Objectives To provide a scientific basis for improving soil quality, we investigated the effects of biochar (BC) on microbial biomass carbon and nitrogen contents and extracellular enzyme activities within soil aggregates, and analyzed the main factors that drive changes in enzyme activities within aggregates.

Methods A 2-year field experiment was conducted in Henan Modern Agricultural Research Base with four treatments: no fertilizer (CK), chemical fertilizers (NPK), biochar (BC), biochar plus chemical fertilizers (BC+NPK). Biochar was prepared by pyrolyzing peanut shells and was applied only once at the beginning of the experiment. NPK fertilizers were applied in each season. Soil samples of the plough layer were collected for determination of nutrient contents, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and enzyme activities in soil aggregates.

Results 1) Compared with CK, NPK increased soil available phosphorus, available K, and NO₃^–-N, BC increased SOC and total N contents, and BC+NPK increased all the aforementioned indicators. 2) Compared with CK, BC reduced MBN in size 2–0.25 mm aggregate and increased MBC/MBN ratio in 2–0.25 mm aggregate. Compared with CK, BC+NPK increased MBC in size >2 mm and 0.25–0.053 mm aggregates by 59.57% and 34.68%, respectively, and increased MBN in the plough layer soil, size >2 mm and 2–0.25 mm aggregates by 17.33%, 42.24% and 19.28%, respectively. Compared with CK, NPK, BC, and BC+NPK treatments increased MBC/SOC in size >2 mm aggregate, but BC and BC+NPK reduced MBC/SOC in plough layer soil, size 2–0.25 mm and 0.25–0.053 mm aggregates. 3) Compared with CK, NPK, BC+NPK increased β-glucosidase, β-cellobiosidase, α-glucosidase, and β-xylosidase activities in size 2–0.25 mm and 0.25–0.053 mm aggregates, whereas only BC+NPK increased the activities of aforementioned four enzymes in size >2 mm aggregate. Compared with CK, NPK and BC+NPK increased urease activity in size >2 mm aggregate and N-acetylglucosaminidase activities in size 0.25–0.053 mm aggregates. Only BC+NPK treatment increased L-leucine aminopeptidase activities in size >2 mm and 0.25–0.053 mm aggregates. Enzyme activities in soil aggregates significantly related to MBC, MBN and MBC/MBN ratio. Enzyme activities in size >2 mm aggregate significantly correlated with MBC/SOC, TN and MBC contents, while enzyme activities in size 2–0.25 mm and 0.25–0.053 mm aggregates significantly correlated with MBC/MBN ratio and MBC content, respectively.

Conclusions Combined application of biochar and chemical fertilizers is suitable for soil carbon sequestration, and soil micro-environment and soil quality improvment, and application of biochar has a longer effect on calcareous soil.