Biogeochemical Process Model Research Articles

Biofilm-NOM-ferrihydrite synergy facilitate the dissimilatory iron bioreduction

Dissimilatory microbial iron reduction, a key biogeochemical process, influences the cycling of iron, hazardous metals such as arsenic, cadmium and chromium, and natural organic matter (NOM) across diverse environments like soils and aquatic systems. Despite its ecological significance, the interaction between ferrihydrite, NOM, and biofilms, particularly the synergy within these conditions, remains unclear. This study explored the bioreduction of ferrihydrite by Shewanella oneidensis MR-1 biofilms, analyzing how varying concentrations of NOM and electron donor affects this process. We developed a kinetic model to quantify iron transformation, revealing that NOM performs dual functions: as a carbon source and an electron shuttle. In carbon-rich environments, NOM acts as an electron shuttle, while insufficient carbon sources, biofilms metabolize NOM for electron transfer to ferrihydrite. The biofilm's extensive surface area facilitates the coprecipitation of NOM and ferrihydrite, enhancing electron transfer efficiency. The study noted a significant conversion of ferrihydrite into goethite and lepidocrocite, particularly at high C/Fe ratios. Additionally, integrating a coupled reactive-diffusion model, we aimed to predict iron transformation and distribution in biofilm systems. This approach emphasizes the importance of the biofilm-NOM-ferrihydrite synergy in iron biogeochemical cycles and the need for incorporating biofilm dynamics in realistic biogeochemical process models. The findings provide a new insight into the biofilm function in mediating the ferrihydrite bioreduction process in different carbon resources in real scenarios.

Development of pedotransfer functions for predicting hydraulic parameters of van Genuchten model by incorporating environmental variables on the Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau (QTP) region lacks large database of soil hydraulic parameters (SHPs) due to difficulty in soil sampling, and time- and labor-consuming measurement, which prevents accurate modeling of hydrological and biogeochemical processes. Pedotransfer functions (PTFs) using easily measurable soil properties may provide an alternative way to indirectly estimate SHPs. In this study, soil properties (bulk density (BD), clay, silt, sand, soil organic carbon, porosity, fitting parameter related to inverse of air entry pressure (α) and soil pore distribution (n), residual and saturated soil water content (θr and θs), and environmental parameters (altitude, slope gradient, annual average precipitation, air temperature, and evaporation) were determined from a north-south transect (ca. 500 km) in Qinghai Province and an east-west transect (ca. 1800 km) in Tibet Autonomous Region for establishing PTFs for SHPs on the QTP. The coefficient of variation of α was the largest, followed by θr, θs, and n, suggesting that α had the greatest spatial variability at the regional scale. There were significant differences in BD among 0–10, 10–20, and 20–30 cm soil layers (p < 0.05), whereas there were no significant differences in α, n, θr, and θs among these soil layers (p > 0.05). The five most important variables from the random forest analysis were selected to establish the PTFs of the SHPs and BD for the 0–30 cm soil layer on the QTP. The established PTFs for predicting SHPs and BD using multiple linear regression analysis were jointly affected by relief, climate, and soil properties. The accuracy of our newly established PTFs with the consideration of gravel content (< 10% or 10–30%) and mattic epipedon (existence or non-existence) was higher than that of the existing PTFs. We developed the first PTFs of SHPs on the QTP, providing a foundation to construct SHPs prediction models for other alpine regions in the world.

