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Title: Evolution of black carbon properties in soil

item PIGNATELLO, JOSEPH - Agricultural Experiment Station, Connecticut
item Uchimiya, Sophie
item ABIVEN, SAMUEL - University Of Zurich
item SCHMIDT, MICHAEL - University Of Zurich

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 3/10/2014
Publication Date: 2/19/2015
Citation: Pignatello, J.J.; Uchimiya, M.; Abiven, S.; Schmidt, M.W.I. 2015. Evolution of black carbon properties in soil. In: Lehmann, J. and Joseph, S., editors. Biochar for Environmental Management: Science, Technology, and Implementation. 2nd Edition. London:Taylor & Frances. pp. 195-234.

Interpretive Summary: Biochar is produced from agricultural wastes under reduced-oxygen conditions at 250-800 °C. Once applied to soil, biochar undergoes a series of physical and chemical changes that can be collectively called aging. Physical (size) change will influence the transport of biochar particles. Biochar will aggregate with a range of soil components including minerals and non-pyrogenic organic matter. Aggregation will change the stability of biochar towards chemical and microbial degradation pathways. Aging will also influence the reactivity of biochar to uptake ions and molecules. This chapter will cover fundamental causes of biochar aging, and influence of biochar aging on its reactivity. Such mechanistic understandings of biochar aging is necessary to predict the long-term agronomic benefits of biochar soil amendment.

Technical Abstract: Black carbon deposited in soil from natural or deliberate wildfires and engineered black carbon products (biochar) intentionally added to soil are known to have significant effects on soil biogeochemical processes and in many cases to influence the yield and quality of crops and to enhance the ability of soils to retain chemical contaminants. The physical-chemical properties of the raw BC materials vary greatly depending on source material and the conditions under which they were formed or made. Once in contact with soil, however, BC undergoes changes in physical-chemical properties over time. Any consideration of their behavior and effects in soil must take into account these changes. That is the aim of this chapter. It will address the peer-reviewed literature as of early 2013. Coverage of physical changes will include fragmentation to smaller particles, aggregation with other soil particles (heteroaggregation), deposition of minerals and non-pyrogenic natural organic matter (NOM), and availability of surfaces and pores to solutes. Fragmentation as a result of physical or biological forces is important in regard to the translocation of BC particles in the soil profile. Heteroaggregation and deposition of mineral phases on the surface can affect BC translocation, stability towards decomposition, and surface activity. Deposition of nonpyrogenic NOM on BC strongly influences surface chemistry and pore availability, which, as we shall see, has important implications for the adsorption of ions and molecules. Coverage of chemical properties will include weathering effects on polyaromatic ring size, selective loss of certain components, concentration of polar functional groups, acidity, surface charge as a function of pH, and reactivity as an adsorbent towards metal ions, dissolved NOM (DNOM), gases and small organic compounds. Incorporation of O through abiotic or enzymatic reactions influences many critical surface chemical attributes, such as polarity, acidity, cation exchange capacity (CEC) and affinity for water and solutes. Issues regarding mineralization of BC and the lifetime of BC in soil are more appropriately considered in other chapters. Claimed attributes of biochar include its ability to retain plant fertilizers and to reduce the bioavailability of organic and inorganic contaminants. Thus, the adsorbent properties of biochar and black carbon towards metal ions and organic compounds are critical to its function in soil and will be covered in depth. The rate and extent of metal sorption strongly depend on biochar-induced changes in solution chemistry. When appropriate conditions are met, certain metal ions strongly bind on biochar by inner-sphere process, and that can lead to surface precipitation of metal nano-crystals that can induce heteroaggregation. Black carbon in general is reputed to be a powerful adsorbent of organic compounds due to its hydrophobic and nanoporous character. The ramifications of combining biochar and soil pesticide applications are covered in other chapter. Biochar amendment has been investigated as a means of stabilizing contaminated soil. Biochar may interfere with chemical communication by adsorbing natural signaling compounds, a result that could be beneficial or not, depending on the organisms and crop involved. Biochar also has appreciable CEC. The charged sites attract metal cations and serve as points of growth of oxide and mixed oxide mineral phases. Incontrovertible evidence exists that weathering of BC in soil alters its adsorbent properties toward organic and inorganic species. Also included in this chapter is a discussion of the inherent reversibility of sorption to BC-a relevant topic in its own right in any discussion of aging, irrespective of weathering effects on the adsorbent.