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Influence of Brackish water aquaculture on Soil Salinisation

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Brackish water aquaculture has been debated severely for its environmental consequences. Mushrooming of such farms on reclaimed mangrove lands and converted agricultural land leads to salinisation of agricultural fields of vicinity. In this study around Bhitarkanika National Park of Orissa, significant correlation between aquaculture farming and high soil salinity has been found. The impact was predominant within a distance of 500 m of the aquaculture pond. The villages having higher aquaculture density were also reported to have tendency of high soil salinity, unsuitable especially for production of paddy-the only crop of the region. Considering other environmental conditions the findings strongly suggest that aquaculture farms play predominant roles in soil salinisation.
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Mitra et al. Int. J. Res. Chem. Environ. Vol. 1 Issue 2 Oct. 2011(166-168)
166
International Journal of Research in Chemistry and Environment
Vol. 1 Issue 2 Oct. 2011(166-168)
ISSN 2248-9649
Research Paper
Influence of Brackish water aquaculture on Soil Salinisation
1
Mitra Rajarshi*,
2
Santra S.C.
1
Department of Environmental Science, Vivekananda College, Thakurpukur, Kolkata (W.B), INDIA.
2
Department of Environmental Science, University of Kalyani, Nadia, W. Bengal , INDIA.
Available online at: www.ijrce.org
(Received 10
th
September 2011, Accepted 20
th
September 2011)
Abstract-Brackish water aquaculture has been debated severely for its environmental
consequences. Mushrooming of such farms on reclaimed mangrove lands and converted
agricultural land leads to salinisation of agricultural fields of vicinity. In this study around
Bhitarkanika National Park of Orissa, significant correlation between aquaculture farming and
high soil salinity has been found. The impact was predominant within a distance of 500 m of the
aquaculture pond. The villages having higher aquaculture density were also reported to have
tendency of high soil salinity, unsuitable especially for production of paddy the only crop of the
region. Considering other environmental conditions the findings strongly suggest that aquaculture
farms play predominant roles in soil salinisation.
Key words:
Bhitarkanika, Brackishwater Aquaculture, Soil salinity.etc.
Introduction
The coastal agricultural lands or soils are
exposed to some specific competitive interactions in
terms of land conversion, which often directly impact
the soil quality. Reclamation of mangroves or swampy
areas adds on the areal extent of agricultural soil. On
the other hand conversion of agricultural lands to
brackish water fisheries and/or shrimp aquaculture has
been a major threat to the coastal agriculture in tropical
countries. Rice farms are favoured site for conversion
to shrimp ponds because they pose several
characteristics well suited for aquaculture
[1]
.
It is generally accepted that the extent of salt affected
soil in coastal areas of tropics is increasing. Salinity
reduces osmotic potential and limits water uptake by
plants. Salinity may also cause specific ion toxicity or
upset the nutrient balance in the soil
[2]
. However, in
our study area of coastal Orissa, in and around
Bhitrakanika Wildlife Sanctuary, both the mangrove
and agricultural lands remain in close vicinity, as in
most of the cases, agricultural lands have been
reclaimed of the mangroves
[3]
. Such kind of reclaimed
lands suffer spatial and temporal variability in soil
salinity and acidity posing threat to the agriculture
[4]
.
The problem seems to be pronounced due to lack of
irrigation facilities.
Mushrooming aquaculture farms intermittently situated
with agricultural lands have been claimed to be another
major cause of soil salinization in the area. Seepage of
saltwater into the adjacent agricultural lands from
aquaculture ponds is well documented and sometimes
it makes cultivations impossible
[5 – 7]
. The seepage
may take place through saltwater leakage, aquaculture
pond overflow and leaching from sludge pile during
rainfall
[8]
. Indirect soil salinization can occur as a
result of irrigation of rice field and orchards from
freshwater canal contaminated with the aquaculture
farm effluent
[9]
.
In the study area the chances of soil salinisation due to
overtopping of river water is ruled out as no such
occurrence was reported by the local people and the
earthen embankments were found to be mostly intact
[10]
. Hence, a predominant role of seepage in soil
salinisation should not be overlooked.
Khan et al. (2000) showed direct correlations between
soil salinity and distance of aquaculture
[7]
. The salinity
and sodicity of soil were found directly proportionate
to the distance from the sea and aquaculture ponds
[11]
.
A buffer of 60 m around aquaculture was suggested to
protect such salinization of soil
[7]
. Taking all those
facts under consideration, the effects of brackish water
aquaculture on adjacent agricultural fields have been
assessed for coastal areas of Bhitarkanika.
Methodology
Salinisation of soil at coastal areas may have
several factors involved. Hence, for assessment of
particular impact of brackish water aquaculture one
Mitra et al. Int. J. Res. Chem. Environ. Vol. 1 Issue 2 Oct. 2011(166-168)
167
needs to be very specific. Seepage being the major
cause of salinisation, more salinity is expected near
aquaculture. With this consideration, the soil salinity
and distance of the sampling point from any
aquaculture have been regressed.
