Print PageOne of the key environmental factors controlling microbial activity is moisture. With drought frequency and severity likely increasing in the future, understanding its effect on terrestrial carbon (C) and nitrogen (N) cycling has become essential for accurately modeling ecosystem response. This water limitation is particularly strong in semi-arid and arid ecosystems such as those found along California’s coast and interior range-lands. Cool, wet winters separated by long, dry summers present some the most challenging conditions for microbial survival and growth.
We studied the drought-associated biogeochemical cycling of carbon and nitrogen in several different Mediterranean ecosystems. Measured parameters include dissolved organic carbon and nitrogen (DOC, DON), microbial biomass carbon and nitrogen, inorganic nitrogen (NH4+, NO3-), and soil respiration. We combined seasonal measurements, to assess the response to natural precipitation variation, with laboratory incubations designed to mimic natural moisture pulse regime variability, specifically, length and amount of water additions.
Results/Conclusions
Unexpectedly, we found that microbial biomass increased during the driest, hottest part of the year. It is unclear if this increase in biomass represents growth of new organisms, or was a result of microbes accumulating internal solutes to avoid desiccation. DOC and DON also accumulated over the dry summer, and then decreased with the onset of the winter rains. The seasonal patterns in NH4+ and NO3- availability suggest that a substantial pulse in net nitrification occurred with the very first winter rain. Under controlled conditions, soil respiration rate correlated with soil moisture, but was not significantly effected by the pattern of soil moisture additions. Also, the initial respiration rate after wetting a dry soil was positively correlated to the duration of soil drought. This suggests that antecedent soil water was more important in the response to pulsed water additions than how the water was introduced to the system.
At the landscape-scale, these abiotic and microbial dynamics may control the amount and composition of C and N export from these ecosystems. Long, dry summers allow organic compounds and nutrients to accumulate, and then a single large precipitation event can lead to a large pulse being released either into the atmosphere (as CO2 or trace N gases), or into ground and streamwater (as dissolved organic matter and inorganic N). These results indicate that there are important interactions between the extent and duration of drought that impact ecosystem C and N cycling.
