Biochar added to soil can improve crop growth and productivity. However, the mechanisms of crop growth improvement by biochar application are not well understood, particularly in the presence of soil-borne pathogens caused by continuous monocropping. Thus, a two-year field experiment was carried out to study the chemical and microbiological response of Lixisols (pH 5.8) to the amendment of biochar and its effect on watermelon productivity and Fusarium wilt disease incidence. Biochar was added alone or together with compost before watermelon transplanting. Mixed application of biochar with compost significantly increased watermelon yield as compared to adding compost or biochar alone. However, biochar had no effects on Fusarium wilt disease incidence in both years. Combined application of biochar with compost significantly increased contents of soil NH₄⁺-N, available phosphorus (P) and available potassium (K). Soil Biolog data indicated that the Shannon-Weaver diversity index and evenness index were increased significantly in the combined application of biochar with the compost treatment. There was a significant positive correlation between watermelon yield and soil NH₄⁺-N, available P, available K, microbial diversity or microbial evenness in the continuous watermelon monocropping system.

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