Response of Soil Organic Carbon Accumulation in Paddy Fields in Fujian Province to Future Temperature Increases

Understanding the effect of the soil carbon "source-sink" in cropland in China under future warming scenarios is the basis for making reasonable carbon neutralization policies. This study focused on the paddy soil in Fujian Province, a typical subtropical region in China including 84 counties (cities and districts). We employed the 1:50000 soil database and biogeochemical process model (DNDC) to simulate the dynamic changes in paddy soil organic carbon under different warming scenarios for the period of 2017-2053. The results indicated that in the context of normal temperature (control run) and 2, 4, and 6℃ of warming, the total amounts of carbon sequestration of paddy soil in Fujian Province were 11.56,9.44, 7.08, and 4.91 Tg, respectively; accordingly, the average annual carbon sequestration rates (expressed by C) were 173, 141, 106, and 74 kg·(hm2·a)-1, indicating that the rate of carbon sequestration was decreasing with the increase in future temperature. However, overall, the paddy field soil in the province was still a "carbon sink" under the warming of 6 (C. We also found that the gleyed paddy soil was mostly affected by the increase in temperature, and the decrease in carbon sequestration rate ranged from 20% to 69% using different treatments. In contrast, the salinized paddy soil was slightly affected, with a 14%-43% decrease in carbon sequestration rates. As for the different administrative regions, Sanming City was the most affected by temperature increase, with the rate of carbon sequestration decreasing by 27%-83% using different treatments. However, it was reduced by only 10%-41% and 14%-42% in Quanzhou and Putian (coastal areas), respectively. Overall, due to different soil properties, fertilization management, and climatic environment, there was a strong variability in the carbon sequestration rates of paddy soil for different soil subtypes and administrative regions in Fujian in response to future climatic warming.

Understanding the spatiotemporal variability in nonpoint source nutrient loads and its effect on water quality in the upper Xin’an river basin, Eastern China

Understanding the spatiotemporal characteristics of major nonpoint sources (NPSs) are crucial for protecting river water quality in drinking water source areas. This study employed a geomorphology-based nonpoint source pollution (GBNP) model to simulate hydrological processes together with NPS pollutant transport and transformation processes from 1980 to 2019 in the upper Xin’an River basin, an important drinking water headwater. The simulations revealed the spatiotemporal characteristics of nitrogen and phosphorus loads and their influences on river water quality. The results showed that the long-term average total nitrogen (TN) and total phosphorus (TP) loads were 3219 kg∙km−2∙yr−1 and 293 kg∙km−2∙yr−1, respectively. The proportional coefficients for TN and TP entering the rivers from the watershed were 0.40 and 0.37. The TN and TP loads during the April to July flood season accounted for 65.0 % and 63.2 % of the annual total, respectively. Annual TN load was significantly correlated with annual runoff depth, soil erosion and TN input in the interannual variation, while the interannual variation of TP load was significantly correlated with annual runoff depth and soil erosion. The long-term average TN and TP loads showed spatial patterns similar to those of land use and runoff depth, respectively. The daily TN concentrations showed a clockwise hysteresis relationship with discharge during flood events, while the daily TP concentrations had a single positive correlation with discharge throughout a whole year. Without atmospheric N deposition, the average TN load would decrease from 3122 kg∙km−2∙yr−1 to 1266 kg∙km−2∙yr−1 in 2000–2009. The TN load from atmospheric deposition accounted for 65 %-71 % of the total load, which implies that river water quality protection must include local basin eco-environmental protection and air pollution control over broader areas. This study showed that the distributed modelling of hydrological and biogeochemical processes is crucial to understand the spatiotemporal variability in nonpoint source nutrient loads over the watershed and is helpful to identifying key factors affecting river water quality.

Seasonal and Morphological Controls on Nitrate Retention in Arctic Deltas

AbstractEstimates of nitrate loading to the Arctic Ocean are limited by the lack of field observations within deltas partly due to logistical constraints. To overcome this limitation, we use a remote sensing framework to estimate retention of nitrate in Arctic deltas. We achieve this by coupling hydrological and biogeochemical process models at the network scale for five major Arctic deltas. Binary masks of delta channels were used to simulate flow direction and magnitude through networks. Models were parameterized using historical and seasonal observations. Simulated nitrate retention ranged from 2.9% to 15% of the incoming load. Retention rates were largest during winter but smallest during spring conditions when increased discharges export large nitrate masses to the coast. Under future climate scenarios, retention rates fall by ∼1%–10%. Arctic deltas have an important effect on the magnitude of nitrate entering Arctic seas and the inclusion of processing in deltas can improve flux estimates.

