KAWAHIGASHI, Masayuki, DO, Nhut Minh, NGUYEN, Bao Ve and ...

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M. Kawahigashi et al. / Pedologist (2012) 458-465
Effective Land and Water Management for Controlling Solutes from
Acid Sulfate Soils in Mekong Delta Paddy Fields
Masayuki KAWAHIGASHI 1 , Nhut Minh DO 2 , Bao Ve NGUYEN 3 and Hiroaki SUMIDA* , 4
1
Department of Geography, Tokyo Metropolitan University, Hachioji Tokyo 192-0397, Japan
2
Agricultural
and Rural Development Science, Kien Giang Agricultural Extension Center,
Rach Gia, Kien Giang, Vietnam
3
College
of Agriculture, Can Tho University, Can Tho, Vietnam
4
College
of Bioresource Sciences, Nihon University, Fujisawa Kanagawa 252-0880, Japan
Keywords: acid sulfate soils, Mekong Delta, land reclamation, paddy field, canal water
Abstract
Actual acid sulfate soils have emerged across large areas of the Mekong Delta due to oxidation of potential acid sulfate
soils following land reclamation for mainly agricultural use. Despite adverse conditions for crop production, due to strong
acidity and high salinity, suitable water management and chemical fertilizer application enables rice to be grown on agricultural
land of actual acid sulfate soils. Changes in the water properties of canals in the double cropping system of a paddy field
in areas of actual acid sulfate soils were monitored to evaluate the effect of water management and fertilizer application
upon water quality. Canal water adjacent to the paddy field was periodically collected, and general chemical properties were
analyzed. The great variation in pH (2.8–6.3) and electric conductivity (EC; 0.1–1.5 dS m -1 ) in the drained water seemed to
be closely associated with water management of adjacent paddy fields. Canal water collected in the growing season had a
low pH and high EC, while a neutral pH and low EC were observed in the fallow season. Values of EC and pH correlated
with major components of solutes such as SO 4 , Ca, Mg, and Al ions. Al concentration in the acidic water was extremely high,
peaking at 260 mg L -1 in the rice crop season. Inundation and drainage for rice cropping is a major controlling factor for the
release of solutes from the soil into drained water. It is important therefore to determine the time lag between initiation of
soil management and the consequent changes in the surrounding water system.
1. Introduction
Vietnam is one of the largest rice producing and rice
exporting countries in the world (FAO, 2009). A large
quantity of Vietnam’s rice is produced in the Mekong
Delta, which constitutes only a small area (12%) of the
country. However, vast tracts of problematic soils occupy
approximately 40% of the total delta area (Xuan et al.
1982). Such affected soils originated in the Holocene
at the same time as the development of the delta coast
line due to regression of the sea (Nguyen et al. 2000).
This geological event led to accumulation of reduced
sulfur materials (pyrite) in the subsoil under reductive
conditions beneath surface alluvium and peat cover. Soil
strongly acidified by oxidation of pyrite is actual acid
sulfate soil, which substantially reduces biological activity
and plant productivity. Actual acid sulfate soil usually
emerges following land degradation after inadequate land
reclamation or land use management.
Land reclamation of potential acid sulfate soils
accompanied by water table depletion affects both
agricultural and aquacultural productivity because of
*Corresponding author: Hiroaki Sumida, E-mail: hsumida@brs.nihon-u.ac.jp, Tel: +81-466-84-3953, Fax: +81-466-84-3952
Received 30 November 2011; accepted 10 January 2012

