Methane flux measurements along a floodplain soil moisture gradient in the Okavango Delta, Botswana

Data-poor tropical wetlands constitute an important source of atmospheric CH 4 in the world. We studied CH 4 fluxes using closed chambers along a soil moisture gradient in a tropical seasonal swamp in the Okavango Delta, Botswana, the sixth largest tropical wetland in the world. The objective of the study was to assess net CH 4 fluxes and controlling environmental factors in the Delta's seasonal floodplains. Net CH 4 emissions from seasonal floodplains in the wetland were estimated at 0.072 ± 0.016 Tg a −1 . Microbial CH 4 oxidation of approximately 2.817 × 10 −3 ± 0.307 × 10 −3 Tg a −1 in adjacent dry soils of the occasional floodplains accounted for the sink of 4% of the total soil CH 4 emissions from seasonal floodplains. The observed microbial CH 4 sink in the Delta's dry soils is, therefore, comparable to the global average sink of 4–6%. Soil water content (SWC) and soil organic matter were the main environmental factors controlling CH 4 fluxes in both the seasonal and occasional floodplains. The optimum SWC for soil CH 4 emissions and oxidation in the Delta were estimated at 50% and 15%, respectively. Electrical conductivity and pH were poorly correlated ( r 2 ≤ 0.11, p < 0.05) with CH 4 fluxes in the seasonal floodplain at Nxaraga. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part1)'.

Carbon and Nitrogen Mineralization in Dark Grey Calcareous Floodplain Soil Is Influenced by Tillage Practices and Residue Retention

Very little is known about the changes that occur in soil organic carbon (SOC) and total nitrogen (TN) under an intensive rice-based cropping system following the change to minimal tillage and increased crop residue retention in the Gangetic Plains of South Asia. The field experiment was conducted for 3 years at Rajbari, Bangladesh to examine the impact of tillage practices and crop residue retention on carbon (C) and nitrogen (N) cycling. The experiment comprised four tillage practices—conventional tillage (CT), zero tillage (ZT), strip-tillage (ST), and bed planting (BP) in combination with two residue retention levels—increased residue (R50%) and low residue (R20%—the current practice). The TN, SOC, and mineral N (NH4+-N and NO3−-N) were measured in the soil at different crop growth stages. After 3 years, ZT, ST, and BP sequestered 12, 11, and 6% more SOC, and 18, 13, and 10% more TN, respectively than the conventional crop establishment practice at 0–5 cm soil depth. The accumulation of SOC and TN was also higher compared to the initial SOC and TN in soil. Among the tillage practices, the maximum SOC and TN sequestration were recorded with ST and with R50% that might be attributed to reduced mineralization of C and N in soil particularly with increased residue retention, since decay rates of potentially mineralizable C was lower in the ST with both the residue retention practices. Increased residue retention and minimum tillage practices after nine consecutive crops has altered the C and N cycling by slowing the in-season turnover of C and N, reducing the level of nitrate-N available to plants in the growing season and increasing retained soil levels of SOC and TN.

Meanders as a scaling motif for understanding of floodplain soil microbiome and biogeochemical potential at the watershed scale

Background Biogeochemical exports from watersheds are modulated by the activity of microorganisms that function over micron scales. Here, we tested the hypothesis that meander-bound regions share a core microbiome and exhibit patterns of metabolic potential that broadly predict biogeochemical processes in floodplain soils along a river corridor. Results We intensively sampled the microbiomes of floodplain soils located in the upper, middle, and lower reaches of the East River, Colorado. Despite the very high microbial diversity and complexity of the soils, we reconstructed 248 quality draft genomes representative of subspecies. Approximately one third of these bacterial subspecies was detected across all three locations at similar abundance levels, and ~ 15% of species were detected in two consecutive years. Within the meander-bound floodplains, we did not detect systematic patterns of gene abundance based on sampling position relative to the river. However, across meanders, we identified a core floodplain microbiome that is enriched in capacities for aerobic respiration, aerobic CO oxidation, and thiosulfate oxidation with the formation of elemental sulfur. Given this, we conducted a transcriptomic analysis of the middle floodplain. In contrast to predictions made based on the prominence of gene inventories, the most highly transcribed genes were relatively rare amoCAB and nxrAB (for nitrification) genes, followed by genes involved in methanol and formate oxidation, and nitrogen and CO2 fixation. Within all three meanders, low soil organic carbon correlated with high activity of genes involved in methanol, formate, sulfide, hydrogen, and ammonia oxidation, nitrite oxidoreduction, and nitrate and nitrite reduction. Overall, the results emphasize the importance of sulfur, one-carbon and nitrogen compound metabolism in soils of the riparian corridor. Conclusions The disparity between the scale of a microbial cell and the scale of a watershed currently limits the development of genomically informed predictive models describing watershed biogeochemical function. Meander-bound floodplains appear to serve as scaling motifs that predict aggregate capacities for biogeochemical transformations, providing a foundation for incorporating riparian soil microbiomes in watershed models. Widely represented genetic capacities did not predict in situ activity at one time point, but rather they define a reservoir of biogeochemical potential available as conditions change.

