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Journal Article•DOI•

Terrestrial c sequestration at elevated co2 and temperature: the role of dissolved organic n loss

Edward B. Rastetter1, Steven S. Perakis2, Gaius R. Shaver1, Göran I. Ågren3•
Marine Biological Laboratory1, United States Geological Survey2, Swedish University of Agricultural Sciences3
01 Feb 2005-Ecological Applications (John Wiley & Sons, Ltd)-Vol. 15, Iss: 1, pp 71-86
TL;DR: In this article, a simple model of carbon-nitrogen (C-N) interactions in terrestrial ecosystems was used to examine the responses to elevated CO2 and to increased CO2 plus warming in ecosystems that had the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) loss.
Abstract: We used a simple model of carbon-nitrogen (C-N) interactions in terrestrial ecosystems to examine the responses to elevated CO2 and to elevated CO2 plus warming in ecosystems that had the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) loss. We postulate that DIN losses can be curtailed by higher N demand in response to elevated CO2, but that DON losses cannot. We also examined simulations in which DON losses were held constant, were proportional to the amount of soil organic matter, were proportional to the soil C:N ratio, or were proportional to the rate of decomposition. We found that the mode of N loss made little difference to the short-term (,60 years) rate of carbon sequestration by the ecosystem, but high DON losses resulted in much lower carbon sequestration in the long term than did low DON losses. In the short term, C sequestration was fueled by an internal redistribution of N from soils to vegetation and by increases in the C:N ratio of soils and vegetation. This sequestration was about three times larger with elevated CO 2 and warming than with elevated CO2 alone. After year 60, C sequestration was fueled by a net accu- mulation of N in the ecosystem, and the rate of sequestration was about the same with elevated CO2 and warming as with elevated CO2 alone. With high DON losses, the ecosystem either sequestered C slowly after year 60 (when DON losses were constant or proportional to soil organic matter) or lost C (when DON losses were proportional to the soil C:N ratio or to decomposition). We conclude that changes in long-term C sequestration depend not only on the magnitude of N losses, but also on the form of those losses.

Summary (2 min read)

Jump to: [1. INTRODUCTION] – [2.1. Initial Dynamic Light Scattering Techniques] – [2.2. Small Angle Light Scattering] – [2.2.3. Argonne Advanced Photon Source] and [2.2.4. Conclusions of SAXS analyses]

1. INTRODUCTION

  • Sandia National Laboratories has been a leader in the development of decontamination technologies for use against chemical and biological warfare (CBW) agents, toxic industrial chemicals and other toxins for use in both the military and civilian arenas.
  • In the case of DF-200, the cleavage at this bond is enhanced by the presence of cationic micelles, which serve to attract and provide a nucleophilic-rich environment of the anionic species hydroxide, hydroperoxicarbonate, and hydroperoxide ions.
  • Data collected under the micellar partition study can be compared to kinetics performance, to deduce how changes in the formulation chemistry impact performance.
  • Potential customers and sponsors include DHS, military agencies (the Defense Threat Reduction Agency, and US Army Chemical Materials Agency), and public health and transportation industries.

2.1. Initial Dynamic Light Scattering Techniques

  • Dynamic Light Scattering (DLS) - Dynamic light scattering measures the Brownian motion of molecules and particles in solution, from which size and size distributions may be determined.
  • Consistent information on micelle size could not be acquired for the surfactant solutions using these dynamic or static light scattering techniques.
  • Effervescence from the breakdown of peroxide (concentrations 3-5%) in solution interfered with the light scattering process, as gas particles passed through the detector cells.
  • In parallel with the internal collection of DLS particle size data, Particle Technology Labs, an industry leader in particle analysis, was contacted to outsource analysis of select surfactant solutions for the determination of micelle size.
  • Through recommendation of a fellow Sandian, UMN Characterization Facility personnel were contacted to perform scoping SAXS and cryo-TEM analyses, discussed in Section 2.2.1 and 2.3.