M. Kawahigashi et al.: Land and water management to control solutes from acid sulfate soils
459
the deterioration of groundwater quality and soil acidity
(Wilson and Hyne, 1997, Shamshuddin et al., 2004,
Gosavi et al., 2004). Despite the adverse conditions for
agricultural productivity, large areas of the Mekong Delta
countryside have been reclaimed for agricultural use by
migrants from urban areas according to the relocation
program planned by the Vietnamese government after the
unification of Vietnam (White et al., 2001). In the Ha Tien
plain on the western back swamp of the Bassac River,
which is a tributary of the Mekong River, canal networks
have been constructed in huge areas of arable land since
1970, deepening the groundwater table and leading to soil
acidification (Vo Quang Minh, pers. comm.). It might take
a long time for regular agricultural land use to ameliorate
the severe acidity and dissolved high-concentration
metals. Actually, the annual rice yield in the Ha Tien plain
is 2–3 t/ha, which is approximately one-third of the rice
yield of alluvial soils along the Bassac River (Kien Giang
Statistical Office, 2005).
Variation in the depth of the groundwater table is
a controlling factor of pH and redox in acid sulfate soils.
Strongly acidic conditions by pyrite oxidation due to
deepening of the water table deteriorate agricultural
fields in the dry season. On the other hand, toxicity due
to reduced Fe under reductive conditions is caused by
the high groundwater table in the wet season. Usually,
both Al and Fe toxicity do not appear simultaneously.
Constraints of rice growth due to excessive Fe and Al
can be avoided by choosing an optimal time (moderately
reduced condition) for sowing rice in actual acid sulfate
soils (Husson et al. 2000). Understanding the periodical
variation in groundwater table, water quality, and crop
growth is useful for effective land use management of
paddy fields.
Modern agriculture in Vietnam requires application
of chemical fertilizer which affects soil reaction and water
quality. Repeated double or triple cropping in a warm
climate using plenty of water provides chemical fertilizer
to agricultural land. Excessive nutrients should be flushed
into the canal system with acids and acidic metals, which
affects aquatic biological activity. Excessive fertilizer
application of actual acid sulfate soils could also make
it difficult to find an optimal time for sowing rice and
managing water supply and drainage. Here, the quality of
tertiary canal water was evaluated in a double cropping
paddy field in the Ha Tien plain to monitor the effect of
fertilizer application and seasonal variation in the water
table on the quality of canal water over two cropping
seasons.
2. Materials and Methods
Sampling Site
The research site was located in the north-western
part of the Mekong Delta, close to the border of Vietnam
and Cambodia. Severely acidic soils (actual acid sulfate
soils) have developed on the Ha Tien plain located in
the western area of the Bassac River in the Longxuyan
Quadrangle area of the Mekong Delta. This area has been
covered with the oldest (6,000 ybp) fluvial deposits in the
Mekong Delta (Nguyen et al., 2000). After removal of the
fluvial cover due to natural and artificial land degradation,
pyritic material (FeS 2 ) changed into strongly acidic
sulfate minerals (jarosite) and/or a sulfuric horizon with a
strong acidic soil reaction. In the Ha Tien plain, including
the research area, actual acid sulfate soils without
jarosite mottles are prevalent (Muhrizal et al., 2006, van
Mensvoort and Tri, 1986). The soil here is classified as
Typic Sulfaquepts according to the USDA soil taxonomy
system (Soil Survey staff 2010). Soil properties are briefly
described in Table 1 (Kawahigashi et al., 2008a).
Agricultural activity in the paddy field
The crop calendar and fertilizer application details
are shown in Fig. 1. Usually, early December and early
May are the rice seeding periods in the study area.
Basal fertilizer was applied two and a half months before
seeding, and additional fertilizer was applied one month
after seeding. Finally, a second application of fertilizer was
applied at the ripening stage. Only chemical fertilizers
were applied. Both basal and additional fertilizers had a
N:P:K ratio of 20:20:15 and were applied at a rate of 100
kg/ha. Seventy kilograms of chemical fertilizer (N:P:K
ratio of 16:16:8) combined with 20 kg of urea were applied
as additional fertilizer. The yield of rice in the research
field was 2–3 ton/ha; a relatively low yield in Kien Giang
Province, which occupies the area from the coast of the
Bassac River to the gulf of Thailand (Kien Giang statistical

