Copper valence dynamics link microbial redox functions to humification during composting

Organic-waste composting supports resource recovery, but microbial links between Cu transformation and humification remain insufficiently resolved. We combined XPS-Auger analysis, metagenomics, day-4 metatranscriptomics, and genome-resolved analysis to compare Cu(I), Cu(II), and mixed-valence treatments receiving equal Cu inputs with a control without added Cu (CK) during lignocellulosic composting. By day 30, surface Cu(I) proportions reached 55.63% and 59.04% in Cu(I) and mixed-valence treatments, respectively, whereas the Cu(II) treatment remained Cu(II)-dominated. On day 4, AA1 accounted for 1.331% of the exported CAZy expression total in CK and 2.085-2.931% in Cu treatments, alongside distinct substrate-transformation and redox profiles. Cu treatments showed higher day-4 transcript levels of carbohydrate-associated MAG204 and Cu-homeostasis-associated MAG674. The Cu(I) treatment had the greatest humic acid (HA) concentration gain (60.09 g kg⁻¹), whereas the mixed-valence treatment had the highest final HA concentration (122.45 g kg⁻¹). The findings support a Cu-valence-associated working model of redox-active humification; extracellular electron transfer remains to be verified by direct electrochemical measurements.