Soil organic matter dynamics: a biological perspective derived from the use of compound-specific isotopes studies

Current attempts to explain the persistence of carbon in soils focuses on explanations such as the recalcitrant plant residues and the physical isolation of substrates from decomposers. A pool of organic matter that can persist for centuries to millennia is hypothesized because of the evidence provided by the persistence of pre-disturbance C in fallow or vegetation change experiments, and the radiocarbon age of soil carbon. However, new information, which became available through advances in the ability to measure the isotope signatures of specific compounds, favors a new picture of organic matter dynamics. Instead of persistence of plant-derived residues like lignin in the soil, the majority of mineral soil is in molecules derived from microbial synthesis. Carbon recycled multiple times through the microbial community can be old, decoupling the radiocarbon age of C atoms from the chemical or biological lability of the molecules they comprise. In consequence is soil microbiology, a major control on soil carbon dynamics, which highlights the potential vulnerability of soil organic matter to changing environmental conditions. Moreover, it emphasizes the need to devise new management options to restore, increase, and secure this valuable resource. - Trends in isotope ecology Soils are the most important interfaces for life on earth. They provide the nutrients and water for plant growth, which in turn is the basis for all heterotrophic life on earth, including humans. Plants are also an important store of carbon, fixing carbon dioxide from the atmosphere and counteracting the human impact on climate change (Friedlingstein et al. 2006). However, human impacts on factors such as land use, which is considered to be the most import human impact on earth, and climate change, decreases the ability of soils to grow plants and to sequester carbon (Canadell et al. 2007; Lobell et al. 2011; van der Molen et al. 2011). The biggest climatic impacts on soils include extreme climate events (Jentsch et al. 2007; Garcia-Herrera et al. 2010), desertification (Lal 2010; Ravi et al. 2011) and soil erosion (Poesen and Hooke 1997; Nearing et al. 2004; Lal et al. 2011). Our knowledge on the reactivity of the fragile surface of our planet, however, is still very limited and soils remain the largest single uncertainty in the global carbon cycle (Canadell et al. 2007). Key to this uncertainty is a lack of basic understanding of the processes involved in stabilization and destabilization of the detrital organic matter added to soils, and how these are influenced by environmental parameters. This paper will briefly summarize how our current understanding of soil organic matter dynamics is evolving, especially through the advent of new results from compound-specific 13C and 14C measurements that can trace specific sources or processes. Overall, a new paradigm is emerging that soil organic matter dynamics and stocks are primarily under biological control, with implications for management of the valuable soil resource. Organic matter, the major product of life, is mainly made from six chemical elements: carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus. Different types of organisms assimilate these elements in characteristic and distinct stoichiometric ratios, depending on the chemistry of structural components, like cell walls and tissues. The extracellular skeleton of plants for example is made from lingo-cellulose type material that mainly contains carbon, oxygen, and hydrogen. The low nitrogen content of lingo-cellulose widens the C/N ratio to characteristic values between 15 and 300 in higher plants. In contrast, microorganisms have a much narrower C/N ratio, i.e., between 5 and 12, as their cell walls are made from nitrogen containing mucopolysaccharides. Element ratios of organic matter are widely used to track the origin of organic matter (Martin and Haider 1971; Turchenek and Oades 1979; Guggenberger et al. 1999; Gleixner et al. 2001; Kogel-Knabner 2002). Carbon, the backbone element of all organic matter, takes several forms in the Earth System. The most oxidized (CO2) and reduced (CH4) forms of carbon are important greenhouse gases that have relatively short lifetimes once they are in the atmosphere (<102 years). For example, the amplitude of the seasonal cycle of atmospheric CO2 indicates that the average CO2 molecule is cycled about once every 6 years through the terrestrial biosphere. The dissolved forms of carbon, mainly dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC), and solid forms of carbon, mainly carbonates and organic carbon in rocks including oil and coal, form larger overall reservoirs in the oceans (104 GtC) and sedimentary rocks (106 GtC) that influence atmospheric CO2 on timescales of hundreds to millions of years, respectively. The terrestrial carbon cycle contains carbon primarily in the form of organic matter, although there is also an inorganic component of soil carbonates. The living terrestrial biosphere contains roughly the same (620 GtC) amount of C as the atmosphere (Fig. 1). Organic matter in soils, made up of dead and decomposing plant tissues as well as the living microbial decomposer community and its residues, contain more than twice as much carbon summing up to about 1,580 GtC (Gleixner et al. 2001). The combination of its relative large pool size in the terrestrial carbon cycle and its relatively fast response brings soil organic matter into the research focus (Amundson 2001; Sugden et al. 2004; Lal 2010). In order to predict the response of these large amounts of potentially reactive carbon to climate change, it is essential to understand the formation, decomposition, and storage of carbon in soils. Plants use atmospheric CO2 to synthesize the structural tissues that form the majority of organic matter in terrestrial ecosystems (Fig. 1). This organic matter is the basis for the formation of soil organic matter. Plants release litter from roots and leaves into and onto the soil. In addition they exude sugars, organic acids, and other low molecular weight compounds into the rhizosphere. High correlations observed at the global scale for carbon stocks in mineral soils with ecosystem net primary productivity (NPP) suggest that plant-derived inputs are driving the soil organic matter formation in Fig. 1 Terrestrial carbon cycle boreal, humid, and tropical forests (Fig. 2). Similar correlations, observed in regional studies from different forest stands in Oregon, also support this general relationship (Sun et al. 2004). However, the direct comparison of the vertical distribution of roots and soil carbon in soil depth profiles from a global dataset (Jobbagy and Jackson 2000) indicates that inputs of new plant materials alone cannot completely explain soil organic matter stocks. Both root and soil carbon distributions are highly correlated and decline exponentially with depth (Fig. 3), but overall declines are steeper than those (...truncated)