Objective  Straw and biochar returning significantly affects soil enzyme activities, and changes in the microdomain environment of soil aggregates at different sizes may be attenuated the intensity of enzyme response, so it is necessary to explore the response of enzyme activities within soil aggregates to straw and biochar returning.

  Method  This study investigated the effects of annual straw returning (SR, 5 t hm⁻²), single application of biochar (BR, 30 t hm⁻²) and combined application of biochar and straw (BS, 5 t hm⁻² + 30 t hm⁻²), on enzyme activities related to soil carbon, nitrogen and phosphorus cycling, which was based on a field location trial for five years in black soil.

  Result  Compared with CK, SR significantly increased polyphenol oxidase and β-1,4-glucosidase activities in all particle sizes with an average increase of 33.4% and 25.6%, while BR significantly increased polyphenol oxidase activities in particles of > 2 mm and < 0.25 mm and β-1,4-glucosidase activities in particle of 0.25 - 2 mm, with an average increase of 30.2%, 67.4% and 44.4%. In terms of nitrogen cycle-related enzymes, BR, SR and BS treatments significantly increased nitrous oxide reductase activities in > 2 mm and < 0.25 mm particle size, nitrate reductase, nitrite reductase and nitric oxide reductase activities in 0.25 - 2 mm particle size. SR significantly increased N-acetyl-β-D aminoglucosidase activities in < 0.25 mm particle size by 35.5%, while BR significantly increased N-acetyl-β-D-aminoglucosidase activities in 0.25 - 2 mm particle size by 42.0%.

  Conclusion  The geometric mean of 16 enzyme activities (GMea) was used as an indicator of the combined enzyme activities, and it was found that straw return significantly increased GMea in soil agglomerates of all particle sizes under the present experimental conditions, while biochar significantly increased GMea in micro-aggregates but decreased GMea in macro-aggregates. Therefore, from the perspective of soil enzyme activities, straw return is more effective in promoting soil bio-fertility in northeastern black soils of China, straw return to the soil is more likely to promote the improvement of soil biological fertility.