# Biogeochemistry

## Current research reports and chronological list of recent articles..

The international scientific journal Biogeochemistry publishes original papers and occasional reviews dealing with biotic controls on the chemistry of the environment, or with the geochemical control of the structure and function of ecosystems.

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Additional research articles see Current Chemistry Research Articles. General information about this topic see biogeochemistry.

## Biogeochemistry - Abstracts

Hourly and daily rainfall intensification causes opposing effects on C and N emissions, storage, and leaching in dry and wet grasslands

### Abstract

Climate change is expected to alter hourly and daily rainfall regimes and, in turn, the dynamics of ecosystem processes controlling greenhouse gas emissions that affect climate. Here, we investigate the effects of expected twenty-first century changes in hourly and daily rainfall on soil carbon and nitrogen emissions, soil organic matter (SOM) stocks, and leaching using a coupled mechanistic carbon and nitrogen soil biogeochemical model (BAMS2). The model represents various abiotic and biotic processes involving 11 SOM pools. These processes include fungal depolymerization, heterotrophic bacterial mineralization, nitrification, denitrification, microbial mortality, necromass decomposition, microbial response to water stress, protection, aqueous advection and diffusion, aqueous complexation, and gaseous dissolution. Multi-decadal modeling with varying rainfall patterns was conducted on nine Australian grasslands in tropical, temperate, and semi-arid regions. Our results show that annual $${\text{CO}}_2$$ emissions in the semi-arid grasslands increase by more than 20% with a 20% increase in annual rainfall (with no changes in the rainfall timing), but the tropical grasslands have opposite trends. A 20% increase in annual rainfall also increases annual $${\text{N}}_2{\text{O}}$$ and NO emissions in the semi-arid grasslands by more than 10% but decreases emissions by at least 25% in the temperate grasslands. When subjected to low frequency and high magnitude daily rainfall events with unchanged annual totals, the semi-arid grasslands are the most sensitive, but changes in annual $${\text{CO}}_2$$ emissions and SOM stocks are less than $$5\%$$ . Intensification of hourly rainfall did not significantly alter $${\text{CO}}_2$$ emissions and SOM stocks but changed annual $${\text{NH}}_3$$ emissions in the tropical grasslands by more than 300%.

Datum: 25.06.2019

Potential effects of the invasive bivalve Corbicula fluminea on methane cycling processes in an urban stream

### Abstract

Lotic methane production, consumption, and flux depends on biogeochemical conditions that vary at fine scales within streams. The invasive bivalve, Corbicula fluminea, is known to affect stream biogeochemistry and may influence methane cycling via bioturbation and respiration. Response of methanogenesis rate, potential rate of methanotrophy, and net methane flux to Corbicula density was tested using intact sediment core incubations. Potential methanotrophy decreased and net methane flux increased with Corbicula density in sediment cores. However, the magnitude of this effect decreased at the highest Corbicula densities tested here. Response of sediment pore water methane concentration to Corbicula density was tested using in situ cage enclosures in a randomized block design over a 50 m stream reach. Block effect was the most significant predictor of in situ pore water methane concentration, and Corbicula density had a positive, marginally significant, interaction with the block effect, indicating that Corbicula’s effect on methane concentration varies with in situ conditions. Our study shows that Corbicula influences methane cycling processes in the laboratory, apparently via their impact on dissolved oxygen, but this effect depends on stream conditions in the field. Further study is needed to evaluate mechanisms of this relationship more fully.

Datum: 15.06.2019

Impacts of an invasive grass on soil organic matter pools vary across a tree-mycorrhizal gradient

