As an efficient filter of airborne Hg compounds, forests retain a significant portion of the Hg deposited to the land. Forested watersheds have been identified as major sources of low-concentration Hg compounds to surrounding streams and lakes. Mercury export from forests is highly variable in both space and time. It is difficult to use field surveys alone to capture the spatiotemporal variation inherent in this variable. Mathematical models are required for improved representation. The objective of this Thesis is to develop and test a monthly dynamic model that can be used to estimate seasonal Hg export from forested watersheds to low-ordered forest streams. The fully developed model consists of four model components: (i) a forest hydrology component, to simulate variation in soil temperature, soil moisture, and stream discharge for input to the other model components; (ii) a forest nutrient cycling and biomass growth component, to simulate forest growth and litter production; (iii) a forest litter decomposition component, to simulate seasonal production of dissolved organic carbon (DOC); and (iv) a monthly DOC and Hg export component to simulate the translocation of DOC and Hg from forested watersheds to low-ordered streams. The Hg-export component incorporates an Hg-to-DOC binding coefficient estimated from a one-time stream survey of Hg and DOC concentrations. Simulations of in-stream Hg concentrations show two main trends: (i) an annual trend, associated with the seasonal (monthly) dynamics of forest litter production, decomposition, and DOC production and export, and (ii) a multiple-year trend, associated with forest harvesting and re-growth patterns of regenerating forests. This study demonstrates that (i) wetland- and conifer-dominated watersheds release a greater amount of Hg to aquatic ecosystems than upland- and deciduous species-dominated watersheds, and (ii) forests nearing maturity, export more Hg than young forests.

This thesis presents the first modeling study on long-term carbon dynamics for the University of Guelph Elora Agricultural Research Station and the Environment Canada Borden Forest Research Station at the daily and half-hourly time-step. The daily version of the CENTURY (DayCENT) model and the Carbon- and Nitrogen-coupled Canadian Land Surface Scheme (CN-CLASS) model were validated for quantifying the effects of agricultural management and component respiration on the carbon budget. DayCENT indicated that conventional tillage (CT) enhanced the annual heterotrophic respiration relative to no-till (NT) by 38.4, 93.7 and 64.2 g C m-2 yr-1 for corn, soybean and winter wheat, respectively. The seasonal variation of total soil organic carbon (SOC) pool was greater in CT than NT due to tillage effects on carbon transfer from the active surface SOC pool to the active soil SOC pool at a rate of 50-100 g C m-2 yr-1. NT accounted for a 10.7 g C m-2 yr-1 increase in the slow SOC pool (20-year turnover time) at a site in Elora, Ontario, Canada. I found that the plant phenology algorithms used in CN-CLASS were not constructed and validated for crop growth, resulting in a high degree of uncertainty in the simulations. Therefore, I designed and tested a new agricultural module for CN-CLASS. The regression analysis indicated that the new crop module improved the net ecosystem productivity (NEP) simulation for a cornfield, with the coefficient of determination (r2) of annual NEP increasing from 0.51 in the original CN-CLASS to 0.78 in the modified version of the model. I verified CN-CLASS to simulate the dynamics of component respiration for tracing the contributions from litterfall, SOC and root respiration in a deciduous mixedwood forest in Borden, Ontario, Canada. The model estimated that the annual ecosystem CO2 respiration was 1366 g C m-2 yr-1, contributed by heterotrophic respiration (57%), maintenance respiration (37%) and growth respiration (6%). The annual accumulated soil respiration was estimated at 782 g C m-2 yr-1, which was dominated by CO2 emissions from soil organic matter (60%). The base respiration rates required further verification based on field measurements. Based on the verified modeling approach in this thesis, the modeling core of DayCENT can be constructed as an integral platform for Agriculture and Agri-Food Canada National Carbon and Greenhouse Gas Accounting and Verification System. The crop phenological module in CN-CLASS allows us to conduct further agricultural studies concerning global carbon budget and environmental change. The validated respiration algorithms in CN-CLASS would be helpful in developing global biological CO2 transport model for tracing emission sources. / Natural Science and Engineering Research Council of Canada

