Nitrogen retention in the riparian zone of watersheds underlain by discontinuous permafrost

Thesis (M.S.) University of Alaska Fairbanks, 2005Riparian zones function as important ecotones for reducing nitrate concentration in groundwater and inputs into streams. In the boreal forest of interior Alaska, permafrost confines subsurface flow through the riparian zone to shallow organic horizons, where plant uptake of nitrate and denitrification are typically high. Two research questions were addressed in this study: 1) how does riparian zone nitrogen retention vary in watersheds underlain by discontinuous permafrost, and 2) what is the contribution of denitrification to riparian zone nitrogen retention? To estimate the contribution of the riparian zone to watershed nitrogen retention, I analyzed groundwater chemistry using an end-member mixing model. To assess the importance of denitrification as a mechanism of nitrogen retention, I conducted field denitrification assays using the acetylene block technique. Over the summer, nitrogen retention averaged 0.75 and 0.22 mmol N m⁻² d⁻¹ in low and high permafrost watersheds, respectively. Compared with the fluvial export of nitrogen, the retention rate of nitrogen in the riparian zone is 10 - 15% of the loss rate in stream flow. Denitrification accounted for a small proportion (3%) of total nitrogen retention in the riparian zone. Variation in nitrogen retention between watersheds did not account for differences in stream nitrate concentration between watersheds.Introduction -- Factors controlling denitrification -- Riparian zones as nutrient filters -- Models of riparian zone function -- Permafrost and hydrology -- Caribou Poker Creeks Research Watershed (CPCRW) -- References -- Nitrogen retention in the riparian zone of watersheds underlain by discontinuous permafrost -- Conclusions -- References

A coupled terrestrial and aquatic biogeophysical model of the Upper Merrimack River watershed, New Hampshire, to inform ecosystem services evaluation and management under climate and land-cover change

Accurate quantification of ecosystem services (ES) at regional scales is increasingly important for making informed decisions in the face of environmental change. We linked terrestrial and aquatic ecosystem process models to simulate the spatial and temporal distribution of hydrological and water quality characteristics related to ecosystem services. The linked model integrates two existing models (a forest ecosystem model and a river network model) to establish consistent responses to changing drivers across climate, terrestrial, and aquatic domains. The linked model is spatially distributed, accounts for terrestrial–aquatic and upstream–downstream linkages, and operates on a daily time-step, all characteristics needed to understand regional responses. The model was applied to the diverse landscapes of the Upper Merrimack River watershed, New Hampshire, USA. Potential changes in future environmental functions were evaluated using statistically downscaled global climate model simulations (both a high and low emission scenario) coupled with scenarios of changing land cover (centralized vs. dispersed land development) for the time period of 1980–2099. Projections of climate, land cover, and water quality were translated into a suite of environmental indicators that represent conditions relevant to important ecosystem services and were designed to be readily understood by the public. Model projections show that climate will have a greater influence on future aquatic ecosystem services (flooding, drinking water, fish habitat, and nitrogen export) than plausible changes in land cover. Minimal changes in aquatic environmental indicators are predicted through 2050, after which the high emissions scenarios show intensifying impacts. The spatially distributed modeling approach indicates that heavily populated portions of the watershed will show the strongest responses. Management of land cover could attenuate some of the changes associated with climate change and should be considered in future planning for the region

Riparian Corridors: A New Conceptual Framework for Assessing Nitrogen Buffering Across Biomes

Anthropogenic activities have more than doubled the amount of reactive nitrogen circulating on Earth, creating excess nutrients across the terrestrial-aquatic gradient. These excess nutrients have caused worldwide eutrophication, fundamentally altering the functioning of freshwater and marine ecosystems. Riparian zones have been recognized to buffer diffuse nitrate pollution, reducing delivery to aquatic ecosystems, but nutrient removal is not their only function in river systems. In this paper, we propose a new conceptual framework to test the capacity of riparian corridors to retain, remove, and transfer nitrogen along the continuum from land to sea under different climatic conditions. Because longitudinal, lateral, and vertical connectivity in riparian corridors influences their functional role in landscapes, we highlight differences in these parameters across biomes. More specifically, we explore how the structure of riparian corridors shapes stream morphology (the river's spine), their multiple functions at the interface between the stream and its catchment (the skin), and their biogeochemical capacity to retain and remove nitrogen (the kidneys). We use the nitrogen cycle as an example because nitrogen pollution is one of the most pressing global environmental issues, influencing directly and indirectly virtually all ecosystems on Earth. As an initial test of the applicability of our interbiome approach, we present synthesis results of gross ammonification and net nitrification from diverse ecosystems

