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

TLDR: 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.

Figures

Table 2: Initial steady state fluxes and associated parameter values for processes altered in this application of the MEL model. Original Value is the value reported in Rastetter et al. (2001). A separate value is given for βDOC for each of the four equations used to simulate DOC loss (M1constant-loss model, M2-proportional-loss model, M3-C:N-based model, and M4-Rm-based model).

Table 1: Model equations, variables, and parameters.

Full text

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

Frequently asked questions

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. 

Q2. What are the future works in "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 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. 

Q3. What can be done to help mask the effects of DON losses?

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

Q4. What is the effect of carbon sequestration in terrestrial ecosystems?

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

Q5. What is the way to test the effects of DON loss on the ecosystem?

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. 

Q6. How long did the gradual-change simulations take to develop?

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. 

Q7. What is the effect of the gradual-change simulations on the sequestration of N?

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. 

Q8. What is the potential for accumulating N by limiting N losses in terrestrial ecosystems?

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. 

Q9. What is the effect of increasing the C:N ratio of soils on the ecosystem?

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. 

Q10. What is the standard model of carbon sequestration in terrestrial ecosystems?

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). 

Q11. How much of the C sequestered in the ecosystems with low DON losses?

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). 

Q12. How long does the C sequestration continue?

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). 

Q13. Why did the models exhibit higher DOC loss?

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

Q14. What is the effect of elevated CO2 on the N stored in the ecosystem?

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).