Feedstock-dependent methane emissions from kitchen-waste hydrochar are mediated by dissolved organic matter...

Converting kitchen waste (KW) into hydrochar is an effective strategy for resource recovery, yet its impact on greenhouse gas emissions after soil application is not well understood. This study evaluated how hydrochars derived from different KW feedstocks affect methane (CH₄) emissions in paddy soil and determined the primary factors driving these changes. In a rice soil column experiment spanning the full growing season, we compared a chemical fertilizer control (CKU) with hydrochars produced from protein (PT), cellulose (CL), skeleton (SK), and starch (ST) feedstocks at two application rates (0.5% and 1% by mass). Compared with CKU, cumulative CH₄ emissions increased significantly under SK and PT treatments, followed by CL, whereas ST hydrochar caused no significant change. Higher application rates consistently amplified these effects. Changes in soil dissolved organic matter (DOM) composition explained the observed CH₄ emission patterns. Hydrochars from SK, PT, and CL feedstocks supplied abundant labile carbon and reduced DOM humification and stability, as indicated by lower humification index (HIX) and Ph/Pp values, thereby enhancing substrate availability for methanogenesis. In addition, SK hydrochar released high NH₄⁺-N in paddy soil, potentially inhibiting CH₄ oxidation. This dual effect resulted in the highest mcrA/pmoA ratio and net CH₄ emissions under the SK treatment. These findings demonstrate that hydrochars effects on CH₄ emissions were strongly governed by physicochemical heterogeneity of the feedstocks and feedstock-driven alterations in the soil organic matter and microbial activity after application rather than the technology itself. Incorporating feedstock-specific considerations into application strategies may reconcile kitchen waste recycling with CH₄ mitigation in paddy agroecosystems.