Estuaries and Coasts (2013) 36:508–518
DOI 10.1007/s12237-012-9499-6
Effects of Increased Salinity and Inundation on Inorganic
Nitrogen Exchange and Phosphorus Sorption by Tidal
Freshwater Floodplain Forest Soils, Georgia (USA)
Mihee Jun & Anne E. Altor & Christopher B. Craft
Received: 11 March 2011 / Revised: 2 December 2011 / Accepted: 20 March 2012 / Published online: 15 May 2012
# Coastal and Estuarine Research Federation 2012
Abstract We investigated the effects of increasing salinity
and inundation on inorganic N exchange and P sorption/
precipitation in soils of tidal freshwater floodplain forests
(TFFF) of coastal Georgia, USA. Our objectives were to
better understand how sea level rise, increasing inundation,
and saltwater intrusion will affect the ability of TFFFs to
retain nitrogen (N) and phosphorus (P). We collected soil
cores (0–5 cm) from three TFFFs that do not currently
experience saltwater intrusion and from one TFFF currently
experiencing saltwater intrusion and measured NH4-N exchange and PO4-P removal over five simulated 6-h tidal
cycles using nutrient-enriched freshwater (30 μM NH4-N
and 5 μM PO4-P). In a second experiment, we exposed soil
cores to three salinities (0, 2, and 5) and two inundation
depths (5 and 10 cm) using the same nutrient enrichment.
When flooded with nutrient-enriched freshwater, soils from
the three TFFFs that do not experience saltwater intrusion
removed inorganic N and P in amounts ranging from 5.2 to
10.7 and 2.3 to 4.4 mg/m2, respectively, and the TFFF soils
experiencing saltwater intrusion removed 2.1 to 3.8 mg
P/m2. However, TFFF soils experiencing saltwater intrusion
released inorganic N to the water column in amounts ranging from 7.1 to 67.5 mg/m2. In the second experiment, soils
from TFFFs not experiencing saltwater intrusion released
NH4-N to the water column when exposed to 2 and 5 salinity,
M. Jun (*)
Gyeongsangnam-do Institute of Health and Environment,
Gyeongsangnam-do 641-825, Republic of Korea
e-mail: [email protected]
A. E. Altor : C. B. Craft
School of Public and Environmental Affairs, Indiana University,
Bloomington, IN 47405, USA
e-mail: [email protected]
and the amount of N released increased with salinity and
number of tidal cycles. In contrast, the same TFFF soils sorbed
two and three times more PO4-P when exposed to 2 and 5
salinity than when exposed to 0 salinity. P removal on a mass
basis was greater under 10 cm of inundation, but the efficiency
of removal was greater under the 5 cm flooding depth. Our
findings suggest that saltwater intrusion caused by sea level
rise will promote N release into the water column through
organic matter mineralization and/or ion exchange and may
promote P sorption, or precipitation of P with metal cations. In
addition, release of N and resulting increased N/P could
exacerbate eutrophication of estuaries in the future.
Keywords Tidal freshwater floodplain forest . N exchange .
P sorption and precipitation . Saltwater intrusion .
Inundation . Sea level rise
Introduction
Tidal freshwater floodplain forests (TFFF) occur in the upper
reaches of river-dominated estuaries where they are inundated
twice daily by astronomical tides. TFFFs are characterized by
annual average salinities of less than 0.5 and are classified as
palustrine, scrub-shrub, and forested wetlands (Cowardin et
al. 1979; Odum 1988). In the USA, TFFFs are estimated to
encompass 200,000 ha, mostly along the southeastern coast
(Doyle et al. 2007). These forests are dominated by bald
cypress (Taxodium distichum [L]) and tupelo gum (Nyssa
aquatica [L]). Above-ground net primary production is estimated to range from 477 to 1,117 g/m2/year (Ozalp et al.
2007). Little is known about the biogeochemistry and nutrient
cycling characteristics of these ecosystems.
Located in low-lying areas at the uppermost reaches of
river-dominated estuaries, TFFFs are vulnerable to rising
Estuaries and Coasts (2013) 36:508–518
sea level and alterations in freshwater input due to global
climate change and human activity in the watershed
(Pezeshki et al. 1990; Doyle et al. 2007; Krauss et al. 2009).
