Soil organic matter turnover is governed by accessibility not recalcitrance

Abstract

Mechanisms to mitigate global climate change by sequestering carbon (C) in different 'sinks' have been proposed as at least temporary measures. Of the major global C pools, terrestrial ecosystems hold the potential to capture and store substantially increased volumes of C in 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 of SOM stored in soils now, with subsequent quantification of how this is affected by management strategies intended to increase SOM concentrations, and used in soil C models for the prediction of the roles of soils in future climate change. An apparently obvious method to increase C stocks in soils is to augment the soil C pools with the longest mean residence times (MRT). Computer simulation models of soil C dynamics, e.g. RothC and Century, partition these refractory constituents into slow and passive pools with MRTs of centuries to millennia. This partitioning is assumed to reflect: (i) the average biomolecular properties of SOM in the pools with reference to their source in plant litter, (ii) the accessibility of the SOM to 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 of SOM dynamics and how this is affecting the development and application of new modelling approaches to soil C.

Microbial community structure mediates response of soil C decomposition to litter addition and warming

“…The importance of physical protection to the stabilization of soil organic matter (SOM) has been understood for decades (Sollins et al, 1996;Krull et al, 2003;Dungait et al, 2012), as has the importance of plant litter chemistry upon the extent of its decay (Melillo et al, 1982;Taylor et al, 1989). However, the role microbial community structure and function plays in soil and litter turnover, and its response to elevated temperatures and N availability, is less understood.…”

Species and Genotype Effects of Bioenergy Crops on Root Production, Carbon and Nitrogen in Temperate Agricultural Soil

“…This estimate of 38-109 kg N 2 O ha −1 year −1 is far greater than current estimates of emissions under both arable (0.9-11.0 kg N 2 O ha −1 year −1 ) and grassland agriculture (1.6-22.0 kg N 2 O ha −1 year −1 ) in the UK [59]. Sequestration in the subsoil may be especially important as such SOC may become protected against further processing [60].…”

Effects of human land use on the terrestrial and aquatic sources of fluvial organic matter in a temperate river basin (The Meuse River, Belgium)

“…Several soil management practices such as tillage or organic fertilizer application are known to stimulate the microbial decomposition of SOM (Luo et al 2010;McLauchlan 2006;Pisani et al 2016). Beyond their role as decomposers, microorganisms are a ubiquitous source of OM to soils through recycling of plant and animal-derived OM and synthesis of new products (Dungait et al 2012;Gleixner 2013;Miltner et al 2012). Consequently, SOM decomposition leads to an enrichment in microbial compounds in residual SOM at the expense of plant-derived compounds (Hobara et al 2014;Kaiser and Kalbitz 2012;Malik and Gleixner 2013).…”