Biogeochemical interactions across trophic groups.

Predicting the response of ecosystems to global environmental change has been challenging in part because ecosystem processes (e.g. nutrient biogeochemical transformations) and properties (e.g. ecosystem productivity) are a product of a complex set of positive and negative feedbacks. As a result, nutrient enrichment of ecosystems often have unexpected / unintended consequences that may vary over temporal and spatial scales. The extent to which nutrient enrichment alters biogeochemical processes depends on the nutrient status of autotrophs and microbes – the two trophic components that can regulate C balance of an ecosystem. Contrary to the conventional wisdom, I recently demonstrated that biological activity in coastal environments is not solely governed by nitrogen availability. I showed that the growth of plants and soil microbes in an ecosystem could be limited by different (separate) nutrients. I found that although the plant community in a pristine salt marsh (in the North Inlet Estuary) was limited primarily by N availability, the bacterial community in the soil was limited by P availability. However, the P limitation of microbial growth had the potential to alter ecosystem-level inputs and outputs of N, thus potentially influencing C fixation, storage and release by N-limited plants. This finding of differential nutrient limitation (DNL) across functionally distinct trophic groups warrants a closer look at the traditional view of ‘one limiting nutrient per ecosystem’, and suggests that the biogeochemical cycling of these elements is intricately coupled, in ways that was previously unrecognized (Sundareshwar et al. 2003, Science). A proposal is currently under review at National Science Foundation’s (NSF’s) ecosystem studies program to investigate if the microbial P limitation observed in this salt marsh is a seasonal phenomenon which occurs during the growing season when the competition for nutrients is the greatest.

 DNL likely is an example of an emergent ecosystem property and is a manifestation of spatio-temporal interactions among the various components within ecosystems. Preliminary evidence suggests that the patterns of DNL described above for a salt marsh also occur in pelagic environments and other ecosystems. This suggests that altering trophic level nutrient limitation by fertilization has major implications for ecosystem properties, global C storage dynamics, and environmental health. A collaborative proposal to study the relationship between DNL and ecosystem C throughput in a salt marsh is also being developed. The proposed study on DNL integrates ecosystem science at three levels. It uses ecological stoichiometric information (such as nutrient limitations) to address biogeochemical questions of key elements, which dictate ecosystem energy balance. Because DNL has been demonstrated in terrestrial, freshwater, and marine systems, the results from this study will provide a framework for understanding how nutrient availability affects ecosystem-scale processes in a variety of systems.

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