TFFF vegetation is sensitive to salinity, which inhibits carbon assimilation in tree seedlings (Pezeshki 1990) and
causes mortality of adult trees (Pezeshki et al. 1990). Saltwater intrusion leads to destruction and conversion of forests
to marsh and open water (Krauss et al. 2007). Saltwater
intrusion into freshwater sediments has various impacts on
biogeochemical processes. For example, salinity intrusion
may increase decomposition of organic matter (Craft 2007)
and alter microbial pathways of organic matter mineralization
in tidal freshwater sediments (Weston et al. 2006). Weston et
al. (2006) reported the primary pathway of C mineralization
shifted from methanogenesis to iron reduction within several
days of saltwater intrusion and to sulfate reduction after
2 weeks. Addition of saltwater to freshwater wetland soil cores
resulted in release of NH4+ and HPO42− due to ion exchange
and to mineralization of organic matter (Weston et al.
2006).
Denitrification in estuarine sediments is strongly
inhibited by increasing salinity (Seo et al. 2008). H2S
produced during SO42− reduction was shown to depress
denitrification in incubations of freshwater pond sediments,
while in saline wetland sediments, H2S addition resulted in
accumulation of NH4-N, possibly via facilitation of dissimilatory nitrate reduction to ammonium (Aelion and Warttinger
2010). Increased salinity has been associated with a shift from
denitrification to dissimilatory nitrate reduction to ammonium
in estuarine sediment cores (Giblin et al. 2010). Increasing salinity (ionic strength) also promotes the release of
NH4+ from estuarine sediments by desorption from cation
exchange sites (Weston et al. 2010) and blockage of exchange
sites with seawater cations (Gardner et al. 1991). In contrast
to numerous studies examining effects of saltwater intrusion into freshwater wetlands, impacts of saltwater intrusion on the biogeochemistry of TFFFs have not been widely
investigated.
In this study, we conducted two experiments, using soil
cores from three TFFFs that are not currently impacted by
saltwater intrusion, and cores from one TFFF that is currently experiencing saltwater intrusion. In the initial experiment, we exposed soils from the four TFFFs to nutrientenriched freshwater over five simulated tidal cycles and
measured concentrations of N and P removed from or
added to the floodwater. In the second experiment, we
exposed soil cores from the three TFFFs not experiencing saltwater intrusion to nutrient-enriched water with three
levels of salinity (0, 2, and 5) and two flooding depths (5 and
10 cm), over five simulated tidal cycles. Our objectives
were to better understand how climate change-driven sea level
rise and salinity increases may affect N and P dynamics in
TFFF soils.
509
Methods
Site Description
We collected soils from TFFF of four rivers the Georgia coast
(USA): the Altamaha, Ogeechee, Satilla, and South Newport
Rivers (Fig. 1). The Altamaha River drains the third largest
watershed on the east coast (35,112 km2) and is the largest
river in Georgia (The Nature Conservancy, http://www.nature.
org). The Altamaha River originates in the Upper Georgia
Piedmont and extends through the Coastal Plain. The Ogeechee River is a low gradient blackwater river that originates in
the Georgia Piedmont but primarily drains the Coastal Plain,
where the majority of its 8,415 km2 watershed lies. The
blackwater Satilla River drains forested wetlands of the coastal plain that contribute substantial dissolved organic carbon to
its waters. The Satilla drains a watershed of approximately
7,348 km2. Discharge in the Ogeechee and Satilla Rivers is
about 25 % of the discharge in the Altamaha River, the median
discharge for which is 250 m3/s (Loomis and Craft 2010).
Over 90 % of the TFFFs in Georgia are associated with these
three rivers. The forests are dominated by bald cypress (T.
distichum [L]), tupelo gum (N. aquatica [L]), and Ogeechee
gum (Nyssa ogeche). The South Newport River (492 km2),
which flows for approximately 69 km through the Coastal
Plain, is unique among our sampling sites in that it currently is
experiencing saltwater intrusion. At this site, the cypress-gum
forest is dying and being replaced by Juncus roemerianus, a
brackish marsh species. Measurements of surface water at the
South Newport River TFFF in March 2009 yielded salinity of
5 (C. Craft, personal observation).
Nitrogen Exchange and P Sorption/Desorption in TFFF
Soils
Two TFFFs on each river were selected for sampling. Ten soil
cores were collected from each TFFF using a 2.3-cm-diameter×5-cm-deep piston corer in January 2009. Five cores were
randomly collected from levee areas adjacent to the main
channel and five from the interior floodplain at each TFFF,
for a total of 80 soil cores. Each core was placed in a labeled
Corning 25-mL centrifuge tube, capped, and stored on ice.
