Dissolved Organic Carbon Dynamics Research Articles

Machine‐Learning Reveals Equifinality in Drivers of Stream DOC Concentration at Continental Scales

AbstractResearch at long‐term catchment monitoring sites has generated a great volume, variety, and velocity of data for analysis of stream water chemistry dynamics. To harness the potential of these big data and extract patterns that are indicative of underlying functional relationships, machine learning tools have advantages over traditional statistical methods, and are increasingly being applied for dimension reduction, feature extraction, and trend identification. Still, as examples of complex systems, catchments are characterized by multivariate factor interactions and equifinality that are not easily identified by most machine‐learning methods. Using dissolved organic carbon (DOC) dynamics as an illustration, we applied a new evolutionary algorithm (EA) to extract geologic, topographic, meteorologic, hydrologic, and land use attributes that were correlated to mean stream DOC concentration in forested catchments distributed across the continental United States. The EA reduced dimensionality of our attribute dataset to identify the combination of factors, and their specific value ranges, that interacted to drive membership in High or Low mean DOC clusters. High mean DOC concentrations were associated with two distinct geographic locations of variable climatic and vegetative conditions, indicating equifinality. Our findings underscore the importance of critical zone structure in mediating hydrological and biogeochemical processes to govern DOC dynamics at the catchment scale. This multi‐scale, pattern‐to‐process approach is being applied to refine hypotheses for process‐based modeling of DOC dynamics in forested headwater streams at catchment to site scales.

GTAR: a new ensemble evolutionary autoregressive approach to model dissolved organic carbon

AbstractThis article explores the forecasting capabilities of three classic linear and nonlinear autoregressive modeling techniques and proposes a new ensemble evolutionary time series approach to model and forecast daily dynamics in stream dissolved organic carbon (DOC). The model used data from the Oulankajoki River basin, a boreal catchment in Northern Finland. The models that were evolved used both accuracy and parsimony measures including autoregressive (AR), vector autoregressive (VAR), and self-exciting threshold autoregressive (SETAR). The new method, called genetic-based SETAR (GTAR), evolved through the integration of state-of-the-art genetic programming with SETAR. To develop the models, high-resolution DOC concentration and daily streamflow (as the external input for VAR) were measured at the same gauging station throughout the ice free season. The results showed that all the models characterize the DOC dynamics with an acceptable 1-day-ahead forecasting accuracy. Use of the streamflow time series as an exogenous variable did not increase the predictive accuracy of AR models. Moreover, the hybrid GTAR provided the best accuracy for the holdout testing data and proved to be a suitable approach for predicting DOC in boreal conditions.

Dynamics of dissolved organic carbon during drought and flood events: A phase-by-stages perspective

Dissolved organic carbon (DOC) is a key water quality parameter that plays a crucial role in controlling aquatic ecosystems and carbon cycling. Understanding DOC dynamics during hydrological extremes (i.e., droughts and floods) helps in managing water quality, but such variability is rarely studied. Furthermore, how differences in DOC concentrations among phase-by-stages of drought/flood affect simulation performances based on hydrological features remains unclear. Here, phase-by-stages of hydrological drought (flood) were divided into intensification (rising) and recovery (falling) periods based on drought peak intensity (flood peak intensity). The long-term (1976–2019) daily discharge and weekly (biweekly) DOC concentrations from four headwater streams with different watershed sizes (from 9.97 to 119.09 ha) in south-central Ontario, Canada, were used to achieve the above aims. The results showed that (i) the average DOC concentration during intensification (rising) stage of drought (flood) was smaller (larger) than during recovery (falling). (ii) Simulations performed better when accounting for phase-by-stages of drought/flood, with reductions in mean absolute percentage error of 32.85 % and 53.59 % for drought and flood events, respectively. These results will help understand the dynamics of DOC during hydrological extremes and improve simulation performance of numerical models for water quality parameters under changing environmental conditions.

