Dryland ecosystems are changing from grass-dominated to shrub-dominated landscapes with significant consequences for regional watersheds and global carbon cycling. This woody-plant encroachment fundamentally changes dryland ecosystem function by altering the hydrology, fire regimes, and carbon storage of semiarid ecosystems. Soil microorganisms that drive biogeochemical and nutrient cycling through the production of exoenzymes facilitate organic matter decomposition and the flux of large amounts of carbon dioxide to the atmosphere.
In this four-year study, we used a warming experiment with added debris as either woodchip or biochar, to simulate different long-term carbon additions that accompany woody plant encroachment in semiarid grasslands. The response of microbial activity and biomass, heterotrophic respiration, and plant biomass was monitored bi-annually. We hypothesized that the temperature manipulations would reduce the activity of seven exoenzymes through dehydration of soil pores and indirectly by reducing microbe-available substrates through lowering plant inputs. We also predicted microbial biomass would be most responsive to seasonal moisture as microbial cells may become dormant during adverse conditions. Overall, reduction in exoenzyme activity may reduce decomposition of coarse woody debris and potentially enhance soil carbon storage in semiarid ecosystems.
Results/Conclusions
On average, infrared mirrors warmed the experimental plots by 5 °C. The passive warming treatment was most pronounced during periods of daily and seasonal temperature maxima. Carbon exoenzyme activity responded negatively to warming following summer monsoons. Contrary to other studies, microbial biomass remained consistent across seasons and in response to warming treatments. Microbial activity (biomass growth and exoenzyme production) was highest in the surface woodchip treatment. Incorporated woodchips significantly lowered soil nutrient availability and reduced overall plant cover, but positively influenced bulk soil respiration. Contrary to expectation, there was higher soil respiration in the absence of plants in the buried woodchip treatments, suggesting a shift in microbial community composition. These results suggest that the microbial community is a large contribution to soil respiration in nutrient-poor conditions. Furthermore, this study shows the shift in woody inputs associated with woody plant encroachment has some significant influences on carbon cycling in semiarid ecosystems because if debris becomes buried, soil carbon loss through soil respiration increases. Woody-plant encroachment, warming, and more variable precipitation are all factors influencing semiarid ecosystems worldwide. Together these disturbances and changes alter substrate inputs to soil microbial communities and therefore have significant potential to influence the rates of carbon and nutrient cycling and storage.