Constraining Carbon and Nutrient Flows in Soil With Ecological Stoichiometry

Abstract

Soil organic matter (SOM) is central to soil carbon (C) storage and terrestrial nutrient cycling. New data have upended the traditional model of stabilization, which held that stable SOM was mostly made of undecomposed plant molecules. We now know that microbial by-products and dead cells comprise unexpectedly large amounts of stable SOM because they can become attached to mineral surfaces or physically protected within soil aggregates. SOM models have been built to incorporate the microbial to mineral stabilization of organic matter, but now face a new challenge of accurately capturing microbial productivity and metabolism. Explicitly representing stoichiometry, the relative nutrient requirements for growth and maintenance of organisms, could provide a way forward. Stoichiometry limits SOM formation and turnover in nature, but important nutrients like nitrogen (N), phosphorus (P), and sulfur (S) are often missing from the new generation of SOM models. In this synthesis, we seek to facilitate the addition of these nutrients to SOM models by (1) reviewing the stoichiometric biasthe tendency to favor one element over another-of four key processes in the new framework of SOM cycling and (2) applying this knowledge to build a stoichiometrically explicit budget of C, N, P, and S flow through the major SOM pools. By quantifying the role of stoichiometry in SOM cycling, we discover that constraining the C:N:P:S ratio of microorganisms and SOM to specific values reduces uncertainty in C and nutrient flow as effectively as using microbial C use efficiency (CUE) parameters. We find that the value of additional constraints on stoichiometry vs. CUE varies across ecosystems, depending on how precise the available data are for that ecosystem and which biogeochemical pathways are present. Moreover, because CUE summarizes many different processes, stoichiometric measurements of key soil pools are likely to be more robust when extrapolated from soil incubations to plot or biome scale estimates. Our results suggest that measuring SOM stoichiometry should be a priority for future empirical work and that the inclusion of new nutrients in SOM models may be an effective way to improve precision.

Multiple distinct, scale‐dependent links between fungi and decomposition

“…Although long neglected, microbial biomass and community composition are increasingly demonstrated as important drivers of biogeochemical process rates (Bradford et al, 2014, 2017; Glassman et al, 2018; Maynard et al, 2018). In accordance, explicit microbial dynamics are becoming a modelling priority (Buchkowski et al, 2019) but approaches vary widely, ranging from a single microbial pool (Moorhead & Reynolds, 1991) to multiple functional groups (Smith & Wan, 2019). Significant challenges remain because while microbial communities are diverse both functionally and taxonomically, successful modelling requires simplification and it is not yet clear which axes of microbial community variation demand focus.…”

Nutrient stoichiometry of a plant-microbe-soil system in response to cover crop species and soil type

“…they tend to be more homeostatic (Cleveland and Liptzin 2007). Ecological stoichiometry represents a helpful conceptual framework to understand the cycling of C, N and P nutrients at different spatial scales (Buchkowski et al 2019;Sterner and Elser 2002).…”

Soil fungal mycelia have unexpectedly flexible stoichiometric C:N and C:P ratios

“…maintain stable C:N:P ratios independently of the soil nutrient status (Persson et al ., 2010). Even though some models allow microbial C:N:P to vary slightly (McGill et al ., 1981; Nicolardot et al ., 2001), homeostatic flexibility is currently only attributed to microbial community shifts, not an actual stoichiometric flexibility in individuals (Buchkowski et al ., 2019). Fixed microbial stoichiometric ratios are interpreted as an indicator of nutrient demands: If microbes need to maintain their narrow C:N ratios, N will be in limiting supply in substrates characterised by wider C:N ratios (Manzoni et al ., 2010).…”