ABSTRACT
Soil organic matter (SOM) represents the largest terrestrial carbon (C) pool being almost three times that present in plant and animal biomass. SOM is not a uniform substance, but occurs in a myriad of di¦erent molecular forms, whose chemical properties and interactions with the abiotic mineral matrix allow them to be placed in conceptual pools. Knowledge of the size and ¥uxes of C from these SOM pools is indispensable to predict C turnover in soils as a function of environmental changes. ¤e amount of C stored in a soil is determined by a dynamic equilibrium between C inputs from primary biomass production and C outputs mainly by mineralization, but also from leaching and erosion. Understanding the controls on this equilibrium is an important scientic issue for two reasons:
Feedback: Large quantities of carbon are stored in SOM, and release of this carbon into the atmosphere as CO2 or methane would have a serious impact on global climate. ¤ere are rst reports that, as the earth warms, SOM is being oxidized at an increasing rate (Bellamy et al., 2005; Powlson, 2005; Davidson and Janssens, 2006; von Lützow and Kögel-Knabner, 2009). Quantifying these carbon cycle-climate feedbacks is di¬cult because of the limited understanding of the processes by which carbon and associated nutrients are transformed or recycled
within ecosystems, in particular within soils, and exchanged with the overlying atmosphere (Heimann and Reichstein, 2008). Below-ground processes in particular are still poorly understood, yet provide a number of potentially important feedbacks in the carbon-cycle-climate system (Sche¦er et al., 2006; Torn and Harte, 2006). It follows that better predictions of the magnitude of soil carbon feedback reactions require an improved mechanistic understanding of processes that protect SOM against decomposition.