Synthetic floodwater was prepared by dissolving
(NH4)2SO4 and KH2PO4 into ultrapure distilled water. In
the laboratory, 20 mL of synthetic floodwater water containing 30 μM NH4-N and 5 μM PO4-P was added to each
centrifuge tube to flood the soil cores to 5 cm depth for 6 h,
simulating a single tidal cycle. These concentrations approximate the molar ratio of NH4-N to PO4-P in (Altamaha) river
waters (Weston et al. 2006). They are higher than measured
concentrations in waters of the three rivers (1–25 μM NH4N, 1–3 μM PO4-P) (Meyer 1992; Austin and Gates 2001;
Hollibaugh 2002; Weston et al. 2003) but are comparable to
510
Estuaries and Coasts (2013) 36:508–518
Fig. 1 Map of study area on
Georgia, USA
concentrations in surface waters of floodplain swamps of the
southeastern US coast (Mitsch and Gosselink 2000). The
supernatant was poured off after each 6-h flooding cycle,
and NH4-N and PO4-P concentrations were analyzed. NH4N was measured spectrophotometrically using the phenol
hypochlorite method (Cataldo et al. 1974), and PO4-P was
determined by the molybdate blue method (Murphy and
Riley 1962). The same procedures were carried out four
more times to examine inorganic N exchange and P sorption
over five simulated 6-h tidal cycles. The experiment was
performed at room temperature (20°C). Field moist cores
were capped and stored in the dark at 4°C for 1 week
between each simulated tidal cycle. In order to verify the
suitability of the synthetic floodwater for simulating natural
floodwater, we conducted a test comparing synthetic and
natural floodwater spiked with 30 μM NH4-N and 5 μM
PO4-P in a sixth simulated tidal cycle using all soil cores.
After storing the cores for 1 week at 4°C, all cores were
flooded with the spiked floodwater for 6 h, and the supernatant was collected and analyzed for N and P as described
above. Exchange and sorption dynamics were compared for
synthetic floodwater from the fifth simulated tidal cycle and
the natural floodwater spiked with N and P from the sixth
simulated tidal cycle.
An additional set of soil cores was collected for physical
and chemical analyses (n02 per wetland, one levee site, one
floodplain site) using a large (8.5-cm-diameter×5-cm-deep)
soil corer. Cores were air-dried, weighed for bulk density,
ground, sieved through a 2-mm mesh screen, and analyzed
for organic carbon, total nitrogen, and total phosphorus.
Total organic C and total N were measured using a PerkinElmer 2400 CHNS analyzer. To measure total P, soils were
digested with HNO3 and HClO4 (Sommers and Nelson
1972), and phosphate was measured colorimetrically by
the molybdate blue method (Murphy and Riley 1962). Bulk
density was calculated from the dry weight per unit volume
Estuaries and Coasts (2013) 36:508–518
(Blake and Hartge 1986) after correcting for moisture content of an air-dried sub-sample that was dried at 105°C.
Effects of Salinity and Flooding Depth on N Exchange
and P Sorption/Desorption in TFFF Soils
To expand on the findings of the first experiment, we investigated the effects of salinity and flooding depth on a second
set of soil cores collected in May 2009 from TFFFs of the
three rivers (Altamaha, Ogeechee, and Satilla) that do not
experience saltwater intrusion. Thirty cores were collected
from the floodplain of each TFFF using a 2.3-cm-diameter×
5-cm-deep piston corer. Each core was placed in a labeled
25-mL Corning centrifuge tube, capped, and stored on ice
until analysis. Five replicate cores were assigned to each of six
treatments for each TFFF. We prepared simulated floodwater
at salinity concentrations of 0, 2, and 5 by adding Instant
Ocean® Synthetic Sea Salt (Cincinnati, OH, USA) to ultrapure distilled water and added the same concentrations of
NH4-N (30 μM) and PO4-P (5 μM) as in the initial TFFF soil
experiment to each treatment. Major ions in Instant Ocean®
by weight include chloride (Cl−) 47.5 %, sodium (Na+)
26.3 %, sulfate (SO42−) 6.6 %, magnesium (Mg2+) 3.2 %,
calcium (Ca2+) 1.0 %, and potassium (K+) 1.0 % (Lebow et al.
