Spatial and seasonal CH4 flux in the littoral zone of Miyun Reservoir near Beijing: the effects of water level and its fluctuation

Effects of Vegetation Types and Soil Properties on Regional Soil Carbon and Nitrogen in Salinized Reservoir Wetland, Northeast China

This study investigated the spatial variability in soil organic carbon (SOC), total nitrogen (TN), soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN) and their possible relationships with other soil properties in the Hongqipao reservoir, which is dominated by different vegetation types. The results showed that there were high spatial variabilities in SOC, TN, SMBC and SMBN, and that the SOC, TN, SMBC and SMBN contents decreased with increasing soil depth in the Hongqipao reservoir. The SOC was significantly positively correlated with TN, SMBC, moisture content (MC) and negatively correlated with carbon to nitrogen ratio (C:N ratio) and bulk density (BD). Soil TN was significantly positively correlated with SMBC, SMBN, MC and negatively correlated with the C:N ratio, BD and pH. The SMBC was significantly positively correlated with SMBN, MC and negatively correlated with the C:N ratio, BD and pH. The SMBN was significantly negatively correlated with the C:N ratio and BD. All of the measures of soil properties in this study could explain the higher significant variability in the response variables (SOC, TN, SMBC and SMBN contents). The generalized additive model (GAM) showed that SOC and TN had different influencing factors in different soil depths. The structural equation model (SEM) showed that vegetation types had a significantly positive effect on TN and SMBN, and the soil depths had a significantly positive effect on SOC and a significantly negative effect on TN and SMBC. This study further suggests that vegetation types play a major role in determining the spatial characteristics of soil carbon and nitrogen, and any changes in the vegetation types in the reservoir may influence the distributions of soil carbon and nitrogen. This may affect the global carbon and nitrogen budgets and the atmospheric greenhouse gas concentration significantly.

Greenhouse gas emissions from riparian zone cropland in a tributary bay of the Three Gorges Reservoir, China

BACKGROUND

A huge reservoir was formed by the Three Gorges Dam in China, which also formed a riparian zone along the bank of the reservoir. In the period of low water-level, the riparian zone in tributary bays of the Three Gorges Reservoir (TGR) was always unordered cultivated, owing to its gentle slope and high soil fertility. This land-use practice creates high potential of generating greenhouse gas (GHG) emissions with periodic water level fluctuation.

METHODS

To evaluate potential GHG emissions from the soil-air interface, the static opaque chamber method was adopted to evaluate the effect of elevations (180 m, 175 m, 170 m and 165 m) and land use types (dry lands, paddy fields and grass fields) from April to September in 2015 and 2016.

RESULTS

The results showed that carbon dioxide (CO2) was the main contributor of GHG emission in riparian zone most likely because of high organic carbon from residues. Furthermore, high soil water content in paddy fields resulted in significantly higher methane (CH4) flux than that in dry lands and grass fields. Compared to grass fields, anthropogenic activities in croplands were attributed with a decrease of soil total carbon and GHG emissions. However, inundation duration of different elevations was found to have no significant effect on CH4 and CO2 emissions in the riparian zone, and the mean nitrous oxide (N2O) flux from dry lands at an elevation of 165 m was significantly higher than that of other elevations likely because of tillage and manure application. The high N2O fluxes produced from tillage and fertilizer suggested that, in order to potentially mitigate GHG emissions from the riparian zone, more attention must be paid to the farming practices in dry lands at low elevations (below 165 m) in the riparian zone. Understanding factors that contribute to GHG emissions will help guide ecological restoration of riparian zones in the TGR.

Dynamics of greenhouse gas emission induced by different burrowing activities of fossorial vertebrates in the Qinghai-Tibetan Plateau alpine meadow ecosystem

Plateau zokor (Myospalax baileyi) and plateau pika (Ochotona curzoniae) are endemic fossorial vertebrates in the Qinghai-Tibetan Plateau alpine meadow ecosystem. Their different burrowing activities together transform soil structure and then significantly change the landscape of meadow ecosystem. However, how their burrowing activities impact greenhouse gas (GHG) emissions and the pattern of GHG emissions between different types of tunnel burrowing still remain obscure. In this study, we conducted in situ measurements quantitatively investigating the impacts of the different burrowing activities of zokors and pikas on three main GHG CO₂, CH₄, and N₂O from an alpine meadow ecosystem in southeastern Qinghai-Tibetan Plateau. Our results showed that zokor hummocks and pika burrows were sources of CO₂ and N₂O and sinks of CH₄. Zokors burrowing increased N₂O in the atmosphere, decreased CO₂, and enhanced CH₄ absorbing, while pikas burrowing increased N₂O in the atmosphere and enhanced CH₄ absorbing. Considering the controversial role of fossorial vertebrates in Qinghai-Tibetan Plateau, this study also shed lights on effective management of animal activities with the aim of stabilizing or increasing ecosystem carbon sequestration.

