Inter-Annual Variability of Area-Scaled Gaseous Carbon Emissions from Wetland Soils in the Liaohe Delta, China

<div><p>Global management of wetlands to suppress greenhouse gas (GHG) emissions, facilitate carbon (C) sequestration, and reduce atmospheric CO₂ concentrations while simultaneously promoting agricultural gains is paramount. However, studies that relate variability in CO₂ and CH₄ emissions at large spatial scales are limited. We investigated three-year emissions of soil CO₂ and CH₄ from the primary wetland types of the Liaohe Delta, China, by focusing on a total wetland area of 3287 km². One percent is <i>Suaeda salsa</i>, 24% is <i>Phragmites australis</i>, and 75% is rice. While <i>S</i>. <i>salsa</i> wetlands are under somewhat natural tidal influence, <i>P</i>. <i>australis</i> and rice are managed hydrologically for paper and food, respectively. Total C emissions from CO₂ and CH₄ from these wetland soils were 2.9 Tg C/year, ranging from 2.5 to 3.3 Tg C/year depending on the year assessed. Primary emissions were from CO₂ (~98%). Photosynthetic uptake of CO₂ would mitigate most of the soil CO₂ emissions, but CH₄ emissions would persist. Overall, CH₄ fluxes were high when soil temperatures were >18°C and pore water salinity <18 PSU. CH₄ emissions from rice habitat alone in the Liaohe Delta represent 0.2% of CH₄ carbon emissions globally from rice. With such a large area and interannual sensitivity in soil GHG fluxes, management practices in the Delta and similar wetlands around the world have the potential not only to influence local C budgeting, but also to influence global biogeochemical cycling.</p></div

Hydrographs for wetland study sites in the Liaohe Delta.

<p>a) Water level patterns for 2012, b) water level patterns for 2013, and c) water level patterns for 2014 from our five wetland sites, including two <i>Phragmites australis</i> sites (Phrag1, Phrag2), two <i>Suaeda salsa</i> sites (Suaeda1, Suaeda2), and one rice paddy site (Rice) in the Liaohe Delta, China. Missing data from Suaeda1 and Suaeda2 at the beginning of 2013, and for Suaeda2 beginning in August of 2014, represent datalogger failure. Consistent water levels <-30 cm for Phrag1 and Rice indicate times when water levels were below pressure transducers embedded in the soils.</p

Location of study sites and aerial distribution of habitat types sampled in the Liaohe Delta, China.

<p>Map highlights 31.6 km² of <i>Suaeda salsa</i> wetlands, 786 km² of <i>Phragmites australis</i> wetlands, and 2464.6 km² of rice paddy wetlands, as well as the location of our five wetland sites, including two in <i>Phragmites australis</i> (Phrag1, Phrag2), two in <i>Suaeda salsa</i> (Suaeda1, Suaeda2), and one in rice paddy (Rice). Aerial distribution data are from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0160612#pone.0160612.ref030" target="_blank">30</a>], and the shape file represents 2011 classifications (China Geological Survey).</p

Salinity, soil temperature, and water table depth versus soil CO₂ and CH₄ fluxes.

<p>a) Soil pore water salinity versus soil CO₂ flux, b) soil temperature versus soil CO₂ flux, c) water table depth (WTD) versus soil CO₂ flux, d) soil pore water salinity versus soil CH₄ flux, e) soil temperature versus soil CH₄ flux, and f) water table depth (WTD) versus soil CH₄ flux by month and site over three years from five wetland sites (two <i>Phragmites australis</i>, two <i>Suaeda salsa</i>, one rice) located in the Liaohe Delta, China. When present, dashed lines depict important thresholds associated with soil pore water salinity <18 PSU (a, d) and soil temperature >18°C (b, e). For d, the polynomial regression has been re-drawn from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0160612#pone.0160612.ref016" target="_blank">16</a>] and superimposed on data collected from the Liaohe Delta. R = Pearson Correlation Coefficient (P < 0.05 for all).</p

Temporal fluctuation in mean (± 1 SE) soil CO₂ and CH₄ fluxes.

<p>a) Soil CO₂ fluxes by sample month over three years, and b) soil CH₄ fluxes by sample month over three years from five wetland sites (two <i>Phragmites australis</i>, two <i>Suaeda salsa</i>, one rice) located in the Liaohe Delta, China. Mean values reflect the responses of six replicate chambers per sampling event per site, each canvassing a 55x55 cm soil area. Fluxes were assumed to be zero from December-March when soils were frozen, as depicted by a straight line between open circles.</p

Nested design ANOVA for soil CO₂ and CH₄ fluxes over three years from among five wetland sites (two <i>P</i>. <i>australis</i>; two <i>S</i>. <i>salsa</i>, and one rice) in the Liaohe Delta, China.

<p>DF_num = numerator degrees of freedom, DF_den = denominator degrees of freedom, MS = Mean Squares, F = F-statistic, P = Probability value (significant if ≤ 0.05).</p

Mean CO₂ fluxes, CH₄ fluxes, and a suite of physico-chemical characteristics of soils (± SE) from five wetland sites in the Liaohe Delta, China collected over three years.

<p>Mean CO₂ fluxes, CH₄ fluxes, and a suite of physico-chemical characteristics of soils (± SE) from five wetland sites in the Liaohe Delta, China collected over three years.</p

Hourly mean (± 1 SE) soil CO₂ and CH₄ fluxes by site.

<p>a) Soil CO₂ fluxes, and b) soil CH₄ fluxes by site over three years from five wetland sites (two <i>Phragmites australis</i>, two <i>Suaeda salsa</i>, one rice) located in the Liaohe Delta, China. Means followed by the same letters are not significantly different at α = 0.05. While these site means and differences represent the general trends persisting across all months sampled, a significant site by date interaction (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0160612#pone.0160612.t001" target="_blank">Table 1</a>) limits interpretation when grouped in the fashion.</p