1999). Instant Ocean® contains 1.65 mM/kg of bicarbonate
(HCO3−) and 0.24 mM/kg of carbonate (CO32−) (Atkinson
and Bingman 1997). We exposed soil cores to two flooding
depths (5 and 10 cm) within each salinity treatment to simulate
increasing inundation caused by sea level rise. The 5-cmdepth treatments received 20 mL of simulated floodwater,
and 10-cm-depth treatments received 40 mL of water. As in
the first experiment, we simulated five successive tidal cycles
by flooding the cores for 6 h per cycle and analyzing the
supernatant for NH4-N and PO4-P after each tidal cycle as
described previously. At the end of the experiment, we measured dissolved organic C (DOC) and inorganic (DIC) in the
supernatant of a subset of samples using a Shimadzu TOCVCPN® total organic carbon analyzer (Tokyo, Japan). One
sample from each river and salinity treatment (n09 total) were
analyzed for total C (TC) and DOC. Dissolved inorganic C
was calculated as the difference between TC and DOC.
At the end of the experiment, pH of soils (40 g of field
moist soil+40 mL of distilled water) was measured with a
Thermo Scientific Orion 290A+™ pH meter. The soils were
then air-dried and passed through a 2-mm-diameter mesh
sieve. Bulk density, total organic C, total N, and total P were
measured as described previously.
Statistical Analyses
Differences in N exchange and P sorption among the
four rivers were examined using repeated measures
analysis of variance (ANOVA) using river, site (two
511
sampling sites per river), and sampling location (levee
vs. floodplain) as main effects. Repeated measures
ANOVA also was used to test for differences in N
exchange and P sorption among the salinity (0, 2, and
5) and depth (5 and 10 cm) treatments over the five
simulated tidal cycles. Two-way ANOVA was used to
test for differences in N exchange and P sorption among
sites and sampling locations on each river. Main effects
means were separated using Ryan–Einot–Gabriel–Welsh
multiple range test (SAS 2002). Paired T tests were
used to compare exchange and sorption dynamics between synthetic floodwater from the fifth simulated tidal
cycle and natural (river) floodwater spiked with N and
P in the sixth simulated tidal cycle. Associations between N exchange/P sorption and soil bulk density,
organic C, total N, and total P were explored using correlation analysis. All tests of significance were conducted
at p≤0.05.
Results
Soil Bulk Density, C, N, and P
The bulk density of TFFF soils (0–5 cm) ranged from 0.20
to 0.67 g/cm3, with the lowest values found in Ogeechee and
Altamaha TFFF soils and the highest value in the Satilla
TFFF soils. Total organic C was the lowest in Satilla TFFF
soils (4.6±0.7 %) and the highest in Altamaha soils (21.8±
6.3 %). Total N was also the lowest in Satilla TFFF soils
(0.29±0.12 %) and the highest in Altamaha soils (1.26±
0.08 %). Total P was the lowest in Satilla (256±105 μg/g)
and the highest in Ogeechee (905±85 μg/g) TFFF soils
(Table 1).
Table 1 Bulk density and nutrient content of TFFF soils (0–5 cm)
TFFF
Bulk density
(g/cm3)
Ogeechee
1
0.20±0.03
2
0.33±0.03
Altamaha
1
0.26±0.10
2
0.20±0.04
Satilla
1
0.56±0.06
2
0.67±0.10
South Newport
1
0.25±0.00
2
0.48±0.04
Total organic
C (%)
Total N
(%)
Total P
(μg/g)
17.7±4.1
12.2±0.8
1.04±0.22
0.89±0.06
820±69
990±12
20.5±9.0
23.0±3.6
1.10±0.39
1.42±0.21
710±186
754±15
6.5±0.4
2.7±1.0
0.40±0.01
0.18±0.06
360±11
152±39
15.0±0.1
8.5±0.6
0.90±0.01
0.57±0.06
688±4
620±98
512
Initial Examination of N Exchange and P Sorption/
Desorption in TFFF Soils
The magnitude of inorganic N exchange from TFFF soils
did not differ in treatments receiving synthetic vs. natural
floodwater, with the exception of Altamaha River soils.