Tyrimų tikslas – nustatyti organinių mulčių ir jų skirtingo storio sluoksnio poveikį dirvožemio savybėms, segetinei florai ir augalų derlingumui. Ginamieji disertacijos teiginiai: 1. Organiniai mulčiai mažina dirvožemio temperatūrą apsaugo dirvožemį nuo staigių temperatūrinių pokyčių, palaiko didesnį dirvožemio drėgnumą, mažina šlyties pasipriešinimą. 2. Organiniai mulčiai gerina dirvožemio agrochemines savybes. 3. Organinai mulčiai nevienodai veikia dirvožemio biologines savybes. 4. Mulčiavimas organinais mulčiais mažina agrocenozių piktžolėtumą. 5. Organiniai mulčiai nevienodai įtakoja žemės ūkio augalų derlingumą. 6. Žolės mulčio įtaka įvairių augalų pasėliams pagal integruoto vertinimo rodiklius yra didesnė negu šiaudų, durpių, bei pjuvenų. / The experimental objective. The study was designed to establish the impact of organic mulches and different mulch layer thickness on segetal flora and crop productivity. Statements to be defended: 1. Organic mulches reduce soil temperature, protect the soil from sharp temperature changes, maintain higher soil moisture content, and reduce shear strength. 2. Organic mulches improve soil agrochemical properties. 3. Organic mulches exert different effects on soil biological properties. 4. Organic mulching reduces weed incidence in crops. 5. Organic mulches have different effects on agricultural crops productivity. 6. According to integrated assessment parameters, the effect of grass mulch on various crop stands was greater than that of straw, peat and sawdust.

Downed woody debris (DWD) is a carbon-rich form of forest litter and plays a unique role in carbon and nutrient cycling. I present a novel modeling approach describing DWD decomposition and nutrient storage in a managed northern hardwood forest. The predicted half-life of DWD carbon was 7 years, less than previously observed in similar northern hardwood forests. A stage-based nutrient model indicated that harvest slash DWD was a net nitrogen and phosphorus sink for eight years following harvest and accumulated calcium during decay. Field observations of respiration and leaching supported model results with a respired C half-life of 8 years, while leached carbon export constituted 1.37% of the respired flux. DWD leachate carbon and nitrogen concentrations were 11× and 2× greater than from litter, respectively, and DWD leachate contributed disproportionately to soil C stocks. This work represents an expedient means of forecasting DWD abundance and partitioning carbon flux from DWD.

Downed woody debris (DWD) is a carbon-rich form of forest litter and plays a unique role in carbon and nutrient cycling. I present a novel modeling approach describing DWD decomposition and nutrient storage in a managed northern hardwood forest. The predicted half-life of DWD carbon was 7 years, less than previously observed in similar northern hardwood forests. A stage-based nutrient model indicated that harvest slash DWD was a net nitrogen and phosphorus sink for eight years following harvest and accumulated calcium during decay. Field observations of respiration and leaching supported model results with a respired C half-life of 8 years, while leached carbon export constituted 1.37% of the respired flux. DWD leachate carbon and nitrogen concentrations were 11× and 2× greater than from litter, respectively, and DWD leachate contributed disproportionately to soil C stocks. This work represents an expedient means of forecasting DWD abundance and partitioning carbon flux from DWD.

