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The following constitute the research activities
Executive
Summary
Nitrous
oxide (N2O) is an important climatically active trace gas whose tropospheric
concentration is currently growing at about 0.3 % yr-1, with implications
for future global temperatures and stratospheric ozone. Known global sources
are several but their combined magnitude cannot account for the observed
tropospheric growth, implying the existence of additional unknown N2O
sources. Soils and the oceans are the largest of the currently recognised
tropospheric N2O sources and for both, microbially mediated nitrification
and denitrification contribute to net N2O production. Current data indicate
that tropical soils and coastal seas are particularly important sites
in this respect but relevant data are few and for the latter are dominated
by studies in temperate latitudes.
Regions likely to make a disproportionately large contribution to
the coastal N2O source are the mangroves that dominate tropical coasts
and are an interface between N-rich tropical soils and coastal seas. Nevertheless,
these regions are excluded from contemporary global source estimates due
to insufficient data. Importantly, new data indicate that mangroves could
contribute as much as 13 % of the currently known global N2O source strength.
Further studies are therefore urgently required in order to interrogate
the robustness of this estimate. Mangroves are very sensitive to global
change and anthropogenic disturbance is linked to enhanced N cycling,
with the clear implication of increased N2O fluxes. Therefore, accurate
projections of tropospheric N2O growth may be problematic in the absence
of relevant data from both pristine and anthropogenically impacted mangrove
sites. The need to improve our understanding of mangrove N cycling and
to evaluate its contribution to the global flux of tropospheric N2O is
therefore paramount and should be a high research priority.
We will examine the mangrove contribution to global
N2O using detailed studies of N fluxes, nitrification and denitrification
rates and associated N2O production and atmospheric flux at carefully
selected contrasting mangrove sites. Using these data we will develop
a generic model of mangrove N cycling and N2O flux that should be of value
in aiding future predictions of tropospheric N2O growth. These outputs
will allow identification of the climatic consequences of mangrove exploitation,
information that will aid strategies for sustainable coastal development.
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Background
and Rationale
Atmospheric
N2O growth and the role of N2O in the global system Nitrous oxide (N2O)
is a long-lived atmospheric trace gas (lifetime ~150 yr) that strongly
influences Earth's climate and the chemical budget of the atmosphere.
Its infrared activity currently accounts for ~12% of enhanced greenhouse
forcing [1] and it impacts the cycles of stratospheric NOx, and O3 [2].
Due to global source-sink imbalances, the tropospheric N2O inventory,
currently ~1525 Tg N and corresponding to a mean tropospheric mixing ratio
(in 1999) of 315 ppbv (cdiac.esd.ornl.edu;sub-directory /pub/ale _gage_
Agage /Agage /monthly), is increasing by ~0.2-0.3 % (i.e ~3 - 4.5 Tg N)
yr-1 [3], reflecting a 30% excess of known emissions over known sinks
[4].
Sources, sinks and uncertainties
Stratospheric photolysis is the principal N2O sink (~10±3 Tg N2O-N yr-1).
The uncertainty is high (±30%) but the estimate is rather well constrained
when compared with perceived global sources (oceans, soils, combustion,
biomass burning and fertilisers), which together cannot adequately account
for annual tropospheric N2O growth [4]. The total source required in this
respect is 10-17.5 Tg N2O-N yr-1, against known sources ~ 4.4-10.5 Tg
N2O-N yr-1 [4]. Clearly, substantial "missing" N2O sources must exist.
The observed N2O growth is widely believed to reflect anthropogenic perturbation.
Hence planning for the effects of global change well into the 21st century
will require robust predictions about how such perturbation will continue
to impact tropospheric N2O growth.
