Moisture controls on plant and microbial contributions to soil respiration in coastal California pine forests

Background/Question/Methods Moisture inputs drive soil respiration dynamics in arid and semi-arid ecosystems. In the Mediterranean climate of California, moisture inputs are almost exclusively derived from wintertime rain. However, in some coastal California ecosystems, warm, dry summers may be ameliorated to some extent by fog and low clouds. Fog can enhance ecosystem moisture availability through fog drip, when cloud water droplets accumulate on vegetation and fall to the ground, and also through shading, by reducing evapo-transpirational losses. This presentation will focus on the importance of these different moisture controls to plant and microbial soil respiration sources and rates in two contrasting Bishop pine (Pinus muricata) stands on Santa Cruz Island, California. Beginning in June 2008, we combined automated chamber measurements of soil respiration with radiocarbon (14C) source partitioning techniques over various timescales. Large differences between the mean 14C signature of CO2 respired by plants versus that respired by microbes permit the in situ separation of soil respiration sources using a mass balance approach.

Results/Conclusions We found that the presence of summertime fog (both through its impacts on drip and shading) was an important control on soil respiration rates and patterns, enabling greater plant and microbial activity in the absence of rain. Over the summer fog period, autotrophic respiration (root and associated microbial respiration) dominated soil respiration rates and patterns, and increased in percent contribution (54-75%) with decreasing soil moisture and increasing soil and air temperature. However, observations on shorter timescales demonstrated that following summertime moisture pulses, heterotrophic respiration (microbial decomposition) was much more dynamic, in some cases contributing up to 90% of the elevated respiration response immediately after the wetting event. Our results suggest that microbial decomposition is more responsive to seasonal and episodic moisture inputs. Thus, alterations to the amount, frequency, and seasonality of moisture pulses likely have very different implications for autotrophic and heterotrophic respiration sources and the total carbon respired in arid ecosystems.