Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: a synthesis

Background. Sediments frequently exposed to dry-wet cycles are potential biogeochemical hotspots for greenhouse gas (GHG) emissions during dry, wet and transitional phases. While the effects of drying and rewetting on carbon fluxes have been studied extensively in terrestrial and aquatic systems, less is known about the effects of dry-wet cycles on N2O emissions from aquatic systems. As a notable part of lotic systems are temporary, and small lentic systems can substantially contribute to GHG emissions, dry-wet cycles in these ecosystems can play a major role on N2O emissions. Methodology. This study compiles literature focusing on the effects of drying, rewetting, flooding, and water level fluctuations on N2O emissions and related biogeochemical processes in sediments of lentic and lotic ecosystems. Results. N2O pulses were observed following sediment drying and rewetting events. Moreover, exposed sediments during dry phases can be active spots for N2O emissions. The general mechanisms behind N2O emissions during dry-wet cycles are comparable to those of soils and are mainly related to physical mechanisms and enhanced microbial processing in lotic and lentic systems. Physical processes driving N2O emissions are mainly regulated by water fluctuations in the sediment. The period of enhanced microbial activity is driven by increased nutrient availability. Higher processing rates and N2O fluxes have been mainly observed when nitrification and denitrification are coupled, under conditions largely determined by O2 availability. Conclusions. The studies evidence the driving role of dry-wet cycles leading to temporarily high N2O emissions in sediments from a wide array of aquatic habitats. Peak fluxes appear to be of short duration, however, their relevance for global emission estimates as well as N2O emissions from dry inland waters has not been quantified. Future research should address the temporal development during drying-rewetting phases in more detail, capturing rapid flux changes at early stages, and further explore the functional impacts of the frequency and intensity of dry-wet cyclesinfo:eu-repo/semantics/publishedVersio

A Conceptual Framework for Understanding the Biogeochemistry of Dry Riverbeds Through the Lens of Soil Science

Intermittent rivers and ephemeral streams (IRES) encompass fluvial ecosystems that eventually stop flowing and run dry at some point in space and time. During the dry phase, channels of IRES consist mainly of dry riverbeds (DRBs), prevalent yet widely unexplored ecotones between dry and wet phases that can strongly influence the biogeochemistry of fluvial networks. DRBs are often overlooked because they do not strictly belong to either domain of soil or freshwater science. Due to this dual character of DRBs, we suggest that concepts and knowledge from soil science can be used to expand the understanding of IRES biogeochemistry. Based on this idea, we propose that DRBs can be conceptually understood as early stage soils exhibiting many similarities with soils through two main forces: i) time since last sediment transport event, and ii) the development status of stabilizing structures (e.g. soil crusts and/or vascular plants). Our analysis suggests that while DRBs and soils may differ in master physical attributes (e.g. soil horizons vs fluvial sedimentary facies), they become rapidly comparable in terms of microbial communities and biogeochemical processes. We further propose that drivers of DRBs biogeochemistry are similar to those of soils and, hence, concepts and methods used in soil science are transferable to DRBs research. Finally, our paper presents future research directions to advance the knowledge of DRBs and to understand their role in the biogeochemistry of intermittent fluvial networks

Limitations of stream restoration for nitrogen retention in agricultural headwater streams

High nutrient loading and channelization reduce the nutrient retention capacity of agricultural streams and lead to increases in nutrient downstream transport. The aim of the current study was to study the effects of channel reconfiguration and riparian reforestation on the nitrogen retention capacity of eutrophic agricultural headwater streams. In addition, we investigated the role of stream sediments as a nitrogen sink or source for the stream ecosystem.We compared two restored reaches with two morphologically pristine and four channelized reaches in an agricultural catchment in the north-east of Austria regarding in-stream ammonium uptake, whole-reach retention of dissolved inorganic nitrogen, potential denitrification enzyme activity, and sedimentary ammonium release.Restored and pristine reaches exhibited significantly shorter ammonium uptake lengths (330m) and larger mass transfer coefficients (2.7×10-5ms-1) than channelized reaches (2500m and 1.1×10-5ms-1, respectively). Increased ammonium uptake was positively correlated with increased transient storage in restored and pristine reaches. Total DIN retention was slightly, though not significantly higher in restored sections (average rates 0.06g DINm-2h-1) and showed signs of temporal nitrogen saturation in all reaches. In general, sediments were characterized by small grain sizes (0.04-0.31mm), high ammonium (60-215μgg-1 DW), and low nitrate concentrations (0.4-5.7μgg-1 DW). Ammonium was released from sediments of all reaches below concentrations of 100μg NH4 +-NL-1 in the overlying water column which shows the high potential of nutrient-rich sediments to act as an internal ammonium source for the stream ecosystem. Potential denitrification was lowest in sediments of restored reaches and significantly increased after nitrate amendment to 3-26mgNm-2h-1.The study reveals that stream sediments, which are loaded with nutrient-rich soil from the agricultural catchment, may limit the effects of stream restoration in agricultural streams. In order to improve the nutrient retention capacity of agricultural streams, reach-scale restoration measures have to be combined with measures in the catchment which reduce nutrient and soil inputs to streams

Highly Integrated and Mobile Sensor System for Dissolved Organic Matter in Stream Ecosystems