Likewise, inorganic P sorption dynamics did not differ
significantly in soils flooded with synthetic vs. natural
floodwater with the exception of Satilla River soils. Both
Altamaha and Satilla TFFF soils removed or sorbed N and P
when they were flooded with either synthetic or natural
floodwater. These results indicate that exchange and sorption/desorption dynamics for inorganic N and P were similar
between synthetic and natural floodwater and support our
use of synthetic floodwater as a medium for investigating
nutrient dynamics in TFFF soils.
Soils of the Ogeechee, Altamaha, and Satilla TFFFs
removed NH 4 -N from nutrient-enriched freshwater
(Fig. 2a). On an area basis, Ogeechee River TFFF soils
removed 8.2±1.1 mg NH4-N/m2 (mean±standard error)
and showed a decreasing trend with consecutive simulated
tidal cycles. Similar to the Ogeechee River, TFFF soils of
the Satilla River removed 10.7±1.7 mg NH4-N/m2during
the first tidal cycle, and N removal decreased to 7.8±1.4 mg
NH4-N/m2 in the fifth tidal cycle. TFFF soils of the Altamaha River removed 7.7±0.6 mg NH4-N/m2 and showed an
increasing trend from the second through the fifth tidal
cycles. In contrast to the three rivers unaffected by saltwater
intrusion, TFFF soils of the South Newport River released
NH4-N to the water column. South Newport River TFFF
soils released 67.5±15.2 mg NH4-N/m2 during the first
simulated tidal cycle (more than 3.5 times the amount of N
added to the simulated floodwater). The amount of NH4-N
released decreased with each successive tidal cycle, declining to 7.11±3.5 mg/m2 by the fifth cycle. The unit
mass of N removed from the water column over the five
tidal cycles was approximately 0.04 gN/m2 in TFFF soils of
the Ogeechee, Altamaha, and Satilla Rivers, equivalent
to 0.03 to 0.04 % of total nitrogen contained in the
soils (Table 2). TFFF soils of the South Newport River
released 0.14 gN/m2, equivalent to 0.10 % of total nitrogen
in the soil (Table 2).
TFFF soils from the four rivers sorbed PO4-P in similar
amounts from nutrient-enriched freshwater (Fig. 2b). Ogeechee River soils sorbed 3.2±0.1 mg PO4-P/m2, and sorption
decreased with consecutive simulated tidal cycles. Mean
(± standard error) PO4-P sorption was 2.6±0.1 mg/m2 in
Altamaha River soils, 3.3±0.2 mg/m2 in Satilla River soils,
and 2.8±0.2 mg/m2 in South Newport River soils. Sorption
of inorganic phosphorus decreased from the first to the
second tidal cycle for all TFFFs and remained relatively
consistent between the second and fifth tidal cycles with
only the Altamaha showing a continuing decrease. Over five
Estuaries and Coasts (2013) 36:508–518
consecutive tidal cycles, the cumulative mass of P sorbed by
TFFF soils of each river ranged from 0.01 to 0.02 g P/m2,
equivalent to 0.12 to 0.19 % of total phosphorus contained
in the soils (Table 2). In the TFFFs that experience saltwater
intrusion, N/P of the supernatant was 5∼24:1, while it was
2∼3:1 in TFFFs unaffected by saltwater intrusion.
There was no difference in N exchange and P sorption
among levee vs. plain sampling locations. For this reason,
we collected samples for the second experiment (below)
from the floodplain location only. There were significant
differences in N exchange and P sorption among TFFF soils
on the same river. Nitrogen exchange differed among the
two sites on all four rivers. Phosphorus sorption differed
among the two sites on the Ogeechee and Satilla Rivers.
Nitrogen exchange and P sorption of TFFF soils were not
correlated with any of our measured soil variables (bulk
density, total organic C, total N and P).
Effects of Salinity and Flooding Depth on N Exchange
and P Sorption/Desorption in TFFF Soils
Exchange of inorganic N by TFFF soils was significantly
affected by salinity (Fig. 3a–c). NH4-N was released from
soils of the Ogeechee, Altamaha, and Satilla River TFFFs
that were flooded with 2 or 5 of salinity. When exposed to 2
salinity, tidal forest soils released 1.4±4.3 (Ogeechee River)
to 4.3±2.9 mg NH4-N/m2 (Altamaha River). Soils exposed
to 5 salinity released more NH4-N, 2.9±3.7 (Satilla River) to
20.0±8.3 mg NH4-N/m2 (Altamaha River). Ogeechee River
soils exposed to 0 salinity removed N from the water column through all tidal cycles (5.0±0.8 mg NH4-N/m2), similar to dynamics observed in our initial experiment. In
contrast, in the 0 salinity treatment, soils of the Altamaha
and Satilla River removed N during the first two tidal cycles
but released N to the water column during the third, fourth,
and fifth cycles. There was no consistent effect of inundation depth on N exchange by TFFF soils (Fig. 3a–c). NH4-N
exchange was negatively associated with salinity (r2 00.40,
p<0.0001). There were no correlations between NH4-N
exchange and soil bulk density, total organic C, total N,
and total P.