### Abstract

Increases in carbon (C) inputs can augment soil organic matter (SOM), or reduce SOM by accelerating decomposition. Thus, there is a need to understand how and why ecosystems differ in their sensitivity to C inputs. Invasive plants that invade wide-ranging habitats, accumulate biomass rapidly, and contribute copious amounts of C to soil can be ideal for addressing this gap. We quantified the effects of the invasive C4 grass, Microstegium vimineum, on SOM in three temperate forests across plots varying in their relative abundance of arbuscular mycorrhizal (AM) versus ectomycorrhizal (ECM) trees. We hypothesized that invasion would differentially affect SOM along the mycorrhizal gradient owing to recognized patterns in nitrogen availability (AM > ECM) and the proportion of unprotected SOM (ECM > AM). Across all sites, M. vimineum was associated with lower particulate organic matter (POM) in ECM-dominated plots, consistent with our hypothesis that invader-derived C inputs should stimulate decomposers to acquire nitrogen from unprotected SOM in soils with low nitrogen availability. However, the pattern of lower POM in the ECM-dominated soils was offset by greater mineral-associated organic matter (MAOM)—and isotopic data suggest this was largely driven by native- rather than invader-derived SOM—implying an invasion-associated transfer of native-derived POM into MAOM. Our results demonstrate a context-dependent shift in the form of SOM in a system with presumably enhanced C inputs. This finding suggests a need to look beyond changes in total SOM stocks, as intrinsic SOM changes could lead to important long-term feedbacks on invasion or priming effects.

Datum: 12.06.2019

Carbon dioxide fluxes of air-exposed sediments and desiccating ponds

### Abstract

Ponds are active components of the global carbon cycle processing and emitting carbon dioxide and methane to the atmosphere. These common habitats frequently experience seasonal water table variations resulting in periodically air-exposed sediments. However, the influence of these events on both the system scale carbon balance and in-pond environmental conditions remains poorly studied. We took advantage of an extraordinarily warm and dry summer to quantify the CO2 efflux from air-exposed sediments and water surfaces in desiccating ponds on Öland, Sweden. Simultaneously, we modelled metabolism and measured environmental variables within the ponds. We found that air-exposed sediments had high CO2 effluxes greatly exceeding that from the water surfaces. Sediment water content influenced the temperature and strongly regulated the CO2 efflux gradually approaching zero as water evaporated. Within the desiccating ponds, respiration was generally higher than gross primary production, but was lower compared to the same ponds with higher water table. These findings highlight the role of periodically air-exposed pond sediments as sites of highly active carbon processes. Not only is this important for the system-scale carbon in ponds, but it may also influence the destiny of buried carbon in lakes subject to climate changes. The environmental conditions within desiccating ponds, most notably high water temperatures and poor oxygen conditions, further iterate the dynamics and extreme nature of ponds.

Datum: 10.06.2019

Fast bacterial succession associated with the decomposition of Quercus wutaishanica litter on the Loess Plateau

### Abstract

Understanding plant litter decomposition in broad-leaved forests is important because it influences the geochemical cycles of nutrients and represents a vital link in the global carbon cycle. Bacteria play an important role in litter decomposition, especially late in the decomposition process, when they become abundant. In this paper, we investigate bacterial community composition and diversity during about 1 year of Quercus wutaishanica litter decomposition using a molecular approach, to fill the gaps in knowledge about bacterial communities during decomposition. The results showed that the phyla Proteobacteria (Alpha and Betaproteobacteria), Actinobacteria, Bacteroidetes, and Acidobacteria were the most dominant throughout the experiment. As decomposition progressed, a dynamic succession of community and diversity was observed for different decomposition periods. Decay stages and seasonal shifts occurred by successful replacement of copiotrophic bacterial groups such as Betaproteobacteria in the early stage. During the entire process of decomposition, litter decomposition selectively stimulated the relative abundance of Alphaproteobacteria (Sphingomonas, Rhizobium, and Methylobacterium) and Bacteroidetes, but reduced the abundance of Massilia (Betaproteobacteria), Acidobacteria, and Actinobacteria. Among the abiotic factors, litter N and P content was the main factor driving the succession of litter bacteria. These results indicate the changes in decomposition stages in terms of the bacterial groups and elucidate the microbial community underpinnings of nutrient cycling in forest ecosystems.

