The carbon budget of planet earth is regulated by the soil compartment in all types of ecosystems. We conducted a first order analysis of soc in November 2017 both in the mangrove dominated Indian Sundarbans and the highly urbanized city of Kolkata with the aim of identifying the natural and anthropogenic contributions of organic carbon in soil. We also attempted to analyze the spatial variation of soc between these two significantly different ecosystems. We observed a comparatively higher mean value of soc in Kolkata (2.06%) than in the Sundarbans (1.25%). The significant spatial variation in soc between Kolkata and the Sundarbans (p < 0.05) may be attributed to anthropogenic stress, which is of greater magnitude in the city of Kolkata. The significant spatial variation in soc between north and south Kolkata (p < 0.05) is due to the efficiency of the drainage system in the north and the magnitude of city limit expansion in the south. In the Sundarban deltaic complex, a natural phenomenon like erosion seems to be a determining factor in the domain of soil carbon dynamics. soc analyses of all major metropolises around the world, of which Kolkata is one, are essential to understand the carbon sequestration potential of urban soils.
Soil is the largest terrestrial pool of organic carbon globally. Research related to soil organic carbon (soc) has grown worldwide, motivated by the potential of the soil to become a manageable sink for atmospheric carbon dioxide and thus to mitigate climate change, and the known benefits of increased soc for the functioning of soils (McBratney et al., 2014). Estimates of soc on a global scale vary between 700 and 3000 Pg C (Bouwman, 1990). Size and changes in the soc pool present major uncertainties for global earth system models used for climate predictions. An accurate estimation of soc stocks is vital for understanding the links between atmospheric and terrestrial carbon (Friedlingstein et al., 2014). Land use patterns, cultivation, crop selection, and other anthropogenic activities could all affect the dynamics of the soc pool. The effects of changing land use on soil carbon stocks are of concern in the context of international policy agendas on greenhouse gas emission mitigation. Estimating and reporting greenhouse gas emissions from anthropogenic sources and removals by sinks has become an important activity of Parties to the United Nations Framework Convention on Climate Change (unfccc) in implementing their commitment under the Convention. Understanding regional distributions of soc is key to stimulating and predicting the influence of global climate change and human activity on the terrestrial carbon cycle.
In this arena of scientific study mangrove forests have been overlooked, mangrove forests develop along the coasts of most major oceans in 118 countries adding between 30 and 35% to the global area of tropical wetland forest over peat swamps alone (fao, 2007; Giri et al., 2011). Mangroves are one of the most threatened and fragile ecosystems in the world with vast potential to absorb and store carbon. The mangroves and the tidal mudflats associated with the forests play a major role in carbon sequestration. Many of the mangrove forests lie close to industrialized and urbanized cities and towns, and often act as the filter zone for the municipal and sewage waste discharged from metropolitan cities. About half of the world’s population now lives in urban areas, which promise a higher quality of life. Many of these urban centers are expanding rapidly, leading to the growth of megacities, which are defined as metropolitan areas with populations exceeding 10 million inhabitants. These concentrations of people and activity are exerting increasing stress on the natural environment, with impacts at urban, regional, and global levels (Molina and Molina, 2004).
In this paper we compare the soc of the Kolkata metropolis with the adjacent Sundarbans mangrove forest with the aim of identifying the role of natural and anthropogenic factors on the soc budget of the ecosystem. Soil samples from 24 stations (12 in the city of Kolkata and 12 in the mangrove dominated Indian Sundarbans) were collected during November 2017 and analyzed for organic carbon percentage. With the data sets of the present study, an attempt has been made to pinpoint an explanation for varying soc doses in two significantly different ecosystems (in terms of natural and anthropogenic factors) situated in the Gangetic plain of West Bengal. These two ecosystems are the highly urbanized city of Kolkata and mangrove dominated deltaic Sundarbans.
The sampling sites of the current research project have been selected primarily on barren areas where there is little or no vegetation cover. This was taken as a precaution in order to maintain uniformity in the sampling design as vegetation plays a major role in contributing soc through litters and detritus, and the overall organic decomposition process is also highly variable as a function of litter biomass. Therefore, in order to avoid the noise created by vegetation in the soc matrix, barren landscapes were selected as sampling sites for the soc matrix. Hence, the role of vegetation does not affect the present case study.
2 Material and Methods
2.1 The Study Area
The metropolitan city of Kolkata (22030’N and 22037’N latitude and 88018’E and 88023’E longitude) with an area of 187.33 square kilometers, and an expanse of 1380.12 square kilometers sustains a total population of approximately 4.5 million. The metropolitan area, including suburbs, has a population of approximately 14.2 million, making it the third most populous metropolitan area in India and the 13th most populous urban area in the world. Kolkata has a subtropical climate with summer monsoons. The annual mean air temperature is approximately 26°C. The summers are hot and humid with temperatures varying in the range of 30’s during dry spells and a maximum temperature that often exceeds 400C (1040 F) during May and June. Winter tends to last for only about two and a half months, with seasonal lows dipping to 90 C to 110 C (48.20 F- 51.80 F) between December and February. The region receives an annual rainfall of about 1,582 mm and most of it in the monsoonal period (August and September). We conducted surveys at 12 stations in this highly urbanized city (Table 1 and Fig. 1). The sampling sites in Kolkata were selected on the basis of the magnitude of anthropogenic influence, as judged by municipal activity. Kolkata has witnessed phase-wise development during pre- and post-independence periods. The northern part of the city is old and highly urbanized, primarily unplanned, lacking modern infrastructure and open spaces. The southern part is well planned with broader roads, large avenues, and open green spaces. The anthropogenic disturbances of recent times have been constituted by the replacing of palatial personal residences with apartments and housing societies.