Im Fokus der Untersuchungen steht das hydrologische Einzugsgebiet der oberen Wilzsch im Westerzgebirge. Veränderte Stoffausträge des bewaldeten und zum Teil vermoorten hydrologischen Einzugsgebietes führen zu erhöhten Stoffkonzentrationen im Hauptzufluss der Trinkwassertalsperre Weiterswiese (Carlsfeld). Seitens der Trinkwasseraufbereitung stellen insbesondere die gestiegenen Gehalte an gelösten organischen Kohlenstoffen (DOC) Mehraufwendungen dar. Im Zuge der Wiedervernässung der Moorflächen kommt es zu relevanten bodenchemischen Veränderungen, die im Abfluss messbar sind. Eine räumliche Betrachtung der Teileinzugsgebiete des hydrologischen Einzugsgebietes der oberen Wilzsch ermöglicht, das Abflussgebiet in verschiedene Herkunftsräume zu gliedern. Dieser raumbezogene Ansatz leistet einen wesentlichen Beitrag in der Herkunftsanalytik von Wässern, deren Fließwege durch Standorte unterschiedlicher Nutzung geprägt sind. Die Arbeit bedient sich hydrochemischen Analysen sowie multitraceranalytischen Verfahren. Die Analyse der stabilen Umweltisotope ²H und 18O am Wasser, 13C am DIC und DOC, 34S und 18O am Sulfat sowie 15N und 18O am Nitrat hilft bei der Identifizierung von Stoffquellen und Transformationsprozessen. Die Arbeit verfolgt das Ziel, den Beitrag der verschiedenen Stoffsysteme am gesamten Stoffaustrag des Vorfluters aufzuzeigen sowie die atmosphärische Grundlast an den Messstandorten nachzuweisen. Dazu wurden von Mai 2009 bis August 2010 fünf Beprobungskampagnen angelegt. Das Messdesign erstreckte sich auf 19 Standorte in ausgewählten Teileinzugsgebieten, die maßgeblichen Einfluss auf die Vorflut haben. Parallel dazu erfolgte die Untersuchung des Niederschlags an der Station Talsperre Weiterswiese. Erfasst wurden die Inhalte von Wässern verschiedener Moorgebiete, Moorwasserpegel unterschiedlich degradierter Moorbereiche, Gräben und Bodendrainagen im Fichtenforst, der Vorfluter Wilzsch und Kranizsch und des Grundwassers im Fichtenforst. Es kann festgehalten werden, dass je nach Lage bzw. Entfernung der Ursprungsgebiete der Anteil des DOC - Exports unterschiedlich stark ist. So sind die DOC - Austräge saisonal sehr verschieden. Im Ergebnis ist der hydrochemische Beitrag - im Besonderen DOC - der unterschiedlichen Stoffsysteme des hydrologischen Einzugsgebietes letztlich am finalen Pegel der Wilzsch messbar. Die vom Basisabfluss gesteuerten silikatischen Standorte im Fichtenforst leisten einen wesentlichen Anteil zum hydrochemischen Milieu der Vorfluter. Die im Untersuchungsgebiet eingeleiteten Renaturierungsmaßnahmen beeinflussen die Trinkwasserqualität in der Talsperre Carlsfeld nicht. Eine Abgrenzung der atmosphärischen Grundlast ist ebenfalls möglich, atmogen eingetragene Stoffe können an den Standorten ausgewiesen werden.

Für die montanen Regionen kontinental Südostasiens (Südwest China, Laos, Kambodscha, Myanmar, Nordost Thailand, Nordwest Vietnam) gibt es nur wenig Informationen über die organische Bodensubstanz (OBS) und ihre Beeinflussung durch Landnutzung, Bewirtschaftung und biophysikalische Eigenschaften. Zum Beispiel ist trotz großflächiger Entwaldung zu Gunsten von Kautschukplantagen der Einfluss dieser Landnutzungsänderung auf OBS Vorräte kaum bekannt. Auch wurde der Einfluss der Terrassierung, wie sie für den Kautschukanbau in montanen Regionen üblich ist, auf die Dynamik der OBS bislang nicht untersucht. Des Weiteren liegen nur begrenzt Informationen über die räumliche Verteilung von OBS Vorräten und die Rolle potentieller Regulationsfaktoren wie Landnutzung, Vegetation, Bodentextur und Topographie vor. Die vorliegende Arbeit zielte auf die genannten Wissenslücken und präsentiert in diesem Kontext drei Studien aus der montanen Region Xishuangannas, Süd Yunnan, China. In den ersten beiden, in einer tropischen Landschaft durchgeführten Studien, habe ich die Änderung des OBS Vorrats durch 1) die Umwandlung von Sekundärwald in Kautschukplantagen und 2) durch den Bau von Terrassen, quantifiziert. Um in der ersten Studie Landnutzungseffekte auf die OBS-Vorräte zu quantifizieren, habe ich den Ansatz der unechten Zeitreihe (space-for-time substitution) genutzt. Ich habe 11 terrassierte Kautschukplantagen im Alter von 5 bis 46 Jahren sowie sieben Sekundärwaldparzellen untersucht. Die Ergebnisse zeigten, dass die Umwandlung