Seasonal variations in metallic mercury (Hg0) vapor exchange over biannual wheat–corn rotation cropland in the North China Plain

Jonas Sommar, Wei Zhu, Lihai Shang, Che-Jen Lin, Xinbin Feng
2016 Biogeosciences  
<p><strong>Abstract.</strong> Air–surface gas exchange of Hg<sup>0</sup> 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&amp;ndash;2013) aiming to capture the flux pattern over essential growing stages of the planting system with a low homogeneous topsoil Hg content ( ∼
more » ... soil Hg content ( ∼ <span class="thinspace"></span>45<span class="thinspace"></span>ng<span class="thinspace"></span>g<sup>&amp;minus;1</sup>). Contrasting pollution regimes influenced air masses at the site and corresponding Hg<sup>0</sup> concentration means (3.3 in late summer to 6.2<span class="thinspace"></span>ng<span class="thinspace"></span>m<sup>−3</sup> in winter) were unanimously above the typical hemispheric background of 1.5&amp;ndash;1.7<span class="thinspace"></span>ng<span class="thinspace"></span>m<sup>&amp;minus;3</sup> during the campaigns. Extreme values in bi-directional net Hg<sup>0</sup> exchange were primarily observed during episodes of peaking Hg<sup>0</sup> concentrations. In tandem with under-canopy chamber measurements, the above-canopy REA measurements provided evidence for a balance between Hg<sup>0</sup> ground emissions and uptake of Hg<sup>0</sup> by the developed canopies. During the wheat growing season covering ∼ <span class="thinspace"></span>2<span class="thinspace"></span>/<span class="thinspace"></span>3 of the year at the site, net field-scale Hg<sup>0</sup> emission prevailed for periods of active plant growth until canopy senescence (mean flux: 20.0<span class="thinspace"></span>ng<span class="thinspace"></span>m<sup>&amp;minus;3</sup>), showing the dominance of Hg<sup>0</sup> soil efflux during warmer seasons. In the final vegetative stage of corn and wheat, ground and above-canopy Hg<sup>0</sup> flux displayed inversed daytime courses with a near mid-day maximum (emission) and minimum (deposition), respectively. In contrast to wheat, Hg<sup>0</sup> uptake of the corn canopy at this stage offset ground Hg<sup>0</sup> emissions with additional removal of Hg<sup>0</sup> from the atmosphere. Differential uptake of Hg<sup>0</sup> between wheat (C<sub>3</sub> species) and corn (C<sub>4</sub> species) foliage is discernible from estimated Hg<sup>0</sup> flux (per leaf area) and Hg content in mature cereal leaves, being a factor of &amp;gt;<span class="thinspace"></span>3 higher for wheat (at ∼ <span class="thinspace"></span>120<span class="thinspace"></span>ng<span class="thinspace"></span>g<sup>&amp;minus;1</sup> dry weight). Furthermore, this study shows that intermittent flood irrigation of the air-dry field induced a short pulse of Hg<sup>0</sup> emission due to displacement of Hg<sup>0</sup> present in the surface soil horizon. A more lingering effect of flood irrigation is however suppressed Hg<sup>0</sup> soil emissions, which for wet soil ( ∼ <span class="thinspace"></span>30<span class="thinspace"></span>% vol) beneath the corn canopy was on average a factor of ∼ <span class="thinspace"></span>3 lower than that for drier soil (&amp;lt;<span class="thinspace"></span>10<span class="thinspace"></span>% vol) within wheat stands. Extrapolation of the campaign Hg<sup>0</sup> flux data (mean: 7.1<span class="thinspace"></span>ng<span class="thinspace"></span>m<sup>&amp;minus;2</sup><span class="thinspace"></span>h<sup>&amp;minus;1</sup>) to the whole year suggests the wheat–corn rotation cropland to be a net source of atmospheric Hg<sup>0</sup>. The observed magnitude of annual wet deposition flux ( ∼ <span class="thinspace"></span>8.8<span class="thinspace"></span>µg<span class="thinspace"></span>Hg<span class="thinspace"></span>m<sup>&amp;minus;2</sup>) accounted for a minor fraction of soil Hg<sup>0</sup> 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 Hg<sup>0</sup> 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 Hg<sup>0</sup> emissions from croplands in the NCP region.</p>
doi:10.5194/bg-13-2029-2016 fatcat:vllt4n2zlfh2vkcyhnhms2kq2a