Soil Carbon, Litter Decomposition and Scaling Theory

Litter decomposition is probably one of the best researched ecosystem processes. It is central to the recycling of plant nutrients within systems and is the energy channel through which much of the soil food web is sustained. The dominant conceptual model of litter decomposition posits that the primary controls on the rate of decomposition are climate, litter quality and decomposer organisms. This conceptual model informs the most common and widely used soil organic matter models, and so our understanding of litter decomposition affects how we think about how soil carbon - a huge carbon store and metric initmately tied to the health of soils - will interact with climaet change and respond to management.  A robust understanding of litter decomposition is therefore essential if we are to steward soils and the carbon cycle in a sustainable manner.

The controls on litter decomposition and soil carbon are hypothesized to operate hierarchically in space, with climate and litter quality co-dominant at regional to global scales; whereas decomposers operate only as an additional local control whose effect is negligible at broader scales. Consequently, decomposers have been omitted as controls from biogeochemical models. Yet evidence that microbial decomposers regulate decomposition rates at regional- to global-scales, independent of climate variables such as temperature and moisture, is generally lacking. One possibility for this lack of evidence is suggested by scaling theory, where the influence of mechanisms that act locally can be obscured in emergent, broad-scale patterns (the Bradford et al. 2016 J. Ecol. paper explains why, and the 2017 Nature Ecology & Evolution paper demonstrates this experimentally) .

Pattern and scale has been described as the central issue in ecology, where the inherent challenge to prediction and understanding lies in the elucidation of mechanisms, which commonly operate at different scales to those on which the patterns are observed. We investigate how such scale mismatches in observation versus mechanism influence our understanding of controls on ecosystem processes, using litter decomposition as a representative process. We do this because litter decomposition is controlled by variables operating at finer scales than those at which the variables are typically measured and evaluated. In particular, the hierarchical model of litter decomposition is conceptually grounded in local (i.e. microsite) dynamics, but has been developed and substantiated with data collected at coarser scales of resolution (typically site-level). We are carrying out a series of field experiments to quantify the effect sizes of different controls on decomposition rates when observed at the same fine spatial grains at which they operate; and how this new understanding affects forecasting of broad-scale decomposition patterns.

The Bradford et al. 2021 Biogeochemistry paper (below) is probably our most complete synthesis of the concepts and new ways of doing research that we feel are essential for moving the field forward, and for improving carbon cycle projections at regional to continental scales, by focusing on local scales! And the Polussa et al. 2021 paper shows how microbial functional differences can manifest (or not!) at both local and regional scales.