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Journal ArticleDOI

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

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)

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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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

Citations
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Journal ArticleDOI
01 Jan 2006-Ecology
TL;DR: The net N accumulation in plant and soil pools at least helps prevent complete down-regulation of, and likely supports, long-term CO2 stimulation of C sequestration, according to compiled data from 104 published papers that study C and N dynamics at ambient and elevated CO2.
Abstract: The capability of terrestrial ecosystems to sequester carbon (C) plays a critical role in regulating future climatic change yet depends on nitrogen (N) availability. To predict long-term ecosystem C storage, it is essential to examine whether soil N becomes progressively limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. A critical parameter to indicate the long-term progressive N limitation (PNL) is net change in ecosystem N content in association with C accumulation in plant and soil pools under elevated CO2. We compiled data from 104 published papers that study C and N dynamics at ambient and elevated CO2. The compiled database contains C contents, N contents, and C:N ratio in various plant and soil pools, and root:shoot ratio. Averaged C and N pool sizes in plant and soil all significantly increase at elevated CO2 in comparison to those at ambient CO2, ranging from a 5% increase in shoot N content to a 32% increase in root C content. The C and N contents in litter pools are consistently higher in elevated than ambient CO2 among all the surveyed studies whereas C and N contents in the other pools increase in some studies and decrease in other studies. The high variability in CO2-induced changes in C and N pool sizes results from diverse responses of various C and N processes to elevated CO2. Averaged C:N ratios are higher by 3% in litter and soil pools and 11% in root and shoot pools at elevated relative to ambient CO2. Elevated CO2 slightly increases root:shoot ratio. The net N accumulation in plant and soil pools at least helps prevent complete down-regulation of, and likely supports, long-term CO2 stimulation of C sequestration. The concomitant C and N accumulations in response to rising atmospheric CO2 may reflect intrinsic nature of ecosystem development as revealed before by studies of succession over hundreds to millions of years.

748 citations


Cites background from "Terrestrial c sequestration at elev..."

  • ...The long-term CO2 stimulation of ecosystem C sequestration on a time scale of decades or longer relies on increases in total ecosystem N stocks (Rastetter et al. 1997, 2005, Luo et al. 2004)....

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TL;DR: In this paper, an Oregon andic soil was sequentially density fractionated at 1.65, 1.85, 2.28, and 2.55 cm −3 and analyzed the six fractions for measures of organic matter and mineral phase properties.
Abstract: In mineral soil, organic matter (OM) accumulates mainly on and around surfaces of silt- and clay-size particles. When fractionated according to particle density, C and N concentration (per g fraction) and C/N of these soil organo-mineral particles decrease with increasing particle density across soils of widely divergent texture, mineralogy, location, and management. The variation in particle density is explained potentially by two factors: (1) a decrease in the mass ratio of organic to mineral phase of these particles, and (2) variations in density of the mineral phase. The first explanation implies that the thickness of the organic accumulations decreases with increasing particle density. The decrease in C/N can be explained at least partially by especially stable sorption of nitrogenous N-containing compounds (amine, amide, and pyrrole) directly to mineral surfaces, a phenomenon well documented both empirically and theoretically. These peptidic compounds, along with ligand-exchanged carboxylic compounds, could then form a stable inner organic layer onto which other organics could sorb more readily than onto the unconditioned mineral surfaces (“onion” layering model). To explore mechanisms underlying this trend in C concentration and C/N with particle density, we sequentially density fractionated an Oregon andic soil at 1.65, 1.85, 2.00, 2.28, and 2.55 g cm −3 and analyzed the six fractions for measures of organic matter and mineral phase properties. All measures of OM composition showed either: (1) a monotonic change with density, or (2) a monotonic change across the lightest fractions, then little change over the heaviest fractions. Total C, N, and lignin phenol concentration all decreased monotonically with increasing density, and 14 C mean residence time (MRT) increased with particle density from ca. 150 years to >980 years in the four organo-mineral fractions. In contrast, C/N, 13 C and 15 N concentration all showed the second pattern. All these data are consistent with a general pattern of an increase in extent of microbial processing with increasing organo-mineral particle density, and also with an “onion” layering model. X-ray diffraction before and after separation of magnetic materials showed that the sequential density fractionation (SDF) isolated pools of differing mineralogy, with layer-silicate clays dominating in two of the intermediate fractions and primary minerals in the heaviest two fractions. There was no indication that these differences in mineralogy controlled the differences in density of the organo-mineral particles in this soil. Thus, our data are consistent with the hypothesis that variation in particle density reflects variation in thickness of the organic accumulations and with an “onion” layering model for organic matter accumulation on mineral surfaces. However, the mineralogy differences among fractions made it difficult to test either the layer-thickness or “onion” layering models with this soil. Although SDF isolated pools of distinct mineralogy and organic-matter composition, more work will be needed to understand mechanisms relating the two factors.

358 citations


Cites background from "Terrestrial c sequestration at elev..."

