Objectives Intensive vegetable cultivation in China is characterized by high nitrogen fertilizer application, multiple crop rotation and harvest. The frequent fertilization practice required by this production system leads to high nitrogen loss inevitably. The application of biochar has been proved of effectively improving soil physical, chemical and biological properties, and affecting the soil nitrogen cycle and crop yield. We studied the effects and mechanisms of different amounts of biochar application on N₂O emission and crop yield.

Methods A field experiment was conducted for continuous two-years in South China, and Indian lettuce and asparagus lettuce were cultivated in turn each year, with multiple harvests in each vegetable during their growing seasons. The experiment was consist of four treatments, including: no nitrogen fertilization control (CK), conventional nitrogen fertilization (NF), conventional nitrogen fertilization plus 20 t/hm² biochar (NB20) and 40 t/hm² biochar (NB40), respectively. The static dark box–gas chromatography was used to measure soil N₂O emissions during the vegetable growing season. Soil samples were collected at the end harvest of the vegetables for measurement of soil physico-chemical properties and enzyme activities, and total vegetable yields were recorded.

Results Compared with NF at the end of two-years of experiment, biochar application significantly increased soil total N, pH, available P and urease activity (P<0.05), and NB20 treatment was recorded significantly higher pH and available P than NB40. The soil N₂O emission fluxes ranged from 42.8 to 7233.0 μg/(m²·h) under NF treatment, and 28.4 to 3188.1 μg/(m²·h) under biochar treatments, depending on temperature, season, and nitrogen fertilization. Compared with NF treatment, NB20 significantly reduced the cumulative N₂O emissions by 64.3%−66.9%, and NB40 reduced that by 55.8%−62.5%, and there was no significant difference between the two treatments in the reduction effect. However, NB20 treatment significantly increased vegetable yield in two years, thereby reduced the unit yield N₂O emissions by 56.4%−74.4%, and increased nitrogen uptake by 13.9%−32.9%, and the annual change was not significant. NB40 was recorded similar yield, unit yield N₂O emission, and nitrogen use efficiency with NB20 in the first year, but significantly lower values in the second year, and also lower values than in the first year of itself, showing significant annual change. Correlation and redundancy analyses showed that soil pH was the main factor affecting N₂O emissions, and available P, pH, and organic carbon were the main factors affecting vegetable yields.

Conclusions Application of biochar in the intensive vegetable production fields could improve soil physico-chemical properties by elevating pH, total N, available P and enzyme activities, thus decrease soil N₂O emissions and increase vegetable yields. Application of 20 t/hm² biochar shows more stable effect in yield and soil pH than 40 t/hm² biochar, so has higher effect in inducing the unit yield N₂O emission and vegetable nitrogen use efficiency, and is recommended for practical use dosage.