Freezing-thawing and acidification are common in nothern soils and have an significant impact in the Humic Substances(HS). The investigation of the changes in HS caused by both factors are of considerable theoretical and practical interest. The study of these changes requires appropriate technique for the preparation of soil humic material. We proposed the cation-exchange resins procedure for the isolation of the HS fractions to study their dfferent response to the freezing and acidification.

The method included three sequential exhausted extractions:1) isolation of the HS-1 fraction by sulfo-resin in Na+ form in distilled water; 2) extraction by carboxylic-resin under the same conditions (HS-2 fraction); 3) extraction by 0.1M NaOH (HS-3) fraction. Conventional emission and excitation fluorscent spectra as well as synchronous-scan excitation were recorded for the qualitative characterization of humic fraction isolated.

This technique was tested in the long-term laboratory experiment dealing with the impact of freezing, acidification and their joint effect on HS. Soil samples were collected from the upper layers of the Tundra soil, Podzolic soil and Gray Forest Soil. The following treatments have been performed during 2 months: 1) freezing at the temperature -12C; 2) acidification to reduce soil pH by 1 pH unit and storing in the refrigerator at +9C; 3) acidifcation followed by freezing at the -12C. The dependence of the changes in the HS recovery, fraction composition and fluorescent characteristics on the different treatments and soil type has been discussed. The result could be useful for the monitoring and prediction of the soil HS behaviour.

Dr. Ludmila T. Shirshova
Institute of Soil Science and Photosynthesis RAS
142292 Pushchino, Moscow Region
Russia
Tel: +095- 923- 35- 58
FAX: +7- 0967- 79- 05 -32
E-mail: sotnikov@issp.serpukhov.su or
        lppolu@dapsas1.weizmann.ac.il