Objectives Application of excess chemical fertilizers and pesticides has deteriorated the soil quality of a jujube orchard in the North Plain of China. Therefore, establishment of a friendly nutrient management is essential for jujube production and maintenance of a favorable soil environment.

Methods A field experiment was conducted consecutively in the jujube orchard, located in the Puyang Forestry Academy, Henan Province from 2013 to 2015. The experimental treatments were composed of four levels of biochar (0, 2.5, 5 and 10 t/hm², namely, C₀, C₁, C₂ and C₃), and three levels of nitrogen fertilizer (300, 450 and 600 kg/hm², namely, N₁, N₂ and N₃), and a control without biochar and nitrogen application. Soil samples of 0–20 cm depth were collected after jujube harvested, the soil microbial biomass, enzyme activities and microbial communities were analyzed.

Results Compared with control, application of biochar significantly affected the contents of soil microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and microbial biomass in soils (P < 0.01). Soil microbial biomass in all biochar treatments was higher than that in C₀, but there was no significant difference among the three N rates when biochar was applied at the rate of 2.5 t/hm². The two highest increases were in C₃N₂ and C₃N₃ treatments, with increase of 208.6% in MBC and 159.4% in MBN, respectively. The soil urease activities were enhanced with the increasing of biochar and nitrogen rates, with the largest increase of 91.7% (P < 0.05). Neither soil alkaline phosphatase activity nor invertase activity was significantly affected by combined application of biochar and nitrogen fertilizer. The biochar, nitrogen and their interaction significantly altered the community of soil bacteria, fungi and actinomycetes. The numbers of bacteria, fungi and actinomycetes were increased by 10.9%–80.4%, 6.6%–143.1% and 50.6%–115.2% compared to the control, respectively. Statistics analysis showed that soil microbial biomass, enzyme activities and microbial quantities had reciprocally significant positive correlations.

Conclusions Combined application of biochar and nitrogen fertilizer has significant and positive effects to the profiles of microbial properties of soil. In this study, the optimum application rates were 10 t/hm² for biochar, and N 300 kg/hm² for nitrogen fertilizer according to the feasible production cost.