Soil dependence of biochar composts in mitigating greenhouse gas emissions: An overlooked biophysical mechanism.

Biochar-manure co-compost (BM) has garnered considerable attention as a promising soil amendment for improving nutrient retention and mitigating the emission of greenhouse gases (GHGs). However, its efficacy often varies largely from one soil to another. By comparing soil CO 2 and N 2 O effluxes, solution chemistry, enzyme activities, and the abundances of N-cycle genes between BM and manure compost (M) across three soils of different texture classes through microcosm incubations, we aimed to develop biophysical mechanics to untangle the soil-dependent efficacy of BM in mediating soil carbon and nitrogen transformation processes. Compared to M addition, BM addition significantly reduced soil CO 2 and N 2 O emissions, but its effectiveness was soil texture-dependent, being strongest for CO 2 and N 2 O in fine-textured clay loam and coarse-textured sand, respectively. Such soil texture-dependent effects of BM versus M were also observed in soil enzyme activities and gene copy numbers of ammonia-oxidizing bacteria and denitrifiers. Our datasets suggest that BM interacted strongly with soil texture to modulate the pore scale-based diffusion of oxygen, thereby creating divergence in the aeration status among soils. Sequentially, soil phenol oxidase activity was greater in BM than in M under more aerobic conditions but no difference between BM and M in oxygen-limited soils. The balance between oxygen depletion due to microbial respiration of organic amendments and oxygen diffusion through soil/biochar pores also shaped the activities of nitrifying and denitrifying prokaryotes differently between coarse- and fine-textured soils. This logic model well-explained the magnitude of change in soil-dependent CO 2 and N 2 O emissions from organic amendments. The knowledge gained in this work will likely have profound practical ramifications for optimizing BM efficacy in mitigating the GHGs emission. Graphic abstract [Display omitted] • Impact of biochar-manure compost (BM) on CO 2 and N 2 O emissions is soil dependent. • BM interacted with soil texture to modulate pore scale-based oxygen diffusion. • Oxygen level regulated by BM addition affected abundance of N-cycling prokaryotes. • A biophysical mechanism is developed to explain the soil dependence of BM efficacy. [ABSTRACT FROM AUTHOR]