COS 125-1 - Stoichiometric controls over nitrogen and phosphorus transformations and loss

Friday, August 12, 2011: 8:00 AM
5, Austin Convention Center
Alan R. Townsend, INSTAAR and Department of Ecology and Evolutionary Biology, University of Colorado at Boulder, Boulder, CO, Philip G. Taylor, Nicholas School of the Environment, Duke University, Durham, NC, William R. Wieder, University of Colorado, Boulder, Boulder, CO, Cory C. Cleveland, Department of Ecosystem and Conservation Sciences, University of Montana, Missoula, MT and Diana R. Nemergut, INSTAAR, Environmental Studies, University of Colorado, Boulder, CO

Interactions between carbon, nitrogen and phosphorus are central to the biogeochemistry of any one of these elements. Yet, models that can explain cross-system interactions are often challenged by unique structural and functional features of different environments. Recently, we showed coherent patterns in organic C – nitrate relationships across a broad array of Earth’s ecosystems, and argued that such coherence can be explained by the relative resource demands of heterotrophic nitrate assimilation, nitrification and denitrification. For example, in most systems, nitrate rises sharply only under microbial resource C:N ratios that should promote nitrification and diminish high rates of anabolic nitrate immobilization. Here, we expand on this conceptual model in three ways. First, we explore key uncertainties in the physiology and dynamics of the microbial community, focusing on the roles of bacterial growth efficiency, biomass plasticity and nitrifier abundance in regulating inverse patterns between organic carbon and nitrate. Second, we describe a similar stoichiometric model that connects global distributions in bioavailable phosphorus to that of dissolved organic carbon. Third, we discuss how our model can be used to understand biogeochemical dynamics at the watershed scale in two disparate landscapes: Niwot Ridge, CO and the Osa Peninsula of southwest Costa Rica. 


Meta-analyses show that bacterial growth efficiency varies between 5 and 80% in a diversity of aquatic ecosystems, with values increasing with greater nutrient availability. This relationship has implications not only for nutrient transformations, but for the fate of organic C.  Meta-analyses also show non-linear inverse relationships between bioavailable P and organic carbon along a hydrologic continuum from soils to the ocean.  This relationship is similar to that for nitrate, but the stoichiometric breakpoints at which P concentrations rise vary widely when compared to C:nitrate ratios, likely reflecting a greater plasticity in microbial biomass C:P (vs C:N) composition.  We also find evidence for strong stoichiometric coupling of N and P when resource ratios exceed probable microbial demand ratios, but once resource ratios for C:P and C:N drop below a theoretical minimum for microbial biomass, nitrate and bioavailable P accumulation become uncoupled.  At the site level, periods of significant nitrate and bioavailable P export in Niwot and Osa watersheds coincide with periods of low relative C availability, and/or periods of substantial hydrologic flushing that may bypass biological controls.  Finally, we show that both the abundance and activity of nitrifiers in multiple soil environments are inversely related to soil C:N ratios.

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