Datum: 06.06.2019

The fate of litter-derived dissolved organic carbon in forest soils: results from an incubation experiment

### Abstract

Despite being a crucial component of nutrient cycling and soil carbon (C) dynamics in forest ecosystems, there is too little information from past studies to discern whether dissolved organic carbon (DOC) exchanges with soil organic carbon or passes unaltered through soils. In this study, we added 13C-labelled litter-derived DOC into different depth soil columns in a 180-day incubation experiment to determine the fate of DOC in soils, and to monitor the changes in DOC composition when it percolates through the soil. The results showed that δ13C values increased in soil microbes, which indicated that some litter-derived DOC was immobilized by soil microbial communities. Approximately 76% of litter-derived DOC was retained in the soil (60% in topsoil and 16% in midsoil). Meanwhile, 18%, 4%, and 3% of litter-derived DOC were mineralized into CO2 in topsoil, midsoil and subsoil respectively. Only 0.04% of litter-derived DOC leached from the soil column (0–60 cm). These results indicated that DOC was mainly retained on soil, and a small portion was mineralized by microorganisms, with minimal leaching. The composition of water soluble soil organic carbon (WSOC) and leachate DOC (LDOC) were similar between the control and treatment. This indicated that the composition of WSOC and LDOC was more similar to soil C than the added DOC, which supports the previously hypothesized dynamic exchange model. These findings provide new insight by showing that most litter-derived DOC is sequestered in forest soils.

Datum: 05.06.2019

The steps in the soil nitrogen transformation process vary along an aridity gradient via changes in the microbial community

### Abstract

Rainfall patterns are predicted to change dramatically in many terrestrial landscapes, including drylands. The most limiting resources for plant growth in arid regions is nitrogen (N) as well as water. A natural aridity gradient provides appropriate candidate conditions for predicting the impacts of changes in rainfall on soil N dynamics. To comprehensively and mechanistically examine soil N dynamics, we focused on the steps of N transformation, their microbial drivers, and the determining soil properties. We divided the N transformation process into three steps: (i) organic matter degradation, (ii) N mineralization, and (iii) nitrification, which are driven primarily by fungi, prokaryotes, and ammonia oxidizers, respectively. Soil samples were collected from three black locust forests with mean annual precipitations ranging from 449 to 606 mm. Along the aridity gradient, all three steps changed while maintaining a balance. The degradation and mineralization steps varied with changes in the soil fungal and prokaryotic communities, respectively. The compositions of these communities were determined by soil substrate quality and quantity; saprotrophs and copiotrophs decreased along the aridity gradient. On the other hand, the abundance of ammonia-oxidizing bacteria, which correlated with the rate of nitrification, was likely determined by soil moisture. Therefore, if precipitation were to decrease, changes in the nitrification step might be the first mechanism to limit plant productivity in semi-arid forests. This limitation would extend to the other steps in the N cycling process via plant–soil feedback. Thus, N cycling dynamics are predicted to achieve new stable states suited to the changed precipitation regime.

Datum: 01.06.2019

Correction to: Characterization of ten extreme disturbance events in the context of social and ecological systems

In the online and printed version of this article a few lines of text and a figure are incorrect. The corrected text and figure are given below

Datum: 01.06.2019

Microbial dormancy improves predictability of soil respiration at the seasonal time scale

### Abstract

Global warming, in combination with altered precipitation patterns, is accelerating global soil respiration, which could in turn accelerate climate change. The biological mechanisms through which soil carbon (C) responds to climate are not well understood, limiting our ability to predict future global soil respiration rates. As part of a climate manipulation experiment, we tested whether differences in soil heterotrophic respiration (RH) driven by season or climate treatment are linked to (1) relative abundances of microbes in active and dormant metabolic states, (2) net changes in microbial biomass and/or (3) changes in the relative abundances of microbial groups with different C-use strategies. We used a flow-cytometric single-cell metabolic assay to quantify the abundance of active and dormant microbes, and the phospholipid fatty acid method to determine microbial biomass and ratios of fungi:bacteria and Gram-positive:Gram-negative bacteria. RH did not respond to climate treatments but was greater in the warm and dry summer than in the cool and less-dry fall. These dynamics were better explained when microbial data were taken into account compared to when only physical data (temperature and moisture) were used. Overall, our results suggest that RH responses to temperature are stronger when soil contains more active microbes, and that seasonal patterns of RH can be better explained by shifts in microbial activity than by shifts in the relative abundances of fungi and Gram-positive and Gram-negative bacteria. These findings contribute to our understanding of how and under which conditions microbes influence soil C responses to climate.

