Field-applied biochar-based MgO and sepiolite composites possess CO capture potential and alter organic C mineralization and C-cycling bacterial structure in fertilized soils.

Agricultural soils contribute a significant amount of anthropogenic CO emission, a greenhouse gas of global environmental concern. Hence, discovering sustainable materials that can capture CO in cultivated soils is paramount. Since the effect of biochar on C mineralization/retention in fertilized soils is unclear, we produced biochar-based MgO and sepiolite-nanocomposites with CO capture potential. The field-scale impacts of the modified-biochars were evaluated on net C exchange rate (NCER) periodically for 3 months in fertilized plots. The effects of the modified-biochar on organic-C mineralization, the activities, and dynamics of C-cycling-related 16S rRNA which are unknown, were investigated. Results revealed an initial rapid and higher cumulative CO emission from the sole fertilizer treatment (F). Unlike the biochar treatment (BF), the successful incorporation of MgO/Mg(OH) nanoparticles into the matrix and surface of biochar, and the potential formation of MgCO with soil CO, mitigated CO emission, especially in the MgO-modified biochar (MgOBF), compared to the sepiolite-biochar treatment (SBF). Compared to F and BF, the higher C retention as MgCO in the modified biochar treatments led to an increase in cellulase activity, stimulation of key C-cycling-related bacteria, and the expression of genes associated with starch, sucrose, amino sugar, nucleotide sugar, ascorbate, aldarate, cellulose, and chitin degradation, thus, increasing organic C mineralization. Among the modified-biochar treatments, higher C mineralization was recorded in SBF, resulting in increased cumulative CO emission, despite its initial capture for up to 42 days. However, MgOBF was effective in capturing soil-derived CO, despite the increased C mineralization compared to biochar. The changes in soil moisture and temperature significantly regulated NCER. Also, the modified biochars positively influenced the distribution of C-cycling-related bacteria by improving soil pH and available nutrients. Among the modified biochars, the observed higher mitigation effect of MgOBF on NCER indicated that it could be preferably applied in agricultural soils.