The impact of agricultural land use on the composition of dissolved organic matter (DOM) and its effects on the aquatic carbon cycle are still largely unknown. A sensor for dissolved DOM in stream ecosystems based on fluorescence measurement was developed. It’s an easy to use handheld optical system for online monitoring of DOM under field-conditions. For the determination of DOM two indices are used, namely the freshness index (BIX) and the fluorescence index (FIX)

Effects of Two-Stage Ditch and Natural Floodplains on Sediment Processes Driven by Different Hydrological Conditions

The two-stage ditch is a river restoration technique that aims at improving the sediment regime and lateral channel connectivity by recreating a small floodplain alongside a stream reach. This study aimed to analyze the efficiency of a two-stage ditch in improving the stream sediment structure and functions under different hydrological conditions (baseflow, post-bankfull, post-flood). Stream sediments were collected in channel sections adjacent to the two-stage ditch, adjacent to a natural floodplain along channelized reaches without inundation areas. Grain sizes, organic matter content and phosphorous (P) fractions were analyzed along with functional parameters (benthic respiration rate and P adsorption capacity, EPC0). The reach at the two-stage ditch showed no changes in sediment texture and stocks, while the floodplain reach showed higher fines and organic matter content under all hydrological conditions. The sediments in degraded reaches were more likely to be P sources, while they were more in equilibrium with the water column next to the natural floodplains and the two-stage ditch. Only functional parameters allowed for assessing the restoration effects on improving the sediment stability and functionality. Due to its sensitivity, the use of P adsorption capacity is recommended in future studies aiming at evaluating the response of river sediments to restoration measures under different hydrological conditions

Dissolved Organic Matter Quality and Biofilm Composition Affect Microbial Organic Matter Uptake in Stream Flumes

Agriculture delivers significant amounts of dissolved organic matter (DOM) to streams, thereby changing the composition and biodegradability of the aquatic DOM. This study focuses on the interactive effects of DOM quality and biofilm composition on the degradation of DOM in a laboratory flume experiment. Half of the flumes were exposed to light to stimulate algal growth, the other half was shaded. Leachates of deciduous leaves, maize leaves, and cow dung were added to the flumes in a single pulse and changes of DOC (dissolved organic carbon) and nutrient concentrations, DOM composition (absorbance and fluorescence data), chlorophyll-a concentrations, bacterial abundances, and enzymatic activities were recorded over a week. DOM was taken up with rates of 50, 109, and 136 µg DOC L−1 h−1 for dung, leaf, and maize leachates, respectively, in the light flumes and 37, 80, and 170 µg DOC L−1 h−1 in the dark flumes. DOC uptake correlated strongly with initial SRP (soluble reactive phosphorus) and DOC concentrations, but barely with DOM components and indices. Algae mostly stimulated the microbial DOC uptake, but the effects differed among differently aged biofilms. We developed a conceptual model of intrinsic (DOM quality) and external (environmental) controlling factors on DOM degradation, with the microbial community acting as biotic filter

staRdom: Versatile Software for Analyzing Spectroscopic Data of Dissolved Organic Matter in R

The roles of dissolved organic matter (DOM) in microbial processes and nutrient cycles depend on its composition, which requires detailed measurements and analyses. We introduce a package for R, called staRdom (“spectroscopic analysis of DOM in R”), to analyze DOM spectroscopic data (absorbance and fluorescence), which is key to deliver fast insight into DOM composition of many samples. staRdom provides functions that standardize data preparation and analysis of spectroscopic data and are inspired by practical work. The user can perform blank subtraction, dilution correction, Raman normalization, scatter removal and interpolation, and fluorescence normalization. The software performs parallel factor analysis (PARAFAC) of excitation–emission matrices (EEMs), including peak picking of EEMs, and calculates fluorescence indices, absorbance indices, and absorbance slope indices from EEMs and absorbance spectra. A comparison between PARAFAC solutions by staRdom in R compared with drEEM in MATLAB showed nearly identical solutions for most datasets, although different convergence criteria are needed to obtain similar results and interpolation of missing data is important when working with staRdom. In conclusion, staRdom offers the opportunity for standardized multivariate decomposition of spectroscopic data without requiring software licensing fees and presuming only basic R knowledge

Current status and restoration options for floodplains along the Danube River

Floodplains are key ecosystems of riverine landscapes and provide a multitude of ecosystem services. In most of the large river systems worldwide, a tremendous reduction of floodplain area has occurred in the last 100years and this loss continues due to pressures such as land use change, river regulation, and dam construction. In the Danube River Basin, the extent of floodplains has been reduced by 68% compared to their pre-regulation area, with the highest losses occurring in the Upper Danube and the lowest in the Danube Delta. In this paper, we illustrate the restoration potential of floodplains along the Danube and its major tributaries. Via two case studies in the Upper and Lower Danube, we demonstrate the effects of restoration measures on the river ecosystem, addressing different drivers, pressures, and opportunities in these regions. The potential area for floodplain restoration based on land use and hydromorphological characteristics amounts to 8102 km2 for the whole Danube River, of which estimated 75% have a high restoration potential. A comparison of floodplain status and options for restoration in the Upper and Lower Danube shows clear differences in drivers and pressures, but certain common options apply in both sections if the local context of stakeholders and societal needs are considered. New approaches to flood protection using natural water retention measures offer increased opportunities for floodplain restoration, but conflicting societal needs and legal frameworks may restrict implementation. Emerging issues such as climate change and invasive non-native species will need careful consideration in future restoration planning to minimize unintended effects and to increase the resilience of floodplains to these and other pressures