European salt marshes diversity and functioning: the case study of the Mont Saint-Michel bay, France

The macrotidal Mont Saint-Michel bay has been studied intensively since 1990. The objectives of this study, supported by the European Union, was to understand various processes underlying the functioning of this hydrosystem with a special focus on organic matter and nutrient fluxes between saltmarshes and marine waters. This paper presents a synopsis of these studies. The tidal flats are unvegetated and primary production is exclusively due to microphytobenthos communities dominated by diatoms. Halophile plant communities colonize the top parts of the tidal flats. Their composition and production vary according to a maturity gradient and sheep grazing. In ungrazed saltmashes, production ranged from 1080 gDW m−2yr−1 in the lower marsh to 1990 gDW m−2yr−1 in the upper marsh whereas it was only 200 to 500 gDW m−2yr−1 in Salicornia spp. dominated pioneer zones and sheep grazed areas. Most of this organic matter (OM) was trapped in situ, processed by fungi and bacteria, and then released seaward via tidal fluxes, groundwater and runoff as particulate OM and nutrients: –497 kg N, –1200/–1000 kg P-PO4 and –9900/–4200 kg inorganic carbon). A small amount of OM was exported to the bay as macrodetritus. Fatty acids and stable isotopes, used as markers, showed that OM produced by the marsh halophytes contributed to the diet of all the tidal flats invertebrates that were studied. Transient fish species were shown to colonize the saltmarshes to forage or graze, exporting about 50 tons POM (DW)y−1. Therefore, it is assumed that the saltmarsh production enhances the production of the whole bay. But the functioning is still poorly known because the nutrient sinks have not all been identified. Part of the nutrients input was provided by precipitation (+327 kg y−1), but the contribution of the catchments was not quantified despite the fact that their influence was shown by the presence of lindane in all the compartments of the system. Dynamics of saltmarshes are mainly influenced by natural sedimentation (1.5 million m3y−1 in the bay), plant community succession, and management (i.e., reclamation and agricultural activities)

Riparian corridors: A new conceptual framework for assessingt nitrogen buffering across biomes

Anthropogenic activities have more than doubled the amount of reactive nitrogen circulating on Earth, creating excess nutrients across the terrestrial-aquatic gradient. These excess nutrients have caused worldwide eutrophication, fundamentally altering the functioning of freshwater and marine ecosystems. Riparian zones have been recognized to buffer diffuse nitrate pollution, reducing delivery to aquatic ecosystems, but nutrient removal is not their only function in river systems. In this paper, we propose a new conceptual framework to test the capacity of riparian corridors to retain, remove, and transfer nitrogen along the continuum from land to sea under different climatic conditions. Because longitudinal, lateral, and vertical connectivity in riparian corridors influences their functional role in landscapes, we highlight differences in these parameters across biomes. More specifically, we explore how the structure of riparian corridors shapes stream morphology (the river's spine), their multiple functions at the interface between the stream and its catchment (the skin), and their biogeochemical capacity to retain and remove nitrogen (the kidneys). We use the nitrogen cycle as an example because nitrogen pollution is one of the most pressing global environmental issues, influencing directly and indirectly virtually all ecosystems on Earth. As an initial test of the applicability of our interbiome approach, we present synthesis results of gross ammonification and net nitrification from diverse ecosystems

Modeling nitrogen loadings from agricultural soils in southwest China with modified DNDC

