22 research outputs found
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Linkages among land use, riparian zones, and uptake and transformation of nitrate in stream ecosystems
Land use alters the physical and biological structure of stream ecosystems and potentially alters their capacity to process nitrogen (N), an essential nutrient that has nearly doubled in abundance on the biosphere
during the past century from human activities. In this dissertation, I quantified uptake and transformation of nitrate (NO₃⁻) in small (≤ third-order) streams and related these dynamics to aquatic ecosystem processes, including primary production and organic matter decomposition, and attributes of riparian zone structure and vegetation composition. I also analyze patterns of stream NO₃⁻ processing among three classes of adjacent land use practices (forest, agriculture, and urban).
In Chapter 2, ambient rates of NO₃⁻ uptake and transformation were measured with 24-hr releases of ¹⁵N-labeled NO₃⁻ in nine stream reaches in the Willamette River Basin of western Oregon during summer low flow (July –
August). Three reaches each were surrounded by forested, agricultural or urban land use. After standardizing reaches to a 500-m length, I estimated that ≥ 20% of tracer ¹⁵NO₃⁻ was taken up by detrital and autotrophic biomass in eight of the reaches. In the remaining stream, which had the largest discharge (120 L s⁻¹) in this study, only 8% of the tracer was taken up in 500 m. Tracer labeling of detritus and autotrophic biomass and a positive correlation (rs=0.81) of uptake with gross primary production suggested that assimilation was the dominant uptake pathway in all streams. Denitrification, dissimilatory reduction of NO₃⁻ to N₂ and N₂O gases, composed 3 – 15% of
¹⁵N budgets over 500 m in two agricultural reaches and in one urban reach dominated by large slowly-turning over pools. However, denitrification was below detection limit at five of the remaining six reaches. This study showed that pathways of stream NO₃⁻ uptake and transformation differed among streams adjacent to three diverse land use practices.
In Chapter 3, I quantified effects of substrate nutritional quality and inorganic N loading (as NO₃⁻) on wood breakdown in western Oregon streams. Short-term (< 2 month) breakdown rates of wood substrates of high nutritional quality (Alnus rubra; red alder) and low quality (Pseudotsuga menziesii; Douglas-fir) increased with dissolved inorganic N (11 to 111 mg N L⁻¹) across six streams (p = 0.04), but this relationship was confounded with concurrent
increases in stream temperature. Across the six streams, breakdown rates of red alder were consistently double that of Douglas-fir. A longer-term study (313 d) in a coniferous forest Oregon Cascades stream suggested effects of increased NO₃⁻ availability on wood breakdown became evident after cellulose
and lignin components of woody tissues began to decompose (> 4 months of incubation). Average breakdown rates substrates enriched with NO₃⁻ were higher than those incubated in low NO₃⁻ conditions, but this difference was not statistically significant. However, microbial biofilm respiration rates and activity of two enzymes involved in the breakdown of woody tissues (beta-glucosidase and phenol oxidase) on red alder had significantly greater responses to NO₃⁻ additions than on Douglas-fir after four
months of incubation in the stream. Results suggest that increases in N loading to streams bordered by riparian forests with fast-growing deciduous species could increase wood breakdown rates. On the other hand, increases to N loading may have a smaller effect on wood breakdown in streams surrounded by long-lived coniferous species.
In Chapter 4, I quantified patterns of stream channel and riparian zone attributes for 72 streams equally distributed among forests or grasslands, agriculture, and urban land use practices on from eight major North American
regions. I also related these patterns to stream NO₃⁻ uptake determined from ¹⁵NO₃⁻ tracer releases. Agricultural and urban streams had a simplified channel structure (low width-to-depth ratio, low variation in stream depth, and high stream banks) relative to forest or grassland streams. Agricultural and urban streams also had a significantly smaller median sediment diameter (D₅₀)
and fraction of benthic sediments composed by silt than in forest and grassland streams. Overstory canopy cover over the channel and in the riparian zone was lowest for agricultural streams but did not significantly differ between forest or grassland streams and urban streams. A multiple regression model showed that stream NO₃⁻ uptake decreased with increasing canopy cover, but also increased with abundance of silt in benthic sediments. This suggested NO₃⁻ uptake was strongly influenced by in-stream primary production and extent of anoxic environments (conducive for denitrification). A multiple regression model for fractional NO₃⁻ uptake by denitrification further supported the concept that extent of anoxic environments influenced overall NO₃⁻ uptake in streams.