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M. Kawahigashi et al. / Pedologist (2012) 458-465
Soil and
horizon
Ta Teng
Depth
Table 1 Physico-chemical properties of soils.
Total pH CEC K Na Ca Mg Base
C org saturation
Exchangeable
Al
Field moisture
content
(cm) (g kg -1 ) (KCl) (cmol c kg -1 ) (%) (g kg -1 ) (%)
Ap 0-6 84.6 3.3 14.9 0.1 0.3 1.2 10.2 79 2.8 21.7
Bwg 6-22 32.3 2.9 13.5 0.1 0.2 0.3 0.2 17 2.8 27.5
Bg1 22-38 51.4 2.8 17.7 0.1 0.3 0.4 2.2 17 3.3 38.4
Bg2 38-56 25.5 2.8 9.5 Tr 0.2 0.2 0.8 13 2.1 20.4
BCg 56-67 7.6 2.9 6.3 Tr 0.1 0.1 0.2 5 1.6 19.2
Cg1 67-94 4.2 2.8 6.8 0.1 0.3 0.2 1.4 20 1.9 20.0
Cg2 94-105 5.2 2.9 7.6 0.1 0.5 0.9 7.4 116 2.0 24.0
Fig. 1 Map of research site in the Mekong Delta. The canal where water
was sampled shows the year of construction (Upper right).
office, 2005).
Canal water
The main canal running in the research area was first
constructed between 1975 and 1979. The perpendicularly
running second canal connecting to the main canal was
completed between 1983 and 1985 (Fig. 2). The third canal,
which is relatively small and shallow, was constructed
between 1998 and 1999. Water samples were periodically
collected from the third canal (N10 26.124’, E104 35.783’)
running through the paddy field in Ta Teng. Excavation of
deposits at the bottom of the canals was a common practice
for maintaining a particular water level and basal water
flow rate in the field. Sampling was done at the lowest tide
of the month to avoid mixing sea water with canal water.
The collected water was kept in a refrigerator at the Kien
Giang agricultural station office until flown to Japan. After
measuring the pH and electric conductivity of water

M. Kawahigashi et al.: Land and water management to control solutes from acid sulfate soils
461
Fig. 2 Crop calendar for the paddy field in Ta Teng in the Mekong delta
Arrows and squares with dotted lines show the 1st rice cultivation, and those with solid lines
show the 2nd crop cultivation.
samples with a glass electrode, samples were immediately
filtered using a 0.2-µm membrane filter (Millipore Ltd.,
Tokyo, Japan) under vacuum. The filtered water samples
were then used for further chemical analysis.
Chemical analysis
Electric conductivity and pH values were measured
with a pH and electric conductivity meter (pH/cond
meter F-54; Horiba, Kyoto, Japan). Basic metals, iron
and aluminum content were determined using atomic
absorption spectrometry (Hitachi Z-5000, Hitachi Hi Tech.,
Tokyo, Japan). Chloride (Cl), nitrate (NO 3 ), phosphate
(PO 4 ), and sulfate (SO 4 ) ion content was measured by
ion chromatography (Compact IC-761; Methorhom,
Herisau, Switzerland). Ammonium (NH 4 ) ion content was
determined by colorimetric methods using phenylphenol–
nitroprusside regent (Rhine et al. 1998). Obtained data
for triplicate extracts were averaged for every analysis.
Total and inorganic carbon content in the soil solutions
was determined using a Shimadzu TOC-5000 analyzer
(Shimadzu, Kyoto, Japan). Dissolved organic carbon
(DOC) content was calculated by subtracting inorganic
carbon from total carbon. The Pearson product-moment
correlation between chemical parameters was evaluated
using Sigma Plot 11.0 (SYSTAT Software Inc., CA).
3. Results and Discussion
The periodically variable pH value (2.8–6.3) probably
changed as a result of agricultural water management.
Water collected in the cultivation period was strongly
acidic, while water pH was neutral in the fallow period
between September and October and between March and
April. The variation in EC values was reciprocal to the
pH values (Fig. 3). This variation in pH and EC indicates
that ionic substances were largely released into the canal
water under acidic conditions. The soil pH from each
horizon (Table 1) was around 3.0. Water extracts from airdried
soils were also strongly acidic (

