Soil Methane and Carbon Dioxide Fluxes from Cropland and Riparian Buffers in Different Hydrogeomorphic Settings

Abstract

Riparian buffers contribute to the mitigation of nutrient pollution in agricultural landscapes, but there is concern regarding their potential to be hot spots of greenhouse gas production. This study compared soil CO₂ and CH₄ fluxes in adjacent crop fields and riparian buffers (a flood-prone forest and a flood-protected grassland along an incised channel) and examined the impact of water table depth (WTD) and flood events on the variability of gas fluxes in riparian zones. Results showed significantly (P < 0.001) higher CO₂ emission in riparian areas than in adjoining croplands (6.8 ± 0.6 vs. 3.6 ± 0.5 Mg CO₂–C ha⁻¹ yr⁻¹; mean ± SE). Daily flux of CO₂ and soil temperature were significantly related (P < 0.002), with Q₁₀ values ranging between 1.75 and 2.53. Significant relationships (P < 0.05) were found between CH₄ daily flux and WTD. Flood events resulted in enhanced CH₄ emission (up to +44.5 mg CH₄–C m⁻² d⁻¹ in a swale) under warm soil conditions (>22°C), but the effect of flooding was less pronounced in early spring (emission <1.06 mg CH₄–C m⁻² d⁻¹), probably due to low soil temperature. Although CH₄ flux direction alternated at all sites, overall the croplands and the flood-affected riparian forest were CH₄ sources, with annual emission averaging +0.04 ± 0.17 and +0.92 ± 1.6 kg CH₄–C ha⁻¹, respectively. In the riparian forest, a topographic depression (<8% of the total area) accounted for 78% of the annual CH₄ emission, underscoring the significance of landscape heterogeneity on CH₄ dynamics in riparian buffers. The nonflooded riparian grassland, however, was a net CH₄ sink (−1.08 ± 0.22 kg CH₄–C ha⁻¹ yr⁻¹), probably due to the presence of subsurface tile drains and a dredged/incised channel at that study site. Although these hydrological alterations may have contributed to improvement in the CH₄ sink strength of the riparian grassland, this must be weighed against the water quality maintenance functions and other ecological services provided by riparian buffers.