Dynamics of dissolved organic carbon in runoff discharge under different rainfall patterns in a representative agricultural catchment

Rainfall-induced runoff transported plentiful terrestrial dissolved organic carbon (DOC) to downstream waters, significantly regulating the biogeochemical cycling of carbon in downstream waters. However, the dynamics of DOC exporting from agricultural catchments in response to different rainfall patterns remain largely unknown. This study aimed to understand the effects of rainfall patterns on the runoff DOC quantity and quality in a representative agricultural catchment in Southwest China. Six rainfall events producing runoff, including three moderate rainfalls, two heavy rainfalls, and one rainstorm, were recorded to investigate the shifts in runoff DOC concentration and its quality indicators, i.e., SUVA254 (indicating the aromaticity content), C:C ratio (denoting the proportion of colored substances to uncolored substances), and E4:E6 ratio (denoting the fulvic acid to humic acid of DOC). The results showed that DOC concentration in runoff discharge was highest in the moderate and heavy rainfalls occurring at the early rainy season, while was much lower in the rainstorm. Exported DOC flux reached 1.82 kg hm-2 h−1 in the rainstorm, which was 3.95–60.67 times that in the other rainfall events. SUVA254 and C:C ratio fluctuated softly during the moderate rainfalls and rainstorm, and changed strongly during the heavy rainfall with longest antecedent dry day. The moderate and heavy rainfalls with longer antecedent dry day and the rainstorm delivered DOC with higher SUVA254 and C:C ratio than the other rainfall events did. Discharge was positively correlated with DOC concentration during rainfall across all rainfall patterns, while was negatively correlated with SUVA254 and C:C ratio. Forward stepwise regression analysis showed that DOC flux per hour was only positively linearly correlated with rainfall intensity. Our results were valuable for understanding the biogeochemical cycling of DOC in analogous catchments worldwide.

Analysis of spatiotemporal variation in dissolved organic carbon concentrations for streams with cropland-dominated watersheds

It remains a challenge to understand how dissolved organic carbon (DOC) is cycled from farmlands to rivers due to the complex interaction between farming practices, the baseflow hydrology of predominantly flat lowlands, and seasonal environmental influences such as snowpack. To address this, field DOC concentrations were measured monthly throughout the year at sub-basin scales across the Chippewa River Watershed, which falls within the Corn Belt of the Midwestern United States. These DOC dynamics in stream water from croplands were benchmarked against the data sampled from hilly forested areas in the Connecticut River Watershed. The Soil Water Assessment Tool (SWAT) simulation was applied to provide potential predictive variables associated with daily baseflow. Our study outlines a framework using the combination of primary field data, hydrological modeling, and knowledge-based reclassification of Land Use/Land Cover (LULC) data to analyze the viability of modeling the spatial and temporal variations of cropland stream DOC concentrations. Calibration of the SWAT model resulted in the overall daily Nash–Sutcliffe model efficiency coefficient (NSE) of 0.67 and the corresponding R2 = 0.89. Our main results show: 1) baseflow DOC concentrations from croplands were substantially higher throughout the year relative to other landcover areas, especially for spring runoff/snowmelt scenarios, 2) an empirical analysis explained ~82 % of the spatial gradient of annual mean observed DOC concentrations, and 3) with the addition of hydrological simulated variables, a linear model explained ~81 % of monthly and 54 % of daily variations of observed DOC concentrations for cropland sub-basins. Our study identified key factors regulating the spatiotemporal DOC concentrations in cropland streamflow; the contribution here promotes to strengthen future analytical models that link watershed characteristics to carbon cycling processes in a large freshwater ecosystem.

