Biogeochemical regime shifts in coastal landscapes: the contrasting effects of saltwater incursion and agricultural pollution on greenhouse gas emissions from a freshwater wetland

Abstract

Many coastal plain wetlands receive nutrient pollution from agricultural fields and are particularly vulnerable to saltwater incursion. Although wetlands are a major source of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O), the consequences of salinization for greenhouse gas emissions from wetlands with high agricultural pollution loads is rarely considered. Here, we asked how saltwater exposure alters greenhouse gas emissions from a restored freshwater wetland that receives nutrient loading from upstream farms. During March to November 2012, we measured greenhouse gases along a ~2 km inundated portion of the wetland. Sampling locations spanned a wide chemical gradient from sites receiving seasonal fertilizer nitrogen and sulfate (SO₄ ²⁻) loads to sites receiving seasonal increases in marine salts. Concentrations and fluxes of CH₄ were low (<100 µg L⁻¹ and <10 mg m⁻² h⁻¹) for all sites and sampling dates when SO₄ ²⁻ was high (>10 mg L⁻¹), regardless of whether the SO₄ ²⁻ source was agriculture or saltwater. Elevated CH₄ (as high as 1,500 µg L⁻¹ and 45 mg m⁻² h⁻¹) was only observed on dates when air temperatures were >27 °C and SO₄ ²⁻ was <10 mg L⁻¹. Despite elevated ammonium (NH₄ ⁺) for saltwater exposed sites, concentrations of N₂O remained low (<5 µg L⁻¹ and <10 µg m⁻² h⁻¹), except when fertilizer derived nitrate (NO₃ ⁻) concentrations were high and N₂O increased as high as 156 µg L⁻¹. Our results suggest that although both saltwater and agriculture derived SO₄ ²⁻ may suppress CH₄, increases in N₂O associated with fertilizer derived NO₃ ⁻ may offset that reduction in wetlands exposed to both agricultural runoff and saltwater incursion.