LAND USE LEGACY REGULATES MICROBIAL COMMUNITY STRUCTURE AND FUNCTION IN TRANSPLANTED CHERNOZEMS
Understanding long-term agricultural influences on soil nutrients and microbial dynamics is often complicated by the heterogeneity of soil environments as well as complex interactions between confounding factors of climate, topography and hydrology. A long-term soil transplant experiment located at the Agriculture and Agri-Food Canada (AAFC) Research Center in Lethbridge, AB provided a unique opportunity to study the long-term effects of agriculture management practices on various soil types, independent of these confounding effects. In 1990, a variety of Chernozemic soils were relocated to a common location and subsequently managed under common conditions. The experiment was a split plot design with soil type and rate of nitrogen (N) fertilizer (0 and 60 kg N ha-1) as the main and subplot factors, respectively and crop residues were removed annually at harvest. A 2012 survey of soil microbial abundance and community structure revealed distinct communities among 10 different soil types, indicating a persistent legacy effect of previous land use, irrespective of the common climatic, topography and management. Next, DNA profiling of archived soils from 1990 and 2011 showed that the contemporary community structures differed from the original 1990 soils but that the degree of divergence was generally linked to the original soil fertility. Thus, although soil fertility of individual transplanted soils changed with common management, variability among soils persisted. Specific differences among transplanted soil microbial community structures were reflected in carbon (C) and N cycling bacteria. For example, nitrifying and denitrifying gene abundance and composition varied among transplanted soils. A 13C stable isotope tracing experiment determined that community composition and function of active cellulose decomposers was different among soils indicating that the subset of the whole soil communities that was active also remained different between transplanted soils, despite common conditions. Overall, soil fertility integrates aspects of pedogenesis, land use and management history which resulted in a legacy effect on the microbial community abundance, composition and function which was more dominant than current conditions. These findings help to broaden our understanding of dominant drivers of agricultural soil ecosystem functioning and microbial communities which will be necessary for predicting the fate of future agricultural systems.
Soil transplant, long-term soil experiment, crop residue removal, microbial community composition, N functional genes, cellulose mineralization, active decomposer community
Doctor of Philosophy (Ph.D.)