Fig. 4. Nitrogen-15 cross polarization with magic angle spinning (CPMAS) 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, 17.5 h; CaHA, 21.6 h); rice–soybean (MHA, 24.3 h; CaHA, 22.7 h); Bicol Integrated Agricultural Research Center (BIARC) control (MHA, 21.9 h; CaHA, 25.3 h); and Long-Term Continuous Cropping Experiment (LTCCE) control (MHA, 19.1 h; CaHA, 21.9 h). A line broadening of 100 Hz was applied. Spinning sidebands can occur at positions marked 
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