Datum: 01.06.2019

Watershed urban development controls on urban streamwater chemistry variability

### Abstract

Datum: 01.06.2019

Labile soil organic carbon loss in response to land conversion in the Brazilian woodland savanna ( cerradão )

### Abstract

Conversion of native vegetation to agriculture may change the carbon cycle by reducing carbon soil storage capacity and increasing CO2 emissions. We aimed to comparatively assess the impact of land use change on labile pools and dynamics of soil organic carbon (SOC) in two land uses (Urochloa pastures and Eucalyptus forestry) relative to the native reference ecosystem (Brazilian woodland savanna, the cerradão), as a function of soil depth and season. For three replicated study sites, each of them including a control area of the native vegetation (Cerrado) and two land uses (Pasture, Eucalyptus), we sampled soil from 0 to 2 m depth in both dry and wet seasons. We quantified dissolved organic carbon (DOC) and microbial biomass carbon (MBC), estimated the microbial quotient (MBC/SOC) and DOC/SOC ratio, and evaluated C dynamics by assessing soil basal respiration and the metabolic quotient (qCO2). Compared with Cerrado, DOC, MBC and MBC/SOC decreased in both Pasture and Eucalyptus. Differences between land uses vanished below 30 cm soil depth. Seasonality affected most analyzed variables, with lower values for DOC, DOC/SOC and qCO2, and slightly higher values for MBC and MBC/SOC in the wet season. In the dry season qCO2 increased in the Eucalyptus topsoil as compared to Cerrado, suggesting higher stress in the microbial community and/or lower decomposition efficiency in Eucalyptus. Overall, our results show that cerradão conversion to pastures and Eucalyptus plantations negatively affects labile pools and dynamics of SOC, with the effects surpassing a strong spatial and seasonal variability in the soil response to land conversion.

Datum: 01.06.2019

Biofilm-specific uptake does not explain differences in whole-stream DOC tracer uptake between a forest and an agricultural stream

### Abstract

Benthic biofilms are often assumed to control terrestrially-derived dissolved organic carbon (tDOC) uptake in streams. We tested this by comparing 13C-enriched ryegrass leachate uptake in an agricultural and a forest stream, hypothesizing that a greater abundance of autotrophic biofilms in the agricultural stream would cause its whole-stream tDOC uptake to be comparatively low. We measured whole-stream and biofilm tDOC tracer uptake, metabolism, bacterial and algal diversity, and nutrient status of benthic epilithic biofilms, and assessed whole-stream hydromorphology. Whole-stream uptake of tDOC was six times lower in the agricultural (3.0 mg m−2 day−1) than in the forest (19.0 mg m−2 day−1) stream, and tDOC uptake velocity indicated lower tDOC demand in the agricultural (1.2 mm min−1) than in the forest (1.9 mm min−1) stream. The agricultural stream differed from the forest stream by slightly lower transient storage capacity and higher benthic biofilm bacterial abundance and production, lower biofilm biomass and lower biofilm molar C:N, C:P, and N:P ratios. Changes in epilithic biofilms contributed little to the differences in whole-stream tDOC tracer uptake between streams, as biofilm tDOC uptake only amounted to 4% and 13% of whole-stream uptake in the forest and agricultural stream, respectively. This comparison of a forest and an agricultural stream suggests that agricultural stressors have the potential to diminish both whole-stream tDOC uptake and uptake efficiency. Furthermore, the weak link between biofilm and whole-stream tDOC uptake implies that benthic biofilms characteristics are poor predictors for human impacts on tDOC uptake in agricultural streams and that hot spots of tDOC uptake are likely situated in the hyporheic zone or in the stream water column.