The sampling of agricultural soil was done at 50
sampling points spread over the area and their distance
from nearest aquaculture farm was recorded. However,
one of the sampling was discarded afterwards and kept
away from the final analysis as a few sampling snag
were found with that.
The number of aquaculture farms present in the village
under study was also recorded for further estimation of
farm density. The farm density had been considered as
a factor of general soil health of the region and
regressed over the salinity distribution data. Soil
salinity was estimated in the soil collected from a
depth of 15 cm. The salinity of soil was expressed as
Electrical Conductivity of 1:5 soil extract (EC
e
) in dS
m
-1
. Based on previous experiments
[12, 13]
the soils
were classified as normal (salinity <1.9 dS m
-1
), saline
soil (salinity ranges between 1.9 dS m
-1
and 3.4 dS m
-1
)
and highly saline (salinity > 3.4 dS m
-1
).
Results and Discussion
The salinity or EC
e
of agricultural soil in
villages around Bhitarkanika ranges between 0.275 and
6.18 dS m
-1
with 76% of the samples represented the
saline soil regime. Among those 49% (of the total) fall
under the high salinity zones i.e. electrical conductivity
is greater than 3.4 dS m
-1
. However, based on the
agricultural land salinity threshold (3dS m
-1
) suggested
by FAO (1999)
[14]
55% of the sampled agricultural
soils were found to be non-suitable for paddy
cultivation.
Villages at and around Bhitarkanika National Park are
delimited by earthen embankments, which have been
found mostly intact throughout. Infrequent flooding
however is reported in a few places without much
significance. It is however presumed that the current
trend of land conversion to brackish water aquaculture
may have a strong hold on soil salinization process.
Introduction of aquaculture farms within the sanctuary
area require saline water transport to the aquaculture
ponds through feeder channels. These, in turn, are the
carrier of irrigation water for the agricultural fields of
the area. Not only the irrigated water but also the salt
water seepage into the field makes the soil saline even
resulting salt encrustations at times. The soil salinity of
the non-agricultural and non-mangrove lands of the
area varied between 2.305 and 3.50 dS m
-1
, which
included soils from different areas of community and
infrastructural utility
[15]
. In contrast, soils from
mangrove forest or degraded mangrove vegetation area
showed salinity between 6.24 and 11.925 dS m
-1
.
Presence of aquaculture in vicinity was correlated with
soil salinity in agricultural fields, as most of the
brackish water shrimp aquaculture farms have been
established in expense of agricultural lands and are
situated intermittent within those.
Significant correlations (p < 0.001) are found among
the soil salinity and distance of sampling point from
the aquaculture farms. However, salinisaing trend up to
a distance of 500 m from aquaculture was noticed.
Software generated salinity zonation suggests that the
high salinity zones are situated mostly at the areas with
higher density of aquaculture farms. While all the
villages with more than 5 aquaculture farms per square
kilometre have been found to fall in the high salinity
zones, on the other hand only 6.25 per cent villages
with an aquaculture density more than 3 per square
kilometre represented low saline soil (Table 1). With
reducing aquaculture density the percentage of high
salinity villages
decreased and subsequently the same
for low salinity villages enhanced (Figure 1).
Conclusion
Salinisation is a problem of the coastal area
due to rapid land conversion practices. The finding
clearly indicates that introduction and storage of saline
water in the aquaculture farm influence salinity of soil
in nearby agricultural fields. Although salt content may
not be the sole factor of soil infertility, but it has
bearing on the productivity. The extent of soil salinity
at vicinity of the aquaculture farms in the study area
shows a similar result obtained for low salinity shrimp
culture, but even this level of soil salinity may render
the soil unsuitable for production
[16]
. In the study area
94 times increase in aqua farming was reported after
1989, most of which are intermingled within the
agricultural fields
[10]
. Finally, inland shrimp farming
represents a situation where significant short-term
economic benefits may be obtained, but at the risk of
creating long-term cumulative environmental impacts.
The particular area of coastal Orissa is facing an
economical crunch due to strict forest conservation
practices promoting economic instability
[17]
. The
tendency of people for land conversion to saline aqua
farming practices seems making the existing coastal
agriculture further vulnerable.
Acknowledgement
The authors are thankful to Prof. Sugata
Hazra of Jadavpur University, for his humble help
during the study.
References
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Table 1 Relationship among aquaculture density and soil salinity.
Soil salinity Zone
Percentage of Villages
(with different aquaculture farm density)
Aquaculture Farm density (km
-
2
)
0 1 >1 >2 >3 >4 >5
High salinity
(>3.4dS m
-1
) 59.9%
69.38%
68.96%
68.75%
77.78%
100%
Moderate salinity
(1.9 to 3.4dS m
-1
) 22.72%
18.36%
20.69%
25%
22.22%
0%
Low salinity
(<1.9dS m
-1
) 18.18%
12.24%
10.34%
6.25%
0%
0%
Figure 1: The proportion of villages in different salinity zones on the basis of aquaculture
density per square kilometre.
