Seasonal variations in metallic mercury (Hg0) vapor exchange over biannual wheat–corn rotation cropland in the North China Plain
Abstract. Air–surface gas exchange of Hg0 was measured in five approximately bi-weekly campaigns (in total 87 days) over a wheat–corn rotation cropland located on the North China Plain (NCP) using the relaxed eddy accumulation (REA) technique. The campaigns were separated over the duration of a full-year period (2012–2013) aiming to capture the flux pattern over essential growing stages of the planting system with a low homogeneous topsoil Hg content ( ∼ 45 ng g−1). Contrasting pollution regimes influenced air masses at the site and corresponding Hg0 concentration means (3.3 in late summer to 6.2 ng m−3 in winter) were unanimously above the typical hemispheric background of 1.5–1.7 ng m−3 during the campaigns. Extreme values in bi-directional net Hg0 exchange were primarily observed during episodes of peaking Hg0 concentrations. In tandem with under-canopy chamber measurements, the above-canopy REA measurements provided evidence for a balance between Hg0 ground emissions and uptake of Hg0 by the developed canopies. During the wheat growing season covering ∼ 2 / 3 of the year at the site, net field-scale Hg0 emission prevailed for periods of active plant growth until canopy senescence (mean flux: 20.0 ng m−3), showing the dominance of Hg0 soil efflux during warmer seasons. In the final vegetative stage of corn and wheat, ground and above-canopy Hg0 flux displayed inversed daytime courses with a near mid-day maximum (emission) and minimum (deposition), respectively. In contrast to wheat, Hg0 uptake of the corn canopy at this stage offset ground Hg0 emissions with additional removal of Hg0 from the atmosphere. Differential uptake of Hg0 between wheat (C3 species) and corn (C4 species) foliage is discernible from estimated Hg0 flux (per leaf area) and Hg content in mature cereal leaves, being a factor of > 3 higher for wheat (at ∼ 120 ng g−1 dry weight). Furthermore, this study shows that intermittent flood irrigation of the air-dry field induced a short pulse of Hg0 emission due to displacement of Hg0 present in the surface soil horizon. A more lingering effect of flood irrigation is however suppressed Hg0 soil emissions, which for wet soil ( ∼ 30 % vol) beneath the corn canopy was on average a factor of ∼ 3 lower than that for drier soil (< 10 % vol) within wheat stands. Extrapolation of the campaign Hg0 flux data (mean: 7.1 ng m−2 h−1) to the whole year suggests the wheat–corn rotation cropland to be a net source of atmospheric Hg0. The observed magnitude of annual wet deposition flux ( ∼ 8.8 µg Hg m−2) accounted for a minor fraction of soil Hg0 evasion flux prevailing over the majority of the year. Therefore, we suggest that dry deposition of other forms of airborne Hg constitutes the dominant pathway of Hg input to this local ecosystem and that these deposited forms would be gradually transformed and re-emitted as Hg0 rather than being sequestered here. In addition, after crop harvesting, the practice of burning agricultural residue with considerable Hg content rather than straw return management yields seasonally substantial atmospheric Hg0 emissions from croplands in the NCP region.