Impact of the addition of black soldier fly larvae on humification and speciation of trace elements during manure composting

This study investigated the effects of livestock manure composting by black soldier fly larvae (BSFL) on humification and the speciation of trace elements (Cu and Zn) using the improved BCR. Six treatments were designed using different manure types (chicken, pig, and cow), either mixed with BSFL (7:1.2 ratio on the basis of fresh weight) or without BSFL (control). Results showed that the range of humic acid/fulvic acid ratio in BSFL treatments was 1.24-1.56 %, without BSFL treatments was 0.40 - 0.47 %. BSFL addition enhanced humic acid production, and as a consequence, the humic acid/fulvic acid ratio of the manure was higher than 1 %. Meanwhile, the sequential extraction of trace elements (TEs) showed that BSFL addition to chicken and pig manure increased the mobilization and bioavailability of TEs, as the added BSFL enhanced their activities. The speciation of TEs differed between Zn and Cu in the BSFL-cow manure system, which the fraction of residue was decreased in Zn but was increased in Cu. The concentration of total Cu and Zn increased among all treatments. Therefore, the results of this study provides the basis for a new process to treat livestock manure and also provides valuable information that could be used to enhance the availability of TEs in the soil through the application of BSFL-amended organic fertilizer.