2.2. Small Angle Light Scattering

  • In addition to cryo-TEM, Small Angle Light Scattering (SAXS) analyses was sought to characterize micelles in solution.
  • For a brief overview of SAXS methodology, refer to the publication authored by Aswal.
  • Several facilities with SAXS competency were identified and contacted.
  • Two of the facilities, the University of Minnesota Characterization Facility and Argonne Advanced Photon Source expressed interest in collecting solution-based micelle characterization data.
  • These independent efforts are described in the following sections.

2.2.3. Argonne Advanced Photon Source

  • The purpose of the study undertaken at the Argonne Advanced Photon Source facility was to perform a controlled experiment, in which SAXS technique was used to characterize the surfactant phase changes (e.g., shape, size, etc.) of micelles following the addition of the components within the standard DF-200 formulation - note that peroxide was not included in this study.
  • Note that the composition of solutions #5 and #6 are nearly the same; solution #5 was prepared in-house at Sandia National Laboratories, and solution #6 was the Part 1 surfactant mixture of the three-part commercial DF-200 product, EasyDecon.
  • The set-up parameters for the experiments were: Photon energy, 12 KeV; Distance of sample to SAXS detector, 2.2 meters; Sample to WAXS detector distance, 48 cm. Solution 2, Solution 3, and Solution 4 displayed two broad peaks, but were not indicative of forming any micelle structure.

2.2.4. Conclusions of SAXS analyses

  • Collectively, the results obtained by the SAXS technique provided insight to the micellar structures and approximate micelle sizes of the key surfactant component within the DF-200 base formulation and a variety of prospective surfactant solutions.
  • The SAXS analyses were performed at three different facilities using differing instrumentation and methods, without the benefit of a standardized test method.
  • Regardless, the micelle sizes were measured to be primarily in the range of 2-3 nm.
  • The baseline data is novel in that it served as the initial indications of the micellar environment of surfactants representative of DF-200 and other prospective CBW decontamination formulations.
  • To be of most value, future test matrices should be expanded to collect micellar characterization data over a range of surfactant, co-solvent and ionic concentrations.

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Figures (2)

