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) [9] . 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).

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.

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.

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 [10] . 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 [11] . 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 [12] . Available K of soil was determined after extraction with from NH₄OAc (pH 7.0) as described by Jackson [11] . The extract was then analyzed for available K by a flame emission spectrometer method at 589 nm by Jackson [11] . 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 [11] . After extraction with NH₄OAc (pH 7.0), available Ca and Mg were determined by titrimetric method as described by Jackson [11] . Organic carbon of the soil sample was estimated by the wet oxidation method of Black [12] . 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 [13] . 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.

The CMI was calculated using the physically fractionated carbon as in Equations (1)-(3) [15] ; on the basis of the Carbon Pool Index (CPI) and the Lability Index (LI) [Equation (1)]. Lability Index was computed from the Lability (L) associated with each soil, which in this case is the fraction of the labile C to non-labile C. The LI is a relative index that gives the fraction (FL) of labile carbon (LC) and non-labile carbon (NLC) from a cultivated field, as compared to a similar fraction (FR) at reference land use [Equation (2)]. This has been applied as a tool for assessing soil quality in terms of increments of total SOC, considering a shift of C to the labile pool as a result of agricultural practices. High CMI suggests high amounts of carbon associated to sand-sized fraction (labile component), thus high soil quality [15] [16] . The CPI compares total SOC (mg∙g⁻¹) from a cultivated soil (C_S) with total SOC [C_R (mg∙g⁻¹)] in a reference soil [CR, Equation (3)].

CMI = CPI × LI × 100 (1)

LI = FL / FR (2)

CPI = C S ( cultivatedsoil ) / C R ( referencesoil ) (3)

With SPSS software version 23 for windows, soil properties of different sites of cultivated soil were analyzed using Analysis of Variance (ANOVA). The ANOVA test also used for significant differences of C and N associated with each of the particle-size fractions. Mean values of soil properties and C and N concentrations in the particle size fraction were compared using the Fisher’s Protected Least Significant of Difference (LSD) at 5% level of significance.

Soil fertility heterogeneity was evident in all fields after stratification by SOC concentration limits, although soil property EC was not significantly different. All other properties such as pH, total SOC, total N, available P, available K⁺ available Na⁺, Ca²⁺ and Mg²⁺, CEC were significantly different across the fields (Table 2). Irrespective of soil fertility status, extractable P was far below the critical concentration of 15 µg∙g⁻¹ [17] .

Routine soil tests in bulk samples indicate that pH, EC, available P, available K⁺ varied little among fields. This is a consequence of inheritance properties of saline soil. In costal soil, soil salinity is usually combined with high pH conditions, due to the presence and enrichment of calcium carbonate in the uppermost soil layers (alkaline saline soils) or to hydrolysis of sodium carbonate (sodic soils). In these regions, shallow water tables and improper irrigation practices often accelerate salinization [19] .

Monovalent cations such as Na⁺ play a very important role in deflocculating clay particles and consequently increasing the tendency to slake, disperse and swell under specific conditions, so that soils become impermeable and ultimately infiltration and percolation of water cease from the upper surface and within the soils [20] [21] . High salt levels often cause poor physical and chemical soil characteristics including low aggregate stability, impervious subsoil layers, low infiltration rates, low hydraulic conductivities, and soil surface crusting [22] [23] .

The data presented in this study express important insights on SOC and N dynamics in saline soil. However, the significant difference of SOC among fields was used to identify most influential SOC fractions in N supply and soil fertility. SOC is widely known for its positive influence on aggregate stability, cation exchange and water holding capacity, infiltration, percolation and consequently on soil productivity [8] .

Clay-sized fractions have shown high SOC and N concentrations, when compared with the sand-sized-fraction in both cultivated and uncultivated soils. The observed trends of SOC in particle-size fractions agree with Lehmann et al. [24] , who found high SOC concentrations in both clay- and silt-sized fractions. The clay fraction is often able to absorb plant-derived aromatics or microbial products [18] across a disparate range of chemical functions and electrical charges. In consequence, the clay is often the richer fraction in SOC, especially in soils with moderate clay contents such as those observed in this study. In addition, some studies in sub-tropical soils assert a direct relationship between SOC and clay contents [8] . However, in the present work, soils have almost the same textural class, which suggests that SOC variability was driven by human-induced practices such as cultivation and use of organic amendments. In fact, base cations such as Na⁺, Ca²⁺, Mg²⁺ cultivated than in uncultivated soils, which can be ascribed to some organic fertilizer applications, although this can hardly occur with enhanced SOC depletion due to aggregate disruption by tillage [25] .