Notwithstanding
the large uncertainties, soils and the oceans are the largest currently
recognised tropospheric N2O sources. For both, nitrification and denitrification
are important to N2O production [5], but denitrification can also be a
net N2O sink [6]. Nitrification-denitrification coupling in soils [7],
marine sediments [8-10] and oceans [11-18] is well known and the associated
N2O production broadly correlates with natural and/or anthropogenic inorganic
N loading [19]. For example, the NO3-/NH4+ ratio of DIN impacts net N2O
production in soils and sediments [20]; freshwater sediments can be a
source or a sink of N2O, depending on N availability as NO3- [21]. In
addition, heterogeneity of microbial ecosystem structure [22, 23] and
specific physico-chemical aspects of the substrate (e.g. porosity, grain
size etc.) also constrain N2O production [24, 25]. Hence, predicting N2O
production is complex and along with a paucity of relevant data presents
a major barrier to accurate source strength quantification. Available
best estimates for soils are that N-rich tropical soils may contribute
~2.2-3.7 Tg N2O-N yr-1 and comparatively N-poor temperate soils up to
1.5 Tg N2O-N yr-1 to the atmosphere [26].
The marine N2O source is also not well constrained, but biologically productive
tropical upwellings and coastal regions seem to be important. The contribution
from the former is well-established [27], but the role of coastal regions
remains debatable for two important reasons. Firstly, data are essentially
restricted to temperate latitudes, excluding potential contributions from
major tropical river-estuarine systems. Secondly, even for the temperate
systems modelling results and observational data diverge. Large-scale
modelling indicates that estuaries contribute only ~ 4-6% of the aquatic
N2O source [5, 28], far lower than the ~ 33% derived from observation
[2, 13, 29-31]. The model estimates [5, 28] express N2O emissions as simple
linear functions based on assumptions regarding the relationships between
DIN loading and the nitrification/denitrification conversion efficiencies
of DIN to N2O. Recent measurements in the Humber indicate that up to 25%
of the terrestrial DIN flux through this estuary may convert to N2O during
sedimentary denitrification [31] (NERC-GANE GST/02/2706). This is a far
larger conversion ratio than the ~3 % maximum global average used in the
models [5, 28]. Clearly, such model results are not well supported by
observation, highlighting the need for more rigorous future modelling
that is constrained by comprehensive data. Such data should include seasonally
averaged, simultaneous estimates of inorganic N fluxes, in situ rates
of nitrification and denitrification and atmospheric N2O fluxes measured
directly. Well-planned observational programmes at relevant sites would
provide working hypotheses amenable to modelling and be a powerful aid
to resolving the current discrepancy.
Sites likely to make a disproportionately large contribution to coastal
N2O fluxes are the N-rich mangroves dominating tropical coasts, but which
are ignored in contemporary global estimates due to a lack of data. This
is especially noteworthy given the comparatively high rates of N2O production
in tropical soils [26] and important new data that indicate a potentially
major mangrove contribution to global N2O, possibly as much as 13 % of
the currently known global source strength [32]. Moreover, mangrove ecosystems
are especially sensitive to global change [33], complicating the prediction
of future N2O fluxes from present data. There is thus a clear need to
improve our understanding of how N cycling in such systems translates
into regional N2O fluxes. In particular, information relating N2O fluxes
to nitrification and denitrification rates, which themselves respond to
such variables as DIN flux and composition, and physico-chemical aspects
of the substrate, is required in order to incorporate mangrove flux data
into and improve global N2O budgets.
Tropical Mangroves: Global Significance, Threats and Role in the
Global N2O cycle.
Tropical
mangroves are a globally important ecological, environmental and socio-economic
resource, yet they form an extremely fragile land-water interface highly
susceptible to global change. These salt tolerant, intertidal ecosystems
cover ~1.8 x 105 km2 globally, fringing ~ 70% of tropical and subtropical
coasts [34] and equivalent to ~ 13 % of the global estuarine area [35].
Mangroves are genetically extremely diverse [36-38], providing important
habitats for numerous marine and terrestrial species [39], including nurseries
for commercial species [40, 41]. They also provide a crucial economic
livelihood [39] and are important to coastal protection [42]. Their intertidal
setting and rapid commercial development [43] subjects mangroves to escalating
climatic and other anthropogenic pressures. Sea level rise is the largest
threat [44-46], and associated socio-economic impacts include increased
flood risk, coastal erosion and inland retreat, saline intrusion and storm
surges [47]. In addition, direct human intervention by river damming and
agricultural and coastal development currently destroys ~ 104 km2 mangrove
yr-1 [39].
Mangrove
waters are characterised by strong physico-chemical gradients, and efficient
element recycling results from hydrodynamic mixing of coastal seawaters
with large river inputs [48]. This makes them important buffer zones in
global element transfer [49], and a particularly important reservoir for
global carbon [50]. Large organic carbon and nutrient fluxes [51-56] fuel
the highest unit area primary productivity of any global ecosystem, >
3000 g C m-2 yr-1 [57] and rapid organic diagenesis [58-63]. Net tidal
export of mangrove nutrients [64-69] and DOC [70, 71] stimulates primary
productivity and impacts food webs far offshore [72, 73]. Such export
could contribute about 4 % (7 x 1011 mol C yr-1) of the globally averaged
terrestrial DOC flux [71]. Recent studies indicate that mangroves could
be a large source of climatically active gases, including N2O [32], DMS
[74] and CH4 [75]. The only available data for N2O puts the potential
mangrove contribution to tropospheric N2O at ~ 0.14-0.56 Tg N2O-N yr-1
[32]. This is up to two orders of magnitude higher than some contemporary
estimates for intertidal areas [76] and could account for 13 % of the
currently known N2O source strength [4]. This potential importance of
mangroves in the global N2O cycle is therefore urgently deserving of further
scrutiny.