Datum: 01.06.2019

Losses of mineral soil carbon largely offset biomass accumulation 15 years after whole-tree harvest in a northern hardwood forest

### Abstract

Changes in soil carbon stocks following forest harvest can be an important component of ecosystem and landscape-scale C budgets in systems managed for bioenergy or carbon-trading markets. However, these changes are characterized less often and with less certainty than easier-to-measure aboveground stocks. We sampled soils prior to the whole-tree harvest of Watershed 5 at the Hubbard Brook Experimental Forest in 1983, and again in years 3, 8, and 15 following harvest. The repeated measures of total soil C in this stand show no net change in the O horizon over 15 years, though mixing with the mineral soil reduced observed O horizon C in disturbed areas in post-harvest years 3 and 8. Mineral soil C decreased by 15% (20 Mg ha−1) relative to pre-harvest levels by year 8, with no recovery in soil C stocks by year 15. Proportional changes in N stocks were similar. The loss of mineral soil C offset two-thirds of the C accumulation in aboveground biomass over the same 15 years, leading to near-zero net C accumulation post-harvest, after also accounting for the decomposition of slash and roots. If this result is broadly representative, and the extent of forest harvesting is expanded to meet demand for bioenergy or to manage ecosystem carbon sequestration, then it will take substantially longer than previously assumed to offset harvest- or bioenergy-related carbon dioxide emissions with carbon uptake during forest regrowth.

Datum: 01.06.2019

Substrate quality and concentration control decomposition and microbial strategies in a model soil system

### Abstract

Soil carbon models typically scale decomposition linearly with soil carbon (C) concentration, but this linear relationship has not been experimentally verified. Here we investigated the underlying biogeochemical mechanisms controlling the relationships between soil C concentration and decomposition rates. We incubated a soil/sand mixture with increasing amounts of finely ground plant residue in the laboratory at constant temperature and moisture for 63 days. The plant residues were rye (Secale cereale, C/N ratio of 23) and wheat straw (Triticum spp., C/N ratio of 109) at seven soil C concentrations ranging from 0.38 to 2.99%. We measured soil respiration, dissolved organic carbon (DOC) concentrations, microbial biomass, and potential enzyme activities over the course of the incubation. Rye, which had higher N and DOC contents, lost 6 to 8 times more C as CO2 compared to wheat residue. Under rye and wheat amendment, absolute C losses as CO2 (calculated per g dry soil) increased linearly with C concentration while relative C losses as CO2 (expressed as percent of initial C) increased with C concentration following a quadratic function. In low C concentration treatments (0.38–0.79% OC), DOC decreased gradually from day 3 to day 63, microbial C increased towards the end in the rye treatment or decreased only slightly with straw amendment, and microbes invested in general enzymes such as proteases and oxidative enzymes. At increasing C levels, enzyme activity shifted to degrading cellulose after 15 days and degrading microbial necromass (e.g. chitin) after 63 days. At the highest C concentrations (2.99% OC), microbial biomass peaked early in the incubation and remained high in the rye treatment and decreased only slightly in the wheat treatment. While wheat lost C as CO2 constantly at all C concentrations, respiration dynamics in the rye treatment strongly depended on C concentration. Our results indicate that litter quality and C concentration regulate enzyme activities, DOC concentrations, and microbial respiration. The potential for non-linear relationships between soil C concentration and decomposition may need to be considered in soil C models and soil C sequestration management approaches.

Datum: 01.06.2019

Litter quantity, litter chemistry, and soil texture control changes in soil organic carbon fractions under bioenergy cropping systems of the North Central U.S.

### Abstract

Soil organic carbon (SOC) storage is a critical component of the overall sustainability of bioenergy cropping systems. Predicting the influence of cropping systems on SOC under diverse scenarios requires a mechanistic understanding of the underlying processes driving SOC accumulation and loss. We used a density fractionation technique to isolate three SOC fractions that are conceptualized to vary in SOC protection from decomposition. The free light fraction (FLF) is particulate SOC that is present in the inter-aggregate soil matrix, the occluded light fraction (OLF) is contained within aggregates, and the heavy fraction (HF) is associated with minerals. We evaluated surface (0 to 10 cm depth) SOC fraction changes from baseline conditions 5 years after biofuel cropping system establishment at two temperate sites with contrasting soil textures. The biofuel cropping systems included no-till maize, switchgrass, prairie, and hybrid poplar. The FLF concentration (g fraction C g bulk soil−1) did not change significantly from baseline levels under any of the cropping systems at either site after 5 years. Except for poplar, OLF concentrations were reduced in all systems at the site with coarse-textured soils and maintained at the site with fine-textured soils. In poplar systems, OLF concentrations were maintained on coarse-textured soils and increased on fine-textured soils. The HF concentrations also increased under poplar on the coarse-textured soil. A structural equation model indicated that OLF concentrations increased with lower litter C:N, and HF concentrations increased with greater litter quantity and lower litter C:N mass ratios. C:N increased over time within all SOC fractions, suggesting that all pools are sensitive to land-use change on sub-decadal timescales. In agreement with modern SOC theory, our empirical results indicate that increasing litter input quantity and promoting plant species with low C:N litter may improve SOC storage in aggregate and mineral-associated soil fractions.