Mobilization of subsurface carbon pools driven by permafrost thaw and reactivation of groundwater flow: a virtual experiment

Permafrost thaw leads to an increase in groundwater circulation and potential mobilization of organic carbon sequestered in deep Arctic sediments (e.g. 3–25 m below surface). Upon thaw, a portion of this carbon may be transported along new groundwater flow paths to surface waters or be microbially transformed or immobilized by in-situ biogeochemical reactions. The fate of thaw-mobilized carbon impacts surface water productivity and global climate. We developed a numerical model to investigate the effects of subsurface warming, permafrost thaw, and resultant increased groundwater flow on the mobilization and reactive transport of dissolved organic carbon (DOC). Synthetic simulations demonstrate that mobilization and groundwater-borne DOC export are determined by subsurface thermo-chemical conditions that control the interplay of DOC production (organic matter degradation), mineralization, and sorption. Results suggest that peak carbon mobilization from these depths precedes complete permafrost loss, occurring within two centuries of thaw initiation with the development of supra-permafrost groundwater flow systems. Additionally, this study highlights the lack of field data needed to constrain these new models and apply them in real-word site-specific applications, specifically the amount and spatial variability of organic carbon in deep sediments and data to constrain DOC production rates for groundwater systems in degrading permafrost. Modeling results point to key biogeochemical parameters related to organic matter and carbon bioavailability to be measured in the field to bridge the gap between models and observations. This study provides a foundation for further developing a physics-based modeling framework to incorporate the influence of groundwater flow and permafrost thaw on permafrost DOC dynamics and export, which is imperative for advancing understanding and prediction of carbon release and terrestrial-aquatic carbon exchange in warming Artic landscapes in the coming decades.

Response of dissolved organic carbon in rainwater during extreme rainfall period in megacity: Status, potential source, and deposition flux

Air pollution and related wet deposition process (rainfall) caused environmental hazards under the climate change, particularly in urban regions, attracting global concern and challenges. Atmospheric dissolved organic carbon (DOC) deposition is not only the major removal pathway of OC from atmosphere, but also a significant influence factor of urban haze pollution and local climate. Although the significance of DOC deposition to carbon budgets is confirmed, the extreme rainfall influenced OC transfers from the atmosphere to land is poorly understood. This study systematically investigated the rainwater DOC in Beijing during the extreme rainfall period of 2021 to explore its status, source, and flux. The results showed that the rainwater DOC concentration level is close to the Asian and global mean level, while the DOC deposition flux was relatively high due to high rainfall amount and rainfall scouring, and the overall declined trend of DOC concentrations was identified. Source identification revealed that human OC emissions (municipal wastes, agricultural and traffic emission) are the primary sources of DOC, whereas ocean and crust source contributed negligibly. This research clarified the extreme rainfall-driven rainwater DOC dynamics in a megacity and supported the evaluation of global carbon budgets.

Carbon limitation may override fine-sediment induced alterations of hyporheic nitrogen and phosphorus dynamics.

The hyporheic zone underneath stream channels is considered a biogeochemical hotspot reducing nutrient loads being transported downstream due to its high surface-to-volume ratio in combination with the hyporheic exchange. However, the effect of environmental stressors such as high amounts of fine sediment (FS; grain size <0.2 mm) on nutrient cycling in the hyporheic zone are not well understood. Physical clogging caused by fine sediment (FS) decreases the hyporheic exchange, thus, diminishing its potential to reduce nutrient loads despite increasing its surface-to-volume ratio. We determined the effect of physical clogging on nutrient cycling based on net change rates of dissolved inorganic nitrogen (DIN; nitrate-N, ammonium-N), soluble reactive phosphorus (SRP), and dissolved organic carbon (DOC) for a sand and gravel hyporheic zone. We performed three experimental runs in 12 flumes with four-week duration each following a factorial design. First, we determined nutrient cycling in sand and gravel in absence of clogging, and then tested the clogging effect for each sediment type under increasing clogging (0-480 g of FS addition increasing by 60 g per level). Without clogging, gravel acted as a source of nitrate-N; and both sand and gravel released SRP. Regardless of the clogging level and the resulting reduced hyporheic exchange, we found no changes in DOC and nitrate-N dynamics but net-release of ammonium-N and SRP for gravel. In contrast, in sand, physical clogging inhibited DOC release for flumes with the higher FS. We propose that not physical clogging but DOC availability limited the nutrient uptake, as molar ratios of DOC to DIN and SRP ranged 1.2-1.5 and 77-191, respectively, indicating severe C limitation of N-uptake and partial C limitation of P-uptake. Our results suggest an interplay between nutrient molar ratios and physical clogging, which emphasize the interactions between hydrology and the stoichiometry of organic carbon, nitrogen and phosphorus in the hyporheic zone.

