Effect of municipal green waste biochar addition on the growth, nitrate content and nitrogen use efficiency of greenhouse pakchoi

【Objectives】 With rapid development of greenhouse vegetable, problems such as low fertilizer use efficiency and nitrate accumulation in vegetables have arisen. Meanwhile, municipal green waste challenges the rapid development of urbanization in China. In this study, biochar made from municipal green wastes are used to investigate the effect of biochar on yield and quality of greenhouse vegetables and the soil nutrient conservation, in order to explore a green practice for urban agriculture. 【Methods】 A pot experiment was conducted in greenhouse andpakchoi was planted as tested material. Five levels of the biochar were added into soil including: C0(no biochar, CK), C1(20 g/kg), C2(40 g/kg), C3(60 g/kg) and C4(80 g/kg). Plant growth, shoot nitrate content, soil total N content and N forms and soil N retention were investigated.【Results】 Biochar amendment significantly increasespakchoi yield.Compared with the control, the yields are increased by up to 75% under the C3 and C4 treatments. There is a positive correlation between the yield and biochar amendment levels. Biochar has significant effect on aboveground and underground biomass which are increased in the harvest index by 2.5% to 9.5%, and the increases are in proportion to the biochar amendment rate. The nitrate contents in shoot ranges from 161 to 256 mg/kg in the biochar addition treatments, which are much lower than that of the control (504 mg/kg).The C1 treatment has the lowest shoot nitrate content, while no difference between the biochar amendment levels. The biochar addition increases soil total N content, decreases the N loss rate from 5.6% of the control to below 3.3% of the biochar treatments, significantly decrease by more than 42%, and enhances the N productivity by over 35% at the same time. The NO-3-N accumulation amounts in soil are decreased by maxmium of 80% with 4% biochar amendment, and the change of NH+4-N is similar to that of NO-3-N, which is decreased by 77%, while there is no correlation between biochar amendment levels and soil NH+4-N and NO-3-N accumulations. There is a decrease trend of NH+4-N/NO-3-N ratio with the increase of the biochar addition. The yields are negatively correlate with soil NH+4-N and NO-3-N contents, positively with the soil total N contents. The shoot nitrate contents significantly correlate with soil NH+4-N and NO-3-N contents, not with soil total N.【Conclusions】 The biochar amendment can improve the soil nutrient retention and reduce vegetable NO-3-N accumulation, increases the greenhousepakchoi yield, decreases the shoot nitrate content, increases the N productivity as a result. Therefore, it is a effective way of treating municipal green waste by making biochar and use them to improve soil fertility. The suitable biochar amendment is 2%-4% of the soil.