Organic Molecules are Deterministically Assembled in River Sediments

Abstract

Abstract Dissolved organic matter (DOM) is central to ecosystem function. A challenge is understanding processes driving variation in DOM chemistry. We examine these processes through a community ecology lens by quantifying the degree to which DOM chemistry is organized via stochastic versus deterministic forces. These forces have been studied using null models applied to aquatic and saturated subsurface DOM. We extend this work into variably inundated riverbed sediments, which are ubiquitous. Among 38 sites across biomes, we found that within-site variation in DOM chemistry is often strongly deterministic, though some sites were dominated by stochasticity. We hypothesized that the strength of determinism would be linked to sediment moisture, and found partial support for this, but in the form of a constraint space. The strength of determinism in many sites fell below the upper constraint boundary. We integrated our results with previous studies to develop a post-hoc conceptual model in which DOM assemblages are hypothesized to be increasingly deterministic moving from the river water column to saturated sediment pore spaces to unsaturated and dry soils/sediments. This conceptual model can guide follow-on work aimed at developing generalizable knowledge of how DOM chemistry is influenced by a combination of stochastic and deterministic processes.