Solid-state 13C NMR spectroscopy has been frequently employed to characterize organic matter. The most-commonly used technique is 13C CP/MAS. In this tech­nique, the magnetization from abundant ‘H is transferred to dilute 1 3C by cross polarization and then 13C magnetization is detected [20]. Its recycle delays depend on 7]H (‘H spin-lattice relaxation time), rather than T1C (13C spin-lattice relaxation time) [15]. Usually, T1H is much shorter than Tf. Therefore, this technique has the advantages of both enhancing sensitivity and shortening recycle delays. The combi­nation of these two factors substantially reduces the measurement time. Despite its advantages, CP has inherent shortcomings which render it unreliable for biochar characterization, especially for quantitative characterization. For CP/MAS, CP efficiency is reduced for nonprotonated carbons, mobile components, or regions having short proton rotating-frame spin-lattice relaxation time (7^) [16]. Note that biochars usually contain significant nonprotonated fused-ring aromatic carbons. Fused-ring aromatic carbons are far away from protons, cannot cross polarize well with ‘H, and are thus difficult to detect by CP. Also, the biochars produced via pyrolysis contain significant radicals which shorten [8]. These two problems

cause significant signal loss, especially for fused-ring aromatics in biochars. Compared with quantitative DP, 13C CP/MAS can only detect approximately 1/3 the aromaticity that ‘ 3C DP/MAS can detect [3]. Solid-state 13C CP/MAS with either regular CP or ramp CP cannot be used to quantify chars [15]. In extreme cases, 13C CP/MAS may not even be able to provide qualitative structural information of chars.