  • ...This distinction is not reflected in current SOM models of SOM dynamics (Jenkinson et al., 1991; Parton et al. 1991; McGuire et al., 1997; Currie, 2003; Rastetter et al., 2005)....

    [...]

01 Jan 2006
TL;DR: In this paper, an Oregon andic soil at 1.65, 1.85, 2.28, and 2.55 g cm was sequentially density fractionated and the six fractions were analyzed for measures of organic matter and mineral phase properties.
Abstract: In mineral soil, organic matter (OM) accumulates mainly on and around surfaces of silt- and clay-size particles. When fractionated according to particle density, C and N concentration (per g fraction) and C/N of these soil organo-mineral particles decrease with increasing particle density across soils of widely divergent texture, mineralogy, location, and management. The variation in particle density is explained potentially by two factors: (1) a decrease in the mass ratio of organic to mineral phase of these particles, and (2) variations in density of the mineral phase. The first explanation implies that the thickness of the organic accumulations decreases with increasing particle density. The decrease in C/N can be explained at least partially by especially stable sorption of nitrogenous N-containing compounds (amine, amide, and pyrrole) directly to mineral surfaces, a phenomenon well documented both empirically and theoretically. These peptidic compounds, along with ligand-exchanged carboxylic compounds, could then form a stable inner organic layer onto which other organics could sorb more readily than onto the unconditioned mineral surfaces (‘‘onion’’ layering model). To explore mechanisms underlying this trend in C concentration and C/N with particle density, we sequentially density fractionated an Oregon andic soil at 1.65, 1.85, 2.00, 2.28, and 2.55 g cm � 3 and analyzed the six fractions for measures of organic matter and mineral phase properties. All measures of OM composition showed either: (1) a monotonic change with density, or (2) a monotonic change across the lightest fractions, then little change over the heaviest fractions. Total C, N, and lignin phenol concentration all decreased monotonically with increasing density, and 14 C mean residence time (MRT) increased with particle density from ca. 150 years to 4980 years in the four organo-mineral fractions. In contrast, C/N, 13 Ca nd 15 N concentration all showed the second pattern. All these data are consistent with a general pattern of an increase in extent of microbial processing with increasing organo-mineral particle density, and also with an ‘‘onion’’ layering model. X-ray diffraction before and after separation of magnetic materials showed that the sequential density fractionation (SDF) isolated pools of differing mineralogy, with layer-silicate clays dominating in two of the intermediate fractions and primary minerals in the heaviest two fractions. There was no indication that these differences in mineralogy controlled the differences in density of the organomineral particles in this soil. Thus, our data are consistent with the hypothesis that variation in particle density reflects variation in thickness of the organic accumulations and with an ‘‘onion’’ layering model for organic matter accumulation on mineral surfaces. However, the mineralogy differences among fractions made it difficult to test either the layer-thickness or ‘‘onion’’ layering models with this soil. Although SDF isolated pools of distinct mineralogy and organic-matter composition, more work will be needed to understand mechanisms relating the two factors. Published by Elsevier Ltd.

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TL;DR: In this article, the authors present a roadmap for how to begin building, applying, and evaluating reliable microbial-explicit model formulations that can be applied in Earth system models (ESMs).
Abstract: Microbes influence soil organic matter decomposition and the long-term stabilization of carbon (C) in soils. We contend that by revising the representation of microbial processes and their interactions with the physicochemical soil environment, Earth system models (ESMs) will make more realistic global C cycle projections. Explicit representation of microbial processes presents considerable challenges due to the scale at which these processes occur. Thus, applying microbial theory in ESMs requires a framework to link micro-scale process-level understanding and measurements to macro-scale models used to make decadal- to century-long projections. Here we review the diversity, advantages, and pitfalls of simulating soil biogeochemical cycles using microbial-explicit modeling approaches. We present a roadmap for how to begin building, applying, and evaluating reliable microbial-explicit model formulations that can be applied in ESMs. Drawing from experience with traditional decomposition models, we suggest the following: (1) guidelines for common model parameters and output that can facilitate future model intercomparisons; (2) development of benchmarking and model-data integration frameworks that can be used to effectively guide, inform, and evaluate model parameterizations with data from well-curated repositories; and (3) the application of scaling methods to integrate microbial-explicit soil biogeochemistry modules within ESMs. With contributions across scientific disciplines, we feel this roadmap can advance our fundamental understanding of soil biogeochemical dynamics and more realistically project likely soil C response to environmental change at global scales.

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  • ...Some first-ordermodels include nutrients with nonlinear functions that capture the costs and benefits of nutrient acquisition [Rastetter et al., 2001, 2005;Wang et al., 2007; Houlton et al., 2008], but generally apply a first-order approach to soil C decomposition [equation (1)]....