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Running head: Role of DON losses in carbon sequestration
Carbon Sequestration in Terrestrial Ecosystems Under Elevated CO
2
and Temperature:
Role of Dissolved Organic versus Inorganic Nitrogen Loss
Edward B. Rastetter
1
, Steven S. Perakis
2
, Gaius R. Shaver
1
, Göran I. Ågren
3
1
-The Ecosystems Center, Marine Biological Laboratory,
Woods Hole, Massachusetts 02543 USA
2
-U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center,
Corvallis, Oregon 97331 USA
3
-Department of Ecology and Environmental Research, Swedish University of Agricultural
Sciences, Box 7072, SE-750 07 Uppsala, Sweden
Key words: Global Climate Change, Carbon Sequestration, Dissolved Organic Nitrogen,
Carbon-Nitrogen Interactions, Ecosystem Models, Terrestrial Ecosystems
Abstract
We used a simple model of carbon-nitrogen (C-N) interactions in terrestrial ecosystems
to examine the responses to elevated CO
2
and to elevated CO
2
plus warming in ecosystems with
the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen (DON) to
dissolved inorganic nitrogen (DIN) loss. We postulate that DIN losses can be curtailed by higher
N demand in response to elevated CO
2
but that DON losses cannot. We also examined
simulations in which DON losses were held constant, were proportional to the amount of soil
1
organic matter, were proportional to the soil C:N ratio, or were proportional to the rate of
decomposition. We found that the mode of N loss made little difference to the short-term (<60
years) rate of carbon sequestration by the ecosystem, but high DON losses resulted in much
lower carbon sequestration in the long term than did low DON losses. In the short term, C
sequestration was fueled by an internal redistribution of N from soils to vegetation and by
increases in the C:N ratio of soils and vegetation. This sequestration was about three times
larger with elevated CO
2
and warming than with elevated CO
2
alone. After year 60, C
sequestration is fueled by a net accumulation of N in the ecosystem and the rate of sequestration
was about the same with elevated CO
2
and warming as with elevated CO
2
alone. With high
DON losses, the ecosystem either sequestered C slowly after year 60 (when DON losses were
constant or proportional to soil organic matter) or lost C (when DON losses were proportional to
the soil C:N ratio or to decomposition). We conclude that changes in long-term C sequestration
depend not only on the magnitude of N losses but on the form of those losses as well.
Introduction
Terrestrial ecosystems are thought to sequester about 25% of the carbon (C) currently
emitted through fossil-fuel burning and land-use change (IPCC 2001). It is hoped that these
ecosystems will continue to be a major sink for C in the future and thereby mitigate further
increases in CO
2
in the atmosphere. However, productivity in terrestrial ecosystems is strongly
constrained by the dynamics of the nitrogen (N) cycle (Vitousek et al. 1998) and C sequestration
will likely require a net accumulation of N in these ecosystems. The input of N to ecosystems
has been widely studied, especially from the perspective of atmospheric N deposition (Galloway
et al. 2003, 1995, Ollinger et al. 1993) and an understanding of the controls on biological N
2
2
fixation is emerging (Cleveland et al. 1999, Rastetter et al. 2001, Vitousek et al. 2002).
However, surprisingly little is known about the form, magnitude, or controls of N losses from
terrestrial ecosystems (Pellerin et al. in press, McDowell 2003, Neff et al. 2003, Aber et al. 2002,
Hedin et al. 1995, Sollins and McCorrison 1981). In this paper we argue that the amount of C
sequestered in terrestrial ecosystems in response to elevated CO
2
depends on the fraction of N
losses that are in the form of dissolved organic N (DON) versus dissolved inorganic N (DIN);
because plants can curtail DIN losses as N demand increases in response to elevated CO
2
, but
plants have little control over DON losses, the potential for accumulating N by limiting N losses
should be small if DON losses are high. Thus, the potential for sequestering C in response to
elevated CO
2
should be small if a large fraction of the N losses are as DON.
Modifications to the Standard Model
Our assessment of C sequestration in relation to DON losses relies upon three
modifications to what has been called "the standard model" of N accumulation in terrestrial
ecosystems (Vitousek et al. 1998). First, as suggested by Vitousek et al. (1998) and Neff et al.
(2003), the standard model needs to be modified to include DON losses. Second, the standard
model needs to be modified to accommodate an increase in N demand by both plants and
microbes in response to elevated CO
2
levels. Finally, the dynamics of DIN in the standard model
have to be modified to reflect the fact that N uptake by microorganisms, N uptake by plants, and
N losses from the ecosystems happen simultaneously rather than sequentially. These changes are
discussed in more detail below.
There are also several assumptions we have made to simplify our analysis. The first
relates to the growing evidence that plants can use organic forms of N (Schimel and Bennett
3
2003, Neff et al. 2003, McKane et al. 2002, Schimel and Chapin 1996, Kieland 1994, Chapin et
al. 1993). We will circumvent this complication by lumping plant-available forms of DON into
the DIN pool and use "DON" to refer only to unavailable forms. By lumping plant-available
forms of DON into the DIN pool, we are also assuming that these forms of DON are available to
soil microbes. We will further simplify our analysis by assuming that any additional DON
available to microbes is retained in the ecosystems and can therefore be lumped with the soil
organic N (Lispon and Monson 1998, Perakis and Hedin 2001). Thus, we assume that the DON
lost from ecosystems is in a form that is unavailable to both plants and microbes. We also
assume that there is no change in the ratio of NH
4
to NO
3
in soil solution so that the DIN losses
can be represented as proportional to the total DIN in soil solution. Finally, we will lump
gaseous N losses (e.g., denitrification) in with DIN losses.
DON losses: Until recently, DON losses from terrestrial ecosystems have been largely
ignored (Goodale et al. 2000, Campbell et al. 2000) and were not incorporated into the standard
model of N accumulation (Vitousek et al. 1998). Estimates that infer total N losses from stream
chemistry indicate that DON losses range from less than 20% to greater than 80% of those losses
(e.g., Perakis and Hedin 2002, Qualls et al. 2002, Buffam et al. 2001, Goodale et al. 2000,
McHale et al. 2000). Because of retention and processing of DON and DIN in the vadose zone,
ground water, riparian areas, and streams (Kroeger 2003, Hedin et al 1998, Newbold et al. 1981,
1982), stream water chemistry probably does not faithfully reflect the chemistry of water leaving
the rooting zone of upland areas. For example, Currie et al. (1996) found that DON accounted
for over 97% of the N in zero-tension lysimeters at the base of the rooting zone of a previously
logged New England forest, whereas Goodale et al. (2000) found that on average DON
4
accounted for only 67% of the N in steams draining previously logged New England forests. In
a southern hardwood forest, Qualls et al. (2002) found N fluxes to be 92% DON in the B
horizon, 75% in the C horizon, and 79% in the stream. In addition, none of these studies
quantify the fraction of DON that might be available to either plants or microbes. Thus, although
DON losses appear to be important, the relative losses of DIN versus DON from upland
ecosystems are far from certain (McDowell 2003). Our purpose here is not to resolve this
uncertainty but rather to assess the consequences of DIN versus DON losses on the potential for
C sequestration in terrestrial ecosystems in response to elevated CO
2
concentrations.
Increased N demand in response to elevated CO
2
: The standard model of N
accumulation is formulated from the perspective of a single limiting resource (i.e., N) and
therefore does not address the effects of other resources, like CO
2
, on N dynamics. An alternate
perspective is provided by the "functional equilibrium hypothesis" (Farrar and Jones 2000,
Chapin et al. 1987, Bloom et al. 1985), which predicts that increased CO
2
concentrations will
free plant resources currently allocated toward C acquisition and allow them to be reallocated
toward the acquisition of other resources like N. This hypothesis has been corroborated in
several studies on tree saplings, in which allocation to fine roots increased in response to
elevated CO
2
(e.g., Tingey et al 2000, Janssens et al. 1998, Prior et al. 1997), and has also been
observed in intact forest stands, although the response is weaker than in studies on saplings
(Pritchard et al 2001, Matamala and Schlesinger 2000). This compensatory reallocation of
internal resources should increase N-uptake potential of plants. In addition, elevated CO
2
should
increase the flux of C to soils in litter and root exudates and thereby increase microbial N
demand (Johnson et al. 2001, Mikan et al. 2000). These responses of plants and microbes to
5
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Plant diversity, CO2, and N influence inorganic and organic N leaching in grasslands.