0
20
40
60
80
100
120
Min 1
>1
>2
>3
>4
>5
% of villages falls under salinity zones
Aquaculture density No./sq.kms
High salinity zone (>3.4 dS/m) Moderate salinity zone(1.9-3.4 dS/m) Low salinity zone (< 1.9 dS/m)
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In the mangrove environment of West Africa, high spatial and temporal variability of soil constraints (salinity and acidity) to rice production is a problem for the transfer and adoption of new agronomic techniques, for land use planning, and for soil and water management. Recently, several National and International Agricultural Centers have initiated research programs to characterize environments where their newly developed technologies have to be applied. However, the mangrove agro- ecosystems in West Africa have not been characterized in a detailed way. Most of the soils in this environment are potential or actual saline acid sulfate soils. The spatial and temporal variability of soil salinity and acidity in these coastal lowlands results from complex interactions between climate, coastal morphology, river hydrology, vegetation, landform and tidal flooding. Diagnosing the occurrence of both potential and actual acid sulfate soils is the first step in land use planning for such areas. But to cope with the intricacies of these soils, understanding the processes of soil salinization and acidification at different scales should be formalized to properly characterize mangrove environments.The main objectives of this thesis were: 1) to give a comprehensive characterization framework for the West African mangrove environments with emphasis on the possibilities of and constraints for rice cultivation; 2) to determine the various causal factors for soil salinization and acidification; 3) to test whether temporal variability of soil chemistry is sufficient to provide a time window of minimum stress during the rice growing period; 4) to relate the response of rice to improved agronomic practices in specific environments and to provide a means to characterize specific rice growing locations, and 5) to test rice varietal responses to saline and acid sods under different agronomic practices and to relate yields and yield components to the nutrient contents in leaves, in order to diagnose physiological disorders.First, a multi-scale approach was developed involving a range from Macro to Micro level based on the pre-conditions of acid sulfate soil formation. The main factors for classification are climate and coastal morphology at Macro scale; hydrology, physiography and vegetation complexes at Meso level; and topography (catena), vegetation species, tidal flooding and sedimentation rate at Micro level. Information from previous process-based studies on acid sulfate soil formation and data from secondary sources were used. Different environments were then distinguished and their characteristics were summarized by ecological zone. Constraints to rice production and potentials for agricultural development were matched with environmental conservation issues.To determine the significance of the causal factors developed in the multiple scale approach, 12 sites were selected along 4 river basins in West Africa, vz. from north to south the Gambia, the Casamance (Senegal), the Geba (Guinea Bissau) and the Great Scarcies (Sierra Leone). Along each river basin 3 sites were selected based on distance from the river mouth. Within a site a strip of land perpendicular to the river was selected for intensive grid sampling (40 by 20 m). Soil samples were taken at each grid node during the dry season of 1991. The relation between causal factors and soil salinization and acidification was determined at Macro and Meso levels by nested ANOVA and yielded a classification of the study area in main ecoregions and sub-environments within watershed. At a detailed scale, geostatistics were applied and zones within catena were defined in terms of their main soil characteristics. A nested statistical approach and geostatistics were used complementarily to disentangle the complexity of the causes of soil salinization and acidification.Temporal variability was studied by monitoring soil solution chemistry at each main landscape unit within the catena. Since the production of rice critically depends on the lowering of salinity and acidity by natural flooding during rainy season, time windows during which soil limitations are minimal were defined and matched with rice varietal duration.The response of rice to different improvement techniques were tested by means of a network of trials in the 1991 and 1992 rainy seasons. The residual effects of lime and phosphate rock (applied in 1991) during 1992 was also evaluated. Lime dressing (2 t ha <sup>-1</SUP>) was found effective whenever dissolved Ca and Mg in the soil were low, and had a clear residual effect in the year after application. Application of phosphate rock did not seem to be effective in general. For iron toxicity, the molar fraction of Fe and (Ca + Mg) in soil solution and in flag leaves were found to be more relevant for diagnosing physiological disorders than the absolute Fe content in the soil solution and in rice flag leaves at panicle initiation.In the 1993 rainy season, rice varietal behavior under different improvement techniques within the main soil limitations in the mangrove environment was tested. Differences in yield and yield components and element contents in flag leaves at panicle initiation were observed between varieties in saline and less saline acid soils. Multiple correlation between rice yields, yield components, element contents in flag leaves at panicle initiation was found to be an effective diagnostic tool for assessing physiological disorders.The approach used in this study provides a logical framework to describe mangrove environments. The multiple-scale can assist in identifying the information required to cater for the needs of various decision-makers and land use planners. It also provides a key to develop technology packages for intensified and sustainable use. It can be used for the extrapolation of site-specific information to geographically different areas, with similar characteristics.
Bhitarkanika: Myth and Reality
  • S Chadha
  • C S Kar
Chadha, S., and Kar, C.S., Bhitarkanika: Myth and Reality; Natraj Publishers, (1988)