Mangrove exploitation and possible consequences for global N2O
Important
aspects of the economic exploitation of mangroves potentially relevant
to global N2O are accelerated nutrient cycling through increased agriculture/aquaculture,
and the stimulation of N fluxes through physical disturbance, such as
during industrial and commercial development. In many mangroves, annual
anthropogenic N inputs exceed natural biological N fixation [77, 78] and
the impacts via increased soil fertility and mangrove growth are well
documented [79, 80]. Enhancement both of N cycling and consequent N2O
emissions has been observed following disturbance to tropical forest [81,
82]. Despite these important effects, mechanisms for processing the increased
N fluxes are not well constrained. Studies of general mangrove N cycling
are relatively few [54, 61, 78, 79] and those targeting nitrification-denitrification
rates are even fewer [32, 82-84]. This is a particularly serious deficiency
in our understanding and the need for further work is particularly urgent
if the large N2O fluxes recently reported for mangroves [32] are typical.
The evidence is that they most likely are: nitrification rates are high
and variable among a range of tropical forest types and denitrification
rates should also be high given sufficient NO3- availability, low O2 diffusion
potentials, and large C inputs from primary production [82]. Although
rate data indicate only a minor denitrification sink for DIN in pristine
mangroves with low NO3- supply [83], such studies are few and subsequent
data support accelerated denitrification through anthropogenic addition
of NO3- [84]. Moreover, the enhancement of mangrove N2O fluxes is an apparently
exponential function of DIN loading [32]. These studies imply that mangrove
denitrification could offset enhanced inputs of anthropogenic N, but at
the expense of an increased atmospheric N2O flux that is inadequately
constrained and even more difficult to predict from existing data. The
important roles of N2O in the global system [4] demand that this situation
be rapidly redressed.
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Scientific
Objectives
We
propose to study the N biogeochemistry and to quantify the resultant atmospheric
N2O fluxes at selected tropical mangrove sites around the Bay of Bengal,
principally along the East Coast of India. Our rationale for selecting
this region has several aspects. Firstly, it is globally significant,
accounting for more than 7% of the global mangrove area. Secondly, it
is highly susceptible to anthropogenic destabilization through demographic
and industrial pressures that caused a 50% size contraction between 1960
and 1990. A consequence of this is that a diversity of mangrove sites
can be readily identified, ranging from pristine to heavily anthropogenically
impacted and hence involving a spectrum of DIN inputs, both in terms of
quality and quantity. In these respects the region can be considered a
microcosm of mangrove environments worldwide. Thirdly, the sites are all
accessible through our collaborators at the Institute for Ocean Management,
Anna University, Chennai, who have the relevant field and analytical expertise
and the means to provide full scientific and logistical project support.
Hence our aim of examining directly the anthropogenic manipulation of
global N2O is realistic within this context.
Our
overarching hypothesis is that tropical mangroves are globally important
sites of N cycling, the implications of which involve both the modification
of inorganic N fluxes to coastal waters and the production and atmospheric
flux of climatically active N2O. These implications are disproportionate
to the geographic extent of mangroves and are likely exacerbated by anthropogenic
activities including direct nutrient discharges and changes in land management
strategy. By specifically focusing on DIN fluxes, rates of nitrification
and denitrification and associated N2O production and atmospheric flux
at carefully selected contrasting sites, we hope to develop a generic
model of nitrogen cycling in mangroves. In so doing we should begin to
more adequately appreciate their contribution to the global N cycle and
climate. Using such knowledge it should be possible to predict the consequences
of continued mangrove exploitation and to incorporate this into strategies
for sustainable coastal management. Within this context we identify the
following specific scientific objectives.
- To use a grid survey approach to make large spatial scale estimates
of seasonal DIN inputs and distribution and of nitrification and denitrification
rates and dissolved N2O concentrations in four (pristine and anthropogenically
impacted) mangrove ecosystems along the E. coast of India and in the
Andaman & Nicobar islands (Bay of Bengal).
- To use the output from objective 1. to identify specific sampling
sites for detailed seasonal measurements of DIN concentrations and
fluxes, nitrification / denitrification rates, dissolved N2O concentrations
and N2O atmospheric fluxes.
- To use objective 1. and 2 outputs to construct and calibrate
a coupled hydrodynamic/mass transport model to simulate nitrification,
denitrification and N2O production and flux for tropical mangrove
systems.
- To exploit the model outputs to evaluate the tropical mangrove
contribution to global N2O, to examine its future evolution in response
to global environmental change and to facilitate formulation of these
outputs into sustainable strategies for coastal management
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Methodology
and Approach
We
will examine four "functioning mangroves". Such systems may be in the
process of impaction but are not yet severely or terminally degraded.