Quantifying the effect of the nitrogen biogeochemical processes on the distribution of ammonium in the riverbank filtration system

Ammonium (NH4+) enrichment of riverbank filtration (RBF) systems is gaining popularity. However, most previous research has concentrated on NO3− removal efficiencies, while the mechanisms of NH4+ enrichment remain unknown. A nitrogen biogeochemical process model was developed for the quantitative analysis of NH4+ enrichment in the Kaladian well field in northwest Songyuan City, NE China. Data from laboratory experiments and in-situ monitoring were used to determine initial values and calibrate the thermodynamic/kinetic parameters representing nitrogen (N) biogeochemical reactions. (1) The NO3− from river was subjected to denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) within 10–14 m of the shore, whereas the NH4+ in groundwater was caused by DNRA, organic nitrogen mineralization (MIN), and mixing with laterally recharged high NH4+ groundwater. (2) DNF and DNRA were regulated by hydrodynamic processes, with the ranges of these processes being more significant in the wet season due to a higher hydraulic gradient. MIN occurred widely throughout the water flow path, with temperature primarily controlling the rates of the three reactions. (3) DNRA activity was relatively higher in the wet season when the water temperature was higher within 10–14 m of the shore. In the wet season, DNRA contributed 25%–30% to NO3− reduction, which was higher than in the dry season (5%–10%). DNRA contributed at least 40% and 15% to NH4+ enrichment in the wet and dry seasons, respectively. (4). Organic N in media gradually released NH4+ into groundwater via MIN and desorption across the entire flow path, with contributions to NH4+ enrichment reaching 75% and 85%, respectively, in the wet and dry seasons.

Prediction of the van Genuchten model soil hydraulic parameters for the 5-m soil profile in China’s Loess Plateau

Soil hydraulic parameters (SHPs) represent a crucial input for modelling of water flow, biogeochemical processes, and plant growth. This modelling depends on an accurate description of the water balance in a critical soil zone. To this end, pedotransfer functions (PTFs) are required to translate basic soil data into SHPs as it is challenging to measure them. This challenge is peculiarly acute for deep soil layers in thick loess deposits, such as in the China’s Loess Plateau Region (CLPR). In this study, we analysed which factors are strongly correlated with SHPs, and developed PTFs for the SHPs. To this end, undisturbed and disturbed soil samples were collected up to 8 m depth at seven sampling sites in the CLPR to measure the soil water retention curve, saturated hydraulic conductivity (Ks), and associated soil and environmental factors. Furthermore, soil core samples in the 5 m soil profile were collected at 243 sites, and the corresponding environmental factors were calculated to estimate the spatial distribution of SHPs across the CLPR using the established PTFs. Redundancy analysis of the collected data in the 0–8-m soil profile revealed that soil texture, soil depth, mean annual precipitation, slope gradient, slope aspect, and elevation were strongly correlated with SHPs. The most correlated variables were used to develop PTFs for SHPs by using stepwise multiple linear regression. The accuracy of the established regional PTFs was significantly higher compared to state-of-the-art in this domain. The spatial distribution of SHPs in the CLPR study area exhibited significant spatial clustering. Generally, SHPs transitioned from the southeast to the northwest, with significant differences between the south and north. Our results provide empirical basis for further quantification of the response of soil hydrological processes to vegetation restoration and land-use change not only in the CLPR, but also in other similar regions.