The SOC concentrations in particle-size fractions are, however, less indicative of SOC retention than the relative pools. The dominant role of the clay-sized fraction in total SOC retention in cultivated soils is shown in Figure 1 and it is clear that SOC bound to silt and especially clay was lost. Similar trends were noted in the 15 - 30 cm depth, although the silt-sized fraction was more important than in the topsoil. Feller et al. [26] , who noted that most SOC lost with cultivation of clayey soils the losses occurred in the <2 μm size class, with no effect of soil mineralogy. Feller et al. [26] also reported that after adoption of fallow, SOC gains occur in the sand- and clay-size pools in clayey soils. The fact that considerable amounts of SOC are stored in the clay sized fraction explains why bulk SOC and clay contents are related [18] . It is noteworthy that most bars in Figure 1 do not reach 100% due to the variable recovery of the total SOC after the particle-sized fractionation, and the non-recovered SOC can be safely ascribed to the clay fraction as proposed by Zinn et al. [18] . The data in the present study supports the stated that SOC and N in silt- and clay-sized fractions are more important to overall soil fertility than the sand-sized fraction. In addition, there is support which stated that the distribution of SOC across particle-size fractions is influenced by cultivation, since the clay-size SOC pool is relatively lower in uncultivated soils.

All cultivated soils had a CMI (Carbon Management Index) < 100%, implying lower amounts of labile C (sand-sized C) and thus a likely poor SOC quality. Organic material applications by farmers in high fertility cultivated soils could not boost the sand-sized C (labile C) to be higher than what was registered in uncultivated soil. This showed that cultivation is a key factor resulting in carbon breakdown and loss; thus, changing to minimal tillage approaches may be attractive for C sequestration [16] . However, the lability indices provide a good basis to detect the changes in active pools of cultivated soils, and this emerged crucial for monitoring short-to medium-term N supply, that would otherwise be difficult using total SOC concentrations and the CPI (Carbon Pool Index). In the present study, potential N limitations can be associated with low N supply from low carbon concentrations associated with sand-sized/labile pool, with or without cultivation. In addition, the cultivated soils in low input systems often lack sufficient residue incorporation, which is critical since this is a key entrance pool for building sand-sized SOC pools. The ratio of labile C to non-labile C (lability) further reflected the aforementioned challenge, and this ratio was far less than 50%, clearly demonstrating the dominance of the non-labile C, and the likely less effectiveness of the active pool in N cycling. In order to improve this labile N pool, judicious use of organic amendments such as cattle manure and adoption of soil conservation practices such as minimal cultivation must be adopted, in accord with the Integrated Soil Fertility Management approach [27] .

The low SOC and N concentrations in the sand-sized fractions, as well as the small relative pools in this size somehow contrast with the often-assumed important role this fraction exerts on N-cycling in the tropics [28] . The computed C/N ratio of the sand fraction in top soil was higher (10 to 14) than in the clay-sized fraction (Table 4), which is coherent with the fact that this fraction is composed of free particulate organic matter. The increase in C/N ratio in silt-sized fraction in the lower soil layer (15 - 30 cm) suggests impacts of cultivation, increasing the distribution of C that does not easily mineralize [29] . In the top soil layer, the high C/N occurs because, in contrast with the clay-sized organic fraction, there is no possibility of forming mineral organic complex in the sand size that could stabilize organic matter [7] . Besides, the low C/N ratio in the clay fraction indicates the high humification degree of constituent organic carbon [7] . Diekow et al. [16] described the sand fraction as the most sensitive pool to soil management, and indeed our data clearly show that this pool was preferentially depleted in cultivated soils, a tendency that has been observed in tropical climate [16] . In consequence, continuous cultivation in the smallholder farming systems can easily disrupt this labile C component, resulting in overall decline in soil quality.

It was also evident that the clay-sized fraction is even more important for the retention of N than for SOC. This can perhaps be related to the mechanisms through which SOC is absorbed by clay minerals, chiefly kaolinite and oxides. Similar results were found in different soils in different parts of the world [30] [31] . The low N concentrations in the silt and especially the sand fractions could be attributed to its mineralization; and the little N released can be easily lost due to the high precipitation patterns in the region, even in the uncultivated soils [32] . This trend suggests that the clay-sized fraction is the main N pool in soils and perhaps the main source of N to plants in low-input, cultivated soils. Consequently, clay-sized SOC can be the fraction most related with chemical fertility, notwithstanding some assertions that this would be a rather recalcitrant or passive pool, unlikely to affect soil productivity within certain agro-ecological zones [33] . Further studies are needed to explore N release mechanisms from the silt and clay fractions to further improve the understanding of N retention and dynamics in the costal saline soils.