Soil organic matter turnover is governed by accessibility not recalcitrance

Abstract

AbstractMechanisms to mitigate global climate change by sequestering carbon (C) in different ‘sinks' have been proposed as at least temporary measures. Of the major globalCpools, terrestrial ecosystems hold the potential to capture and store substantially increased volumes ofCin soil organic matter (SOM) through changes in management that are also of benefit to the multitude of ecosystem services that soils provide. This potential can only be realized by determining the amount ofSOMstored in soils now, with subsequent quantification of how this is affected by management strategies intended to increaseSOMconcentrations, and used in soilCmodels for the prediction of the roles of soils in future climate change. An apparently obvious method to increaseCstocks in soils is to augment the soilCpools with the longest mean residence times (MRT). Computer simulation models of soilCdynamics, e.g. RothC and Century, partition these refractory constituents into slow and passive pools withMRTs of centuries to millennia. This partitioning is assumed to reflect: (i) the average biomolecular properties ofSOMin the pools with reference to their source in plant litter, (ii) the accessibility of theSOMto decomposer organisms or catalytic enzymes, or (iii) constraints imposed on decomposition by environmental conditions, including soil moisture and temperature. However, contemporary analytical approaches suggest that the chemical composition of these pools is not necessarily predictable because, despite considerable progress with understanding decomposition processes and the role of decomposer organisms, along with refinements in simulation models, little progress has been made in reconciling biochemical properties with the kinetically defined pools. In this review, we will explore how advances in quantitative analytical techniques have redefined the new understanding ofSOMdynamics and how this is affecting the development and application of new modelling approaches to soilC.