In contrast to N, P sorption increased with increasing
salinity, with significant differences between the three salinity treatments (Fig. 4a–c). Soils flooded with 5 salinity
sorbed three to four times more inorganic P than soils
flooded with 0 salinity. Sorption of PO4-P generally decreased with repeated tidal cycles in Ogeechee and Altamaha River TFFF soils. No consistent pattern in P sorption
was observed among tidal cycles in Satilla River TFFF soils.
Inundation depth also affected inorganic P sorption in 2 and
5 salinity treatments. Soils flooded to a depth of 10 cm
sorbed more PO4-P than soils flooded to a depth of 5 cm,
regardless of salinity treatment (Fig. 4a–c), presumably
Estuaries and Coasts (2013) 36:508–518
513
Fig. 2 Sorption/desorption of a
NH4-N and b PO4-P from soils
of TFFF of the Georgia (USA)
coast flooded with nutrientenriched freshwater over five
simulated tidal cycles. Different
letters represent significant differences (p<0.05) among TFFF
soils within a simulated tidal
cycle according to the
Ryan–Einot–Gabriel–Welsch
multiple range test
because there was twice as much P in floodwater of the 10-cm
flooding treatment (40 mL of solution) than in the 5-cm flooding treatment (20 mL of solution). PO4-P sorption increased
from the 0 to 5 salinity treatments (r2 00.77, p<0.0001) and
decreased with flooding depth (r2 00.43, p<0.0001). N/P of
the supernatant was 2∼4:1 in the 0 salinity treatment, and it
Table 2 Nitrogen and phosphorus pools in TFFF soils (0–5 cm)
and cumulative exchange/sorption from simulated floodwater
over five tidal cycles, extrapolated
to an area basis
TFFF
Ogeechee
Altamaha
Satilla
South
Newport
Total N in soil
(g/m2)
128
145
89
134
increased proportionately in the 2 (4∼10:1) and 5 (4∼13:1)
salinity treatments. As with N, no correlations were observed
between P sorption and soil bulk density, total organic C, total
nitrogen, and total phosphorus.
Dissolved inorganic carbon concentrations in the supernatant increased with increasing salinity (Table 3). DIC was
N removed from (+) or
released to (−) simulated
floodwater (g/m2)
0.04
0.04
0.04
−0.14
Total P in
soil (g/m2)
12.0
8.4
7.9
11.9
P sorbed from simulated
floodwater (g/m2)
0.02
0.02
0.01
0.01
514
Estuaries and Coasts (2013) 36:508–518
Fig. 3 Effect of salinity and flooding depth on NH4-N exchanged by
soils of a Ogeechee, b Altamaha, and c Satilla Rivers during five
successive simulated tidal cycles. Different letters represent significant
differences (p<0.05) among salinity treatments for each flooding depth
and tidal cycle, according to the Ryan–Einot–Gabriel–Welsch multiple
range test
significantly higher in the 5 salinity treatment than in the 0
salinity treatment. There was no effect of salinity on DOC
concentrations in the supernatant (Table 3). pH of the supernatant increased from weakly acidic–circumneutral (5.2
to 6.0) in the 0 salinity treatment, to circumneutral (6.5 to
7.0) in the 5 salinity treatment (Table 3).
2007). Sustained intrusion of saline water into TFFFs due to
sea level rise is expected to alter the physical and biogeochemical characteristics of these ecosystems (Doyle et al.
2007; Hackney et al. 2007). We observed release of NH4-N
from tidal freshwater forest soils exposed to 2 and 5 salinity,
and both removal and release of N when soils were exposed
to freshwater (Fig. 3a–c). Other studies have shown that
increasing salinity leads to release of NH4-N from estuarine
sediments (Seitzinger et al. 1991; Rysgaard et al. 1999).