[...]

Feike A. Dijkstra1, Jason B. West1, Sarah E. Hobbie1, Peter B. Reich1, Jared J. Trost1 •
University of Minnesota1
01 Feb 2007-Ecology
TL;DR: The results suggest that plant species richness, elevated [CO2], and N deposition alter DIN loss primarily through changes in biological N demand, suggesting that, even with high biological demand for N, substantial amounts of N can be lost as DON.
Abstract: In nitrogen (N)-limited systems, the potential to sequester carbon depends on the balance between N inputs and losses as well as on how efficiently N is used, yet little is known about responses of these processes to changes in plant species richness, atmospheric CO2 concentration ((CO2)), and N deposition. We examined how plant species richness (1 or 16 species), elevated (CO2) (ambient or 560 ppm), and inorganic N addition (0 or 4 gm � 2 � yr � 1 ) affected ecosystem N losses, specifically leaching of dissolved inorganic N (DIN) and organic N (DON) in a grassland field experiment in Minnesota, USA. We observed greater DIN leaching below 60 cm soil depth in the monoculture plots (on average 1.8 and 3.1 g Nm � 2 � yr � 1 for ambient N and N-fertilized plots respectively) than in the 16-species plots (0.2 g Nm � 2 � yr � 1 for both ambient N and N-fertilized plots), particularly when inorganic N was added. Most likely, loss of complementary resource use and reduced biological N demand in the monoculture plots caused the increase in DIN leaching relative to the high-diversity plots. Elevated (CO2) reduced DIN concentrations under conditions when DIN concentrations were high (i.e., in N-fertilized and monoculture plots). Contrary to the results for DIN, DON leaching was greater in the 16-species plots than in the monoculture plots (on average 0.4 g Nm � 2 � yr � 1 in 16-species plots and 0.2 g Nm � 2 � yr � 1 in monoculture plots). In fact, DON dominated N leaching in the 16-species plots (64% of total N leaching as DON), suggesting that, even with high biological demand for N, substantial amounts of N can be lost as DON. We found no significant main effects of elevated (CO2) on DIN or DON leaching; however, elevated (CO2) reduced the positive effect of inorganic N addition on DON leaching, especially during the second year of observation. Our results suggest that plant species richness, elevated (CO2), and N deposition alter DIN loss primarily through changes in biological N demand. DON losses can be as large as DIN loss but are more sensitive to organic matter production and turnover.

