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.

Factors controlling the transport of mobile colloids through soils are poorly understood yet have major environmental impacts. This study attempted to identify the relative significance of two physical factors controlling movement of different illitic clays through columns of various stable, nonreactive soils:  (i) pore size distribution of the soil matrix and (ii) average size of mobile colloids. The columns were leached with various clay suspensions, and colloid concentrations were measured in relation to effluent solution chemistry. No single factor dominated colloid mobility through the soils tested:  pore size distribution exerted some control, but so too did size of the mobile colloids (partly affected by cation status). The most mobile colloids in low-charge soils may simply be fine enough to escape physical filtration by the soil matrix. This applies mainly to near-saturated soils, because as soils dry, greater salt concentrations and particle interactions increase mobile-particle sizes and thus increase the degree of possible physical filtration.