Impacts of moisture, soil respiration, and agricultural practices on methanogenesis in upland soils as measured with stable isotope pool dilution

Authors: BREWER, PAUL - Colorado State University; Calderon, Francisco; Vigil, Merle; VON FISCHER, JOSEPH - Colorado State University

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/13/2018
Publication Date: 12/20/2018
Citation: Brewer, P., Calderon, F.J., Vigil, M.F., Von Fischer, J. 2018. Impacts of moisture, soil respiration, and agricultural practices on methanogenesis in upland soils as measured with stable isotope pool dilution. Soil Biology and Biochemistry. 127:239-251. https://www.sciencedirect.com/science/article/pii/S0038071718303092?via%3Dihub.
DOI: https://doi.org/10.1016/j.soilbio.2018.09.014

Interpretive Summary: Soils can be sources or sinks of atmospheric methane, which is a powerful greenhouse gas. Typically, waterlogged soils are net producers of methane, while drier soils tend to absorb methane from the atmosphere. However, this pattern is not universal, given that upland soils can sometimes be net sources of methane due to the slow movement of oxygen into the soil. For this project, we wanted to test the idea that methane production in agricultural soils is mainly determined by soil moisture and soil organic matter. We studied the relationships between soil properties, methane production, and in the laboratory, comparing conventional, no-till, and organic managements, under different soil moistures. Our results show that organically-managed soils tended to produce more methane than conventionally managed soils within six weeks of incubation, while soil moisture had a stronger effect on later measurements.

Technical Abstract: Nutrient availability, carbon cycling, and greenhouse gas fluxes vary greatly between oxic and anoxic environments. Anoxic microsites can exist in upland soils and host an array of anaerobic processes, including methanogenesis, but little is known about the controls of anoxic microsite formation, persistence, and biogeochemical impact. In agricultural systems, there is reason to expect that management practices could alter anoxic microsite distributions and thus affect redox-sensitive processes like greenhouse gas and nitrogen fluxes. Based on work at larger scales (e.g., wetlands, sediments), where anoxia is driven by the balance of O2 diffusion and consumption, we hypothesize that microsite methanogenesis is primarily affected by soil moisture and organic matter. To test this hypothesis, we examine the relationships between soil properties, methanogenic microsite dynamics, and biogeochemical effects in a full factorial laboratory experiment over soil source (semi-arid and mesic ecosystems), agricultural practice (conventional, no-till, and organic), and moisture (10 to 90% water-filled porespace, WFPS). We measured methanogenesis as an indicator of anoxia over five months in an incubation of soil cores. Consistent with the hypotheses from larger scales, our results show that organically-managed soils had higher rates of methanogenesis than conventionally tilled soils, and that rates of methanogenesis were greatest in soils above 70% WFPS. However, effects of site on methanogenesis were comparatively minor. Interestingly, rates of methanogenesis changed over time with early methanogenesis (6th week of incubation) correlated with agricultural practice and soil respiration while later rates dates (21st week) were better predicted by WFPS. Methanogenic persistence was also associated with elevated WFPS, and this relationship was especially strong in no-till soils. Because methanogenesis was associated with high CO2 flux and WFPS, at different times in the incubation it appears that anoxic microsite dynamics can result from changes in the underlying biological and physical mechanisms driving anoxia (i.e., O2 consumption and diffusion). Higher rates of methanogenesis were associated with higher soil NH4+, less NO3-, more N2O emissions, and less microbial biomass. Moreover, anoxic microsites and their impacts may form and persist more often in wet or high SOC soils, such as those under organic or no-till agricultural production.