Fig. 1. Carbon-13 cross polarization and magic angle spinning (CPMAS) with total suppression of sidebands (TOSS) nuclear magnetic resonance (NMR) spectra of mobile humic acids (MHA) and calcium humates (CaHA) from four soils varying in intensity of irrigated rice cropping: maize (MHA, 2.4 h; CaHA, 3.1 h); rice–soybean (MHA, 2.4 h; CaHA, 2.2 h); Bicol Integrated Agricultural Research Center (BIARC) control (MHA, 2.6 h; CaHA, 6.9 h); and Long-Term Continuous Cropping Experiment (LTCCE) control (MHA, 0.9 h; CaHA, 2.4 h).