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    [...]

  • ...…are typically only a small fraction of total ecosystem N pools (Chapin et al. 2002, Van Breemen et al. 2002), persistent N leaching losses can, in the long-term, significantly reduce accumulation of N and C in plant biomass and soil organic matter (Vitousek et al. 1998, Rastetter et al. 2005)....

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TL;DR: This work examines processes controlling equilibrium and transient nutrient dynamics at three separate timescales in a model of a nutrient cycling through plants and soil in terrestrial ecosystems to help guide empirical and experimental studies.
Abstract: Nutrient availability often limits primary production, yet the processes governing the dynamics of nutrient limitation are poorly understood. In particular, plant‐available (e.g., nitrate) versus plant‐unavailable (e.g., dissolved organic nitrogen) nutrient losses may have qualitatively different impacts on nutrient limitation. We examine processes controlling equilibrium and transient nutrient dynamics at three separate timescales in a model of a nutrient cycling through plants and soil. When the only losses are from the plant‐available nutrient pool, nutrient limitation at a long‐term equilibrium is impossible under a wide class of conditions. However, plant biomass will appear to level off on a timescale controlled by plant nutrient turnover (years in grasslands, decades to centuries in forests), even though it can grow slowly forever. Primary production can be nutrient limited in the long‐term when there are losses of plant‐unavailable nutrients or when the mineralization flux saturates with...

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Nitrogen dynamics in a small arctic watershed: retention and downhill movement of 15N

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Yuriko Yano1, Gaius R. Shaver1, Anne E. Giblin1, Edward B. Rastetter1, Knute J. Nadelhoffer2 •
Marine Biological Laboratory1, University of Michigan2
01 May 2010-Ecological Monographs
TL;DR: In this paper, the authors examined short and long-term nitrogen dynamics and availability along an arctic hillslope in Alaska, USA, using a stable isotope of nitrogen (15 N), as a tracer.
Abstract: We examined short- and long-term nitrogen (N) dynamics and availability along an arctic hillslope in Alaska, USA, using a stable isotope of nitrogen ( 15 N), as a tracer. Tracer levels of 15 NH4 þ were sprayed once onto the tundra at six sites in four tundra types: heath (crest), tussock with high and low water flux (mid- and footslope), and wet sedge (riparian). 15 N in vegetation and soil was monitored to estimate retention and loss over a 3- year period. Nearly all 15 NH4 þ was immediately retained in the surface moss-detritus-plant layer, and .57% of the 15 N added remained in this layer at the end of the second year. Organic soil was the second largest 15 N sink. By the end of the third growing season, the moss-detritus-plant layer and organic soil combined retained � 87% of the 15 N added except at the Midslope site with high water flux, where recovery declined to 68%. At all sites, non-extractable and non- labile-N pools were the principal sinks for added 15 N in the organic soil. Hydrology played an important role in downslope movement of dissolved 15 N. Crest and Midslope with high-water-flux sites were most susceptible to 15 N losses via leaching, perhaps because of deep permeable mineral soil (crest) and high water flow (Midslope with high water flux). Late spring melt season also resulted in downslope dissolved- 15 N losses, perhaps because of an asynchrony between N release into melt water and soil immobilization capacity. We conclude that separation of the rooting zone from the strong sink for incoming N in the moss- detritus-plant layer, rapid incorporation of new N into relatively recalcitrant-soil-N pools within the rooting zone, and leaching loss from the upper hillslope would all contribute to the strong N-limitation of this ecosystem. An extended snow-free season and deeper depth of thaw under warmer climate may significantly alter current N dynamics in this arctic ecosystem.

