Fig. 1. Cross-polarization magic angle-spinning total sideband suppression ¹³C NMR spectra of humic acids in a Norfolk soil under different tillages: (1A) conservation tillage treatment (CnT1, 0–5 cm; CnT2, 5–10 cm; and CnT3, 10–15 cm); (1B) conventional tillage treatment (CT1, 0–5 cm; CT2, 5–10 cm; and CT3, 10–15 cm).