[Effects of combined application of biochar and nitrogen fertilizer on soil quality of summer maize-winter wheat system]

The effects of nitrogen fertilizer and biochar on topsoil quality in the drylands of northwest China were studied in field trials for two years. A split plot design with two factors was adopted, with five nitrogen rates (0, 75, 150, 225 and 300 kg N·hm^-2) as main plots, and two rates of biochar (0 and 7.5 t·hm^-2) as submain plots. We collected soil samples at 0-15 cm depth after two years of winter wheat-summer maize rotation and measured physical, chemical, and biological properties. The minimum data set (MDS) was established by using principal component analysis and correlation analysis to analyze the responses of soil quality to nitrogen fertilizer and biochar addition. The results showed that the combined application of nitrogen fertilizer and biochar could improve soil physical properties, increase macroaggregate content, reduce soil bulk density, and increase soil porosity. Both fertilizer application and biochar application had a significant effect on soil microbial biomass carbon and nitrogen. The application of biochar could improve soil urease activity and the contents of soil nutrients and organic carbon. Six out of 16 indicators (urease, microbial biomass carbon, total phosphorous, total nitrogen, pH, and available potassium) relating to soil quality were used to construct MDS, and soil quality index (SQI) was calculated. The variation range of SQI was 0.14-0.87, with that of 225 and 300 kg N·hm^-2 nitrogen application combined with biochar application being significantly higher than other treatments. Soil quality could be significantly improved by application of nitrogen fertilizer and biochar. Interactive effect was observed, which was particularly stronger under high nitrogen application rate.