|Olk, Daniel - Dan|
Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/27/2012
Publication Date: 7/27/2012
Citation: Mao, J., Johnson, R.L., Lehmann, J., Olk, D.C., Neves, E.G., Thompson, M.L., Schmidt-Rohr, K. 2012. Abundant and stable char residues in soils: Implications for soil fertility and carbon sequestration. Environmental Science and Technology. 46(17):9571-9576. Interpretive Summary: Carbon contributes to several favorable soil properties, including nutrient retention and aggregation. Charcoal-like material is attracting increasing attention recently as a form of stable carbon in many soils. However, little is known about the internal structure of charcoal-like material, which would be helpful information for understanding its activity and persistence in soil. Working with Brazilian soils that had received charcoal-like material through human activity for centuries, we discovered that charcoal-like material remains extremely abundant in these soils, and it is made of small-sized structures having high amounts of negative charges. This result agrees with our earlier work on Iowa prairie soils. Together, these results suggest that charcoal-like material might have this small-sized structure in many soils and that its high amount of negative charges might contribute substantially to soil behavior. This information will help researchers understand the activity and stability of carbon in soil and determine the benefit of adding charcoal-like materials to soil during agricultural production.
Technical Abstract: Large-scale application of biochar to soil has been proposed as a method to sequester carbon from the atmosphere while enhancing soil fertility, particularly in the tropics. Bringing this approach to fruition would greatly benefit from understanding which char structures are stable and contribute to soil fertility. Terra Preta soils, anthropogenic dark earths in Amazonia enriched with char >800 years ago, contain stable char residues, but nuanced knowledge of Terra Preta char residues has been lacking. Aromaticity values estimated by EXAFS and nonquantitative cross-polarization NMR have suggested a moderate char fraction (~25-50% of organic carbon). Here, using quantitative 13C NMR, we show instead that most (87 ± 4%) of the organic carbon in the deeper horizons of two Terra Preta soils consists of oxidized char residues, with fused aromatic rings decorated by COO groups. We also estimate the size of the aromatic clusters by two NMR approaches. We find that the Terra Preta char residues are similar to those in fertile, temperate-zone prairie soils, where char was generated by recurring pre-settlement prairie fires, suggesting that oxidized char residues containing ~5 aromatic rings and 5 carboxyl groups represent a particularly stable form of carbon in the soil environment.