von Sekundärwald in Kautschukplantagen eine Abnahme der OBS Vorräte von 37.4 Mg C ha-1 im Bereich bis zu einer Tiefe von 1.2 m hervorrief; diese Abnahme entsprach 19% des ursprünglichen OBS Vorrats im Sekundärwald. Im Oberboden nahm der OBS Vorrat exponentiell ab; in den ersten 5 Jahren nach der Landnutzungsänderung war die Abnahme am stärksten, nach ca. 20 Jahren hat sich ein Gleichgewicht eingestellt. Der mittlere OBS-Verlust von 37.4 Mg C ha-1 war viel höher als literaturbasierte Schätzwerte für Änderungen der oberirdischen Kohlenstoffvorräte, welche zwischen einem Verlust von 18 Mg C ha-1 und einer Steigerung von 8 Mg C ha-1 liegen. Im Gegensatz zur IPCC tier 1-Methode, die davon ausgeht, dass OBS Vorratsänderungen bei einer Umwandlung von Wald zu Kautschuk gleich 0 sind, zeigen meine Ergebnisse, dass OBS-Verluste in Betracht gezogen werden müssen, um potentiell große Fehler bei der Schätzung von Kohlenstoffflüssen von Ökosystemen zu vermeiden. Terrassierte Kautschukplantage bestehen aus schmalen Terrassen mit einer Baumreihe, die sich mit ursprünglichen geneigten Flächen abwechseln. Bei der Konstruktion der Terrassen wird Boden vom Hang abgetragen, und so eine innere Kante der Terrasse entsteht (Entnahmebereich); der entfernte Boden wird dann auf den Hang unterhalb der Grabungsfläche aufgehäuft und bildet die äußere Kante der Terrasse (Ablagebereich). Die zweite Studie untersucht den Einfluss der Terrassierung auf OBS Vorräte in 5, 29 und 44 Jahre alten Plantagen. In jeder Plantage habe ich die Terrassen systematisch in den verschiedenen Bodenverteilungszonen beprobt, die ursprünglichen Hangflächen zwischen den Terrassen diente als Referenz. Die Ergebnisse dieser Studie zeigten, dass die Terrassierung die OBS Vorräte der 5 Jahre alten Plantage nicht beeinflusst hat. In den 29 und 44 Jahre alten Plantagen wurden jedoch in 0-1.2 m Tiefe höhere OBS Vorräte auf den Terrassen als auf den Referenzflächen beobachtet. Der positive Effekt der Terrassierung auf die OBS Vorräte in den beiden älteren Plantagen wurde auf die Erholung des OBS Vorrats im freiliegenden Oberboden des Entnahmebereichs, und die teilweise Erhaltung von OBS im begrabenen Boden des Ablagebereichs erklärt. Die Erholung der OBS Vorräte im Entnahmebereichen konnte durch die Aufnahme neuer OBS des freiliegenden Unterbodens in Form von Wurzeln und Laubfall sowie durch die Sedimentation von erodiertem Oberbodenmaterial des Oberhangs erklärt werden. Zusammenfassend zeigen die Ergebnisse, dass Terrassierung die Verluste von OBS verringern kann; ohne die Anlage von Terrassen könnte der Verlust von OBS durch die Umwandlung von Wald zu Kautschukplantagen größer sein. In der dritten Studie, durchgeführt in einer subtropischen Landschaft, habe ich die aktuellen OBS Vorräte pro dominanter Landnutzung quantifiziert und die Beziehungen zwischen OBS und Landnutzung, sowie Vegetation, Bodentextur und Topographie untersucht. In einem 10.000 Hektar großen Gebiet habe ich 28 ein Hektar große Probeflächen in Wäldern mit geschlossenem und offenem Kronendach, Teeplantagen und Buschland ausgewählt. Die OBS-Vorräte in einer Tiefe von 0-0.9 m waren unter den höchsten der Region: 228.6 ± 19.7 (SE) Mg C ha-1 in Wäldern mit geschlossenem Kronendach, 200.4 ± 15.5 Mg C ha-1 in Wäldern mit offenem Kronendach, 197.5 ± 25.9 Mg C ha-1 in Teeplantagen und 236.2 ± 13.7 Mg C ha-1 im Buschland. OBS Konzentrationen und Vorräte unterschieden sich nicht signifikant zwischen den Landnutzungstypen. Mehr als 50% der gesamten Varianz der OBS wurde innerhalb der ein Hektar großen Flächen beobachtet und war abhängig von der Variabilität der Grundfläche der Bäume, Kohlenstoffvorrat der Streuauflage und der Geländeneigung. Diese Ergebnisse illustrieren die Bedeutung lokaler Prozesse auf die Variabilität von OBS Vorräten in einer montanen Landschaft. Die Ergebnisse aller drei hier vorgestellten Studien tragen zu einem besseren Verständnis von OBS Vorräten und deren Dynamik in einer schnellen Änderungsprozessen ausgesetzten Region bei. Darüber hinaus bilden sie eine potentielle Grundlage für weitere Studien über Änderungen von Ökosystemdienstleistungen in montanen Regionen des kontinentalen Südostasiens.