Soil water content and pH drive archaeal distribution patterns in sediment and soils of water-level-fluctuating zones in the East Dongting Lake wetland, China

Archaea play a vital role in Earth's geochemical cycles, but the factors that drive their distribution between sediments and water-level-fluctuating zones in the East Dongting Lake (EDL) wetland are poorly understood. Here, we used Illumina MiSeq to investigate the variation in the soil archaeal community structure and diversity among sediments and four water-level-fluctuating zones (mudflat, sedge, sedge-Phragmites, and Phragmites) in the EDL wetland. Diverse archaeal assemblages were found in our study, Crenarchaeota, Euryarchaeota, and ammonia-oxidizing and methanogenic subset were the dominant groups, and all their abundances shifted from sediment to water-level-fluctuating zones. The principal coordinates analysis and cluster analysis showed that the overall archaeal community structure was separated into two clusters: cluster I contained nine samples from sediment, mudflat, and sedge zones, whereas cluster II contained six samples from sedge-Phragmites and Phragmites zones. Archaeal diversity was significantly highest in sediment and lowest in Phragmites zone soils. The Mantel test showed that the variation in archaeal community structure was significantly positively correlated with soil water content and pH. The relative abundances of Crenarchaeota and Nitrososphaerales decreased with soil water content, while Euryarchaeota and Methanomicrobiales increased with soil water content. The relative abundance of Methanomicrobiales significantly decreased with pH (R² = 0.34-0.48). Chao 1, observed operational taxonomic units, Shannon index, and Simpson index all correlated significantly positively with water content (R² = 0.40-0.60), while Shannon and Simpson indexes both correlated significantly negatively with pH (R² = 0.20-0.37). Our results demonstrated that the variations in the archaeal community structure were markedly driven by soil water content and pH in the EDL wetland. Our findings suggested that archaeal communities shifted among sediment and four water-level-fluctuating zones, highlighting that the spatiotemporal heterogeneity of greenhouse gas flux in small scale should be taken into account for accurate prediction of greenhouse gas emissions in the Dongting Lake area, especially on the background of climate change and human activities.

Contrasting methane emissions from upstream and downstream rivers and their associated subtropical reservoir in eastern China

Subtropical reservoirs are an important source of atmospheric methane (CH4). This study investigated the spatiotemporal variability of bubble and diffusive CH4 emissions from a subtropical reservoir, including its upstream and downstream rivers, in eastern China. There was no obvious seasonal variation in CH4 emissions from the main reservoir, which increased slightly from the first half year to the next half year. In the upstream river, CH4 emissions were low from February to June and fluctuated widely from July to January due to bubble activity. In the downstream river, CH4 emissions were lowest in February, which was possibly influenced by the low streamflow rate from the reservoir (275 m3 s-1) and a short period of mixing. There was spatial variability in CH4 emissions, where fluxes were highest from the upstream river (3.65 ± 3.24 mg CH4 m-2 h-1) and lowest from the main reservoir (0.082 ± 0.061 mg CH4 m-2 h-1), and emissions from the downstream river were 0.49 ± 0.20 mg CH4 m-2 h-1. Inflow rivers are hot spots in bubble CH4 emissions that should be examined using field-sampling strategies. This study will improve the accuracy of current and future estimations of CH4 emissions from hydroelectric systems and will help guide mitigation strategies for greenhouse gas emissions.

Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis

Collectively, reservoirs created by dams are thought to be an important source of greenhouse gases (GHGs) to the atmosphere. So far, efforts to quantify, model, and manage these emissions have been limited by data availability and inconsistencies in methodological approach. Here, we synthesize reservoir CH₄, CO₂, and N₂O emission data with three main objectives: (1) to generate a global estimate of GHG emissions from reservoirs, (2) to identify the best predictors of these emissions, and (3) to consider the effect of methodology on emission estimates. We estimate that GHG emissions from reservoir water surfaces account for 0.8 (0.5-1.2) Pg CO₂ equivalents per year, with the majority of this forcing due to CH₄. We then discuss the potential for several alternative pathways such as dam degassing and downstream emissions to contribute significantly to overall emissions. Although prior studies have linked reservoir GHG emissions to reservoir age and latitude, we find that factors related to reservoir productivity are better predictors of emission.