Catchment Dissolved Organic Carbon Transport: A Modeling Approach Combining Water Travel Times and Reactivity Continuum

AbstractQuantifying the transfer of organic carbon from the terrestrial to the riverine ecosystems is of crucial importance to fully appreciate the carbon cycle at the catchment, regional and global scales. In this study, we propose a framework for modeling the flux of dissolved organic carbon (DOC) from hillslopes to stream and river networks which couples a transport model based on travel time distributions with the reactivity continuum (RC) approach to model DOC degradation. We test the model by applying it to the Plynlimon catchments (UK) exploiting both weekly and high‐frequency (7‐hr interval) time‐series. We use information about chloride to get an independent estimate of water travel times using the framework of StorAge Selection functions. Following the RC model, the composition and the degradation of DOC along the flowpaths, and its consequent concentration in the streamflow, is described assuming that DOC is composed by a mixture of compounds that follows a continuous spectrum of reactivity. For the high‐frequency data set, the model is able to reproduce DOC streamflow concentrations and to capture the complex hysteretic relation between DOC concentration and discharge. Weekly data are instead not frequent enough to properly describe DOC dynamics in this catchment. The distribution of the age of the water comprised in the streamflow proves thus a key variable to predict the quantity and the reactivity of the DOC exported from soils, and the effect of hydrologic variability on this process.

Dissolved Organic Carbon Along a Meridional Transect in the Western North Pacific Ocean: Distribution, Variation and Controlling Processes

Dissolved organic carbon (DOC) in the ocean is one of the largest reduced and exchangeable organic carbon pools on Earth and plays important roles in carbon cycling and biogeochemical processes in the ocean. Here, we report the concentrations and distributions of DOC in water samples collected along a meridional transect in the western North Pacific (NP) Ocean in November 2019. Concentrations of DOC ranged from 33-102 μM, were higher in surface water, decreased rapidly with depth to 1,000 m, and then remained relatively constant. The labile fraction of DOC accounted for 20-40% of the surface bulk DOC and was respired very rapidly in the upper 200 m depth. The semi-labile fraction of DOC accounted for 15-20% of the surface bulk DOC and was exported downward and turned over at water depths of 200-2,000 m. The formation of NP Intermediate Water (NPIW) in the Kuroshio Extension (KE) region is a major process carrying some surface semi-labile DOC down. The Low concentrations of DOC (33-44 μM) were present in the entire water column below 1,000 m along the transect in the NP. Primary production and microbial consumption played major roles in the concentration and distribution of DOC in the euphotic zone, and hydrodynamic mixing and circulation of different water masses appear to be dominant factors controlling the distribution and dynamics of DOC in the deep water of the western NP.

The channel‐source hypothesis: Empirical evidence for in‐channel sourcing of dissolved organic carbon to explain hysteresis in a headwater mountain stream

