Global Patterns and Controls of Nutrient Immobilization on Decomposing Cellulose in Riverine Ecosystems

Abstract

Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low‐nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low‐nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature‐dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature. Collectively, we demonstrated that exogenous nutrient supply and immobilization are critical control points for decomposition of organic matter.

Parsing spatial and temporal variation in stream ecosystem functioning

“…For example, in a recent study of over 500 streams with minimal human impacts, organic‐matter decomposition rates in streams ranged by more than two orders of magnitude (Tiegs et al, 2019). This wide variation in decomposition rates represented in this and other studies was driven by environmental and other factors, including: temperature (Griffiths & Tiegs, 2016), nutrient concentrations (Costello et al, 2022; Woodward et al, 2012), pH (Colas et al, 2019; Dangles et al, 2004), microbial community structure (Burdon et al, 2020; Fell et al, 2021), and interactions among these variables (Ferreira & Chauvet, 2011). Individually, these factors are known to exert strong constraints on processing rates, but how they all interact to determine variation among streams and over time remains a considerable knowledge gap.…”

“…However, relationships between water temperature and decomposition rates were more elusive, although we did find a relationship between the coefficient of variation of air temperature during the incubation period and decomposition rates. Concentrations of dissolved nutrients are widely known to govern decomposition rates in streams and other habitats (e.g., Costello et al, 2022; Ferreira et al, 2020). The study presented here only made use of nutrient data and other site descriptors from a single year (2016), which may not have been representative of other years, hence its limited explanatory power.…”

From organismal physiology to ecological processes: Effects of nutrient enrichment and salinity variation in a freshwater ecosystem

“…Finally, measuring microbial activity through an organic-matter decomposition assay (after Tiegs et al 2019) allows the quantification of the capacity of the simplified community to process organic carbon when exposed to nutrient enrichment and saltwater input. Increased nutrient concentrations have become a central theme in freshwater ecosystems (Vitousek et al 2009), and decay rates are known to vary along nutrient enrichment gradients, with slower rates at high and low nutrient concentrations (Woodward et al 2012;Costello et al 2022). Both phototrophic periphyton biomass and heterotrophic microbial activity were measured to assess the effect of combined stressors on ecosystem processes (Young et al 2008).…”

A stream‐to‐sea experiment reveals inhibitory effects of freshwater residency on organic‐matter decomposition in the sea

“…freshwater processing during downstream transport. Once treated as simple transport conduits (Leopold 1994), rivers are now viewed as hotspots in the landscape for biogeochemical activity (sensu McClain et al 2003), including organic-matter retention and transformation (Webster et al 1999;Tank et al 2010;Costello et al 2022). Microbial activity in rivers is typically maintained by well-oxygenated water, constant nutrient delivery via stream flow, and a supply of organic matter from overland transport and direct infall.…”