Morphological Features of Soils in the Yelshanka River Floodplain Under Conditions of Urbotechnopedogenesis

The river system on the territory of Volgograd is represented by eight small rivers. Currently, in the territory of Volgograd there is an active inclusion of valley and floodplain landscapes in the urban area. Under conditions of the anthropogenic impact, the normal regime of valley and floodplain landscapes is disturbed. As a result of changes in the hydrological regime of floodplains and valleys, various morphological transformations of soil cover occur, such as the formation of specific iron and carbonate new formations, etc. As a result of economic activities, modern alluvial soils and sediments are often buried under the thickness of the anthropogenic material. The relevance of the study is due to the high recreational value of valley and floodplain landscapes. The authors analyze 6 soil profiles set up in different parts of the Yelshanka river floodplain. The description of soil sections was performed with the use of the Field Qualifier of Soils of Russia. Soil profiles were named in accordance with the Russian Soil Classification and Diagnosis (2004) and the Field Qualifier of Soils of Russia (2008). In Volgograd, as in most cities in Russia and abroad, the study of soils of floodplain landscapes is not systematic, and in some cases is limited to a certain area of research. The purpose of the work was to monitor the morphological state of the floodplain soil cover of the Yelshanka River in order to determine the degree of soils transformation in the area as a result of urbotechnopedogenesis. The studies conducted at the site allow considering that the urban soils of recreational areas are a consequence of the combined effect of zonal natural and climatic and urbanogenic factors of soil formation. Studies of soils of floodplain landscapes in the urban environment make it possible to monitor the state of natural landscapes and their transformation in conditions of urbotechnopedogenesis, as well as the development of the ecological framework of the city.

Impact of fluvial flooding on potentially toxic element mobility in floodplain soils

&lt;p&gt;Climate projections suggest that rainfall events will become more frequent and intense, which may lead to more widespread flooding. Floodplains can be used to help reduce the magnitude of floods downstream by storing excess flood water, thus making them useful for flood risk management. This means that floodplains are subjected to repeated drying and rewetting, which has implications for biogeochemical cycling of chemical elements in floodplain soils.&lt;/p&gt;&lt;p&gt;Floodplains have been considered a sink for contaminants in urban catchments, where high river flows transport contaminated sediments downstream and deposit them onto the floodplain topsoil. With increasing flooding frequency and duration, floodplains may become sources of legacy pollution through desorption of contaminants into soil porewater or resuspension of particulate matter into the overlying floodwater. Therefore, flooding could re-mobilise potentially toxic elements (PTEs) such as Cadmium (Cd), Copper (Cu), Chromium (Cr), Nickel (Ni), and Lead (Pb) that are present in the floodplain soil as a result of historic deposition. Mobilising PTEs in floodplain soils may cause adverse ecological impacts for soil microorganisms, plants, and both terrestrial and aquatic fauna.&lt;/p&gt;&lt;p&gt;The mobility of PTEs from the floodplain soil can increase or decrease due to the net effect of five key processes that influence dispersion and accumulation; 1) soil redox potential for which decreases &amp;#160;can directly alter the speciation, and hence mobility, of redox sensitive PTEs (e.g. As and Cr), 2) soil pH for which an increase usually reduces the mobility of metal cations (e.g. Cd&lt;sup&gt;2+&lt;/sup&gt;, Cu&lt;sup&gt;2+&lt;/sup&gt;, Ni&lt;sup&gt;2+&lt;/sup&gt;, Pb&lt;sup&gt;2+&lt;/sup&gt;), 3) dissolved organic matter which can mobilise PTEs were strongly bound to soil particles, 4) iron (Fe) and manganese (Mn) hydroxides undergo reductive dissolution, releasing adsorbed and co-precipitated PTEs, and 5) reduction of sulphate which immobilises PTEs due to precipitation of metal sulphides.&lt;/p&gt;&lt;p&gt;We took a field-based approach; extracting soil pore waters from a floodplain downstream of a typical urban catchment in southeast England before, during and after a flooding event. During the flood, there was increased mobility of Cd and Pb, and decreased mobility for Cu and Cr, compared to the mobility before flooding. After the flood, Ni mobility increased, whereas the other PTEs had lower mobility than they had prior to the flood. We also measured explanatory variables (e.g. pH, redox, Fe and Mn) that might explain the changes in mobility of PTEs that we found. Reductive dissolution of Mn is a possible mechanism for the increased mobility of Cd and Pb and redox likely played a role in the reduced Cr mobility.&lt;/p&gt;&lt;p&gt;Flooding did not influence the mobility of all PTEs in the same way. The duration of flooding is thought to influence the mobilisation due to the length of time for key processes to take place. It is therefore difficult to predict what PTEs might be mobilised into the environment with any given flooding event, further work is required to identify which soil properties should be measured in order to improve our capability to predict how a flooding event will influence the mobility of individual PTEs in geochemically contrasting floodplain soils.&lt;/p&gt;

