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Soil Organic Fractions in Cultivated and Uncultivated Soils of Costal Area in Bangladesh

Ipsita Sarkar
Soil, Water and Environment Discipline, Khulna University, Khulna, Bangladesh.

Md. Zulfikar Khan*
Soil, Water and Environment Discipline, Khulna University, Khulna, Bangladesh.

Md. Hanif
Soil, Water and Environment Discipline, Khulna University, Khulna, Bangladesh.

Abstract/ Executive Summary:

Assessment of soil organic matter fractions can be instrumental in understanding the causes of limited nitrogen supply, and thus soil fertility restoration. A study was conducted in cultivated and uncultivated saline soil, in order to assay soil organic carbon (SOC), its particle-size fractions and their influence on cultivation and soil fertility at Sundarbans costal area in Bangladesh. Soil samples were taken from the 0 - 15 and 15 - 30 cm depths from four cultivated fields and from four nearby sites in a native mangrove forest as references. Soil samples were physically fractionated into sand (2000-50 μm), silt (50-2 μm) and clay (<2 μm). Total SOC and N were analyzed in bulk samples and each size fraction, and the Carbon Management Index (CMI), a widely used indicator of soil quality, was calculated for each field. The CMI in cultivated soils was far below the 100% in reference soils, reaching 38.16%, 25.70%, 32.21% and 34.43% in Field 1, Field 2, Field 3 and Field 4 respectively. SOC and N concentrations decreased in particle size separates in the order clay > silt > sand. The SOC pool and N in the clay-sized fraction were correlated to soil fertility indicators. More N was stored in the silt + clay size fractions, a generally more stable pool, than in the more labile sand-sized pool. The SOC pool in sand size fractions was far below in cultivated soils than in a reference uncultivated soil. Thus, the sand-sized pool emerged as the most likely cause of limited N supply in cultivated soils.

Keywords: Particle Size Fraction, Soil Organic Carbon, Nitrogen, Saline Soil

Location: Joymany ghol and nearby Sundarbans area of Mongla Upozilla, Bagerhat district

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Soil salinity, Soil Physical Properties

Objectives:

The main objectives of the research work were: 1) to assess which soil fraction contains the highest level of SOC, 2) to identify the SOC fractions most influential in N supply and soil fertility and 3) To understand how these fractions are affected by cultivation in saline soil.

Methodology:

2.1. Study Area

The study was undertaken at Joymany ghol and nearby Sundarbans area of Mongla Upozilla, Bagerhat district which is situated beside the Posur River and directly connected to Bay of Bangle. This study was carried out to investigate the present situation of organic content with salinity in the forest and nearby cultivated areas. This area is selected because about 53% of the coastal areas of Bangladesh are affected by salinity. Agricultural land use in these areas is very poor, which is much lower than country’s average cropping intensity. Salinity causes unfavorable environment and hydrological situation that restrict the normal crop production throughout the year. The factors which contribute significantly to the development of saline soil are tidal flooding during the wet season (from June to October), direct inundation by saline water, and upward or lateral movement of saline ground water during the dry season (from November to May). According to the farmers most soils of this area had been under cultivation for more than 15 years. But most of the land is now fallow since Aila* (Severe Cyclonic Storm Aila was the worst natural disaster to affect Bangladesh since Cyclone Sidr in November 2007. The second tropical cyclone of the 2009 North Indian Ocean cyclone season, Aila formed over the Bay of Bengal on May 23. A relatively strong tropical cyclone, it caused extensive damage in Bangladesh).

2.2. Collection of Soil Samples

The soil samples were collected from four different agricultural field and natural sites. The natural forest sites were bench-marked as reference sites. From each selected site, soil samples were taken from 0 - 15 cm (topsoil) and 15 - 30 cm (sub soil) depths, using an auger. The sampling sites along with the cropping pattern were shown in Table 1.

2.3. Preparation of Soil for Chemical Analysis

The collected soil samples were air dried by spreading on separate sheet of papers after it was transported to the laboratory. After drying in air, the larger aggregates were broken gently by crushing it by a wooden hammer. A portion of the crushed soils was passed through a 2.0 mm sieve. The sieved soils were then preserved in plastic bags and labeled properly. These were later used for various chemical analyses. The chemical analyses of these collected soil samples were carried out in the laboratory of the Soil Science Discipline, Khulna University.

