Bacterial diversity and functions in soils amended with biochar and organic fertilizers

Abstract

Organic farming relies on the activities of bacterial communities for optimal soil productivity. Understanding the responses of bacterial communities to different soil amendments, including organic fertilizers and biochars, can provide information for soil management. This thesis firstly describes the bacterial diversity and functions that are central to soil processes of organic soils and their responses to fertilizers with different C/N ratios. Secondly, this thesis analyzes the effect of a mineral-enhanced biochar (MEB) on the bacterial community of organically amended soil. Finally, the thesis investigates the direct interactions between biochar and bacteria that underpin metabolic processes in the soil. Bacterial taxa that are resilient to different fertilizers were identified and defined as the core community of organic soil. The phyla Bacteroidetes and Planctomycetes, the family Cytophagaceae and the class Acidobacteria-5 were thus found in organic soil regardless of the type of fertilizer being applied. These core bacterial taxa were further linked to the functional potential of organic soil. The C/N ratio of fertilizer was also found to have a positive correlation with microbial N assimilation in organic soil. MEB was applied in combination with compost to soil and this resulted in synergistic effects on soil properties. Specifically, the soil nitrate content was increased, which correlated with an enrichment of bacterial nitrifiers due to the MEB addition. As a consequence, plants produced larger leaves, which demonstrates that MEB could be used to manipulate specific agricultural outcomes in organic farming. To understand the detailed mechanism that supports the beneficial effects of biochar and MEB, a novel method was developed to visualize the in situ interactions between bacteria and surfaces on a single-cell level. Distinct bacterial communities were found to exist on the surface of biochar and MEBs compared to surrounding soils and surface-associated bacteria were found to have the capacities to fix carbon dioxide using chemolithotrophic processes. This provides a bacterial mechanism on how biochar and MEBs can drive carbon sequestration into the soil environment. Together, the discoveries and models presented in this thesis provide new insights into the functions of soil microbiomes in organically amended soil.