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M. Kawahigashi et al. / Pedologist (2012) 458-465
Fig. 3 Changes in pH and EC in canal water
Fig. 5 Changes in concentration of Na and Cl in the canal
water
Fig. 4 Changes in concentration of Ca, Mg, and Al in the canal
water
Variations in Ca and Mg concentrations in the water
were similar (Fig. 4); these cations probably leached into
the water. Monovalent base cations showed different
trends to divalent cations (Fig. 5). Concentration of Na
and Cl showed a different trend to that of pH and EC.
Both Na and Cl concentration was high in the dry spring
season, indicating intrusion of sea water into the canal.
Potassium, showing a similar concentration trend to Na,
was a minor constituent in the canal water (Data not
shown). Al was also largely dissolved in the acidic canal
water. In particular, the Al concentration peaked in the
summer seasons (Fig. 5). The concentration observed in
early August was extremely high (265 ppm), enough to
be toxic to an aquatic biome. Dominant cations showed
trends in relation to changing pH and EC. Major cation
concentrations were relatively high in the strongly acidic
water compared with the neutral canal water. Higher
concentrations of acidic and basic metals were typically
due to the ionic composition of actual acid sulfate soils
(Hartikinen and Yli-halla, 1986).
Despite the high potential to release dissolved Fe from
the deeper soil horizons by water extraction (Kawahigashi
et al., 2008a, b), the concentrations of Fe was very low in
the canal water. Only the extremely acidic water (pH

M. Kawahigashi et al.: Land and water management to control solutes from acid sulfate soils
463
(Fig. 6) and because of high correlation coefficients
between the sulfate ion and these cations (Table 2).
The change in concentration of Cl relative to Na could
have been caused by sea salt. Despite the great amount
of phosphate application as a chemical fertilizer, the
phosphate concentration in the canal water was extremely
low. Fixation of phosphate ion onto active Al is a potential
means of controlling phosphate concentration. Phosphate
uptake by plants is another possible explanation of the low
concentration.
Ammonium compounds are a major ingredient of
chemical fertilizers in the studied paddy field. However,
variation in ammonium concentration does not seem to
correspond to fertilizer application or plowing (Fig. 7).
The ammonium concentration has a significant negative
correlation with pH (Table 2). There was a negligible
concentration of ammonium in the neutral canal water,
while a high concentration of ammonium was observed in
the acidic canal water. Usually, ammonium salt is easily
dissolved in water and is stable under acidic conditions.
About one month after inundation for rice seeding, the
ammonium concentration peaked. Acidic water from
the fallow field was probably flushed into the canal with
fertilizer-derived ammonium. Absorption of ammonium
by rice plants is also a possible route for decreasing the
ammonium concentration after the cultivation period.
Ammonium from applied fertilizer can be easily
oxidized to nitrite and nitrate in the fallow period by
nitrification bacteria under oxidative and neutral to
weakly acidic soil conditions. However, strongly acidic
soil conditions inhibit the nitrification process (Roelofs
1983). A large quantity of reduced sulfur and iron in soils
can consume soil oxigen, preventing ammonium oxidation
for the production of nitrates (Straub et al., 1996, Nielsen
and Nielsen 1998). Therefore, nitrate cannot be produced
via the nitrification process in acid sulfate soils (Fig. 7).
The dissolved carbon concentration is shown in Fig.
8. Although there was a significant positive correlation
between pH and DIC or DOC (Table 2), the variation of
both carbon concentrations did not correlate with that
of pH. A high concentration of both DOC and DIC was
observed twice in the research period when pH was
Fig. 6 Changes in concentration of SO 4 in the canal water
Fig. 7 Changes in concentration of NH 4 and NO 3 in the canal
water
Table 2 Correlation between pH, EC, and solute concentration. Statistical significance is indicated by *,**, and *** for p < 0.05, 0.01,
and 0.001, respectively.
SO 4
Ca Mg Al Fe NH 4
DOC DIC
pH -0.629** -0.629** -0.429 -0.459 -0.460 -0.684** 0.673** 0.783***
EC 0.894*** 0.939*** 0.799*** 0.797*** 0.765*** 0.729** -0.570* -0.424

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M. Kawahigashi et al. / Pedologist (2012) 458-465
Fig. 8 Changes in concentration of DOC and DIC in the canal
water
drainage of surface water in the ripening stage of rice.
The strongly acidic water contained a considerably high
concentration of Al, which probably affected aquatic biota.
However, canal waters could not immediately reflect soil
solution properties.
There seems to be an interval of a few months
between the soil solution and drainage of water into
the canal. Repetition of inundation and drainage for
rice cropping is the major controlling factor for solute
production and release from the soil into the water. It is
thus important to determine the time lag between the
initiation of soil management activities and the response
in the surrounding water system.
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