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TL;DR: In this article, a review of recent studies of climate-forest relationships with emphasis on indications and mechanisms of change during recent decades is presented, where the authors identify and mitigate some conditions that increase vulnerability to climate change in the forest sector.
Abstract: Climate is a critical factor affecting forest ecosystems and their capacity to produce goods and services. This chapter reviews published studies of climate-forest relationships with emphasis on indications and mechanisms of change during recent decades. Effects of climate change on forests depend on ecosystem-specific factors including human activities, natural processes, and several dimensions of climate (temperature, drought, wind, etc.). Indications of recent climate-related changes in ecosystem processes are stronger in boreal forests than in other domains. In contrast, constraints on adaptive capacity that increase vulnerability to climate change are generally more severe in subtropical and tropical forests than in temperate and boreal domains. Available information is not sufficient to support a quantitative assessment of the ecological, social and economic consequences of recent forest responses to human influences on climate. The complexity of natural and human systems is a formidable barrier to impact quantification and predictability. For example, effects of land use practices and invasive species can overshadow and interact with effects of climate change. Nevertheless, substantial progress has been made in defining mechanisms of climate-change impacts on forest ecosystems. Knowledge of impact mechanisms enables identification and mitigation of some of the conditions that increase vulnerability to climate change in the forest sector.

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References
More filters
Journal ArticleDOI
TL;DR: In this paper, the authors studied the response of a southern US pine forest to elevated CO2 for 5 years (1997-2001) and observed no statistically significant change in the gross or net rate of inorganic N mineralization and immobilization in any soil horizon under elevated CO 2.
Abstract: Empirical and modeling studies have shown that the magnitude and duration of the primary production response to elevated carbon dioxide (CO2) can be constrained by limiting supplies of soil nitrogen (N). We have studied the response of a southern US pine forest to elevated CO2 for 5 years (1997–2001). Net primary production has increased significantly under elevated CO2. We hypothesized that the increase in carbon (C) fluxes to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization. We tested this hypothesis by quantifying the pool sizes and fluxes of inorganic and organic N in the forest floor and top 30 cm of mineral soil during the first 5 years of CO2 fumigation. We observed no statistically significant change in the gross or net rate of inorganic N mineralization and immobilization in any soil horizon under elevated CO2. Similarly, elevated CO2 had no statistically significant effect on the concentration or flux of organic N, including amino acids. Microbial biomass N was not significantly different between CO2 treatments. Thus, we reject our hypothesis that elevated CO2 increases the rate of N immobilization. The quantity and chemistry of the litter inputs to the forest floor and mineral soil horizons can explain the limited range of microbially mediated soil–N cycling responses observed in this ecosystem. Nevertheless a comparative analysis of ecosystem development at this site and other loblolly pine forests suggests that rapid stand development and C sequestration under elevated CO2 may be possible only in the early stages of stand development, prior to the onset of acute N limitation.

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TL;DR: Results from this study suggest modest, if any, increases in ecosystem-level root productivity in CO2-enriched environments.
Abstract: Root dynamics are important for plant, ecosystem and global carbon cycling. Changes in root dynamics caused by rising atmospheric CO2 not only have the potential to moderate further CO2 increases, but will likely affect forest function. We used FACE (Free-Air CO2 Enrichment) to expose three 30-m diameter plots in a 13-year-old loblolly pine (Pinus taeda) forest to elevated (ambient + 200 m LL ‐1 ) atmospheric CO2. Three identical fully instrumented plots were implemented as controls (ambient air only). We quantified root dynamics from October 1998 to October 1999 using minirhizotrons. In spite of 16% greater root lengths and 24% more roots per minirhizotron tube, the effects of elevated atmospheric CO2 on root lengths and numbers were not statistically significant. Similarly, production and mortality were also unaffected by the CO2 treatment, even though annual root production and mortality were 26% and 46% greater in elevated compared to ambient CO2 plots. Average diameters of live roots present at the shallowest soil depth were, however, significantly enhanced in CO2-enriched plots. Mortality decreased with increasing soil depth and the slopes of linear regression lines (mortality vs. depth) differed between elevated and ambient CO2 treatments, reflecting the significant CO2 by depth interaction. Relative root turnover (root flux/live root pool) was unchanged by exposure to elevated atmospheric CO2. Results from this study suggest modest, if any, increases in ecosystem-level root productivity in CO2-enriched environments.

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Journal ArticleDOI
TL;DR: In this article, the nitrogen retention due to both biotic and abiotic reactions was analyzed in terms of organic matter properties in agricultural and forest soils and it was shown that the major factor determining the fraction of added inorganic nitrogen that is immobilized is the ratio between soil carbon and nitrogen concentration.
Abstract: Summary The organic matter of the soil has a large potential to retain inorganic nitrogen by means of both biotic (microbially mediated) and abiotic (chemical) reactions. We derive one equation with which we analyse the nitrogen retention due to these mechanisms in terms of organic matter properties. We first of all show how to separate gross mineralization from immobilization. We then show that our equation can reproduce studies of microbially–mediated or purely inorganic immobilization. We then apply the theory to soils of different characteristics (typical agricultural and forest soils) and find that the major factor determining the fraction of added inorganic nitrogen that is immobilized is the ratio between soil carbon and inorganic nitrogen concentration.

42 citations

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).