Datum: 01.05.2019

The effect of a shift from macrophyte to phytoplankton dominance on phosphorus forms and burial in the sediments of a shallow hard-water lake

### Abstract

In shallow lakes, increasing phosphorus (P) loading has often been accompanied by a shift from a clear-water, macrophyte-dominated state to a turbid state featuring phytoplankton dominance. The effect of a regime shift on P burial and P fractions in lake sediments, however, is poorly understood. We used sediment cores from a eutrophic hard-water lake (Lake Gollinsee, Germany) that had undergone a regime shift (in approximately 1917) to investigate the effect on the accumulation rate of P and on changes in P forms. The cores were dated using Hg contents and radioisotopes (210Pb, 137Cs, and 241Am). A combination of total organic carbon to total nitrogen ratios (TOC:TN), δ13TOC values, X-ray fluorescence calcium (Ca) counts, and sediment colour clearly distinguished sediment layers that were deposited during periods of macrophyte or phytoplankton dominance. The accumulation rate of total P (TP) in the sediments was 1.8 times higher after the regime shift and was associated with changes in the distribution of P fractions. The proportions of loosely-(NH4Cl-extracted TP) and Ca-(HCl-extracted TP) bound P decreased significantly, whilst the proportions of biogenic P (NaOH-extracted NRP) and aluminium-bound P (NaOH-extracted SRP) increased significantly. A higher dry mass deposition rate, reduced burial of stable Ca-P complexes, and increased contents and proportions of the mobile iron-bound (BD-extracted TP) and biogenic P fractions in the near-surface sediment layers are assumed to have enhanced the internal cycling of P and hence to have helped to maintain a state of phytoplankton dominance.

Datum: 01.05.2019

Bioconcentration of Cd and Zn in the soils of an uncontaminated forest in the Quebec Laurentians

### Abstract

Fine-scale trace element (TE) concentration patterns exist in soils that were never exposed to the direct deposition of TE in the past. We aimed to understand the role of vegetation, in particular trees, in defining these spatial patterns in the soil horizons of an uncontaminated forest ecosystem. Living tree leaves and surface soil samples (L, F, and B horizons) were collected from a ca. 1400 m2 plot in the Hermine watershed where a strong gradient in the abundance of Yellow birches (Betula alleghaniensis—BA) exists. TE analysis showed that BA leaves had up to 13 times more Cd and Zn than the other tree species. Significantly similar spatial patterns were detected between the density of BA trunks, the abundance of BA leaves in the litter (L) layer, and Cd and Zn concentrations in the F horizon. The opposite trend was found in the upper B horizon, where Cd and Zn were depleted under a denser BA canopy. We submit that soil depletion could be associated with the preferential phytoextraction of Cd and Zn by BA over a period of several decades. Our study stresses the role of biological cycling by trees on the profile redistribution and the generation of patterns of soil TE in quasi-pristine forests.