Spatial analysis of riverine microplastic in a Rhine floodplain soil in Germany

&lt;p&gt;The Rhine River flows through six European countries and is in exchange with diverse land use forms and human activities that potentially release microplastics (MPs). The Rhine interacts permanently with its surrounding banks and floodplains by changing water-levels. Several studies have documented the presence of MPs in the Rhine along its course as well as in its tributaries. However, the spatial distribution of MPs due to certain flood events in alluvial floodplains remains widely unclear. The knowledge about the amount and distribution of MPs and on their potential entry pathways into Rhine floodplains is essentially important for an ecological risk assessment. In this study, we analysed the amount and distribution of MPs in a floodplain soil in the nature reserve Merkenich-Langel, in the northern periphery of Cologne (Germany). We hypothesize that MPs are transported by the Rhine and are deposited at the site during flood events. For spatial analysis we used the MIKE software (DHI A/S, H&amp;#248;rsholm Denmark) merged with a digital terrain model of the study site to analyse past flood events and their potential deposition of MP. We chose three sampling transects located within the past flooded area each with three sampling spots with increasing distance and elevation to the river. Samples were taken from two different soil depths (0&amp;#8211;5 cm and 5&amp;#8211;20 cm) and the samples of the three sampling spots and same depth were combined to one mixed soil sample per transect. MP concentrations were analysed via ATR-FTIR and &amp;#181;-FPA-FTIR spectroscopy after density separation and enzymatic-oxidative purification. We found an increase of MP concentration per kg of dry soil in the depth 5&amp;#8211;20 cm with increasing distance to the river ranging from 25.612 particles/kg to 85.076 particles/kg. The results of MP concentration in 0&amp;#8211;5 cm topsoil layer will be compared to the concentration in the soil depth of 5&amp;#8211;20 cm. We correlate these results to the frequency of flood events.&lt;/p&gt;

Prediction of Spatial Distribution of Organic Carbon in Lower Brahmaputra Active Floodplain Soils of Bangladesh

Soil acts as a large reservoir of Organic Carbon (OC) but the amount varies significantly with space and time. Thus, soil analysis and interpretation of spatial variability of Soil Organic Carbon (SOC) are keys to site-specific management. The study aimed to characterize the spatial variability of SOC in an active floodplain. Soil samples were collected in three major landform categories (natural levee, back slope, marsh land) from the lower Brahmaputra River floodplain and then analyzed for SOC measurement in the laboratory. The measured data were then analyzed for spatial variability interpretation using descriptive statistics and geo-statistical analysis. The study found that the amount of SOC varies with landform variation, soil texture and distance between sample points. The topsoil of marsh land has the highest (1.41%), back slope holds a moderate amount (1.15%) and the natural levee has the lowest (0.75%) amount of SOC. The amount of clay particles at the top layer was found to be positively correlated to the SOC whereas in the same layer of sand and silt showed a negative correlation. The geo-statistical analysis illustrated the nugget effect. Low (<1%) SOC is commonly found in the agricultural soils of Bangladesh which was corroborated in this study; moderate (1.1%) SOC was found in the floodplain. This study aimed to provide an insight into spatial variability to assist in predicting SOC in the active floodplain; consequently, the interpretation of spatial variability analysis can be implemented for site specific management strategies and to calculate carbon stock in floodplain soils. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 8(2), 2019, P 33-40