The object of this study was to explore the effect of biochar on Loess Plateau calcareous arable soil organic carbon mineralization under the condition of with or without nitrogen-fertilizer addition. Maize straw-derived biochars were prepared at 400 and 800°C (BC400 and BC800), respectively. After physicochemical property determination, biochars were thoroughly mixed with soil at the mass ratio of 1% and 2% respectively, and a control without biochar addition was also arranged. Each treatment was carried out with 6 replicates, half of which were subjected to nitrogen-fertilizer addition. After 1-week preincubation, 50-day indoor static soil incubation experiment was carried out consecutively. Results showed that compared with BC400, pH, carbon content, alkaline oxygen-containing functional group, and specific surface area of BC800 significantly increased, while dissolved organic carbon, nitrogen, hydrogen, oxygen contents, carboxyl, carbonyl, and acidic oxygen-containing functional group significantly decreased. In the treatments without nitrogen-fertilizer addition, maize straw-derived biochar inhibited CO 2 emission, especially for BC800-1% and BC800-2%. However, in the treatments with nitrogen-fertilizer addition, the biochar promoted CO 2 emission, especially for BC400-1% and BC400-2%. Moreover, the pyrolysis temperature of biochar preparation showed more crucial impact on CO 2 emission than its application rate. CO 2 emission was significantly correlated with total organic carbon/inorganic nitrogen (TOC/IN) and dissolved organic carbon/inorganic nitrogen (DOC/IN). However, DOC/IN explained the variance of CO 2 emission with a higher degree than TOC/IN. Carbon/nitrogen mass ratio was the key factor in controlling CO 2 emission from biochar-amended soil, and DOC/IN showed more significant effect on CO 2 emission than TOC/IN.

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