AbstractCatchment hydrologists have long puzzled over the question: How can catchments rapidly generate storm flows and pulses of solutes in response to storm events? Conceptual models viewing catchments as composed of discrete source areas generating flow at unique time scales and with unique chemical characteristics have been used to explain the observed changes in flow and water chemistry. Surprisingly, those conceptual models usually do not treat the stream channel as one of the potential source areas. Here, we propose the channel‐source hypothesis in which the stream itself should be considered as a potential source with the same rigour as other contributing areas. We pose this in the spirit of the scientific use of the word: a hypothesis is not a proven idea but rather a provisional supposition serving as the basis for further study. We suggest that the channel should be considered as a potential source for dissolved organic carbon (DOC). Channels store substantial amounts of organic matter, and stream ecologists have long studied stream carbon cycling. From those studies, we know that leaching and decomposition can generate DOC from particulate organic carbon (POC). Further, POC is stored in channel ‘dead‐zones’—regions of low flow velocity ‐ that can be activated as flow velocity increases, thus releasing accumulated DOC during storms. All catchments are different; there is no reason to assume that channel sources are always important, in every catchment, in every storm. Thus, the channel‐source hypothesis does not replace existing conceptual models. Instead, it adds another potential mechanism that may explain DOC dynamics observed in streams. The channel‐source hypothesis has substantial implications for catchment studies examining sources of DOC in stream water or using DOC as a tracer to determine the locations of, and proportional contributions of, different source areas for streamflow generation.

Sediment-Derived Dissolved Organic Matter Stimulates Heterotrophic Prokaryotes Metabolic Activity in Overlying Deep Sea in the Ulleung Basin, East Sea

The effects of benthic dissolved organic carbon (DOC) flux on the dynamics of DOC in the deep continental margins (200 – 2000 m depth) is poorly understood. We investigated heterotrophic prokaryotes (hereafter bacteria) production (BP) and the bio-reactive properties of sediment-derived dissolved organic matter (SDOM) to elucidate microbially mediated cause-effect relationships regarding the rapid consumption of dissolved oxygen (DO) and accumulation of humic-like fluorescent DOM (FDOMH) in the deep-water column (750 – 2000 m depth range) of the Ulleung Basin (UB) in the East Sea. BP in the deep water (2.2 μmol C m-3 d-1) of the UB was among the highest reported for various deep-sea sites. The high DOC concentration (55 μM) likely supported the high BP seen in the deep-water column of the UB. Concentrations of DOC and C1 component of the FDOMH, which is indicative of microbial metabolic by-products, were 13-fold and 20-fold greater, respectively, in pore water than in the overlying bottom water, indicating that the sediment in the continental margins is a significant source of DOM in the overlying water column. Fine-scale water sampling revealed that BP near the sediment (0 – 30 m above the seafloor; 2.78 μmol C m-3 d-1) was 1.67 times higher than that measured in the water column above (30 – 100 m above the seafloor; 1.67 μmol C m-3 d-1). In addition, BP increased in the bottom water incubation amended with SDOM-containing pore water (PW). The results demonstrated that SDOM contains bio-reactive forms of DOM that stimulate heterotrophic microbial metabolism at the expense of oxygen in the bottom water layer. The accumulation of C1 component in both PW-amended and unamended bottom water incubation (i.e., without an extra DOM supply from sediment) further indicated that refractory DOM is produced autochthonously in the water column via heterotrophic metabolic activity. This explains in part the microbially mediated accumulation of excess FDOMH in the deep-water column of the UB. Overall results suggest that the benthic release of bio-reactive DOM may be of widespread significance in controlling microbial processes in the deep-water layer of marginal seas.

Spatiotemporal dynamics of dissolved organic carbon in a subtropical wetland and their implications for methane emissions

Wetlands represent about one-third of the global soil organic carbon storage and are the largest natural source of atmospheric methane, a powerful greenhouse gas. Understanding the factors that influence the carbon dynamics in wetlands, and their influence on methane emission, is therefore important. Dissolved organic carbon (DOC) is a major form of carbon export from wetlands and plays an essential role in the aquatic carbon cycling process. However, constraints on spatial and temporal dynamics of DOC in vertical peat layers and their relationship to wetland methane emissions are still rare. Here we investigated spatiotemporal changes in porewater DOC concentration and optical properties (SUVA254, E2/E3, E4/E6) in surface (0–10 cm) and deep (20–30 cm, 50–60 cm, 100–110 cm) peat layers in a subtropical wetland complex, central China, with the aim to explore DOC dynamics and their relationship to methane emissions from wetlands. DOC dynamics and environmental controls were investigated in the context of high-resolution environmental monitoring, including air temperature, rainfall, water table depth, water chemistry, and methane emissions. DOC of surface peat layers generally had higher concentrations, higher aromaticity, lower humification, and greater temporal variations compared to deeper peat layers. During summer floods and high water table conditions, DOC concentrations in the surface peat increased significantly, whilst DOC exports from rivers and lakes also increased synchronously. In contrast, DOC concentrations and spectral properties of deep peat layers remained constant across the sampling months and water table fluctuations. Methane emissions were positively correlated with air temperature and DOC concentration in the porewater of surface peat layers, and negatively correlated with water table depth. Our results suggest that DOC concentration in the porewater of surface peat layers may also have affected the flux of methane emission from wetlands.

