced AA content, even as overall soil MBC increased (~35%). The increase in absolute amounts and mol% of certain AA (eg: Taurine, glutamine, tyrosine, phenylalanine) in the driest treatment (−10 MPa) were similar in both soils regardless of amendment suggesting a common mechanism underlying the energy intensive acclimation across soils. MBC and EPS, both increased ~1.5X and ~3X due to drying and especially drying associated with amendment. Overall major pools of C and N based microbial metabolites are dynamic to drying (drought), and thus have implications for earth’s biogeochemical fluxes of C and N, perhaps costing 4–7% of forest fixed photosynthetic C input during a single drying (drought) period.
Highlights
Soil drying and drought are considered major disturbances that effect the physiological function of microbes that can resonate to global scale carbon (C) and nutrient biogeochemistry
The mole percentage of amino acids (AA) in Sumter and Marietta soil were, as expected, very different (MRPP; p < 0.000001) and are analyzed and presented separately to highlight the differences associated with amendment and drying
The shift of AA in similar ways in two different soils suggests comparable mechanisms of microbial acclimation to drying. Despite this similarity, the overall mass of AA was ~ 2X greater in Marietta than Sumter (MRPP: p < 0.00001), suggesting the size of the response may depend on the size of the microbial biomass
Summary
Soil drying and drought are considered major disturbances that effect the physiological function of microbes that can resonate to global scale carbon (C) and nutrient biogeochemistry. The adaptation of microbes to water potential decline, for example, by AA-osmolyte production, can be used to build cellular proteins, comprising ~50% bacterial cell dry weight and a large component of soluble organics in soil. Polysaccharides (and proteins), exuded from the cell membrane are used to create exopolysaccharides and extracellular polymeric substances[11,18] that support microbial adaptation to dessication To exemplify this latter response, upon starvation or other stress, gram-negative Myxoccoccus xanthus will form a polysaccharide spore-like fruiting body of ~106 cells[19], and so like many other biofilm forming bacteria utilize polysaccharides to cope with stresses and regain activity when environmental conditions allow. The production of osmolytes during low water potential has been shown to be highly regulated by the presence of organic molecules and nutrients[23], the availability of C and N near the root-zone[5] may help determine how soil microbes respond to variability of water availability and water deficit stress, and may help to explain the variable reports explaining microbial responses to soil drying
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