Saltwater intrusion can cause changes in soil physicochemical properties and shifts in microbial pathways that affect
ammonium exchange and organic matter mineralization in
soils and sediments. Using freshwater marsh sediment cores
as plug flow reactors in the laboratory, Weston et al. (2006)
found that microbial pathways of organic matter mineralization were dramatically affected by salinity intrusion.
While more than 60 % of organic matter was oxidized by
Discussion
Anaerobic decomposition is the dominant pathway of organic matter mineralization in tidal freshwater floodplain
forest wetland soils due to frequent inundation (Anderson
and Lockaby 2007). Aerobic decomposition occurs on the
soil surface and can occur in oxygenated sediments when
the water table is low, or where bioturbation or radial oxygen loss creates local aerobic zones (Bowden 1987; Craft
Estuaries and Coasts (2013) 36:508–518
515
Fig. 4 Effect of salinity and flooding depth on PO4-P sorption by soils
of a Ogeechee, b Altamaha, and c Satilla Rivers during five successive
simulated tidal cycles. Different letters represent significant differences
(p<0.05) among salinity treatments for each flooding depth and tidal
cycle, according to the Ryan–Einot–Gabriel–Welsch multiple range
test
methanogenesis under freshwater conditions, salinity of 10
caused a rapid shift from methanogenesis to iron reduction,
followed by sulfate reduction as the dominant mineralization
pathway within 2 weeks. A subsequent study by Weston et al.
(2011) reported that salt water intrusion into tidal freshwater
marsh soil cores increased rates of organic matter decomposition, via sulfate reduction and methanogenesis, resulting in a
loss of soil organic C. The total amount of CO2 and CH4
emitted in soil cores exposed to saline water was ∼37 %
greater than that of freshwater soil controls (Weston et al.
2011).
Sulfate reduction has been estimated to account for the
oxidation of large amounts of soil organic matter in coastal
marine and salt marsh sediments (Jorgensen 1977; Howes et
al. 1984). It is thought that sulfate reduction releases NH4-N
by mineralizing N in soil organic matter. However, Jacobson
Table 3 pH and dissolved carbon in supernatant of 0, 2, and 5 g/L salinity treatments following five simulated tidal cycles
Salinity (g/L)
TFFF
0
pH
2
5
0
2
DOC (mg/L)
5
0
DIC (mg/L)
2
5
Ogeechee
Altamaha
Satilla
5.99
5.84
5.22
6.64
6.72
6.09
7.01
6.98
6.52
0.64
0.99
1.36
0.66
0.89
1.02
1.5
2.05
0.66
1.84
2.48
1.45
4.14
3.99
2.67
0.5
0.84
0.89
516
et al. (1987) suggested that NH4-N release may be linked to
fermentation and hydrolysis reactions rather than to sulfate
reduction or methanogenesis, though compounds produced
from these reactions might be utilized to complete mineralization of sedimentary organic matter by sulfate reducing
bacteria (Burdige 1991). Howarth and Giblin (1983) found
that mineralization of organic matter in salt marshes predominantly occurred through sulfate reduction and related fermentative processes. Joye and Hollibaugh (1995) determined that
hydrogen sulfide produced from sulfate reduction in sediment
slurries inhibited nitrification and denitrification, which could
result in increased nitrogen availability.
Ion exchange and ion pairing can also contribute to NH4-N
release when TFFF soils are exposed to increased salinity.
Cations in sea salts (Ca2+, Mg2+, and Na+) can block adsorption sites of soils and sediments, and coupling of NH4+ with
anions such as SO42− can prevent adherence of NH4+ to soil
particles (Gardner et al. 1991; Rysgaard et al. 1999). Concentrations of dissolved inorganic carbon (DIC) increased with
increasing salinity in our experiment, which supports the
interpretation that the release of NH4-N is linked at least in
part to increased mineralization of organic matter. Our artificial seawater, Instant Ocean, contains some bicarbonate
(HCO3−) in it. However, it accounts for only 2–4 % of the
DIC measured in our 2 (0.045 mg C/L) and 5 (0.11 mg C/L)
salinity treatments, which supports the idea that most of the
increase in DIC in our salinity treatments is the result of C
mineralization.