Up to 59 small tundra lakes were sampled in the uplands east of the Mackenzie Delta, NWT, Canada, in order to assess the impact of permafrost thaw on pelagic nutrient concentrations and biota. Permafrost thaw did not affect the concentrations of nitrogen, phosphorus or organic carbon in the water column of the study lakes. Instead, nitrogen and organic carbon concentrations were positively related to relative catchment size, and phosphorus concentrations were negatively related to maximum lake depth. Lakes affected by permafrost thaw did have lower water colour. In lakes not affected by thaw, where plankton production could have been light limited due to high water colour, the TP-chlorophyll a relationship was weaker than in lakes affected by thaw, where light limitation was probably weaker. A model selection analysis for chlorophyll a concentration indicated water colour as the best predictor variable in unaffected lakes, but nitrogen and phosphorus as the best predictors in thaw-affected lakes. This result, in particular, suggested a significant shift in the processes governing productivity in thaw-affected lakes. In a smaller subset of lakes, chlorophyll a concentrations were lower in lakes affected by actively degrading permafrost than in lakes affected by stabilized thaw scars or in unaffected lakes. In contrast, zooplankton abundance was lowest in lakes with stabilized thaw scars. Bacterioplankton abundance was not different across the gradient of permafrost thaw. The differences in phytoplankton and zooplankton abundance between active and stable thaw scar lakes did not display a gradient of response that mirrored the degree of permafrost thaw activity. Because sampling did not include higher trophic levels, including macroinvertebrates and fish, it was difficult to determine how these biomass patterns arose. However, detectable differences existed in the morphometry of the lakes that might have affected habitat conditions for several species. Deep, near-shore lake-bottom pits occurred in lakes affected by permafrost thaw, while unaffected lakes were generally deepest at their centre. The pits increased the effective depth of the thaw-affected lakes, and allowed for thermal stratification where it might not have otherwise occurred. Future research should explore the habitat-related impacts of permafrost thaw on adjacent and higher trophic levels in order to better understand the fundamental shifts in trophic structures that appear in lakes affected by permafrost thaw.

The fate and behaviour of hydrophobic organic compounds (e.g. pesticides) in soils are largely controlled by sorption processes. Recent findings suggest that the chemical properties of soil organic carbon (OC) significantly control the extent of sorption of such compounds in soil systems. However, currently there is no practical tool to integrate the effects of OC chemistry into sorption predictions. Therefore, the K [subscript]oc model, which relies on the soil OC content (foc), is used for predicting soil sorption coefficients (K[subscript]d) of pesticides. The K[subscript]oc model can be expressed as K[subscript]d = K[subscript]oc × foc, where K[subscript]oc is the OC-normalized sorption coefficient for the compound. Hence, there is a need for a prediction tool that can effectively capture the role of both the chemical structural variation of OC as well as foc in the prediction approach. Infrared (IR) spectroscopy offers a potential alternative to the K[subscript]oc approach because IR spectra contain information on the amount and nature of both organic and mineral soil components. The potential of mid-infrared (MIR) spectroscopy for predicting K[subscript]d values of a moderately hydrophobic pesticide, diuron, was investigated. A calibration set of 101 surface soils from South Australia was characterized for reference sorption data (K[subscript]d and K[subscript]oc) and foc as well as IR spectra. Partial least squares (PLS) regression was employed to harness the apparent complexity of IR spectra by reducing the dimensionality of the data. The MIR-PLS model was developed and validated by dividing the initial data set into corresponding calibration and validation sets. The developed model showed promising performance in predicting K[subscript]d values for diuron and proved to be a more efficacious than the K[subscript]oc model. The significant statistical superiority of the MIR-PLS model over the K[subscript]oc model was caused by some calcareous soils which were outliers for the K[subscript]oc model. Apart from these samples, the performance of the two compared models was essentially similar. The existence of carbonate peaks in the MIR-PLS loadings of the MIR based model suggested that carbonate minerals may interfere or affect the sorption. This requires further investigation. Some other concurrent studies suggested excellent quality of prediction of soil properties by NIR spectroscopy when applied to homogenous samples. Next, therefore, the performance of visible near-infrared (VNIR) and MIR spectroscopy was thoroughly compared for predicting both foc and diuron K[subscript]d values in soils. Some eleven calcareous soils were added to the initial calibration set for an attempt to further investigate the