Our sites have contrasting anthropogenic impacts (pristine, agricultural
pollution, salt pan activity etc). The Andaman/Nicobar Island Mangroves
cover 1200 km2 and will be our "control site". This is the most pristine
ecosystem in India and is naturally shielded from strong winds and waves,
although tidal currents are strong. The Muthupet Mangrove is classified
as having low to medium anthropogenic impact. It covers ~ 68 km2, of
which ~10% is thick forest and ~ 20% is submerged. A tendency towards
eutrophication in some areas due to increased nutrient fluxes reflects
aquaculture effluent discharge. Pichavaram-Coleroon is an estuarine
mangrove of medium to high anthropogenic impact covering ~14 km2. About
53 % is barren saline soil, ~11 % is mudflat and ~10% is dense mangrove,
the remainder being halophytes. Freshwater inputs are in decline, causing
modification of the tidal regime. Aquaculture activities have recently
intensified. Major nutrient enrichments are from agricultural (diffuse
inputs) and domestic (point source) wastes, with additional fluxes from
tree felling. The Sunderbans Mangrove is strongly anthropogenically
impacted. It is a complex, densely forested estuarine ecosystem covering
~1.2 x 104 km2, with a strong salinity gradient. Aquaculture and deforestation
is extensive. Intense cyclonic storms occur during August-November.
Inundation by seawater occurs during high spring tides and by freshwater
during the monsoon.
Sampling
and analytical protocols
We
plan a 24-month field season to give full seasonal coverage at each
site. Water and sediment samples will be collected in 16 large-scale
seasonal grid surveys (1 per season at each site; ~ 2-5 samples km-2)
as follows: post monsoon (Jan-Mar), summer (Apr-Jun), pre-monsoon (Jul-Sep)
and monsoon (Oct-Dec). The surveys will give a regional site characterisation
and identify specific locations for more detailed examination of N2O
production rates and fluxes. Planned sampling intensities are essentially
a function of mangrove geographic size. Required grid samples per survey
are Pichavaram,~ 100; Muthupet, 20-40; Andaman/Nicobar,~ 100; Sunderbans
~100+ (~ 1500 samples in total for the four seasons). Waters will be
analysed in situ for "controlling variables" (S, T, pH, Eh, turbidity,
DO etc.) using a "Horiba" multi probe. Aliquots for the analysis of
dissolved N2O (GC) and DIN (NO3-, NO2-& NH4+ by standard colorimetry)
will be collected and stored for later analysis at IOM-Anna, along with
samples for the analysis of bulk sediment characteristics (C, N, grain
size spectra etc). At the detailed sampling sites identified in the
surveys, sediment cores (6 x 24-cm acrylic liners) will be collected
for porewater nutrient analysis. In situ sediment N2O fluxes will be
determined using acrylic chambers (50 x 40 x 70 cm) inserted ~10 cm
into the sediment to enclose ~ 40 dm3 bottom water and ~ 100 dm3 air.
Chamber air will be circulated with a small D.C. pump and automatically
sampled every 15 min. using gas tight vials. We will also measure water-air
N2O exchange using a free-floating gas exchange tank [85]. N2O in transects
and at the detailed sampling sites will be determined by automated,
high precision gas chromatography [86]. Simultaneous in situ measurements
of nitrification and denitrification will be made using acrylic core
liners (25 x 7 cm) inserted into the sediment. 15NO3- labeled overlying
water will be added and time-series samples collected and stored for
later "nitrogen isotope pairing" N2O analysis [87] in Newcastle, using
a state-of-the-art isotope ratio mass spectrometer (Europa 20:20), coupled
to a TG-2 preparation unit capable of returning the 15N abundance of
atmospheric N2O. Air, water and soil temperatures and flux chamber water
levels will be monitored routinely.
Development of the mechanistic model
A
coupled hydrodynamic/mass transport model will simulate nitrification,
denitrification and N2O production for permanently exposed, periodically
submerged (tidal/seasonal) and permanently submerged mangrove sediment.
Required data to be furnished from fieldwork are as follows: dissolved
N2O, DIN and "state variables" (i.e. S,T, pH, Eh, DO, BOD, turbidity,
water residence times). Nitrification and denitrification simulations
both require water residence time (for estimates of bacterial removal
[88-90]), DO, S and T. Nitrification will also require NH4+ and turbidity,
and denitrification will require NO3- [88, 91, 92]. The chosen model
will routinely generate water residence times through an implicit velocity
distribution calculation. The model will also account for both "one-dimensional"
dry season flow (i.e. flow along a main channel only) and "two-dimensional"
wet season flow (inputs via a number of one-dimensional tributaries)
[93]. N2O production will be simulated from the model outputs.
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