Dissolved Organic Carbon (DOC) in Ground Ice on Northeastern Tibetan Plateau

Ground ice in permafrost stores substantial amounts of dissolved organic carbon (DOC) upon thaw, which may perpetuate a carbon feedback in permafrost regions, yet little is known to date about the dynamics of DOC and source variability of ground ice on the Tibetan Plateau. Here, the high-resolution data of DOC in ground ice (4.8 m in depth) from two permafrost profiles on the Northeastern Tibetan Plateau (NETP) were firstly presented. We quantified the DOC concentrations (mean: 9.7–21.5 mg/L) of ground ice and revealed sizeable—by a factor of 7.0–36.0—enrichment of the ground ice relative to the other water elements on the TP. Results indicated remarkable depth differences in the DOC of ground ice, suggestive of diverse sources of DOC and different sequestration processes of DOC into ice during permafrost evolution. Combined with DOC and carbon isotopes (δ13CDOC), we clarified that decomposition of soil organic matter and leaching of DOC from organic layers and surrounding permafrost sediments are the important carbon sources of ground ice. The DOC sequestration of ground ice in the upper layers was related to the active layer hydrology and freeze–thaw cycle. However, the permafrost evolution controlled the decomposition of organic carbon and sequestration of DOC in the deep layers. A conceptual model clearly illustrated the dynamics of DOC in ground ice and suggested a significant impact on the carbon cycle on the NETP. The first attempt to explore the DOC in ground ice on the NETP is important and effective for further understanding of carbon cycle under permafrost degradation on the Tibetan Plateau.

High-frequency monitoring enables operational opportunities to reduce the dissolved organic carbon (DOC) load in Germany’s largest drinking water reservoir

ABSTRACT Rising dissolved organic carbon (DOC) is interfering with drinking water production. While strategies for DOC removal during water treatment have been successfully implemented, the potential for DOC load reduction by optimized reservoir operation is not yet fully explored, mainly constrained by data paucity on real-time DOC dynamics. In this study, we utilized the emerging in situ high-frequency (HF) monitoring technique for DOC and developed a simulation–operation framework that promotes DOC mitigation in Germany’s largest drinking water reservoir. Rappbode Reservoir is embedded in a network of smaller upstream reservoirs from which Königshütte Reservoir delivers the most water but can also be operated as a bypass system. Using high-frequency monitoring of DOC concentrations and discharge at the inflows and outflows, we constructed a mass balance model that simulated the DOC dynamics in the reservoir, allowing us to explore alternative operation regimes that deliver the same amount of water but a lower DOC load. Our results show that, through rapid decision-making that enables bypassing of water with high DOC concentrations around the drinking water reservoir, the optimized operation regime is able to reduce DOC load of the drinking water reservoir by 25 ± 3%. Therefore, our proposed operational strategy to minimize DOC loading to reservoirs is promising.

