One of the key environmental factors controlling microbial activity is moisture. Cool, wet winters separated by long, dry summers present some the most challenging conditions for microbial survival and growth. In coastal bishop pine forests on California’s Channel Islands, previous work has shown that fog can provide an important moisture source sustaining tree growth during the dry summers in this Mediterranean climate. However, little is known about how moisture inputs from fog affect soil microbial dynamics in these ecosystems. Starting in May 2008, we measured soil moisture, microbial biomass carbon and nitrogen, and soil inorganic nitrogen, root/microbial respiration, and dissolved organic carbon and nitrogen under tree canopies, and in open spaces, in two bishop pine stands on Santa Cruz Island, California. Measurements were made roughly every other month, and are ongoing.

Results/Conclusions

Soil water content was consistently higher under tree canopies compared to open spaces during the dry summer of 2008 (6.1±0.8 compared to 2.9±0.4% determined gravimetrically under trees and in the open respectively), with transient increases due to individual fog events. We found roughly a 3-fold decrease in soil NO₃^- (0.80±0.12 to 0.26±0.11 µg NO₃^--N g^-1 dry soil) and a commensurate 3-fold increase in NH4⁺ concentrations (from 0.58±0.10 to 1.79±0.59 µg NH4⁺-N g^-1 dry soil) over the summer dry season regardless of cover type. Microbial biomass, DOC, and DON showed no consistent patterns related to fog events or soil cover type. Root and associated microbial respiration comprised most of the soil respiratory flux and increased from 54 to 75% over the course of the dry summer. However, observations on shorter timescales demonstrated that following summertime fog moisture pulses, heterotrophic respiration (microbial decomposition) contributed up to 90% of the elevated respiration response immediately after a wetting event. The decrease soil NO₃^- may be due to plant uptake stimulated by fog water inputs while soil NH₄⁺ dynamics may be related to microbial physiological response to increasing soil drought interspersed with transient moisture pulses. Soil respiration partitioning suggests that soil microbial activity is highly pulse dependent, but biomass data suggest that microbial populations remain relatively static. Simulated moisture pulse additions should provide the sampling time-scale to resolve these relatively short-term microbial dynamics. While the pulse dependent nature of microbial dynamics is not yet fully understood in this ecosystem, it appears that summertime fog inputs can stimulate microbial activity and affect the seasonal carbon and nitrogen cycling.