A New Dynamic Modeling Approach to Predict Microbial Methane Generation and Consumption in Marine Sediments

Methane, as a clean energy source and a potent greenhouse gas, is produced in marine sediments by microbes via complex biogeochemical processes associated with the mineralization of organic matter. Quantitative modeling of biogeochemical processes is a crucial way to advance the understanding of the global carbon cycle and the past, present, and future of climate change. Here, we present a new approach of dynamic transport-reaction model combined with sediment deposition. Compared to other studies, since the model does not need the methane concentration in the bottom of sediments and predicts that value, it provides us with a robust carbon budget estimation tool in the sediment. We applied the model to the Blake Ridge region (Ocean Drilling Program, Leg 164, site 997). Based on seafloor data as input, our model remarkably reproduces measured values of total organic carbon, dissolved inorganic carbon, sulfate, calcium, and magnesium concentration in pore waters and the in situ methane presented in three phases: dissolved in pore water, trapped in gas hydrate, and as free gas. Kinetically, we examined the coexistence of free gas and hydrate, and demonstrated how it might affect methane gas migration in marine sediment within the gas hydrate stability zone.

Estimate greenhouse gas emissions from water-saving and drought-resistance rice paddies by deNitrification-deComposition model

Flooded rice fields have been confirmed as a major anthropogenic source of atmospheric methane (CH4). Avoiding continuous flooding is an effective practice to mitigate the CH4 emissions originating from paddy fields. However, the contradiction between the high yield and water consumption of paddy rice limits the mitigation potential of greenhouse gas (GHG) emissions through a dramatic reduction in water usage. Recently, a new rice type, water-saving and drought-resistance rice (WDR), has been developed to satisfy both the rice yield and quality with a high water use efficiency and good drought resistance. WDR can be planted under dry cultivation, similar to upland crops. A biogeochemical process model, DNDC (i.e., DeNitrification-DeComposition) was used to evaluate the effect of agricultural practices (F-R as paddy rice under flooding management and D-WDR as WDR under dry cultivation) on GHG emissions from paddy fields in Anhui Province, China. The results are as follows: 1) the DNDC model attained a good performance when simulating the rice yield, seasonal cumulative CH4 and N2O emissions, and GWP; 2) the mitigation potential of the D-WDR system was higher than 90% while maintaining a high rice yield; 3) if rice paddies in Anhui can be replaced by D-WDR system, the GHG mitigation potential could reach 16.92 Mt CO2-eq, and the increased N2O emissions of the WDR system could offset a small fraction (7.6%) of the GHG radiated forcing benefit gained by the decrease in CH4 emissions. Shifting rice production from paddy rice to WDR has a remarkable potential to balance the relationship between CH4 mitigation and rice yield maintenance.

Mathematical modeling of zooplankton productivity in the Azov Sea in summer on a high-performance computer system

The work is devoted to modeling of eurythermal and stenothermal zooplankton populations productivity in the Azov Sea in summer. The discretization of the problem was carried out using difference schemes of a higher order of accuracy, taking into account the filling of the calculated cells. The grid equations obtained as a result of the proposed mathematical model of biological kinetics discretization were solved by a modified adaptive variable-triangular method, which has a higher convergence rate in comparison with the existing methods of the variational type. A parallel algorithm for solving the problem on a high-performance computing system was developed, which made it possible to significantly reduce the time of the numerical solution of the problem. With the help of mathematical modeling, the analysis of various ways of development of the Azov Sea ecosystem was carried out, the influence of the gelatinous invasive on the production and de-struction processes of phyto- and zooplankton was researched. The developed software module, oriented to a multiprocessor computing system, can be effectively used for predictive modeling of interrelated hydrodynamic and biogeochemical processes in a shallow water body in summer.

Are there memory effects on greenhouse gas emissions (CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;) following grassland restoration?