Dissolved organic carbon concentration and biodegradability across the global rivers: A meta-analysis

Riverine dissolved organic carbon (DOC) exerts a vital role in the global carbon cycle. Although substantial studies have explored DOC dynamics in aquatic ecosystems, it remains unknown about the patterns and drivers of riverine DOC concentration and biodegradability at the global scale. Here, by synthesizing 396 data points from 42 published literatures worldwide, we explored the spatial variations and controls of riverine DOC concentration and biodegradability. Our results revealed that the DOC concentration varied largely across the global rivers, with an average concentration of 10.4 mg L−1. The variations of DOC concentration were influenced by mean annual precipitation, vegetation type, soil type, permafrost degradation and river order. Furthermore, our results illustrated that the riverine DOC also had high biodegradability, showing an average of 16.4% loss within less than 90 days. The biodegradability of DOC was dominantly regulated by inorganic nutrients and DOC composition, but the controlling factors exhibited significant differences between small streams and large rivers. For small streams, DOC composition was the primary driver influencing DOC biodegradability, while for large rivers, nitrogen availability became the dominant factor affecting DOC biodegradability. This study highlights the high concentration and biodegradability of riverine DOC, which could exert an important role in the feedbacks between the global carbon cycle and climate warming.

Integrating McGill Wetland Model (MWM) with peat cohort tracking and microbial controls

Peatlands store a large amount of organic carbon and are vulnerable to climate change and human disturbances. However, ecosystem-scale peatland models often do not explicitly simulate the decrease in peat substrate quality, i.e., decomposability or the dynamics of decomposers during peat decomposition, which are key controls in determining peat carbon's response to a changing environment. In this paper, we incorporated the tracking of each year's litter input (a cohort) and controls of microbial processes into the McGill Wetland Model (MWMmic) to address this discrepancy. Three major modifications were made: (1) the simple acrotelm-catotelm decomposition model in MWM was changed into a time-aggregated cohort model, to track the decrease in peat quality with decomposition age; (2) microbial dynamics: growth, respiration and death were incorporated into the model and decomposition rates are regulated by microbial biomass; and (3) vertical and horizontal transport of the dissolved organic carbon (DOC) were added and used to regulate the growth of microbial biomass. MWMmic was evaluated against measurements from the Mer Bleue peatland, a raised ombrotrophic bog located in southern Ontario, Canada. The model was able to replicate microbial and DOC dynamics, while at the same time reproduce the ecosystem-level CO2 and DOC fluxes. Sensitivity analysis with MWMmic showed increased peatland resilience to perturbations compared to the original MWM, because of the tracking of peat substrate quality. The analysis revealed the most important parameters in the model to be microbial carbon use efficiency (CUE) and turnover rate. Simulated microbial adaptation with those two physiological parameters less sensitive to disturbances leads to a significantly larger peat C loss in response to warming and water table drawdown. Thus, the rarely explored peatland microbial physiological traits merit further research. This work paves the way for further model development to examine important microbial controls on peatland's biogeochemical cycling.

Changes in water‐extractable organic matter in tropical forest and agricultural soils as detected by the DRIFT spectroscopy technique

AbstractWater‐extractable organic matter (WEOM) drives many fundamental functions which determine soil equilibria, acting as a useful probe to highlight the effect of land management on overall C cycling. The ecological importance of WEOM is emphasized in tropical environments characterized by humid climate and short OC turnover. The low OC concentration and the physical–chemical conditions associated to water dissolved compounds determine an analytical issue for the characterization of WEOM. The present experiment aimed to test the diffuse reflectance infrared Fourier transformation (DRIFT) spectroscopy as a rapid tool to evaluate the dynamic of WEOM in tropical soils under different soil organic matter (SOM) managements. The influence of SOM composition on WEOM fraction was evaluated in a laboratory incubation of two tropical soils treated with different organic amendments. The superficial layers of forest and sugarcane Ultisol were incubated for 1 year with 10 mg C g−1 soil of litter from either native tropical forest or sugarcane crop residues. The sugarcane residues promoted a fast increase of dissolved organic carbon (DOC) followed by a subsequent decrease in both soils, whereas the soils amended with forest litter showed DOC dynamics similar to control samples. The DRIFT analysis unveiled changes in the chemical profile of WEOM mainly reflected on the preservation of dissolved hydrophobic compounds. The results indicate the DRIFT technique as an efficient tool to monitor the chemical changes of WEOM f as related to soil use and organic inputs, and to support a comprehensive understanding of the interaction of WEOM in agro‐ecological and environmental processes.

