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In order to ascertain the sustained influence of the application of biochar on sloping farmland in black soil region of Northeast China, field runoff plots with 1.5°, 3° and 5° of typical slops in the region was chosen, where the 4-years experiments were conducted, and soil structure and soil nutrients were observed. In 2016, before the start of the experiment, biochar derived from corn straw was applied with 75t/hm2 in the soil, and a control group was without biochar application on each slope plots, counted up to six plots. Biochar was not applied in subsequent years. The results showed that the application of biochar can increase soil gas phase and liquid phase ratio, improve soil aeration, improve soil threephase ratio, and then increase porosity by 2.83%~5.56% and decrease bulk density by 1.89%~3.62%. The contents of organic matter, ammonium nitrogen and available potassium in the soil were significantly increased after the application of biochar, increased by 9.54%~18.21%, 21.35%~28.02%, and 11.99%~22.71%, respectively. And the effects of above was weakened with time. A random forest regression model was used to evaluate the comprehensive fertility grading indexes and fit the regression equation to predict the indexes of 2020—2022. Comparing fertility changes, it was concluded that the reasonable and effective life for one time application of biochar on soil fertility in the region would be 6~7 years.