Abstract. A 5-year greenhouse gas (GHG) exchange study of the three major gas species (CO2, CH4 and N2O) from an intensively managed permanent grassland in Switzerland is presented. Measurements comprise 2 years (2010 and 2011) of manual static chamber measurements of CH4 and N2O, 5 years of continuous eddy covariance (EC) measurements (CO2–H2O – 2010–2014), and 3 years (2012–2014) of EC measurement of CH4 and N2O. Intensive grassland management included both regular and sporadic management activities. Regular management practices encompassed mowing (three to five cuts per year) with subsequent organic fertilizer amendments and occasional grazing, whereas sporadic management activities comprised grazing or similar activities. The primary objective of our measurements was to compare pre-plowing to post-plowing GHG exchange and to identify potential memory effects of such a substantial disturbance on GHG exchange and carbon (C) and nitrogen (N) gains and losses. In order to include measurements carried out with different observation techniques, we tested two different measurement techniques jointly in 2013, namely the manual static chamber approach and the eddy covariance technique for N2O, to quantify the GHG exchange from the observed grassland site. Our results showed that there were no memory effects on N2O and CH4 emissions after plowing, whereas the CO2 uptake of the site considerably increased when compared to pre-restoration years. In detail, we observed large losses of CO2 and N2O during the year of restoration. In contrast, the grassland acted as a carbon sink under usual management, i.e., the time periods 2010–2011 and 2013–2014. Enhanced emissions and emission peaks of N2O (defined as exceeding background emissions 0.21 ± 0.55 nmol m−2 s−1 (SE = 0.02) for at least 2 sequential days and the 7 d moving average exceeding background emissions) were observed for almost 7 continuous months after restoration as well as following organic fertilizer applications during all years. Net ecosystem exchange of CO2 (NEECO2) showed a common pattern of increased uptake of CO2 in spring and reduced uptake in late fall. NEECO2 dropped to zero and became positive after each harvest event. Methane (CH4) exchange fluctuated around zero during all years. Overall, CH4 exchange was of negligible importance for both the GHG budget and the carbon budget of the site. Our results stress the inclusion of grassland restoration events when providing cumulative sums of C sequestration potential and/or global warming potential (GWP). Consequently, this study further highlights the need for continuous long-term GHG exchange observations as well as for the implementation of our findings into biogeochemical process models to track potential GHG mitigation objectives as well as to predict future GHG emission scenarios reliably.

Mathematical modeling of phytoplankton populations evolution in the Azov Sea

The paper describes the construction of a three-dimensional mathematical model of biogeochemical processes, considering the salinity and temperature influence on the phytoplankton populations development. The paper proposes a new difference scheme for solving convection-diffusion-reaction problems at large values of the Peclet grid number (2<Pe≤20), which is a linear combination of the central and the upwind leapfrog difference schemes. The three-layer difference scheme is more accurate than the traditional upwind leapfrog difference scheme for problems where convection prevails over diffusion. The construction of discrete equations for solving the problem of biogeochemical cycles on the basis of the scheme considering the filling of cells is described. The stationary regimes of phytoplankton dynamics problem were researched considering the transformation of phosphorus, nitrogen and silicon forms. Results of software complex, which allows to simulate biogeochemical processes in the Azov Sea, were described. The software package allows to forecast the dynamics of the Azov Sea ecosystem development in the conditions of modern salinization.

Understanding Environmental Changes in Temperate Coastal Seas: Linking Models of Benthic Fauna to Carbon and Nutrient Fluxes

Coastal seas are highly productive systems, providing an array of ecosystem services to humankind, such as processing of nutrient effluents from land and climate regulation. However, coastal ecosystems are threatened by human-induced pressures such as climate change and eutrophication. In the coastal zone, the fluxes and transformations of nutrients and carbon sustaining coastal ecosystem functions and services are strongly regulated by benthic biological and chemical processes. Thus, to understand and quantify how coastal ecosystems respond to environmental change, mechanistic modelling of benthic biogeochemical processes is required. Here, we discuss the present model capabilities to quantitatively describe how benthic fauna drives nutrient and carbon processing in the coastal zone. There are a multitude of modeling approaches of different complexity, but a thorough mechanistic description of benthic-pelagic processes is still hampered by a fundamental lack of scientific understanding of the diverse interactions between the physical, chemical and biological processes that drive biogeochemical fluxes in the coastal zone. Especially shallow systems with long water residence times are sensitive to the activities of benthic organisms. Hence, including and improving the description of benthic biomass and metabolism in sediment diagenetic as well as ecosystem models for such systems is essential to increase our understanding of their response to environmental changes and the role of coastal sediments in nutrient and carbon cycling. Major challenges and research priorities are (1) to couple the dynamics of zoobenthic biomass and metabolism to sediment reactive-transport in models, (2) to test and validate model formulations against real-world data to better incorporate the context-dependency of processes in heterogeneous coastal areas in models and (3) to capture the role of stochastic events.

