The Effect of Chemical Fumigation and Biofumigation on the Soil Nitrogen Cycle, Soil Respiration, and the Soil Microbial Community

Abstract

Chemical fumigation and biofumigation are used to reduce soil-borne diseases in potato crop production systems. However, soil fumigation may impact non-targeted soil microorganisms, consequently altering essential soil processes. In a first microcosm experiment, chemical fumigation with chloropicrin (CP) or metam sodium (MS) either used alone or combined with an organic amendment (OA) and biofumigation with mustard plant residues were assessed for their effects on key soil nitrogen (N) processes, the abundance of N-related microorganisms, and soil bacterial diversity. Biofumigation did not affect net N mineralization, nitrification, or nitrifier and denitrifier abundance, but increased denitrification. Chemical fumigation used alone did not affect net N mineralization. All chemical fumigant treatments inhibited nitrification and decreased nitrifier abundance compared to biofumigation. Only CP fumigation decreased denitrifier abundance. Biofumigation transiently decreased bacterial evenness at early time points. Conversely, CP fumigation had an enduring impact on bacterial diversity, decreasing species evenness and yielding the most dissimilar community. Unexpectedly, MS fumigation combined with an OA delayed the flux of soil respiration compared to amended non-fumigated soil, indicating chemical fumigation may impact plant residue decomposition. This was further investigated in a second microcosm experiment, in which chemical fumigation (with CP or MS) combined with an OA of contrasting carbon (C) availability were assessed for their effects on soil respiration, soil inorganic N concentrations, and soil bacterial and fungal diversity. When combined with an OA, chemical fumigation decreased soil respiration, regardless of C availability, and apparent plant residue decomposition compared to amended non-fumigated soil. Chemical fumigation with MS only affected bacterial diversity when combined with a labile OA. MS fumigation did not affect fungal diversity. Chemical fumigation with CP used alone or combined with an OA decreased bacterial and fungal species richness and evenness and yielded the most dissimilar bacterial and fungal community compared to all other treatments. Overall, the results of this thesis suggest that 1) chemical fumigation had a greater impact on the soil N cycle, N-related microorganisms, and bacterial diversity than biofumigation, and 2) chemical fumigation reduced soil respiration, indicative of reduced OA decomposition, and altered microbial diversity regardless of soil OA or C availability.