Microbial Processes

Core Areas: 

Most recent studies of microbial processes and microbial ecology at the Sevilleta National Wildlife Refuge (SNWR) have focused on the semiarid grassland biome. These studies can be broadly classified into process, phylogenetic and modeling categories.

Process studies have included two investigations of the role of photodegradation in the decomposition of surface plant litter. Litter of diverse composition decomposes more rapidly when exposed to sunlight with mass loss rates increasing by 25-100% relative to shaded or UV-filtered controls. Analyses of litter composition, microbial ecoenzymatic activity and microbial community composition indicate that solar radiation affects all components of the decomposition process but the magnitude of these effects vary in relation to the composition and surface area to mass ratio of the litter.

Other process studies have focused on the nitrogen cycle. A tracer study using 15N-labeled glutamate and nitrate showed that nitrogen can be translocated between grasses and biotic crusts through fungal networks. The finding is significant because biotic crusts are sites of N fixation and atmospheric N deposition, while soil N reserves are relatively small. Other studies have used biocides to examine the role of fungi and bacteria in the nitrification and denitrification processes. These studies indicate that fungi play a larger role in the redox transformation of nitrogen than do bacteria.

Analyses of microbial ecoenzymatic activity in SNWR grassland soils have shown that oxidative and proteolytic enzyme activities are high compared to soils from mesic ecosystems. In part, these trends reflect the alkaline pH of arid soils, but other factors including selective stabilization of enzymes on mineral surfaces and the specific contributions of Ascomycete fungi play a role. These activities limit the accumulation of soil organic matter and increase biotic nitrogen cycling, reinforcing the symbiotic interaction between biotic crusts and plants.

A series of phylogenetic studies have focused on the fungal communities of plants, biocrusts, soil and litter. Using molecular, microscopy and culture methods in combination, these studies show that the fungal communities in every major compartment of the grassland ecosystem are overwhelmingly dominated by the Ascomycota, with most of taxa falling within 2-3 orders. Because of their tolerance for heat, drought and UV radiation these fungi displace Glomeromycota and Basidiomycota taxa that provide a large fraction of the symbiotic and saprotrophic services in mesic ecosystems. Other studies have focused on the fungi associated with climate-induced conifer mortality and fungal community responses to increased nitrogen deposition.

Findings from studies at the SNWR have catalyzed the development of new models for ecosystem and microbial community organization. Sporadic moisture availability imposes a pulsed pattern on biotic activity in arid ecosystems. Variable moisture thresholds uncouple biogeochemical processes. Earlier formulations of the pulse-reserve paradigm for arid ecosystem organization did not include specific microbial functions. Studies at the SNWR have led to the an expanded version of the paradigm that incorporates fungi and enzymes as elements of ecosystem function. Studies at Sevilleta have also been valuable in the development of ecoenzymatic theory, which links ecological stoichiometry and metabolic theory to model the biogeochemical equilibrium of heterotrophic microbial communities.