Carbon dioxide
Mean soil CO2 efflux (mg CO2-C m-2 h-1) for the reference, buffer, and no buffer sites for the entire sampling period (June to September) was 65.7 ± 29.9, 76.0 ± 39.1, 71.5 ± 40.9, respectively (Figure 4A). Based on the linear mixed effects (LME) model, CO2emissions were not significantly different between treatments (Table 2). However, over the sampling period, CO2 emissions were, on average, 16% higher at the buffer sites and 9% higher at the no buffer sites compared to the reference sites, respectively. Mean soil CO2 emissions peaked in the middle of the growing season, with 85.9 ± 38.5 mg CO2 m-2h-1 in July and 83.6 ± 42.0 mg CO2m-2 h-1 in August. Mean soil CO2 emissions were the lowest in the spring followed by the autumn, with 53.1 ± 24.78 mg CO2m-2 h-1 in June and 56.9 ± 33.9 mg CO2 m-2 h-1 in September. CO2 fluxes were not significantly different between local groundwater conditions. On average, over the study period CO2 efflux was 64.9 ± 37.6 at DIS sites and 74.9 ± 36.3 mg CO2 m-2 h-1 at ND areas. Soil temperature, soil moisture, and depth to the groundwater table were significant predictors of CO2 fluxes according to the LME models, with a marginal r2 of 0.16, 0.11, and 0.06, respectively. For every degree increase in soil temperature, the mean CO2 flux rate increased by 8.13 mg. For every percent increase in soil moisture, the mean CO2 flux rate decreased by 0.58 mg. For every cm increase in depth to the groundwater table, the mean CO2flux rate increased by 0.48 mg.
Table 2. Pairwise difference between Treatment levels (reference, R; buffer, B; and no buffer, NB) using Tukey’s HSD post hoc test for the linear mixed effects models explaining the dynamics of carbon dioxide, methane, and nitrous oxide fluxes, respectively. All models included the autocorrelation term “AR1(Week + 0 | Site/Chamber)”. Bolded comparison indicates a significant effect at p < 0.05.
Model Comparison Est. SE p
CO2 ~ Treatment R - B -8.65 11.2 0.72
R - NB -1.86 11.4 0.99
B - NB 6.79 11.4 0.82
CH4 ~ Treatment R - B -6.65 6.82 0.59
R - NB -22.35 6.96 < 0.01
B - NB -15.71 6.97 0.07
N2O ~ Treatment R - B 0.33 0.14 0.06
R - NB 0.39 0.14 0.02
B - NB 0.06 0.14 0.90