Historical precipitation regime and drying-rewetting interval alters grassland soil microbial community dynamics

Background/Question/Methods

Climate change scenarios predict the North American Great Plains to experience decreases in annual average precipitation and fewer but more intense storms. These trends will cause greater variation in water availability likely affecting microbially mediated soil carbon cycling. Our goal was to address how drying interval and differences in historical precipitation regime affect grassland soil microbial responses to wet-up. Soils with historically (17y) different precipitation timing regimes at Konza Prairie Biological Station – “Ambient” conditions reflecting precipitation occurring on site and “Altered” conditions that have periods between rainfall extended 50% over ambient intervals with all rainfall from that period applied in one pulse – were collected. These soils were incubated in the laboratory and dried to 3 different soil volumetric water contents (25%, 16%, 9% VWC; short, intermediate, and long interval treatments). Sterile tap water was added to bring soils to the time-zero VWC (39%, control incubations held constant at this level). We hypothesized that after dry-down and subsequent wet-up, drought resilient taxa would increase in abundance corresponding with higher microbial biomass C (MBC) and carbon use efficiency (CUE) relative to control soils. We also expected that long interval Altered soils would hold the highest proportion of resilient taxa and exhibit higher CUE.

Results/Conclusions

Microbial respiration rates significantly increased following wet-up in all drying interval treatments (P < 0.01). Respiration rates were higher in long interval soils post wet-up (mean 24.1 μg CO₂-C·g^-1 soil·h^-1) compared to intermediate and short interval soils (8.4, 5.5 μg CO₂-C·g^-1 soil·h^-1; P < 0.01). After wet-up, long interval Ambient soils had a higher respiration rate and metabolic quotient (qCO₂) compared to long interval Altered soils (respectively 33.4 > 14.8 μg CO₂-C·g^-1 soil·h^-1, P = 0.02; 0.04 > 0.02 μg CO₂-C· μg^-1 microbial biomass-C g^-1 soil h^-1, P = 0.03).  Over the full incubation period (13d), less total carbon was mineralized (5.9 mg CO₂-C·g^-1 soil) in long interval compared to control, intermediate and short interval incubations (respectively 6.9, 7.3, 7.8 mg CO₂-C·g^-1 soil; P < 0.01), but historical precipitation regime did not affect total carbon mineralized (P = 0.20). These preliminary data support the hypothesis that microbial communities are functionally resilient to drought stress but extended drying periods increase microbial efficiency and slow carbon mineralization rates after wet-up.  Analysis is ongoing and additional data will evaluate relative abundance of drought tolerant and resilient taxa and their correlation to changes in MBC and CUE across drying interval treatments and Ambient versus Altered soils.