Dr Bobbi Helgason1, Dr Jennifer Town2, Mr Kehinde Adams1, Dr Edward Gregorich3
1University Of Saskatchewan, Saskatoon, Canada, 2Agriculture and Agri-Food Canada, Saskatoon, Canada, 3Agriculture and Agri-Food Canada, Ottawa, Canada
Sustainable cropping systems rely on diverse crop rotations to maintain balanced fertility and critical soil functions, including organic matter turnover. We evaluated soils from two long-term field experiments comparing continuous cereals with cereals in diverse crop rotations. The first is a humid corn-based system (Ontario, Canada) and the second is a semi-arid wheat-based system (Saskatchewan, Canada), both of which have exhibited a yield boost of at least 20% in the cereals grown in diverse rotations. Using multiple indicators of soil function and DNA amplicon-based microbial community profiling we found significant differences in soil organic matter composition and microbial community structure and activity. Effects of diverse crop rotations were greater in the corn vs. wheat system, with bigger changes in the relative abundances of different bacterial and fungal taxa, extracellular enzyme activities and available nutrient concentrations. Soil organic matter characterization showed an accumulation of lignin, increased syringyl:vanillin ratio and greater mineralizable C in continuous wheat and corn but no difference in amino sugar content, indicating that there was faster turnover of recent plant C inputs in the diverse rotations but no differences in microbial necromass accumulation. A soil incubation study using ¹³C labelled corn and wheat residues to track the microbial decomposition of residue C revealed differences in active decomposers between rotation treatments. Similar to our field survey, these rotation-induced differences were greater in the corn compared to the wheat system. We hypothesize that this is due to moisture stress being a stronger determinant of microbial decomposer community structure and function than residue quality in the semi-arid climate. Our work demonstrates that changes in residue quality, realized through crop rotation, impact soil organic matter composition and soil function and explicitly considers microbial functions as both causative and indicative of changing soil fertility and organic matter dynamics.
Bobbi Helgason is a soil microbial ecologist whose research program focusses on how soil bacteria and fungi affect soil function and plant growth, with a focus on agroecosystems. Her research program focusses on how soil microorganisms interact with climate and agricultural management practices to affect soil functioning and crop productivity. Her work combines field and laboratory research using leading edge methodologies in microbial ecology, stable isotope probing, and soil energetics to link microbial abundance, community structure and activity with soil biogeochemical processes and plant-microbe interactions