Freshwater pearl mussels from northern Sweden serve as long-term, high-resolution stream water isotope recorders

Abstract. The stable isotope composition of lacustrine sediments is routinely used to infer Late Holocene changes in precipitation over Scandinavia and, ultimately, atmospheric circulation dynamics in the North Atlantic realm. However, such archives only provide a low temporal resolution (ca. 15 years), precluding the ability to identify changes on inter-annual and quasi-decadal timescales. Here, we present a new, high-resolution reconstruction using shells of freshwater pearl mussels, Margaritifera margaritifera, from three streams in northern Sweden. We present seasonally to annually resolved, calendar-aligned stable oxygen and carbon isotope data from 10 specimens, covering the time interval from 1819 to 1998. The bivalves studied formed their shells near equilibrium with the oxygen isotope signature of ambient water and, thus, reflect hydrological processes in the catchment as well as changes, albeit damped, in the isotope signature of local atmospheric precipitation. The shell oxygen isotopes were significantly correlated with the North Atlantic Oscillation index (up to 56 % explained variability), suggesting that the moisture that winter precipitation formed from originated predominantly in the North Atlantic during NAO+ years but in the Arctic during NAO− years. The isotope signature of winter precipitation was attenuated in the stream water, and this damping effect was eventually recorded by the shells. Shell stable carbon isotope values did not show consistent ontogenetic trends, but rather oscillated around an average that ranged from ca. −12.00 to −13.00 ‰ among the streams studied. Results of this study contribute to an improved understanding of climate dynamics in Scandinavia and the North Atlantic sector and can help to constrain eco-hydrological changes in riverine ecosystems. Moreover, long isotope records of precipitation and streamflow are pivotal to improve our understanding and modeling of hydrological, ecological, biogeochemical and atmospheric processes. Our new approach offers a much higher temporal resolution and superior dating control than data from existing archives.

Reassessing Southern Ocean Air-Sea CO2 Flux Estimates With the Addition of Biogeochemical Float Observations.

New estimates of pCO2 from profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project have demonstrated the importance of wintertime outgassing south of the Polar Front, challenging the accepted magnitude of Southern Ocean carbon uptake (Gray et al., 2018, https://doi:10.1029/2018GL078013). Here, we put 3.5 years of SOCCOM observations into broader context with the global surface carbon dioxide database (Surface Ocean CO2 Atlas, SOCAT) by using the two interpolation methods currently used to assess the ocean models in the Global Carbon Budget (Le Quéré et al., 2018, https://doi:10.5194/essd-10-2141-2018) to create a ship‐only, a float‐weighted, and a combined estimate of Southern Ocean carbon fluxes (<35°S). In our ship‐only estimate, we calculate a mean uptake of −1.14 ± 0.19 Pg C/yr for 2015–2017, consistent with prior studies. The float‐weighted estimate yields a significantly lower Southern Ocean uptake of −0.35 ± 0.19 Pg C/yr. Subsampling of high‐resolution ocean biogeochemical process models indicates that some of the differences between float and ship‐only estimates of the Southern Ocean carbon flux can be explained by spatial and temporal sampling differences. The combined ship and float estimate minimizes the root‐mean‐square pCO2 difference between the mapped product and both data sets, giving a new Southern Ocean uptake of −0.75 ± 0.22 Pg C/yr, though with uncertainties that overlap the ship‐only estimate. An atmospheric inversion reveals that a shift of this magnitude in the contemporary Southern Ocean carbon flux must be compensated for by ocean or land sinks within the Southern Hemisphere.