Through these studies, I demonstrated that attributes of riparian zone structure and vegetation composition can strongly influence NO₃⁻ uptake and transformation in stream ecosystems by controlling organic matter dynamics. I also have shown that riparian zone attributes vary significantly among three different land use types (forest or grassland, agriculture, and urban). Similarly, pathways of NO₃⁻ uptake and effects of NO₃⁻ on wood breakdown did or were
expected to differ among different land use types / riparian zone characteristics. However, other factors besides riparian attributes, particularly level of nutrient loading, alteration of stream channel physical structure, and basin position of the stream, must be considered in concert when evaluating effects of land use on riparian zone and stream ecosystem structure and
function
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Fall directions and breakage of riparian trees along streams in the Pacific Northwest
In the Pacific Northwest, regulatory agencies have recently
implemented management strategies for restoration and maintenance of wood
recruitment to streams over time. This allochthonous organic material is a
critical component in the geomorphic and biological structure of forested
streams. Mathematical models are commonly used to evaluate long-term
wood dynamics in stream ecosystems. Accuracy and applicability of these
models in a wide range of forest, geomorphic and regional conditions are
limited by current understanding of tree fall patterns. I quantified fall directions
and breakage of riparian trees at 21 second- to fourth-order stream sites in the
Pacific Northwest, USA. These patterns influence rates, amounts and spatial
distribution of wood delivered to streams.
Riparian tree fall directions strongly differed by species, rooting position
away from the stream and side slope steepness adjacent to the channel. Though average tree fall directions oriented towards stream regardiess
species, rooting position, and side slope, variance of fall directions significantly
declined between valley bottoms and hillslopes and between gentle and steep
side slopes. Standard deviation of fall directions, as calculated from circular
statistical techniques, was two times greater on gentle slopes (0-10%
steepness) than on extremely steep slopes (> 90% steepness) (± 80° versus ±
40°). A similar pattern occurred based on rooting position away from the
stream (valley bottom versus hillslopes). Valley constraint, forest structure,
forest age and region (west of the Cascades Mountains crest versus the
Interior Columbia Basin) were not associated with variation in fall directions
(p>0.10 in all cases). Several sites with relatively thin trees (height/dbh > 50
rn/rn) had overall upstream fall directions.
Over half (58%) of the riparian trees measured in this study did not
break when they fell. Of riparian trees that were broken, 62% of breaks could
not be attributed to physical features of the stream or riparian zone. Percent
of trees with breaks slightly increased with forest age (p=0.08). More
significantly, percent of trees with fall breaks increased with tree height
(p<0.0001). Models indicated slight differences among species except for
western larch (Larix occidental/s Nutt.), which had a significantly higher rate of
breakage than other species. Trees that broke when they fell typically
produced 2 to 3 pieces (geometric mean of 2.7 pieces with 42% coefficient of
variation) and the first break typically occurred at half height (average of 0.50 with ± 0.24 standard deviation; proportion of height from base to top).
Effects of species type were insignificant on pieces produced or location of
first break.