effect of carbonate minerals on sorption. MIR spectroscopy was clearly a more accurate predictor of foc and K[subscript]d in soils than VNIR spectroscopy. Close inspection of spectra showed that MIR spectra contain more relevant and straightforward information regarding the chemistry of OC and minerals than VNIR and thus useful in modelling sorption and OC content. Moreover, MIR spectroscopy provided a better (though still not great) estimation of sorption in calcareous soils than either VNIR spectroscopy or the K[subscript]oc model. Separate research is recommended to fully explore the unusual sorption behaviour of diuron in calcareous soils. In the last experiment, two dimensional (2D) nuclear magnetic resonance/infrared heterospectral correlation analyses revealed that MIR spectra contain specific and clear signals related to most of the major NMR-derived carbon types whereas NIR spectra contain only a few broad and overlapped peaks weakly associated with aliphatic carbons. 2D heterospectral correlation analysis facilitated accurate band assignments in the MIR and NIR spectra to the NMR-derived carbon types in isolated SOM. In conclusion, the greatest advantage of the MIR-PLS model is the direct estimation of Kd based on integrated properties of organic and mineral components. In addition, MIR spectroscopy is being used increasingly in predicting various soil properties including foc, and therefore, its simultaneous use for K[subscript]d estimation is a resource-effective and attractive practice. Moreover, it has the advantage of being fast and inexpensive with a high repeatability, and unlike the K[subscript]oc approach, MIR-PLS shows a better potential for extrapolating applications in data-poor regions. Where available, MIR spectroscopy is highly recommended over NIR spectroscopy. 2D correlation spectroscopy showed promising potential for providing rich insight and clarification into the thorough study of soil IR spectra. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1415416 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

Sediment diagenesis in aquatic systems is usually understood to be controlled by the concentrations of both organic carbon and the oxidant. However, the concept that sediment respiration may be limited by the supply of organic carbon, even in systems with moderate concentrations of organic carbon in the water column, has yet to be fully explored. Typically we assume that a direct coupling between water column and sediment diagenesis processes occurs and the chemical evolution of porewater and surface water are linked through fluxes of chemical species across the sediment-water interface. While the dynamics of supply of particulate organic carbon (POC) to the sediments via plankton deposition and resuspension, has previously been examined, the fate of dissolved organic carbon (DOC) once in the sediments, has rarely been investigated. A series of experiments comprising batch tests, microcosms and sediment cores were conducted on sediment and water from four diverse field sites in which sediment respiration was considered to be carbon limited. Three sites were oligotrophic, acidic lakes and the fourth an oligotrophic coastal embayment. During each experiment dissolved organic carbon was added and measurements were undertaken of solutes that were considered participants in diagenetic processes. While each system differed in its chemical, biological and geological makeup, a key commonality was the rapid onset of anoxic conditions in the sediments irrespective of the overlying water oxygen concentrations, indicating lack of direct coupling between biogeochemical processes in the water column and sediments. Also, similar apparent DOC remineralisation rates were observed, measured solute fluxes after the addition of DOC indicated adherence to the ecological redox sequence, and increased ammonium concentrations were measured in the overlying waters of the acidic microcosms. In marine system experiments it was noted that diagenetic respiration, as indicated by decreasing concentrations of oxygen in the overlying water, increased rapidly after labile DOC was added. To explore the influence of geochemical processes on sediment respiration, a diagenetic model was tested against the laboratory data. The model was able to capture the rapid changes observed in the microcosms after addition of DOC in both the marine and acidic systems experiments. The model has the potential to serve as an essential tool for quantifying sediment organic matter decomposition and dissolved chemical fluxes. This work has focussed our attention on the control of DOC availability on sediment respiration and thus its ultimate control on solute fluxes across the sediment water interface. The results highlight the need to understand and quantify the supply of DOC to the sediment (as POC or already as the dissolved form), its transport through the sediment and its eventual remineralisation. This understanding is critical for improved management of aquatic systems, possibly even in systems where water column organic carbon is plentiful but sediment respiration is constrained by high organic carbon turnover rates in the water column and a resulting low flux of organic carbon to the sediment.