Drainage gradient versus seasonal cycles: Differential response of microbial community composition to variations in soil moisture

Authors: BURGESS, CHRISTOPHER - Oregon State University; MYROLD, DAVID - Oregon State University; MUELLER, DAVID - Oregon State University; WANZEK, TOM - Oregon State University; Moore, Jennifer; KASSCHAU, KRISTIN - Oregon State University; KLEBER, MARKUS - Oregon State University

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2024
Publication Date: 10/23/2024
Citation: Burgess, C., Myrold, D., Mueller, D., Wanzek, T., Moore, J.M., Kasschau, K.D., Kleber, M. 2024. Drainage gradient versus seasonal cycles: Differential response of microbial community composition to variations in soil moisture. Soil Science Society of America Journal. 88(6):2123-2134. https://doi.org/10.1002/saj2.20780.
DOI: https://doi.org/10.1002/saj2.20780

Interpretive Summary: The composition and activities of soil microorganisms provide important ecosystem services, including food and fiber production, water filtration and storage, and carbon accrual. Soil microbes are sensitive to changes in environmental conditions including soil moisture due to the flow and availability of resources (carbon and nutrients) and oxygen availability, necessary for growth and maintenance. Three neighboring soils that differed only in their drainage class were sampled monthly over a 12-month period. Soils in this region are exposed to heavy precipitation during the cool, wet winters and dry down during the warm, dry summers. Thus, the combined effects of drainage class and seasonal precipitation patterns created a unique combination to determine how strong the variation in soil moisture and oxygen needs to be to induce a shift to a compositionally and, potentially, functionally distinct microbial community. Soil bacterial diversity and community composition were characterized using molecular sequencing tools for each monthly soil sample. The number of different microbial taxa (a-diversity) varied little throughout the year across all three soils supporting the notion that microbial communities are adapted to seasonal fluctuation is soil moisture. We did find that the composition of the bacterial community in the poorly drained soils was distinct from the well-drained and moderately well-drained soil. The poorly drained soil is exposed to a period of water saturation that limits oxygen availability, which is the most probable cause setting that microbial community apart from the better drained soils which do not experience this level of oxygen limitation. Our investigation suggests that the soil microbial community is likely adapted to seasonal fluctuations in soil moisture; however, if soil drainage was altered enough to impact seasonal oxygen availability beyond which microbial communities adapted to, then short-term fluctuations in the environmental driver "soil moisture availability” would induce a persistent change in the community composition.

Technical Abstract: The temporal variation of microbial community composition was assessed at monthly time steps for one year in three neighboring and closely related Mollisols spanning a gradient in drainage class. This was done to distinguish between natural oscillations in the community composition and lasting adaptations to environmental change. Specifically, there is little knowledge about how strong an external influence / signal needs to be to induce a microbial community to persistent change its composition. To isolate soil water availability as a controlling factor, we selected three soils sharing the same soil order (fine-silty, superactive Argixerolls/Argialbolls), topography, temperature (mesic), and moisture (xeric) regimes, and land use history (continuous grassland for the past 20 years) but differing in drainage class (well drained, poorly drained, and moderately well drained). Variations in microbial diversity were quantified by monitoring both alpha and beta diversity. Within individual soils, a-diversity varied little throughout the year and did not show a trend towards a state change. While the three soils developed under the same climate regime and vegetation/land use; they developed distinct community composition, turnover and metabolic capabilities. We posit that an annually recurring shift in soil redox state induced by a 100 - day period of intermittent water saturation in the poorly drained soil is the most probable cause setting that microbial community apart from those in the better drained soils. Our investigation suggests the existence of thresholds of prolonged water saturation beyond which microbial communities adapted to short-term oscillations in the environmental driver "soil moisture availability", thereby inducing a persistent change in the community composition.