Saturday, 15 July 2006

Some Unique Characteristics of Humic Acid in the Vertisols of Central China.

Shanmei Wu, Nanjing Agricultural Univ/Univ of California, Berkeley, CA 94705, Xirong Gao, Nanjing Agricultural Univ, Nanjing, 210095, China, Changqing Lu, Institute of Soil Science, Nanjing, 210000, China, and Qing Xu, Cotra Costa College, San Pablo Ave, San Pablo, CA 94806.

Three representative Vertisols in the Shandong, Henan, and Hubei provinces were selected for the study. The clay content (<0.002mm) was 48.34 55.36% dominant in montmorillonite. The COLE was 0.14 0.17. Soil pH was 6.51 7.78. Soil organic carbon (g/kg) was from 6.6 9.8 in A horizons and from 9.211.9 in subsoil horizons (Bss). The CEC was between 30.34 and 44.44 cmol(+)/kg. Humus in A and or Bss horizons were characterized by chemical, elemental analysis, and infrared spectrophotometer methods. Pollen analysis was also done in the study. Results showed that the humic acid was dominate in the humus extracted. The humic acid-fulvic acid ratio (HA/FA) averaged 1.17(0.26) in A layer and 2.02 (0.22) in Bss horizon, which was close to the Mollisols in NE China. Humin averaged 20.70% in the organic matter, and some unextratable humus were also present. Results from the visible spectrum showed that the E4 (m) was 2.53(0.13) and 3.17(0.32) in A horizon and Bss horizon, respectively. The latter was higher than that in the mollisols. The E4/E6 was between 4.48 (0.11) and 4.60 (0.16). The data above implied that the degree of aromaticity of the humic acid was very high, especially in the Bss horizon.

Further studies of Bss horizons by the elemental analysis, we found that the average content(ash free) of H in the humic acid was only 2.21% which was lower than the weathered coal (2.24-3.14 %) and the C was high at 60.55%, higher than that in the the weathered coal(55.95-60.21%) The C/H atomic ratio was high between 2.24 and 2.38. The latter one might be the highest value being found among other soils, and also higher than that in the weathered coal(1.55-2.32%). The data implied that the degree of armaticity of the humic acid was very high. The O% averaged high at 35.75 (1.19), and N % was low from 2.30 to 3.70 depending on the depth of the soil samples collected. The layer near the A horizon possessed higher value of atomic N %. Therefore, the C/N atomic ratio was high between 18 and 30 as compared to some published data which was between 13.2 and 17.4. The results indicated the humid acid had the structural characteristics of condensed aromatic rings and poor in nitrogen content.

The infrared spectrum showed the structural composition of the humus. Results showed that phenolic hydroxyl groups, hydroxyl group, conjugated double bond of aromatic family (C=C), and free carboxyl group were the strongest bands in the humic acid. The humin had stronger absorption band of CH and CH3 in aliphatic group than that found in humic acid. The absorption bands of montmorillonite at 1050-1030 and 880-875 were detected in the humic acid and humin. The bands were stronger in humin than those in the humid acid. The finding proved that the presence of humus-montmorillonite complex in the Vertisols.

Pollen analysis found that Ceratopteris, Sphagnum, Polypodiaceae, Pinus and Artemisia were present in the Bss samples. It denoted that the paleo-waterlogged and meadow-swampy environment existed where organic matter accumulated and humified prior to the formation of the Vertisols. The humus further condensed as the land lifted, water reduced, and grew the terrestrial vegetations.


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