Soil aggregate isolation method affects measures of intra-aggregate extracellular enzyme activity

• Method of aggregate isolation affected measured extracellular enzyme activity.
• Wet-sieving aggregates increased potential absolute enzyme activity.
• Dry and optimal moisture aggregate fractions exhibited similar enzyme activities.
• Different aggregate isolation methods will address different ecological hypotheses.

Within soil aggregates, binding of organic matter is known to occlude it from microbial attack. Within aggregate fractions of different sizes, microbial communities and activities have also been shown to differ. As a result the soil physical structure, organic inputs and microbial activity together impact the rate at which organic matter is decomposed and stored within soil. However, methods developed for isolating soil aggregates may affect subsequent biological assays. In this study, we sought to understand how enzyme activity within soil aggregates is influenced by aggregate isolation methodology, including wet, dry, and ‘optimal moisture’ sieving procedures within two contrasting ecosystems (a corn agroecosystem and a 2-yr old, diverse planted tallgrass prairie). Mass distribution of aggregates from wet-sieving was skewed toward small macroaggregates (250–1000 μm) and microaggregates (<250 μm), but the distribution of dry and optimal moisture aggregates was highly skewed toward large macroaggregates (>2000 μm). Wet-sieved macroaggregates (>1000 μm) had greater aggregate potential enzyme activity (nmol substrate h−1 g−1 dry aggregate) than smaller aggregate fractions and whole soil, particularly for C-cycling enzymes cellobiohydrolase and β-glucosidase. Also, wet-sieved aggregates from corn systems had higher potential cellobiohydrolase and β-glucosidase activity than aggregates isolated from prairie. Neither of these relationships was observed in dry and optimal moisture aggregates, suggesting that elevated activities are characteristic of water-stable aggregates and possibly stimulated by soil rewetting. The proportional contribution to total enzyme activity observed in water-stable microaggregates accounted for 46–62% of whole soil activity; although water-stable large macroaggregates (>2000 μm) had greater aggregate enzyme activity, they contributed a minority of overall soil activity. In contrast, the proportional contribution of large macroaggregates comprised 70–78% of whole soil activity when dry sieved and 38–66% under optimal moisture sieving. Wet-sieving soil aggregates is most useful to examine long-term changes in soil organic matter and microbial activity between soil types. Optimal moisture and dry sieved aggregates may be useful alternatives to more closely capture short-term in situ measures of seasonal and intra-annual soil microbial activity.