Datum: 01.05.2019

Denitrification in the river network of a mixed land use watershed: unpacking the complexities

### Abstract

River networks have the potential to permanently remove nitrogen through denitrification. Few studies have measured denitrification rates within an entire river network or assessed how land use affect rates at larger spatial scales. We sampled 108 sites throughout the network of the Fox River watershed, Wisconsin, to determine if land use influence sediment denitrification rates, and to identify zones of elevated sediment denitrification rates (hot spots) within the river network. Partial least squares regression models identified variables from four levels of organization (river bed sediment, water column, riparian zone, and watershed) that best predicted denitrification rates throughout the river network. Nitrate availability was the most important predictor of denitrification rates, while land cover was not always a good predictor of local-scale nitrate concentrations. Thus, land cover and denitrification rate were not strongly related across the Fox River watershed. A direct relationship between denitrification rate and watershed land cover occurred only in the Wolf River sub-watershed, the least anthropogenically disturbed of the sub-watersheds. Denitrification hot spots were located throughout the river network, regardless of watershed land use, with hot spot location being determined primarily by nitrate availability. In the Fox River watershed, when nitrate was abundant, river bed sediment character influenced denitrification rate, with higher denitrification rates at sites with fine, organic sediments. These findings suggest that denitrification occurring throughout an entire river network, from headwater streams to larger rivers, can help reduce nitrogen loads to downstream water bodies.

Datum: 01.05.2019

Shining light on the storm: in-stream optics reveal hysteresis of dissolved organic matter character

### Abstract

The quantity and character of dissolved organic matter (DOM) can change rapidly during storm events, affecting key biogeochemical processes, carbon bioavailability, metal pollutant transport, and disinfection byproduct formation during drinking water treatment. We used in situ ultraviolet–visible spectrophotometers to concurrently measure dissolved organic carbon (DOC) concentration and spectral slope ratio, a proxy for DOM molecular weight. Measurements were made at 15-minute intervals over three years in three streams draining primarily agricultural, urban, and forested watersheds. We describe storm event dynamics by calculating hysteresis indices for DOC concentration and spectral slope ratio for 220 storms and present a novel analytical framework that can be used to interpret these metrics together. DOC concentration and spectral slope ratio differed significantly among sites, and individual storm DOM dynamics were remarkably variable at each site and among the three sites. Distinct patterns emerged for storm DOM dynamics depending on land use/land cover (LULC) of each watershed. In agricultural and forested streams, DOC concentration increased after the time of peak discharge, and spectral slope ratio dynamics indicate that this delayed flux was of relatively higher molecular weight material compared to the beginning of each storm. In contrast, DOM character during storms at the urban stream generally shifted to lower molecular weight while DOC concentration increased on the falling limb, indicating either the introduction of lower molecular weight DOM, the exhaustion of a higher molecular weight DOM sources, or a combination of these factors. We show that the combination of high-frequency DOM character and quantity metrics have the potential to provide new insight into short-timescale DOM dynamics and can reveal previously unknown effects of LULC on the chemical nature, source, and timing of DOM export during storms.

Datum: 01.05.2019

Groundwater nitrate removal in riparian buffer zones: a review of research progress in the past 20 years

### Abstract

This review evaluates research in the past 20 years focusing on groundwater nitrate removal in the riparian zones of agricultural watersheds. Studies have reported a large range in the magnitude of groundwater and nitrate fluxes to buffers in different hydrogeologic settings. An earlier focus on buffers with shallow subsurface flow has expanded to include sites with deep flow paths and groundwater-fed overland flow. Nitrate removal efficiency and the width required for removal have been linked to riparian sediment texture and depth to an impervious layer. Denitrification has been identified as the dominant mechanism of nitrate removal based on evidence that this process occurs at depth in many buffers which contain buried organic-rich deposits. Several studies have assessed the cumulative effect of riparian buffers on nitrate removal at the watershed scale. Despite considerable research progress areas of uncertainty still remain. Buffers with coarse-textured sediments located in landscapes with upslope sand aquifers have received most attention. In contrast, few sites have been analysed in weathered bedrock and glacial till landscapes. Many studies have reported nitrate removal efficiency based on nitrate concentrations rather than measuring groundwater fluxes which assess the magnitude of nitrate removal. More information is needed on interactions between riparian hydrological flow paths and biogeochemical processes. Further research is recommended on the effect of riparian zone nitrate removal at the watershed scale and long-term monitoring with respect to buffer restoration, the ability to sustain nitrate removal and responses to land use and climate change.

Datum: 01.05.2019

Category: Current Chemistry Research

Last update: 28.03.2018.

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