Abstract: A field experiment was conducted to examine the effects on soil fertility and enzyme activities in paddy field after six years of one-split rice straw-derived biochar [0 (BC₀), 7.5(BC₁), 15(BC₂), 22.5(BC₃) t·hm^-2] and rice straw (3.75 t·hm^-2, STR) application. The results showed that soil organic carbon, available phosphorus and rapidly available potassium concentrations significantly increased, by 34.6%, 12.4% and 26.2%, respectively. Soil pH and soil bulk density were significantly reduced, but total nitrogen content had no significant difference compared with BC₀. Biochar addition significantly increased the activities of soil urease and acid phosphatase. The soil fluorescein diacetate (FDA hydrolase) and arylsulfatase activity were inhibited to varying degrees. Among them, BC₂ treatment increased soil urease activity by 36.5%. The soil acid phosphatase activity increased with the increases of biochar application rate, which was positively correlated with soil available phosphorus concentration. FDA hydrolase and urease activity had positive correlation with soil available potassium content, while soil acid phosphatase and arylsulfatase activity had positive correlation with soil bulk density. After six years, soil dehydrogenase and polyphenol oxidase activity significantly increased by 48.8% and 27.5%, respectively, while catalase activity significantly decreased when compared with control BC₀. STR treatment increased activities of soil urease, FDA hydrolase, dehydrogenase, acid phosphatase and arylsulfatase significantly, while decreased the catalase and polyphenol oxidase activities by 23.4% and 15.9%, respectively.

Key words: biochar, straw returning, soil fertility, paddy soil, soil enzyme activity