Modeling of biogeochemical processes in a barrier island freshwater lens (Spiekeroog, Germany)

Freshwater lenses present valuable water resources on barrier islands. Yet, the biogeochemical processes that control the groundwater quality of these freshwater lenses and how they are affected by the prevailing groundwater dynamics is largely unexplored. In this study we investigated the biogeochemistry of a barrier island freshwater lens with a focus on understanding and quantifying organic matter mineralization, sulfur cycling, and chemical fluxes to the land-ocean interface. We analyzed a comprehensive set of hydrogeochemical field data from Spiekeroog Island (Germany), including stable sulfur isotope signatures of dissolved sulfur species, with a reactive transport modeling approach. Tritium-Helium groundwater ages were used to constrain the hydrogeochemistry as a function of residence time. Our results revealed that freshwater lenses can act as archives for anthropogenic pollution, conserving the high sulfur loads associated with historic atmospheric deposition. We observed two distinct (hydro)biogeochemical patterns, which we attribute to a heterogeneous distribution of reactive organic matter. Those patterns were well replicated by two separate reactive transport models that considered the variations in organic matter reactivity. Simulation and field results demonstrated that net sulfur cycling is dominated by microbial sulfate reduction and subsequent iron sulfide precipitation. In the absence of dissolved oxidants, we attribute the observed high stable sulfur isotope fractionation between dissolved sulfate and sulfide of up to 67‰ to low (<20 pmol mL−1 d−1) microbial sulfate reduction rates. We show that older groundwater becomes progressively enriched in ammonium and phosphate due to the mineralization of organic matter, and we speculate that lens-derived nutrient fluxes may be important for the benthic zone at local groundwater discharge sites, at least seasonally in spring and summer.

Assessing the Sustainable Development of Bioenergy from Cassava within “Water-Energy-Food” Nexus Framework in China

Bioenergy from cassava is a promising alternative energy source for both energy supply and the mitigation of greenhouse gases. However, major global trends, such as climate change and competing landuse patterns, pose substantial risks to the sustainable development of bioenergy. The main purpose of this study was to assess the sustainable development of bioenergy from cassava, considering landuse change and climate change with a biogeochemical process model within the “water-energy-food” nexus framework. The results showed that the land resources that were suitable for the development of cassava bioenergy have continuously decreased in China since 1990. At the same time, the climate has also undergone significant changes, with temperature showing an increasing trend, and precipitation showing a decreasing trend. With the influences of both landuse change and climate change, the total bioenergy of cassava showed a downward trend. In China, the potential bioenergy production for the year 1990, 2000, and 2010 was 6075 PJ, 5974 PJ, and 4399 PJ, respectively. Compared to 1990, the bioenergy production in 2010 decreased by 1676.40 million GJ, which equals 57 million tons of standard coal. In addition, the water footprint of bioenergy from cassava was discussed. After considering changes to landuse, climate, and water footprint, it was concluded that Guangxi was the most suitable place to develop cassava bioenergy, followed by Fujian, Guangdong, and Yunnan.