These results have several important implications for riparian zone
management of forested streams in the Pacific Northwest. Topographic
influences on tree growth patterns and rooting conditions could affect stream
wood loading rates and patterns where riparian forest dynamics dominate
input pathways. Stream sections with side slopes > 90% on both banks
potentially receive 1 .5 times more falling trees (by numbers) than sections with
slopes 0-10% and 2.5 times more than what is expected from a random fall
pattern. Breakage by falling trees on impact needs to be considered in
simulations of wood recruitment from riparian forests. Large channel-spanning
wood pieces that have potential to form pools and jams also have the highest
probability to break when initially falling into the channel. A simple model of
wood recruitment based on field data and riparian forest dynamics of an oldgrowth
forest in the West Cascades of Oregon indicated that fall breakage
might reduce numbers of channel spanning wood pieces entering a 10-m wide
stream by 7 to 79% over a 100-year time period. At the same time, breakage
may amplify absolute number of wood pieces entering a stream channel by
1.1 to 1 .8 times. Overall, results presented in this thesis may improve
accuracy and calibration of stream wood recruitment models for use in riparian
management in the Pacific Northwest
Intentional versus unintentional nitrogen use in the United States : trends, efficiency and implications
© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeochemistry 114 (2013): 11-23, doi:10.1007/s10533-012-9801-5.Human actions have both intentionally and unintentionally altered the global economy of nitrogen (N), with both positive and negative consequences for human health and welfare, the environment and climate change. Here we examine long-term trends in reactive N (Nr) creation and efficiencies of Nr use within the continental US. We estimate that human actions in the US have increased Nr inputs by at least ~5 times compared to pre-industrial conditions. Whereas N2 fixation as a by-product of fossil fuel combustion accounted for ~1/4 of Nr inputs from the 1970s to 2000 (or ~7 Tg N year−1), this value has dropped substantially since then (to <5 Tg N year−1), owing to Clean Air Act amendments. As of 2007, national N use efficiency (NUE) of all combined N inputs was equal to ~40 %. This value increases to 55 % when considering intentional N inputs alone, with food, industrial goods, fuel and fiber production accounting for the largest Nr sinks, respectively. We estimate that 66 % of the N lost during the production of goods and services enters the air (as NO x , NH3, N2O and N2), with the remaining 34 % lost to various waterways. These Nr losses contribute to smog formation, acid rain, eutrophication, biodiversity declines and climate change. Hence we argue that an improved national NUE would: (i) benefit the US economy on the production side; (ii) reduce social damage costs; and (iii) help avoid some major climate change risks in the future.This work resulted from a workshop
supported by NSF Research Coordination Network Awards
DEB-0443439 and DEB-1049744 and by the David and Lucille
Packard Foundation
Nitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake
We measured uptake length of 15NO3− in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO3− uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban-urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO3− concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO3− uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (SWtot). Uptake length increased with specific discharge (Q/w) and increasing NO3− concentrations, showing a loss in removal efficiency in streams with high NO3− concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO3− removal. The fraction of catchment area as agriculture and suburban-urban land use weakly predicted NO3− uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO3− uptake lengths via directly increasing both gross primary production and NO3− concentration. Gross primary production shortened SWtot, while increasing NO3− lengthened SWtot resulting in no net effect of land use on NO3− removal
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A Stable Isotope Tracer Study of the Influences of Adjacent Land Use and Riparian Condition on Fates of Nitrate in Streams
The influence of land use on potential fates of nitrate (NO3-) in stream ecosystems, ranging from denitrification to storage in organic matter, has not been documented extensively. Here, we describe the Pacific Northwest component of Lotic Intersite Nitrogen eXperiment, phase II (LINX II) to examine how land-use setting influences fates of NO3- in streams. We used 24 h releases of a stable isotope tracer ((NO3)-N-15-N) in nine streams flowing through forest, agricultural, and urban land uses to quantify NO3- uptake processes. NO3- uptake lengths varied two orders of magnitude (24-4247 m), with uptake rates (6.5-158.1 mg NO3-N m(-2) day(-1)) and uptake velocities (0.1-2.3 mm min(-1)) falling within the ranges measured in other LINX II regions. Denitrification removed 0-7% of added tracer from our streams. In forest streams, 60.4 to 77.0% of the isotope tracer was exported downstream as NO3-, with 8.0 to 14.8% stored in wood biofilms, epilithon, fine benthic organic matter, and bryophytes. Agricultural and urban streams with streamside forest buffers displayed hydrologic export and organic matter storage of tracer similar to those measured in forest streams. In agricultural and urban streams with a partial or no riparian buffer, less than 1 to 75% of the tracer was exported downstream; much of the remainder was taken up and stored in autotrophic organic matter components with short N turnover times. Our findings suggest restoration and maintenance of riparian forests can help re-establish the natural range of NO3- uptake processes in human-altered streams.Keywords: Oregon,
Organic matter storage,
Streams,
Spiraling,
Land use,
N-15,
Isotope tracer,
Denitrification,
Nitroge
Nitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification
We measured denitrification rates using a field 15NO3− tracer-addition approach in a large, cross-site study of nitrate uptake in reference, agricultural, and suburban-urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (SWdenn) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land-use categories, likely because of the wide range of conditions within each region and land use. N2 production rates far exceeded N2O production rates in all streams. The fraction of total NO3− removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NH4+ concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling SWden were specific discharge (discharge / width) and NO3− concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (Uden) and NO3− concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis-Menten equation. Although Uden increased with increasing NO3− concentration, the efficiency of NO3− removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO3− load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO3− concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO3− concentration
The regional and global significance of nitrogen removal in lakes and reservoirs
Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Biogeochemistry 93 (2009): 143-157, doi:10.1007/s10533-008-9272-x.Human activities have greatly increased the transport of biologically available N through
watersheds to potentially sensitive coastal ecosystems. Lentic water bodies (lakes and
reservoirs) have the potential to act as important sinks for this reactive N as it is
transported across the landscape because they offer ideal conditions for N burial in
sediments or permanent loss via denitrification. However, the patterns and controls on
lentic N removal have not been explored in great detail at large regional to global scales.
