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[AGE05-03] How important is soil aggregation in regulating the activity of enzymes involved in the depolymerization of soil organic nitrogen?
Keywords:Soil aggregate, Extracellular enzyme, Nitrogen minralization, Depolymerization
In the present study two different soils from grassland (GL) and arable land (AL) respectively were air-dried and sieved for isolation of three fractions (4.75-2mm, 2-0.25mm and 0.25-0.063mm) of soil aggregates. For aggregate soil samples, three fractions were mixed based on the following proportion: 24g of 4.75-2 mm, 24g of 2-0.25 mm and 6g of 0.25-0.063 mm fraction. Corresponding de-aggregated soil samples were prepared by physical disruption with a pestle and mortar. To quantify N mineralization rate, anaerobic incubations of the soils at 26 were conducted for 10 days. Using the soils harvested from the aggregated and de-aggregated samples, potential protease and chitinase activities were determined using colorimetric assays.
The first hypothesis was supported for both land uses as N mineralization rates were significantly higher in de-aggregated soils than in aggregated ones (P < 0.05). The second hypothesis was supported in the case of the GL soil: significant correlations (P < 0.05; r > 0.89) between N mineralization rate and protease and chitinase activities were detected in de-aggregated soil whereas correlations for aggregated soils were weaker and not significant (P > 0.05; r >0.7). For AL soils, only chitinase activity was reliably above the limit of detection of the assay. There were no significant correlations between the chitinase activity and N mineralization rate for both treatments, however, there was a positive correlation (r > 0.7) for de-aggregated soils.
The stronger correlations for the GL de-aggregated soil indicated that soil proteases and chitinases had access to, and depolymerized, organic polymer N which was physically protected before de-aggregation. We concluded that the difference between two land uses could be attributed to tillage effects on the relative location of extracellular enzymes and their substrates.