Oak SavannaSedgwick

Biogeochemistry of Drought

SedgwickThe world is a dry place: roughly 1/3 of the terrestrial land surface has arid, semi-arid, or Mediterranean climates that are characterized by long droughts (Gurrevitch et al. 2002). Climate models also suggest that drought is likely to become more prevalent with climate warming (Wetherald & Manabe 2002). However, the biogeochemistry of the dry season has usually been studied only implicitly– as “antecedent conditions” that regulate the pulses of biological activity 3 that occur with the early rains (Davidson 1992; Hungate et al. 1997; Xu et al. 2004; Smolander et al. 2005) or the chemical characteristics of streamflow (McKee et al. 2000; Wellington & Driscoll 2004; Welter et al. 2005). However, rarely have the biogeochemical processes that occur during the dry season been studied explicitly to understand what creates the conditions at the beginning of the wet, “growing season.”

In California, summer can go for 6 months without any rain, and even during the spring and fall, rains can be intermittent. SedgwickWhile a number of studies have examined the soil biogeochemistry of California (Jones & Woodmansee 1979; Jackson et al. 1988; Schimel et al. 1989a; Stark & Firestone 1996; Dahlgren et al. 1997; Higgins et al. 2002; Balser & Firestone 2005; Henry et al. 2005), they have largely focused on the growing season. In California this is the cool, wet, winter period when plants are actively growing and when the conditions for field work are bearable. During the summer, temperatures can exceed 40° C and the sun is intense. It has always been assumed that the “non-growing season” was a period of dormancy and mere survival.

We naturally expected, therefore, that in California the winter growing season would be a season of high soil microbial biomass, active respiration, and net N mineralization, but that biomass and activity would drop during the dry summer. Oak SavannaSurprisingly, however, in both annual and perennial grass plots, we observed patterns at odds with that. Over the summer, microbial biomass increased (Parker and Schimel 2006), net mineralization rates shifted from negative during the winter to positive during the summer, the N pool shifted from NO3 domination during the winter to NH4 + during the summer, nitrification potentials increased during the summer and even denitrification potentials more than doubled. These surprising results beg an explanation. Why, at a time when activities are lowest and conditions appear worst, does it appear that many groups of organisms are doing best? We hypothesize that these surprising ecosystems-level summertime dynamics result from two micro-scale phenomena: a) the physiology of microbial drought survival and b) the hydrological connectivity/disconnectivity of the “microbial landscape.” As soils dry, microbes experience direct physiological stress, resource limitation from drying, and hydrological disconnections in their environment. For example, at soil matric potentials down to -0.6 MPa, NH4 + diffusion is a greater limitation on nitrification than is physiological stress (Stark & Firestone 1995). At water potentials of -10 MPa (easily attainable in dry soil), the only available water is in capillaries and water films of <1 μm diameter (Harris 1981; Papendick & Campbell 1981) and possibly held within the matrix of extracellular polysaccharides (Roberson et al. 1993), creating potentially extreme limitations for resource diffusion and movement. On the other hand, microbes may experience reduced predation pressure (Gorres et al. 1999) because microbial predators also rely on a “connected” landscape for foraging.

Alan DoyleTo date, our research has focused specifically on grassland dynamics, and we found very comparable microbial dynamics under invasive Mediterranean annual grasses and under native California perennial grasses. In this project our main focus will be on intensive research on annual grassland to evaluate the specific mechanisms that produce the surprising microbial and biogeochemical patterns we observed. Grasslands in California cover over 10 million hectares (Jackson 1985), are dominated by annual grasses, and are important ecosystems in the State. The mechanisms we will evaluate likely occur in other ecosystem types as well, and so this project should have value in understanding dry-season biogeochemistry broadly.