Hydrological and catchment controls on event‐scale dissolved organic carbon dynamics in boreal headwater streams

AbstractHydrological events transport large proportions of annual or seasonal dissolved organic carbon (DOC) loads from catchments to streams. The timing, magnitude and intensity of these events are very sensitive to changes in temperature and precipitation patterns, particularly across the boreal region where snowpacks are declining and summer droughts are increasing. It is important to understand how landscape characteristics modulate event‐scale DOC dynamics in order to scale up predictions from sites across regions, and to understand how climatic changes will influence DOC dynamics across the boreal forest. The goal of this study was to assess variability in DOC concentrations in boreal headwater streams across catchments with varying physiographic characteristics (e.g., size, proportion of wetland) during a range of hydrological events (e.g., spring snowmelt, summer/fall storm events). From 2016 to 2017, continuous discharge and sub‐daily chemistry grab samples were collected from three adjacent study catchments located at the International Institute for Sustainable Development‐Experimental Lakes Area in northwestern Ontario, Canada. Catchment differences were more apparent in summer and fall events and less apparent during early spring melt events. Hysteresis analysis suggested that DOC sources were proximal to the stream for all events at a catchment dominated by a large wetland near the outlet, but distal from the stream at the catchments that lacked significant wetland coverage during the summer and fall. Wetland coverage also influenced responses of DOC export to antecedent moisture; at the wetland‐dominated catchment, there were consistent negative relationships between DOC concentrations and antecedent moisture, while at the catchments without large wetlands, the relationships were positive or not significant. These results emphasize the utility of sub‐daily sampling for inferring catchment DOC transport processes, and the importance of considering catchment‐specific factors when predicting event‐scale DOC behaviour.

Is a simple model based on two mixing reservoirs able to reproduce the intra-annual dynamics of DOC and NO3 stream concentrations in an agricultural headwater catchment?

Agriculture disturbs the biogeochemical cycles of major elements, which alters the elemental stoichiometry of surface stream waters, with potential impacts on their ecosystems. However, models of catchment hydrology and water quality remain relatively disconnected, even though the observation that dissolved organic carbon (DOC) and nitrate (NO3−) have opposite spatial and temporal patterns seems relevant for improving our representation of hydrological transport pathways within catchments. We tested the ability of a parsimonious model to simultaneously reproduce intra-annual dynamics of stream flow, DOC and NO3− concentrations using 15 years of daily data from a small headwater agricultural catchment (AgrHyS observatory). The model consists of an unsaturated reservoir, a slow reservoir representing the groundwater and a fast reservoir representing the riparian zone and preferential flow paths. The sources of DOC and NO3− are assumed to behave as infinite pools with a fixed concentration in each reservoir that contributes to the stream. Stream concentrations thus result from simple mixing of slow and fast reservoir contributions. The model simultaneously reproduced annual and storm-event dynamics of discharge, DOC and NO3− concentrations in the stream, with calibration KGE scores of 0.77, 0.64 and 0.58 respectively, and validation KGE scores of 0.72, 0.58 and 0.43 respectively. These results suggest that the dynamics of these concentrations can be explained by hydrological transport processes and thus by temporally variable contributions from slow (NO3− rich and DOC poor) and fast reservoirs (DOC rich and NO3− poor), with a poor representation of the biogeochemical transformations. Unexpectedly, using the concentration time series to calibrate the model increased uncertainty in the parameters that control hydrological fluxes of the model. The legacy storage of NO3− resulting from agricultural history in the studied catchment supports the assumption that the main DOC and NO3− sources behave as infinite pools at the scale of several years. Nevertheless, reproducing the long-term trends in solute concentration would require additional information about DOC and NO3− trends within the reservoirs.