Objectives The paper studied the effects of biochar on soil microbial population, the activities of enzymes related to nitrogen metabolism, yield and sugar content of sugar beet under saline-alkali stress, in order to clarify the mitigation effect of biochar on saline-alkali stress and the possibility of reducing nitrogen fertilizer input.

Methods A pot experiment was conducted at the Experimental Station of Northeast Agricultural University in Harbin, Heilongjiang Province in 2018. The sugar beet cultivar of KWS0143 was used as the test material, and neutral and alkaline salts were added to make a saline-alkali stress soil for the experiment. There were total of six treatments, among them, saline-alkali soil applied with N 180 kg/hm² was used as control (CK), and in the other five treatments, biochar were added in rate of 30 g/kg soil, and N were applied in rates of 180, 162, 144, 126 and 108 kg/hm² in turn. The seedling emergence rate of sugar beet was measured when cotyledon was fully expanded. The population of soil bacteria and fungi were measured every 20 days since the six-leaf stage of sugar beet, and the activities of nitrate reductase (NR), glutamine synthetase (GS) and glutamate synthetase (GOGAT) in leaves were analyzed at the same time. Sugar content was analyzed after harvest.

Results The population of soil fungi and bacteria were significantly increased by the application of biochar under saline-alkali stress. BC+N180 treatment significantly increased the amount of actinomycetes, BC+N126 treatment increased the amount of bacteria, but BC+N162 treatment was more suitable for the survival of fungi in the soil. After the application of biochar, the activities of enzymes were significantly improved, and the activities of NR in BC+N162 and BC+N144 treatments were higher than that in CK. Except the 117th and 138th day after sowing, the activity of GS in BC+N144 treatment was significantly higher than that in control, and the activity of GOGAT in BC+N144 treatment was always significantly higher than that in CK. Except the 53th and 138th day after sowing, the activity of GOGAT in BC+N126 treatment was significantly higher than that in CK. Applying biochar increased the sugar beet root yield and sugar content significantly, except that in BC+N108 treatment. At the base of applying biochar, BC+N162 treatment obtained similar root and sugar yield as BC+N180 treatment, and significantly higher root and sugar yield than BC+N144 and BC+N108 treatments.

Conclusions The application of biochar is effective to alleviate the saline-alkali stress on the population of soil microorganisms and increase the activities of enzymes related to the metabolism of beet nitrogen, and improve the yield and sugar content consequently. Under the condition of the experiment, the application of 3% biochar in soil can save 10%–20% of nitrogen fertilizer input.