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Stoichiometry of Ecoenzymatic Activities and Decomposition Modeling

Ecological stoichiometric theory (EST) uses elemental ratios to predict nutrient retention and biomass production on subcellular to ecosystem scales. The complementary metabolic theory of ecology (MTE) uses thermodynamics to predict metabolic activity across all levels of biological organization. EST and MTE evaluations at higher levels of ecological organization are complicated because ecological systems are controlled by non-equilibrium flows of both materials (nutrients) and energy (carbon) and it is difficult to know which is predominant in a specific context. 

For heterotrophic microbial communities, the acquisition of carbon and nutrients from organic sources is mediated by ecoenzymes (enzymes not contained by membranes of living cells). Ecoenzymatic activity is an intersection of EST and MTE because enzyme expression is a product of cellular metabolism specifically regulated by environmental nutrient availability.

The activities of four enzymes that catalyze the hydrolysis of assimilable products from the principal environmental sources of C, N and P (β-1,4-glucosidase (BG), which generates glucose (C) from cellulose and other beta-linked glucans; β-1,4-N-acetyglucosaminidase (NAG), which generates glucosamine (C and N) from chitin and peptidoglycan, and leucine aminopeptidase (LAP), which hydrolyzes leucine (C and N) and other hydrophobic amino acids from the N-terminus of polypeptides; alkaline/acid phosphatase (AP) which hydrolyzes phosphate (P) from phosphosaccharides and phospholipids) show similar scaling relationships in soil and sediment microbial communities with a mean lnBG:ln(NAG+LAP):lnAP ratio near 1:1:1. These ratios provide a functional measure of the threshold elemental ratio (TER) at which control of community metabolism shifts from resource to energy flow.

Ecoenzymatic stoichiometry can also be extended to carbon composition by relating ratios of BG and phenol oxidase (POX) activities. The mean lnBG:lnPOX ratio for soil approximately 0.8. The ratio is negatively correlated with the abundance of recalcitrant carbon. Lower carbon availability as the result of increasing recalcitrance increases the potential for carbon (energy) limitation of microbial growth, leading to a reduction in TER.The lnBG/lnPOX ratio, in combination with ecoenzymatic ratios of relative N and P availability, provide easily measured metrics for TER. Ecoenzymatic stoichiometry has the potential to facilitate further development and application of decomposition models in the context of ecological theory.




Abiotic Decomposition in Aridland Ecosystems

Decomposition models that use climatic and litter composition variables as surrogates for microbial activity underestimate decomposition rates in arid ecosystems because abiotic processes facilitate the physical and chemical degradation of plant litter.  Unlike microbial degradation, the photodegradation of surface litter is not restricted to transient intervals of high water availability.  Consequently, organic matter decomposition is less moisture dependent than primary production. Belowground, alkaline soils with reserves of stabilized enzymes may also facilitate decompositon.  This disparity between production and decomposition potentials increases with aridity, limiting the accumulation of soil organic matter and the capacity of the system to immobilize and retain nutrients.  There is a need for decomposition models that include both biotic and abiotic mechanisms to accurately represent the carbon dynamics of aridland systems.



Plant–Microbial Interaction

Nitrogen enrichment increases plant productivity and reduces plant species richness in many terrestrial communities. Microbial community structure and function link N availability and plant community response. Increased N availability affects the composition and biomass of soil microbial communities along with extracellular enzyme activities, root colonization, and rates of soil organic matter mineralization. However, these investigations have generally been conducted in systems where plant species turnover is slow, so their role in mediating plant community changes is unclear.

In arid ecosystems, symbiotic microorganisms also may be particularly important in plant community responses to climate change. Fungal endophytes colonize all parts of the dominant plants. The functional role of these organisms is a topic of active investigation. Demonstrated functions include heat and drought tolerance, translocation of carbon and nitrogen, and denitrification. Other climate associated responses like conifer dieoffs may be mediated or accentuated by transmission of pathogenic fungi by beetles. The extent to which climate changes influence plant–microbe interaction and ultimately plant community composition is a topic amenable to investigation at several scales including rapidly developing metagenomic and metatranscriptomic approaches.


Other Lab Research Interests
  • Simulation modeling of decomposition processes.
  • Formation and fate of dissolved organic matter.
  • Biogeochemistry of extracellular organic matter transformations.
  • Mechanisms of soil carbon storage.