Phosphorus removal from the water column was observed in TFFF soils of all rivers (Fig. 2b), and removal
increased with increasing salinity (Fig. 4a–c). It is known
that inorganic phosphate retention in soil is controlled by
aluminum, iron, calcium, magnesium, and organic matter
(Richardson 1985; Walbridge and Lockaby 1994; Reddy et
al. 1999; Darke and Walbridge 2000), and mechanisms of
phosphorus exchange in soils include adsorption/desorption,
ion exchange, and precipitation/dissolution (Reddy et al.
1999). We hypothesize that PO4-P removal in our flooded
TFFF soils was promoted by precipitation of inorganic P
with Ca- and Mg-containing minerals in sea salts. In addition, reduction of ferric iron may have provided sites for P
sorption (Reddy et al. 1999) and increasing anion exchange
capacity due to increased pH of saline water (Reddy and
Delaune 2008).
In the experiment described here, the amount of PO4-P
removed from the water column was greater in soil treatments flooded to 10 cm depth than in those flooded with
5 cm for all levels of salinity. This probably was due to the
greater mass of P that was available in the 10- vs. the 5-cm
treatments (6.2 μg P in 40 mL vs. 3.1 μg P in 20 mL simulated
floodwater, respectively).
Phosphorus removal also showed a decreasing trend
across the five successive simulated tidal cycles (Fig. 4).
Estuaries and Coasts (2013) 36:508–518
When wetland soils containing Fe are flooded, ferric phosphate minerals can be reduced to ferrous iron, releasing P
back into the water column (Darke et al. 1997; Reddy and
Delaune 2008). Wright et al. (2001) reported increasing P
availability after flooding in floodplain forest soil of the
Ogeechee River, Georgia. The ability of soils to remove P
from the water column decreases as exchange sites become
occupied and availability of minerals that can form complexes with phosphorus declines. Precipitation of iron sulfides in wetlands flooded with sulfate rich water can reduce
the potential for phosphorus to complex with iron (Reddy
and Delaune 2008). The fact that P retention was greater in
saline treatments than in freshwater treatments suggests that
sulfides in the solution may not have affected the availability of iron to form complexes with P.
In conclusion, TFFF soils from three rivers that currently
do not experience saltwater intrusion removed inorganic N
and sorbed P when exposed to nutrient-enriched freshwater
over five simulated tidal cycles. However, TFFF soils exposed to nutrient-enriched saline water released inorganic N
to the water column. Phosphorus removal increased with
increasing salinity, possibly due to higher concentrations of
Ca2+ and Mg2+, with which it may have precipitated.
While our experimental results represent short-term
observations, the dynamics exhibited reveal patterns of
response that can be anticipated with saltwater intrusion
into TFFF soils. Little research on impacts of sea level
rise has been conducted in these ecosystems, and longerterm experiments and field studies will help to corroborate and
expand upon the findings presented here. Accelerated sea
level rise driven by climate change is expected to increase
saltwater intrusion into tidal freshwater marshes and
forests (Craft et al. 2009). Saltwater intrusion will likely
result in the release of NH4+ into the water column via mineralization of soil organic matter, desorption from cation exchange sites, and potentially from increased dissimilatory
nitrate reduction to ammonium, all of which may increase N
loading and water column N/P and exacerbating eutrophication of estuaries and marine ecosystems downstream
(Howarth 1988; Paerl 2009). In contrast, sorption of inorganic P may increase in TFFF soils exposed to saltwater
intrusion. Longer-term studies are needed to examine
whether removal of phosphorus is a persistent phenomenon,
or whether prolonged anaerobic conditions and exposure
to sea water would result in re-release of P to the water
column.
This study provides documentation of some of the
biogeochemical changes that may be expected as rising
sea level leads to salinity intrusion into tidal freshwater
forests. Other changes that may be expected include decreased denitrification (Aelion and Warttinger 2010), increased DRNA (Giblin et al. 2010), and conversion of TFFF
to oligohaline marsh (Connor et al. 2007).
Estuaries and Coasts (2013) 36:508–518
Acknowledgments We thank John M. Marton and John Carswell,
the Satilla River Keeper for their assistance in the field. We appreciate
the constructive comments of two anonymous reviewers of the manuscript. This research was supported by the US Department of Energy
through grant #TUL-563-07/08 and the National Science Foundation
grant #OCE-9982133 to the Georgia Coastal Ecosystems Long Term
Ecological Research program. This is contribution number 1004 from
the University of Georgia Marine Institute.
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