Distribution pattern of amidohydrolase activities among soil aggregates: Effect of soil aggregates isolation methods

The effect of aggregate isolation methods on distribution patterns of N-cycling enzymes within soil aggregate is not well understood. In this study, the effects of wet and dry sieving methods on organic C (OC) content and amidohydrolase activities (Urease, l-glutaminase and l-asparaginase) were determined for six aggregate sizes (4-2, 2-1, 1-0.5, 0.5-0.25, 0.25-0.05 and <0.05 mm) isolated from five grassland soils. The distribution of wet-sieved aggregates was skewed toward microaggregates (0.25-0.05 mm) and silt and clay fractions (<0.05 mm), while the contrary was found with dry sieving. Wet-sieved macroaggregates (>0.25 mm) had higher OC content and potential amidohydrolase activities than other aggregate sizes but not a specific size of macroaggregates was consistently higher in all soils studied. No significant differences in OC and amidohydrolase activity were also observed between dry-sieved macroaggregates. Although dry-sieved 0.25-0.05 mm fraction had generally higher amidohydrolase activities than the other aggregate sizes, the distribution pattern of urease (URE) activity within dry-sieved aggregates was different among soils studied. Unlike the dry sieving method, microaggregates and silt and clay fractions separated by wet sieving had a major contribution to the total OC content and amidohydrolase activities in all soils. Both sieving methods altered the amidohydrolase activities, causing either losses or even increases depending on the soil and the enzyme studied. The significant difference between grassland soils in terms of OC content and enzyme activity was observed in wet- and dry-sieved aggregates, although it was more pronounced in wet-sieved large macroaggregates. Overall, sieving methods resulted in different OC content and amidohydrolase activities in soil aggregates, however, wet sieving showed greater ability to reveal significant differences in terms of aggregate potential enzyme activity compared to dry sieving. Wet sieving was also most capable to examine long-term changes in organic matter and enzyme activity between soil types.