Characterization of two humic acid fractions and their contribution to soil nitrogen supplying capacity of tropical lowland rice soils.
1996
Nguyen Bao Ve
The MHA [mobile humic acids] and CaHA [calcium humates] were extracted from 14 tropical lowland rice soils for chemical characterization and N mineralization studies under anaerobic and aerobic conditions. For all soils, the MHA had a low C concentration, lower C/N ratio, and lower optical density at a wavelength of 465 nm (E sub 465) than CaHA. These parameters can be equated with the degree of humification of humic acids [HA]. Humification of MHA and CaHA varied greatly among soils and it was greatest in soils where the HA had accumulated. Nitrogen mineralization was 2-5 times greater per unit N for the MHA than CaHA and varied greatly among soils extracted for HA. Nitrogen mineralized from MHA and CaHA was negatively correlated with their C and N content per unit soil, Esub465, and C/N ratio in both anaerobic and aerobic incubations. The relatively poor prediction of potential soil N mineralization by SOC [soil organic C] and TN [total N] was improved by considering the C and N per unit soil, E sub 465, and C/N ratio of MHA and CaHA. The MHA properties were better predictors of potential soil N mineralization than CaHA properties. Whether MHA properties together with SOC and TN would constitute a reliable indicator of plant N uptake needs to be tested under greenhouse and field condition.
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