Degradation of water quality has been widely observed in China, and loadings of nitrogen (N) and other nutrients from agricultural systems play a key role in the water contamination. Process‐based biogeochemical models have been applied to quantify nutrient loading from nonpoint sources at the watershed scale. However, this effort is often hindered by the fact that few existing biogeochemical models of nutrient cycling are able to simulate the two‐dimensional soil hydrology. To overcome this challenge, we launched a new attempt to incorporate two fundamental hydrologic features, the Soil Conservation Service curve and the Modified Universal Soil Loss Equation functions, into a biogeochemistry model, Denitrification‐Decomposition (DNDC). These two features have been widely utilized to quantify surface runoff and soil erosion in a suite of hydrologic models. We incorporated these features in the DNDC model to allow the biogeochemical and hydrologic processes to exchange data at a daily time step. By including the new features, DNDC gained the additional ability to simulate both horizontal and vertical movements of water and nutrients. The revised DNDC was tested against data sets observed in a small watershed dominated by farmlands in a mountainous area of southwest China. The modeled surface runoff flow, subsurface drainage flow, sediment yield, and N loading were in agreement with observations. To further observe the behaviors of the new model, we conducted a sensitivity test with varied climate, soil, and management conditions. The results indicated that precipitation was the most sensitive factor determining the rate of N loading from the tested site. A Monte Carlo test was conducted to quantify the potential uncertainty derived by variations in four selected input parameters. This study demonstrates that it is feasible and effective to use enhanced biogeochemical models such as DNDC for quantifying N loadings by incorporating basic hydrological features into the model framework

Effects of management practices on water yield in small headwater catchments at Cordillera de los Andes in southern Chile

In several parts of the world, drinking water is obtained from springs in natural and managed mountainous forests. Since forests regulate quality as well as quantity of water, the effects of forest-management activities on water yield are an important subject of study. The objective of this study was to evaluate the effects of forest management on water yield in managed and unmanaged temperate native rainforests in the Andean range of southern Chile. The study area is located in San Pablo, a forest reserve of 2,184 ha located at the Andean range of southern Chile (39º 35’ S, 72º 07’ W, 600-925 m a.s.l.). From April 2003 to October 2008, seven experimental small catchments were monitored for rainfall, throughfall, stemflow, soil water infiltration, soil water percolation and runoff. In 2002, one catchment with a secondary deciduous forest was managed, through thinning, causing a reduction in basal area by 35% whereas the other one remained unthinned as control. Both watersheds are adjacent and are located at 600 – 720 m of elevation on deep loam textured volcanic soils (100 - 120 cm). In November 2006, a watershed covered with evergreen old-growth forests was thinned extracting 40% of the total basal area whereas another adjacent catchment remained unthinned as control. Both watersheds are located at 725 – 910 m a.s.l. and have the same aspects. The effects of management of deciduous secondary forests showed that for the period April 2003-March 2007, the mean value of the increase in total annual streamflow was 12.7%, ranging from 10.9% to 14.6%. Thinning of the evergreen old-growth forest increased the streamflow for the period November 2006-October 2008 with 6.1%, ranging from 4.4% to 7.8%, with greater differences during summertime (15.7 to 206%)

Representation of dissolved organic carbon from land to river system in JULES model

