Abundant and stable char residues in soils: Implications for soil fertility and carbon sequestration

Abstract

Large-scale soil application of biochar may enhance soil fertility, increasing crop production while also sequestering atmospheric carbon. But reaching these outcomes requires an understanding of the relationships among biochar structure, stability, and contribution to soil fertility. Using advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy, we show that the char residues in terra preta soils are composed of ~5 fused aromatic rings substituted by COO- groups that significantly increase the soils' cation-exchange capacity and thus the retention of plant nutrients. We also show that in highly productive, grassland-derived US soils, char generated by presettlement fires is structurally comparable to that of the terra preta soils. These oxidized char residues represent a particularly stable, abundant, and fertility-enhancing form of soil organic matter.