Numerical Modeling of Long-Term Biogeochemical Processes and Its Application to Sedimentary Bed Formation in Tokyo Bay

Even though models of water quality have become increasingly detailed over time, their applicability to analyze long-term effect on sedimentary bed formation is yet to be clarified. Hence, an integrated, layer-resolved, process-based, sediment–water coupled, long-term robust, three-dimensional (3D) ecosystem model, including realistic sedimentary and pelagic processes, was developed. The constituents of the integrated model included a multi layered ecosystem model, a quasi-three dimensional hydrodynamic model, a wave hindcasting model, and a sediment deposition and erosion model. Because numerical modeling difficulties arise in setting the initial conditions, especially for a sedimentary bed, this novel modeling approach suggests to initiate the model with the simplest initial conditions of no particulate organic carbon content (POCC) and uniform inorganic sediment distribution. The model was applied to Tokyo Bay and quasi-steady state POCC in the sediment was obtained through long-term computation with realistic sedimentary and pelagic processes. Wave induced bed shear stress (WBSS) is critical for sediment erosion and spatial movement of sediment pollution, while a stable morphology is determined by the balance between the current induced bed shear stress (CBSS) and WBSS in Tokyo Bay. This novel modeling approach, with the simplest sedimentary initial conditions and realistic sedimentary and pelagic processes, provides a great tool for long-term ecosystem modeling in future studies.

Expanded diversity of microbial groups that shape the dissimilatory sulfur cycle

A critical step in the biogeochemical cycle of sulfur on Earth is microbial sulfate reduction, yet organisms from relatively few lineages have been implicated in this process. Previous studies using functional marker genes have detected abundant, novel dissimilatory sulfite reductases (DsrAB) that could confer the capacity for microbial sulfite/sulfate reduction but were not affiliated with known organisms. Thus, the identity of a significant fraction of sulfate/sulfite-reducing microbes has remained elusive. Here we report the discovery of the capacity for sulfate/sulfite reduction in the genomes of organisms from 13 bacterial and archaeal phyla, thereby more than doubling the number of microbial phyla associated with this process. Eight of the 13 newly identified groups are candidate phyla that lack isolated representatives, a finding only possible given genomes from metagenomes. Organisms from Verrucomicrobia and two candidate phyla, Candidatus Rokubacteria and Candidatus Hydrothermarchaeota, contain some of the earliest evolved dsrAB genes. The capacity for sulfite reduction has been laterally transferred in multiple events within some phyla, and a key gene potentially capable of modulating sulfur metabolism in associated cells has been acquired by putatively symbiotic bacteria. We conclude that current functional predictions based on phylogeny significantly underestimate the extent of sulfate/sulfite reduction across Earth’s ecosystems. Understanding the prevalence of this capacity is integral to interpreting the carbon cycle because sulfate reduction is often coupled to turnover of buried organic carbon. Our findings expand the diversity of microbial groups associated with sulfur transformations in the environment and motivate revision of biogeochemical process models based on microbial community composition.

Simulating the dependence of aspen (Populus tremuloides) on redistributed snow in a semi‐arid watershed

AbstractIn semi‐arid mountainous regions across the western United States, the distribution of upland aspen (Populus tremuloides) is often related to heterogeneous soil moisture subsidies resulting from redistributed snow. As temperatures increase, interactions between decreasing snowpack and future trends in the net primary productivity (NPP) of aspen forests remain uncertain. This study characterizes the importance of heterogeneously distributed snow water to aspen communities in the Reynolds Creek Critical Zone Observatory located in southwestern Idaho, USA. Net primary productivity of three aspen stands was simulated at sites spanning elevational and precipitation gradients using the biogeochemical process model Biome‐BGC and precipitation data adjusted to account for drifting snow. Compared to a spatially homogeneous precipitation distribution, Biome‐BGC simulations accounting for redistributed precipitation were in better agreement with previous simulations of snow accumulation and soil moisture field measurements. During drought years, simulations below the largest drifts that included wind‐redistributed snow resulted in NPP values nearly 77% higher than simulations assuming uniform precipitation. However, during wet years (and at sites with higher total precipitation), increased effective precipitation resulting from drifting snow did not have a significant role in aspen productivity. In these cases, soil moisture was found to be non‐limiting even in the absence of redistributed snow. Increased water availability from snow drifts often exceeded the storage capacity of the soil and contributed little to plant available water used later in the growing season.