Climate change effects on soil microbial communities: Altered precipitation and invasion in a chaparral system

Background/Question/Methods

In arid and semiarid systems episodic rain events play fundamental roles in determining microbial activity and resulting effects on ecosystem level biogeochemical cycling. During dry conditions, overall soil microbial activity decreases, slowing decomposition rates and allowing for litter accumulation and with a rain pulse, microbial activity is rapidly stimulated. Additionally, indirect effects of climate change via invasion by exotic species also affect belowground communities through a combination of litter inputs, root exudates, and competition with microbes for nutrients. However, it is unclear how altered precipitation regimes coupled with changing community composition will affect soil microbial dynamics. To test this, we performed a wet-up and litter-addition experiment on soils collected following a 2-year rainfall manipulation where plots were subjected to low (0% or 50%), ambient (100%), or high (150% or 200%) rainfall. High and low water pulse treatments were applied to soils over eight weeks in combination with litter additions from two native and two exotic species found at the site. Microbial respiration, microbial biomass, and enzyme activities were measured following the pulses. 

Results/Conclusions

In general, microbial activity was higher for soils receiving a large moisture pulse than for soils receiving a low moisture pulse showing highlighting that decomposition is often moisture limited in arid and semiarid systems. Microbial activity was higher for soils with exotic litter additions than for soils with native litter additions. Exotic species at the site have more labile litter than natives and stimulated activity as indicated by microbial respiration throughout and microbial biomass at the conclusion of the experiment. Soils originating from different field rainfall treatments had no difference in respiration responses to a moisture event. Microbial communities may be more responsive to current conditions rather than retaining legacy effects of precipitation regimes. These results show that altered rainfall regimes in combination with invasion by exotic species are capable of altering belowground processes as indicated by differences in microbial activities.