Seasonal variability of CO2, CH4, and N2O content and fluxes in small agricultural reservoirs of the northern Great Plains

Abstract

Inland waters are important global sources, and occasional sinks, of CO2, CH4, and N2O to the atmosphere, but relatively little is known about the contribution of GHGs of constructed waterbodies, particularly small sites in agricultural regions that receive large amounts of nutrients (carbon, nitrogen, phosphorus). Here, we quantify the magnitude and controls of diffusive CO2, CH4, and N2O fluxes from 20 agricultural reservoirs on seasonal and diel timescales. All gases exhibited consistent seasonal trends, with CO2 concentrations highest in spring and fall and lowest in mid-summer, CH4 highest in mid-summer, and N2O elevated in spring following ice-off. No discernible diel trends were observed for GHG content. Analyses of GHG covariance with potential regulatory factors were conducted using generalized additive models (GAMs) that revealed CO2 concentrations were affected primarily by factors related to benthic respiration, including dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), dissolved organic carbon (DOC), stratification strength, and water source (as δ18Owater). In contrast, variation in CH4 content was correlated positively with factors that favoured methanogenesis, and so varied inversely with DO, soluble reactive phosphorus (SRP), and conductivity (a proxy for sulfate content), and positively with DIN, DOC, and temperature. Finally, N2O concentrations were driven mainly by variation in reservoir mixing (as buoyancy frequency), and were correlated positively with DO, SRP, and DIN levels and negatively with pH and stratification strength. Estimates of mean CO2-eq flux during the open-water period ranged from 5,520 mmol m−2 year1 (using GAM-predictions) to 10,445 mmol m−2 year−1 (using interpolations of seasonal data) reflecting how extreme values were extrapolated, with true annual flux rates likely falling between these two estimates.

Differential Controls of Greenhouse Gas (CO2, CH4, and N2O) Concentrations in Natural and Constructed Agricultural Waterbodies on the Northern Great Plains

“…Surfacewater conductivity is highly correlated with sulfate concentrations (adj. R 2 = 0.83) in these systems and previous analyses (Jensen et al, 2022), suggesting that sulfate concentrations (up to 3,883 mg L −1 in reservoirs; 6,687 mg L −1 in ponds) can be high enough to inhibit methanogenesis in freshwaters (Lovley & Klug, 1983). Similar negative relationships between CH 4 concentrations and conductivity are known from other reservoirs in our study region , as well as natural wetlands in North America and Europe (Gauci et al, 2004).…”

“…Mean air concentrations were 413.09 μatm for CO 2 , 1.868 μatm for CH 4 , and 0.333 μatm for N 2 O. Diffusive flux was calculated using the average gas transfer velocity as measured directly on comparable agricultural reservoirs in 2017 (k 600 -CO 2 = 1.50 m d −1 ; k 600 -CH 4 = 1.64 m d −1 , Webb, Hayes, et al, 2019; k 600 -N 2 O = 1.64 m d −1 ) following procedures of and Jensen et al (2022).…”

“…Small inland waterbodies are well‐known hotspots of greenhouse gas (GHG) emissions (Cole et al., 2007; Tranvik et al., 2009) owing to their cumulative abundance in many regions (Downing et al., 2006) and because they often release carbon dioxide (CO 2 ) and methane (CH 4 ) at higher rates than larger inland waters (Downing, 2010). However, GHG concentrations are extremely variable, both spatially and temporally in these small surface waters (Deemer et al., 2016; Downing, 2010; Holgerson & Raymond, 2016; Jensen et al., 2022), with some sites acting as net GHG sinks (Webb, Hayes, et al., 2019; Webb, Leavitt, et al., 2019). Measurements of all three major GHGs (CO 2 , CH 4 , and nitrous oxide (N 2 O)) are providing insights into spatial and temporal patterns of GHG flux (Huttunen et al., 2002; Jensen et al., 2022; Liikanen et al., 2006; Whitfield et al., 2011), but it is not currently known whether different water body types (natural vs. artificially constructed) respond similarly to underlying mechanisms controlling gas fluxes.…”

“…However, GHG concentrations are extremely variable, both spatially and temporally in these small surface waters (Deemer et al., 2016; Downing, 2010; Holgerson & Raymond, 2016; Jensen et al., 2022), with some sites acting as net GHG sinks (Webb, Hayes, et al., 2019; Webb, Leavitt, et al., 2019). Measurements of all three major GHGs (CO 2 , CH 4 , and nitrous oxide (N 2 O)) are providing insights into spatial and temporal patterns of GHG flux (Huttunen et al., 2002; Jensen et al., 2022; Liikanen et al., 2006; Whitfield et al., 2011), but it is not currently known whether different water body types (natural vs. artificially constructed) respond similarly to underlying mechanisms controlling gas fluxes. These uncertainties lead to questions of the accuracy of regional upscaling models and estimates of the importance of small surface waters in calculations of net global warming potential of inland waters (Webb, Hayes, et al., 2019; Webb, Leavitt, et al., 2019).…”

“…Of the limited research to date, studies from Australia (Grinham, Albert, et al., 2018; Ollivier et al., 2018, 2019), and India (Panneer Selvam et al., 2014) show that small agricultural reservoirs typically emit CO 2 , CH 4 , and N 2 O to the atmosphere. In contrast, work on the northern Great Plains in Canada suggests that while all reservoirs are sources of CH 4 , more than half of sites act as sinks for CO 2 (52%) and N 2 O (67%) (Webb, Hayes, et al., 2019; Webb, Leavitt, et al., 2019), with predictable seasonal patterns of gas flux (Jensen et al., 2022), suggesting high variation among global agricultural regions.…”