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  • ...The importance of dissolved-N losses is increasingly recognized for many terrestrial ecosystems (Vitousek et al. 1998, Stepanauskas et al. 2000, Perakis and Hedin 2002, Kawahigashi et al. 2004, Rastetter et al. 2004, 2005)....

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Controls over Soil Nitrogen Pools in a Semiarid Grassland Under Elevated CO2 and Warming

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Yolima Carrillo1, Yolima Carrillo2, Feike A. Dijkstra1, Elise Pendall2, Jack A. Morgan, Dana M. Blumenthal •
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Additional excerpts

  • ...Key words: elevated CO2; warming; grasslands; climate change; nitrogen; dissolved organic nitrogen; nitrate; ammonium; dissolved organic carbon; moisture; temperature; 13C....

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James N. Galloway, John D. Aber, Jan Willem Erisman, Sybil P. Seitzinger, Robert W. Howarth, Ellis B. Cowling, B. Jack Cosby 
01 Apr 2003-BioScience
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Joshua P. Schimel1, Jennifer N Bennett1, Jennifer N Bennett2•
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01 Mar 2004-Ecology
TL;DR: A complete new conceptual model of the soil N cycle needs to incorporate recent research on plant–microbe competition and microsite processes to explain the dynamics of N across the wide range of N availability found in terrestrial ecosystems.
Abstract: Until recently, the common view of the terrestrial nitrogen cycle had been driven by two core assumptions—plants use only inorganic N and they compete poorly against soil microbes for N. Thus, plants were thought to use N that microbes “left over,” allowing the N cycle to be divided cleanly into two pieces—the microbial decomposition side and the plant uptake and use side. These were linked by the process of net mineralization. Over the last decade, research has changed these views. N cycling is now seen as being driven by the depolymerization of N-containing polymers by microbial (including mycorrhizal) extracellular enzymes. This releases organic N-containing monomers that may be used by either plants or microbes. However, a complete new conceptual model of the soil N cycle needs to incorporate recent research on plant–microbe competition and microsite processes to explain the dynamics of N across the wide range of N availability found in terrestrial ecosystems. We discuss the evolution of thinking abou...

2,126 citations

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John D. Aber, William H. McDowell, Knute J. Nadelhoffer, Alison H. Magill, G. M. Berntson, Mark Kamakea, Steven G. McNulty, William S. Currie, Lindsey E. Rustad, Ivan J. Fernandez 
01 Nov 1998-BioScience
TL;DR: In the US, nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. as mentioned in this paper, 2003), while acid acid deposition is increasing.
Abstract: N itrogen emissions to the atmosphere due to human activity remain elevated in industrialized regions of the world and are accelerating in many developing regions (Galloway 1995). Although the deposition of sulfur has been reduced over much of the United States and Europe by aggressive environmental protection policies, current nitrogen deposition reduction targets in the US are modest. Nitrogen deposition remains relatively constant in the northeastern United States and is increasing in the Southeast and the West (Fenn et al. in press). The US acid deposition effects

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Gene E. Likens
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1,613 citations

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01 Sep 2004-Biogeochemistry
James N. Galloway, Frank Dentener, Douglas G. Capone, Elizabeth W. Boyer, Robert W. Howarth, Sybil P. Seitzinger, G. P. Asner, Cory C. Cleveland, P. A. Green, Elisabeth A. Holland, David M. Karl, Anthony F. Michaels, John H. Porter, Alan R. Townsend, Charles J. Vörösmarty 
Frequently Asked Questions (14)
Q1. What are the contributions mentioned in the paper "Running head: role of don losses in carbon sequestration carbon sequestration in terrestrial ecosystems under elevated co2 and temperature: role of dissolved organic versus inorganic nitrogen loss" ?

