A review of the environmental behavior and effects of black carbon in soils and sediments

Abstract

Soils and sediments are major global sinks of black carbon (BC). BC in soils and sediments has complex environmental behaviors and effects. Because of its ability to resist decomposition, BC captures and sequesters carbon from the bio-atmospheric cycle, which, in the long term, reduces greenhouse gas emissions and thus mitigates the greenhouse effect. Meanwhile, resistant BC adds organic matter to soils, thus enhancing soil fertility. Because of its high adsorption capacity, BC adsorbs organic pollutants, heavy metals, and nutrients thus decreasing pollution risks and further enhancing soil fertility. Based on analysis of the concept of BC, it is concluded that BC should be studied in the context of its environmental effects. Variation in the formation of BC leads to its varied properties and classification. Several frequently used methods for identifying sources of BC are summarized. Δ14 C and stratigraphic analysis can be used to identify BC sources (biomass, fossil fuel or rock); δ13 C analysis can distinguish between C3 and C4 plant sources, and fresh water and ocean organisms; particle size analysis can identify local and exotic sources; while analysis of the ratio of polycyclic aromatic hydrocarbon (PAH) isomers, the ratio of BC to total organic carbon, the ratio of benzene polycarboxylic acids, and morphological properties can identify biomass and fossil fuel sources. Transfers of BC between soils/sediments and other pools as well as those within soil and sediment pools are discussed. Global BC reserves in pools and the fluxes between pools are estimated, and it is noted that dissolved BC eroded from soils and transferred by rivers is an important part of the BC global cycle. Decomposition and stability of BC in soils and sediments are discussed in relation to fluxes of greenhouse gases between the land surface and atmosphere, the stable carbon pool in soils, and carbon cycle models in soil. Especially, coupled mineralization between BC and native or added labile organic carbon in soil is introduced, and adsorption and related mechanisms of organic matter, heavy metal and nutrient retention by BC in soils and sediments are also summarized. Three adsorption mechanisms of PAHs by BC are described: adsorption on surfaces, capture in micropores, and occlusion inside the BC structure. Correlations between BC and PAH concentrations should not be simply explained as a result of adsorption, since environmental adsorption of PAHs by BC is affected by complex factors. Finally, some future directions in BC research are recommended. More fundamental surveys of BC concentrations and its distribution in different kinds of soils/ sediments in different areas are needed. More studies should concentrate on key processes related to the stability of BC in soils/ sediments such as coupled mineralization between BC and labile organic carbon, losses of particulate and dissolved soil BC, and transformations between BC and other forms of stable carbon in soil. More studies are also needed on BC's specific surface area, porosity, functional groups, and surface morphology, and their roles in the environmental behavior of BC, with a change from concentration measurements or correlation analysis to mechanistic analysis. More application studies should focus on the possibility of applying manufactured BC as part of agricultural management.