The lateral transfer of organic carbon along the terrestrial-aquatic continuum is an important link in the global carbon (C) cycle and an important process which should not be ignored when assessing or modelling changes in terrestrial and aquatic C budgets. The amounts of C exported from terrestrial ecosystem into the inland water network have so far only coarsely been estimated by closing a budget based on observed fluvial C exports to the coast and the still poorly constrained estimates of inland water CO2 evasion and C burial in aquatic sediments. The representation of lateral C transfers in Earth System models (ESMs) will arguably help to improve the representation of soil C cycling and its response to climate change and atmospheric CO2 increase. A first and critical step in that direction is to include processes of production and export of dissolved organic carbon (DOC) in soils. Hence, in the first part of my thesis I developed an extension of the Joint UK Land Environment Simulator (JULES-DOCM) that integrates a representation of DOC production in terrestrial ecosystems based on incomplete decomposition of organic matter, DOC decomposition within the soil column, and DOC export to the river network via leaching. Our results showed that the model is able to reproduce the DOC concentration and controlling processes including leaching to the riverine system which is fundamental for integrating terrestrial and aquatic ecosystems. In the second part of my thesis, I optimized JULES-DOCM for global scale application by recalibrating two key processes controlling soil DOC concentrations: the rate of DOC production associated with soil organic carbon decomposition and the rate of DOC decomposition for the locations where observations were available. Then I used JULES-DOCM with these optimised parameters to simulate the global distribution of soil DOC concentrations and DOC leaching fluxes from soils to rivers. For the third part of my thesis, I used JULES-DOCM to simulate spatial-temporal trends in DOC inputs from soil to the river system from 1860 to 2010 at global scale, quantifying the impacts of major environmental drivers such as CO2 fertilization, climate and land use change. At the global scale, CO2 fertilization was identified as the main controller, followed by climate and land use change. Contrary to general assumptions, we find land use changes to only play a minor role in driving the changes in DOC leaching. In the last of my work I used JULES-DOCM and three representative concentration pathways (RCPs), RCP 2.6, RCP 4.5 and RCP 8.5 in order to estimate the future of terrestrial transported DOC flux to the river system. We find the increase of the atmospheric CO2 concentration as the main reason of the future increase of transported terrestrial DOC. In this thesis, I focussed on the detailed representation of soil DOC cycling and leaching, and simulated the historical and future trend of it. However, future work should include the fate of exported DOC in the river system as well as the exports of dissolved inorganic C and particulate organic C from soils to complete the representation of lateral C exports through the terrestrial aquatic continuum

ORIGINS AND SEASONAL VARIATION OF DISINFECTION BYPRODUCT PRECURSORS

Disinfection byproducts (DBPs) are formed from the disinfectant (e.g., chlorine) reacting with components of natural organic matter (NOM) in water drawn from surface water supplies, and are considered as the cause of potential serious human health problems. DBP precursors originate in large reservoirs from at least three types of sources: (1) watershed or allochthonous, (2) algal or autochthonous, and (3) bottom sediments or benthic. The properties of the NOM and the DBP precursor content of that NOM are unique to each source. The first objective of this dissertation was to use temporal and spatial water quality data from a drinking water reservoir to shed light on autochthonous and benthic sources of NOM and DBP precursors. The second objective of this dissertation was to identify the seasonal variation and spatial fate of DBP precursors in a drinking water system located in a temperate environment where seasonal variations of surface water quality and water temperature are considerable. Organic matter released from plants is quite likely the most important fraction as potential DBP precursors, especially in heavily forested catchments. However, very few studies have been conducted on plant leachate as DBP precursors. The third objective of this dissertation was to characterize the organic matter that is released by plants, and examine their potentials to form DBPs under light, dark, and dark-with-biocide conditions. The final objective of this dissertation was to determine the comparative significance of DBP (i.e., trihalomethanes, dihaloacetic acids, and trihaloacetic acids) precursors released from profundal sediments of a water supply impoundment under aerobic, hypoxic, and anaerobic conditions

Sources, sinks and stores of Nitrogen and Phosphorus associated with public water supply and the vadose zone

Reactive Nitrogen (N) and Phosphorus (P) in the environment remain a considerable problem for both ecosystems and drinking water quality. Although public water supply processes and the vadose zone are well known components of the anthropogenically perturbed hydrological cycle, the impact of these components on N and P cycling is poorly understood. The aim of this thesis was to improve the understanding of sources, sinks and stores of N and P associated with public water supply and the vadose zone, to support the development of integrated nutrient management approaches. Mains water leakage is shown to be an important source of P, which will increase in importance in the future. Mains water leakage of P has significant temporal variability associated with winter burst events, summer shrink-swell leakage and active leakage control. Mains water leakage has also been shown to be an important source of N in urban areas, contributing up to 20% of all N loads. Abstraction for public water supply has been shown to be a considerable temporary sink of N, equivalent to up to 39% of denitrification. The unsaturated zone has also been shown to be an important store of nitrate, with the quantity of nitrate stored in the vadose zone being equivalent to 200% of estimates of inorganic N stored in soils globally. The results of this thesis have important implications for the development of integrated nutrient management approaches. The sources, sinks and stores associated with public water supply and the vadose zone quantified in this thesis should be considered in future macronutrient budgets and models. Continued use of existing models which do not consider these additional N and P sources, sinks and stores should be tempered with the knowledge gained from this research