Gain in carbon: Deciphering the abiotic and biotic mechanisms of biochar-induced negative priming effects in contrasting soils.

The biochar-induced priming effects (PEs) were investigated by applying maize straw (C4) derived biochar to eight C3 soils, with a gradient of pH and a sub-gradient of soil organic carbon (SOC). To decipher the physicochemical and microbial mechanisms, we adopted C-isotopic analysis, high-throughput sequencing and multivariate statistical analyses such as random forest (RF) and structure equation modeling (SEM). Negative and neutral PEs were observed up to -48.5% of relative PEs during 28 days of incubation. All the acidic soils exhibited negative PEs, so as the neutral Alfisol and alkaline Aridisol, which had a suppression effect on SOC mineralization accounted for -29.4 and -32.0% of relative PEs. Among all abiotic factors, soil silt-clay fraction and the initial pH values play the most important roles in PEs determination through directly inhibiting PEs by protection SOC and indirectly shaping bacterial communities respectively. On the whole community level, biochar treatments defined much less microbiome (0.6% and 1.2% for variance of bacterial and fungal community) than soil types (93.5% and 83.3% respectively) across soils. Thus, the initial community (i.e., bacteria alpha-diversity and copiotrophic bacteria as revealed by SEM) of different soils might be more critical for PE prediction. Furthermore, co-occurrence network analysis indicated out-competition of fungi by bacteria with increase of mutual exclusion and decrease of fungal occupancy. This could exacerbate negative PEs in soils with lower bacterial alpha-diversity and dominance by copiotrophys due to less functional complementary for recalcitrant SOC decomposition.