[Effects of Biochar Application Amount and Frequency on Yellow Soil Nutrients and Key Enzyme Activities]

Yellow soil, predominant in the southern regions of China, constitutes 28% of the nation's cultivated land. Exploring the regulatory mechanisms of biochar on nutrient levels and key enzyme activities in yellow soil holds significance for soil health restoration and food security. This study investigated the effects of different amounts and frequencies of biochar application on yellow soil nutrients and enzyme activities. The experiment involved two biochar application frequencies： ① a once'application amount of biochar at rates of 5 （B5）, 10 （B10）, 20 （B20）, and 50 （B50） t·hm^-2, and ② a consistent application of an equivalent total amount of biochar over three years at rates of 5 （B5-3）, 10 （B10-3）, 20 （B20-3）, and 50 （B50-3） t·hm^-2. A control treatment （CK） without biochar application （0 t·hm^-2） was included. The results of the study showed that the application of biochar in the yellow soil agriculture ecosystem significantly increased soil pH and electrical conductivity （EC）. Moreover, it improved the contents of alkaline hydrolysis nitrogen （AN）, available phosphorus （AP）, and organic carbon （SOC）. Furthermore, the activities of urease, phosphatase, sucrase, and catalase in the soil were increased by the application of biochar. A comparison between the two application modes showed that the continuous application of biochar resulted in higher soil nutrient levels and enzyme activities. No significant interactive effects of different biochar amounts and frequencies exist on soil pH, EC, AN, SOC, phosphatase, sucrase, and catalase activities. However, interactive effects were observed on AP, available potassium （AK）, and urease activity. The structural equation model elucidated that biochar had a direct negative effect on the activities of soil urease, catalase, phosphatase, and sucrase. Nevertheless, it indirectly promoted enzyme activity by enhancing soil nutrient levels. The application of biochar in yellow soil agriculture ecosystems consistently enhanced the soil's comprehensive fertility under both application modes. Among the once-application models, B10 emerged as the most effective treatment, exhibiting a higher comprehensive fertility score than that of other treatments. Specifically, compared to those in the control （CK）, soil pH, EC, AN, AP, AK, SOC, urease, phosphatase, sucrase, and catalase activities in B10 increased by 8.31%, 13.28%, 22.18%, 10.68%, 21.49%, 15.40%, 44.79%, 16.62%, 35.68%, and 16.62%, respectively. In contrast, for the continuous three-year application of biochar, treatment B50-3 had the highest comprehensive fertility score. Compared to that in CK, soil pH in B50-3 increased by 15.11%, EC by 26.26%, AN by 11.02%, AP by 43.30%, AK by 17.29%, SOC by 36.68%, urease by 20.44%, phosphatase by 3.71%, sucrase by 18.38%, and catalase activities by 16.62%. In summary, the application of biochar in yellow soil farmland could effectively enhance soil fertility by increasing soil nutrient content and enzyme activities. Further, this study revealed that the effect of continuous annual application of biochar surpassed that of once-application of biochar, indicating the long-term benefits of this practice in maintaining and improving soil health and food security.