A Trait-Based Framework Linking Biogeochemical Control Points to Organic Matter Chemistry

Abstract:

Ecosystem control points are formalized here with a quantitative approach known as control point influence (CPI). CPI compares contributions of biogeochemical hot spots/moments to system-scale aggregated rates, and is elevated when hot spots/moments are common enough and have high enough rates to drive aggregated rates. The CPI approach focuses on the shape of biogeochemical rate distributions through space and/or time, instead of maximum rates. This provides an opportunity to develop a transferable and mechanistic framework in which CPI is a system-scale biogeochemical trait. To drive towards transferable/mechanistic understanding we link CPI to organic matter (OM) chemistry in the hyporheic zone due to mounting evidence that OM chemistry has strong biogeochemical influences in these systems. A mechanistic understanding of the link between OM chemistry traits and CPI is lacking, however. Drawing from ecology, we consider the relative influence of stochasticity over the assembly of organic molecules into an OM pool to be a functionally relevant trait. In turn, we used two simulation models to (1) link environmental disturbances to stochasticity in OM chemistry and (2) link OM stochasticity to CPIs. The simulations indicated that (1) increasingly frequent disturbances can cause OM chemistry to be less stochastic, and (2) that CPI can be a unimodal function of OM stochasticity. The models are extensions of ecological theory and are conceptually analogous to an ecological system in which the environment selects for particular biological taxa. In turn, increasing environmental variation led to strong selection and lower stochasticity in the assembly of the OM pool. Changes in OM stochasticity are, in turn, connected to CPI; higher OM stochasticity increases the probability of hot spots/moments. This effect is non-monotonic such that too much stochasticity results in an unstructured system with consistently low rates and thus low CPI. Linking OM traits summarized through ecological concepts to biogeochemical traits provides a theoretical, trait-based framework that is transferable across systems and can be used to better understand mechanisms governing influences of hot spots/moments.