Dissolved organic matter composition may be a key modifier of ecosystem-scale macronutrient reactivity and flows across the terrestrial - aquatic continuum

In my talk, I propose that stoichiometric imbalances between microbial metabolic needs and carbon (C) : nitrogen (N) : phosphorus (P) ratios affect reactive macronutrient flows between ecosystems and in landscapes, much like how stoichiometric imbalances of macronutrients affect organism growth and nutrient cycling at smaller scales. More specifically, I hypothesize that the mismatch between microbial C : N : P ratios and biologically reactive macronutrient ratios modulates macronutrient retention and export. When microbial C : N : P matches nutrient availability, reactive macronutrients should be retained or transformed, reducing downstream transport. Conversely, stoichiometric imbalances between microbial C : N : P and reactive macronutrient C : N : P lead to excess reactive macronutrients being exported to downstream ecosystems

These stoichiometric imbalances are strongly modified by dissolved organic matter (DOM) quantity and especially by DOM composition, which defines the microbial reactivity of DOM. With laboratory microcosm and stream mesocosm experiments, colleagues and myself provide first mechanistic evidence for the importance of DOM composition for the stoichiometric modification of macronutrient flows. Furthermore, comparing global published C : N : P data from soils, lakes, and marine ecosystems, we find evidence that microbial activity uniformly modulates reactive DOM and macronutrient ratios across environments, affecting macronutrient cycling and flows, with probable secondary effects on ecosystem functioning and eutrophication. 

The proposed concept links small-scale mechanistic understanding to ecosystem-scale patterns of macronutrient cycling in inland-water ecosystem networks. This cross-scale perspective highlights the need for integrated stoichiometric experimental and monitoring research to better understand reactive macronutrient cycling and flows, with high potential for improved macronutrient management.