The authors used a simple model of carbon-nitrogen ( C-N ) interactions in terrestrial ecosystems to examine the responses to elevated CO2 and to elevated CO2 plus warming in ecosystems with the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen ( DON ) to dissolved inorganic nitrogen ( DIN ) loss. The authors also examined simulations in which DON losses were held constant, were proportional to the amount of soil The authors postulate that DIN losses can be curtailed by higher N demand in response to elevated CO2 but that DON losses can not. 

The best potential for testing their ideas in a timely manner would be to experimentally manipulate ecosystems where the masking effects of within-ecosystem responses are likely to be small relative to the effects of DON losses to determine if there is a trend toward high C sequestration with low DON losses relative to DIN losses. Thus the manipulations should be on ecosystems where the C: N ratio of vegetation is low ( i. e., close to the C: N ratio of soils so that the redistribution of N has a smaller effect ), where the vegetation is unlikely to increase in woodiness ( i. e., to avoid the masking effects of increasing C: N ratios ), and where the total throughput of DON plus DIN is high ( i. e., a high potential to sequester N ). Their aim in this paper has been to examine how considering the relative magnitudes of DON versus DIN losses might influence assessments of potential C sequestration in terrestrial ecosystems. Their conclusions are that it is vital to quantify these fluxes at least in regards to evaluations of the long-term potential for C sequestration. 

In addition, increases in plant and soil C:N ratios can contribute to the withinecosystem responses and help mask the effects of DON losses. 

Terrestrial ecosystems are thought to sequester about 25% of the carbon (C) currentlyemitted through fossil-fuel burning and land-use change (IPCC 2001). 

The best potential for testing their ideas in a timely manner would be to experimentally manipulate ecosystems where the masking effects of within-ecosystem responses are likely to be small relative to the effects of DON losses to determine if there is a trend toward high C sequestration with low DON losses relative to DIN losses. 

With high DON losses, N gains and losses were small during the first 100 years of all the simulations, and the dynamics in the gradual-change simulations generally lagged behind those in the instantaneous-change simulations by about two decades. 

Increases in plant and soil C:N ratios contributed less to C sequestration, but in amounts proportionately equivalent to their contributions in the instantaneous-change simulations. 

In this paper the authors argue that the amount of C sequestered in terrestrial ecosystems in response to elevated CO2 depends on the fraction of N losses that are in the form of dissolved organic N (DON) versus dissolved inorganic N (DIN); because plants can curtail DIN losses as N demand increases in response to elevated CO2, but plants have little control over DON losses, the potential for accumulating N by limiting N losses should be small if DON losses are high. 

Because the C:N ratio of soils is about 25 and that of plants is about 143 (initial C:N values), this redistribution of N results in a net increase in the amount of C stored per unit N in the ecosystem. 

Their assessment of C sequestration in relation to DON losses relies upon threemodifications to what has been called "the standard model" of N accumulation in terrestrial ecosystems (Vitousek et al. 1998). 

On average, the ecosystems sequestered only about 1 kg C m-2 between years 60 and 1000 or about 7% of the C sequestered during the first 60 years and 6% of the C sequestered in the ecosystems with low DON losses (Fig. 1). 

Sequestration of C continues for the duration of all low-DON-loss simulations, although at a rate that is only about 17% of that during the first 60 years (Fig. 1,Table 3). 

Because of the explicit linkages between DOC and DON in the various model structures,simulations with higher DON loss also exhibit higher DOC loss. 

with a combination of elevated CO2 and warming, increases in woody tissues and the consequent increase in plant C:N ratio contributed significantly to an increase the C stored per unit N in the ecosystem (Fig. 2).