2.4. Analytical Procedure for Soil Analysis

Measurements of selected physical and chemical properties of the soils were carried out in triplicate according to conventional methods. Electrical conductivity (EC) of soil was estimated by EC meter (Jackson EC meter). The ratio with of soil and water was 1:5 and the result was converted to the ratio of 1:1 (soil: water) as suggested by USDA. pH value of the soil samples was measured by Jackson pH meter maintaining the ratio of soil to water was 1:2.5 as suggested by Jackson. The CEC of the soils were determined by extracting the soil with ammonium acetate solution (NH₄OAc at pH 7) along with replacing the ammonium in the exchange complex by 2 M KCL solution. The displaced ammonium was distilled in for CEC determination in a way similar to the detection of available ammonium nitrogen. Available K of soil was determined after extraction with from NH₄OAc (pH 7.0) as described by Jackson. The extract was then analyzed for available K by a flame emission spectrometer method at 589 nm by Jackson. Available Phosphorus was extracted from the soil with 0.5 M NaHCO₃ (Olsen’s Method) at pH 8.5 after extraction Molybdophosphoric blue colour method was employed for determination of phosphorus. After extraction with NH₄OAc (pH 7.0), available Ca and Mg were determined by titrimetric method as described by Jackson. Organic carbon of the soil sample was estimated by the wet oxidation method of Black. The underlying principle of this method is to oxidize the organic matter with an excess of K₂Cr₂O₇ in presence of concentrated H₂SO₄ and to titrate the residual K₂Cr₂O₇ solution with 0.5 M FeSO₄ by Jackson. Total nitrogen content of soil samples was determined by micro-Kjeldahl method as described by Jackson [11] through the digestion of organic matter with concentrated H₂SO₄ acid, H₂O₂ and catalyst mixture of potassium sulphate (K₂SO₄), copper sulphate (CuSO₄) and selenium powder in the ratio of 10:1:1. After completion of digestion, 40% NaOH was added with digestion mixture for distillation. The distillate was received in 2% boric acid (H₃BO₃) solution with 4 drops of mixed indicator of bromocresol green (C₂₁H₁₄O₅Br₄S) and methyl red (C₁₀H₁₀N₃O₃) solution. Finally, the distillate was titrated with standard H₂SO₄ (0.01 N) until the color changed from green to pink.

Source of Information: Journal of Agricultural Chemistry and Environment, 8, 129-144.

Article Link (Online Journal): doi: 10.4236/jacen.2019.83011.

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

This study has been conducted to identify the soil fraction containing highest level of soil organic carbon along with influential effect in Nitrogen supply and soil fertility as well as to understand how these fractions are affected by cultivation in saline soil. In this study indicate that the soil organic carbon and nitrogen were significantly decreased after cultivation within the cultivated fields and cultivation decreases the organic matter in soils. It was also observed that silt-sized and clay-sized fraction contained more ^OC that sand fractions and these two fractions were significantly different within the cultivable fields. In both cultivated and uncultivated soils, SOC concentrations in the clay-sized fractions were at least fourth times as high as that of sand-sized fractions and at least three times for N. The C/N ratio in clay-sized fraction of cultivated soils was on average 9 to 6, lower than what was registered in sand-sized fraction (10 to 14). The C/N ratio was 7 - 10 in the silt-sized fraction. The clay-sized fraction was the largest SOC pool in cultivated soils, ranging between 46.27% - 56.63% in both depths. However, in the uncultivated soils, the clay-sized SOC pool was generally smaller with slightly more total SOC stored in the sand fraction. In the lower depth, there was a general increase in the importance of the silt-sized SOC pool. In addition, the clay-sized fraction holds most of the total SOC and N in cultivated soils, and is probably the most important size fraction affecting soil fertility in general. On the contrary, low SOC and N concentrations in the sand-sized fraction can be a common cause of N limitations in cultivated and even uncultivated saline soils. Practices that preserve the clay- and silt-sized C organic fraction while increasing SOC pools in the sand size, are the best options to enhance N supply and crop productivity in the studied soils.

Knowledge Management: Journal