INFLUENCE OF SALINE-SODIC SOIL SALINITY ON THE STRUCTURAL CHARACTERISTICS OF HUMIC ACID IN SOIL AGGREGATES

This study aimed to investigate the effect of the salinity of saline-sodic soils on the structural characteristics of humic acid (HA) in soil aggregates of various grain sizes. Saline-sodic soils in the Songnen Plain with light, moderate and high salinity, referred to as Light Saline(LS), Middle Saline(MS), and Heavy Saline(HS) soils, respectively, were collected and fractionated into > 2 mm, 2-0.25 mm, 0.25-0.053 mm and < 0.053 mm grain-size fractions by wet sieving. HA was extracted and qualitatively characterized by a variety of means, including infrared (IR) spectra, fluorescence spectra and thermodynamic properties. The results showed that (1) the content of the water-stable aggregate of < 0.25 mm grain size (micro-aggregate) was greater than 75% in each soil sample, regardless of soil salinity; (2) total organic carbon (TOC) content was higher in the < 0.053 mm grain-size fraction than in any other grainsize fraction; (3) the ratio of the summed intensities of HA IR peaks at 2920 cm(-1) and 2850 cm(-1) to the intensity of the HA peak at 1720 cm(-1) and at 1620 cm(-1), which were respectively referred to as (2920+2850)/1720 and (2920+2850)/1620 for simplicity, decreased in the order of Light Saline(LS) > Middle Saline(MS) > Heavy Saline(HS); (4) the intensity of fluorescence peaks D, C and A decreased in the order of LS > MS > HS, and peak D was the highest of the three HA peaks for each grain-size fraction of LS soils; (5) the high-temperature to moderate-temperature heat loss ratio of HA extracted from different saline-sodic soils decreased in the order of HS > MS > LS; and (6) for soil micro-aggregates (< 0.25 mm), the high-temperature to moderate-temperature heat release ratio, the high-temperature heat release, the high-temperature weight loss, and the high-temperature to moderate-temperature weight loss ratio all decreased in the order of HS > MS > LS. The findings provide basic data for the improvement and utilization of saline-sodic soils in the Songnen Plain.