Globally, soil organic matter (SOM) contains more than three times as much carbon as either the atmosphere or terrestrial vegetation. Yet it remains largely unknown why some SOM persists for millennia whereas other SOM decomposes readily-and this limits our ability to predict how soils will respond to climate change. Recent analytical and experimental advances have demonstrated that molecular structure alone does not control SOM stability: in fact, environmental and biological controls predominate. Here we propose ways to include this understanding in a new generation of experiments and soil carbon models, thereby improving predictions of the SOM response to global warming.Understanding soil biogeochemistry is essential to the stewardship of ecosystem services provided by soils, such as soil fertility (for food, fibre and fuel production), water quality, resistance to erosion and climate mitigation through reduced feedbacks to climate change. Soils store at least three times as much carbon (in SOM) as is found in either the atmosphere or in living plants 1 . This major pool of organic carbon is sensitive to changes in climate or local environment, but how and on what timescale will it respond to such changes? The feedbacks between soil organic carbon and climate are not fully understood, so we are not fully able to answer these questions 2-7 , but we can explore them using numerical models of soil-organic-carbon cycling. We can not only simulate feedbacks between climate change and ecosystems, but also evaluate management options and analyse carbon sequestration and biofuel strategies. These models, however, rest on some assumptions that have been challenged and even disproved by recent research arising from new isotopic, spectroscopic and molecularmarker techniques and long-term field experiments.Here we describe how recent evidence has led to a framework for understanding SOM cycling, and we highlight new approaches that could lead us to a new generation of soil carbon models, which could better reflect observations and inform predictions and policies.

Abstract. This review highlights the ubiquity of black carbon (BC) produced by incomplete combustion of plant matehal and fossil fuels in peats, soils, and lacusthne and marine sediments. We examine vahous definitions and analytical approaches and seek to provide a common language. BC represents a continuum from partly charred matehal to graphite and soot particles, with no general agreement on clear-cut boundares. Formation of BC can occur in two fundamentally different ways. Volatiles recondense to highly graphitized soot-BC, whereas the solid residues form char-BC. Both forms of BC are relatively inert and are disthbuted globally by water and wind via fluvial and atmosphehc transport. We summarize, chronologically, the ubiquity of BC in soils and sediments since Devonian times, differentiating between BC from vegetation fires and from fossil fuel combustion. BC has important implications for various biological, geochemical and environmental processes. As examples, BC may represent a significant sink in the global carbon cycle, affect the Earth's radiative heat balance, be a useful tracer for Earth's fire history, build up a significant fraction of carbon buhed in soils and sediments, and carry organic pollutants. On land, BC seems to be abundant in dark-colored soils, affected by frequent vegetation burning and fossil fuel combustion, thus probably contributing to the highly stable aromatic components of soil organic matter. We discuss challenges for future research. Despite the great importance of BC, only limited progress has been made in calibrating analytical techniques. Progress in the quantification of BC is likely to come from systematic intercomparson using BCs from different sources and in different natural mathces. BC identification could benefit from isotopic and spectroscopic techniques applied at the bulk and molecular levels. The key to estimating BC stocks in soils and sediments is an understanding of the processes involved in BC degradation on a molecular level. A promising approach would be the combination of short-term laboratory expehments and long-term field thals.

[1] Black carbon (BC), the product of incomplete combustion of fossil fuels and biomass (called elemental carbon (EC) in atmospheric sciences), was quantified in 12 different materials by 17 laboratories from different disciplines, using seven different methods. The materials were divided into three classes: (1) potentially interfering materials, (2) laboratory-produced BC-rich materials, and (3) BC-containing environmental matrices (from soil, water, sediment, and atmosphere). This is the first comprehensive intercomparison of this type (multimethod, multilab, and multisample), focusing mainly on methods used for soil and sediment BC studies. Results for the potentially interfering materials (which by definition contained no fire-derived organic carbon) highlighted situations where individual methods may overestimate BC concentrations. Results for the BC-rich materials (one soot and two chars) showed that some of the methods identified

Traditionally, the selective preservation of certain recalcitrant organic compounds and the formation of recalcitrant humic substances have been regarded as an important mechanism for soil organic matter (SOM) stabilization. Based on a critical overview of available methods and on results from a cooperative research program, this paper evaluates how relevant recalcitrance is for the long-term stabilization of SOM or its fractions. Methodologically, recalcitrance is difficult to assess, since the persistence of certain SOM fractions or specific compounds may also be caused by other stabilization mechanisms, such as physical protection or chemical interactions with mineral surfaces. If only free particulate SOM obtained from density fractionation is considered, it rarely reaches ages exceeding 50 y. Older light particles have often been identified as charred plant residues or as fossil C. The degradability of the readily bioavailable dissolved or water-extractable OM fraction is often negatively correlated with its content in aromatic compounds, which therefore has been associated with recalcitrance. But in subsoils, dissolved organic matter aromaticity and biodegradability both are very low, indicating that other factors or compounds limit its degradation. Among the investigated specific compounds, lignin, lipids, and their derivatives have mean turnover times faster or similar as that of bulk SOM. Only a small fraction of the lignin inputs seems to persist in soils and is mainly found in the fine textural size fraction (<20 lm), indicating physico-chemical stabilization. Compound-specific analysis of 13 C : 12 C ratios of SOM pyrolysis products in soils with C3-C4 crop changes revealed no compounds with mean residence times of > 40-50 y, unless fossil C was present in substantial amounts, as at a site exposed to lignite inputs in the past. Here, turnover of pyrolysis products seemed to be much longer, even for those attributed to carbohydrates or proteins. Apparently, fossil C from lignite coal is also utilized by soil organisms, which is further evidenced by low 14 C concentrations in microbial phospholipid fatty acids from this site. Also, black C from charred plant materials was susceptible to microbial degradation in a short-term (60 d) and a long-term (2 y) incubation