Fig. 2. The relationship between the amount of humic acid N mineralized and the amount of N contained in the humic acid fraction per kg extraction soil for the mobile humic acid (MHA) fraction and the calcium humate (CaHA) fraction that were extracted from seven tropical lowland rice soils and incubated in the (a) Tanhoi and (b) IRRI soils under anaerobic conditions at 30°C for 3 and 6 wk. * and ** indicate significance at 5 and 1% levels, respectively.