In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale,
global model of lentic N removal called NiRReLa (Nitrogen Retention in Reservoirs and
Lakes). The NiRReLa model incorporates small lakes and reservoirs than have been
included in previous global analyses, and also allows for separate treatment and analysis
of reservoirs and natural lakes. Model runs for the mid-1990s indicate that lentic systems
are indeed important sinks for N and are conservatively estimated to remove 19.7 Tg N
yr-1 from watersheds globally. Small lakes (< 50 km2) were critical in the analysis,
retaining almost half (9.3 Tg N yr-1) of the global total. In model runs, capacity of lakes
and reservoirs to remove watershed N varied substantially (0-100%) both as a function of
climate and the density of lentic systems. Although reservoirs occupy just 6% of the
global lentic surface area, we estimate they retain approximately 33% of the total N
removed by lentic systems, due to a combination of higher drainage ratios (catchment
surface area : lake or reservoir surface area), higher apparent settling velocities for N, and
greater N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of
NiRReLa suggests that, on-average, N removal within lentic systems will respond more
strongly to changes in land use and N loading than to changes in climate at the global
scale.The NSF26 Research Coordination Network on denitrification for support for collaboration
(award number DEB0443439 to S.P. Seitzinger and E.A. Davidson). This project was
also supported by grants to J.A. Harrison from California Sea Grant (award number
RSF8) and from the U.S. Geological Survey 104b program and R. Maranger (FQRNT
Strategic Professor)
Stream denitrification across biomes and its response to anthropogenic nitrate loading
Author Posting. © The Author(s), 2008. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature 452 (2008): 202-205, doi:10.1038/nature06686.Worldwide, anthropogenic addition of bioavailable nitrogen (N) to the
biosphere is increasing and terrestrial ecosystems are becoming increasingly N
saturated, causing more bioavailable N to enter groundwater and surface waters.
Large-scale N budgets show that an average of about 20-25% of the N added to the
biosphere is exported from rivers to the ocean or inland basins, indicating
substantial sinks for N must exist in the landscape. Streams and rivers may be
important sinks for bioavailable N owing to their hydrologic connections with
terrestrial systems, high rates of biological activity, and streambed sediment
environments that favor microbial denitrification. Here, using data from 15N
tracer experiments replicated across 72 streams and 8 regions representing several
biomes, we show that total biotic uptake and denitrification of nitrate increase with
stream nitrate concentration, but that the efficiency of biotic uptake and
denitrification declines as concentration increases, reducing the proportion of instream
nitrate that is removed from transport. Total uptake of nitrate was related
to ecosystem photosynthesis and denitrification was related to ecosystem
respiration. Additionally, we use a stream network model to demonstrate that
excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate
that is exported to receiving waters and reduces the relative role of small versus
large streams as nitrate sinks.Funding for this research was provided by the National Science
Foundation
Magna Carta, the Rule of Law and the Limits on Government
This paper surveys the legal tradition that links Magna Carta with the modern concepts of the rule of law and the limits on government. It documents that the original understanding of the rule of law included substantive commitments to individual freedom and limited government. Then, it attempts at explaining how and why such commitments were lost to a formalist interpretation of the rule of law from 1848 to 1939. The paper concludes by arguing how a revival of the substantive commitments of the rule of law is central in a project of reshaping modern states