The Sundarban mangrove ecosystem (between 21013’N and 22040’N latitude and 88003’E and 89007’E longitude) covers about one million hectares across the deltaic complex of the Rivers Ganga, Brahmaputra and Meghna, and is shared between Bangladesh (62%) and India (38%), constituting the world’s largest coastal wetland. Enormous amounts of sediment carried by the rivers contribute to its expansion dynamics. The ecosystem borders Bangladesh in the east and the Hooghly River (a continuation of the River Ganga) in the west. The Dampier and Hodges line is in the north, and the Bay of Bengal is to the south. The important geomorphology of the deltaic Sundarbans includes beaches, mudflats, coastal dunes, sand flats, estuaries, creeks, inlets and mangrove swamps (Chaudhuri and Choudhury, 1994). The air temperature is moderate due to its proximity to the Bay of Bengal in the south. Average annual maximum air temperature is around 350C. The summer (pre-monsoon) extends from the middle of March to mid-June, and the winter (post-monsoon) from mid-November to February. The monsoon usually sets in around the middle of June and lasts up to the middle of October. Rough weather with frequent cyclonic depressions occurs between mid-March and mid-September. Average annual rainfall is around 1920 mm. Average humidity is about 82% and is more or less uniform throughout the year. With regards to the flora, 34 true mangrove species and 62 mangrove associate species have been documented in the Indian Sundarbans (Mitra, 2000). As in the Kolkata study, 12 stations were selected in this deltaic complex to identify the soc status in the intertidal mudflats (Table 2 and Fig. 2). Soils from intertidal mudflats of Indian Sundarbans were analyzed from three different sectors (western sector, eastern sector and central sectors) during December 2017. We observed significant spatial variations in soc, which may be due to natural and anthropogenic factors. The natural factors include erosion due to tidal effects, wave actions, and current pattern among others. The anthropogenic factors are related to human activities like the dumping of waste materials and deforestation (Pal, 2018).
2.2 Sampling and Analysis
Sampling plots of 10 m × 5 m were considered for each station. Stratified random sampling of soils was carried out with 15 soil samples collected at each station. The large sample size was managed by effective homogenization. One of the most common methods used to obtain a homogeneous sample is through grinding and sieving the soil to a desired particle-size (generally <2 mm) followed by random sampling. This method assumes that the initial material is ground without preference to any given factor, such as color, and that the sample becomes sufficiently homogenized during grinding and sieving. To further eliminate possible heterogeneity within the sample and to reduce the sample size to the desired quantity for a given analysis, subsamples were obtained by “spooning” (Carver, 1961). Shovel digging was done to collect the samples from sampling sites both in Kolkata city and the Sundarbans delta.
During sampling in the Sundarbans delta, care was taken to collect the samples within the same distance from the estuarine edge, tidal creeks, and the same micro-topography. Under such conditions, spatial variability of external parameters such as tidal amplitude and frequency of inundation (Ovalle et al., 1990), input of material from the adjacent bay/estuary and soil granulometry and salinity (Lacerda et al., 1993; Tanizaki, 1994) were minimal. In the Sundarbans, collection was done during low tide. Both in the Sundarbans and in Kolkata the uppermost centimeter of the soil layer, which frequently includes debris and freshly fallen litter as well as dumped garbage, was discarded. In the laboratory, the collected samples were carefully sieved and homogenized to remove roots and other plant and animal debris prior to oven-drying them to constant weight at 1050 C. Total organic carbon was analyzed by a standard method (Walkley and Black, 1934).
2.3 Statistical Analysis
The spatial difference of soc was evaluated through anova between the city of Kolkata and the deltaic complex of the Indian Sundarbans. Moreover, the spatial variation was evaluated between the stations of north and south Kolkata. In the deltaic Sundarbans many of the stations are stressed by anthropogenic factors and climatic disturbances while some stations are within the reserved forest area. Cyclonic storms such as Aila have also played a major role in changing the ecosystem dynamics of the Sundarbans in recent years. Finally, we also evaluated the spatial variation of soc between these two zones in the Sundarbans. All statistical calculations were performed with spss 9.0 for Windows.
3 Results and Discussion
The reservoirs of soc can act as sources or sinks of atmospheric carbon dioxide depending on land use practices, climate, texture, and topography (Vesterdal et al., 2002; Zinn et al., 2005; Homann et al., 2004; Lal and Shukla, 2005). In the highly urbanized city, the land is exposed to intense human activity such as traffic, construction and garbage dumping from markets, household, hospitals and more. The study by the International Council for Local Environmental Initiatives (South Asia), supported by the British High Commission in India, revealed that residential houses are responsible for more than one-fourth of carbon dioxide emission, and commercial establishments in Kolkata and together with industrial units and vehicles they contribute close to three-quarters of the city’s emissions.1 The city of Kolkata, located on the eastern banks of the River Hooghly and approximately 120 kilometers from the Bay of Bengal, is noted for its rapid growth in urbanization. The population has increased from 9,194,018 in 1981 to 11,021,918 in 1991, and finally to 13,216,546 in 2001. This yields a growth rate of 1.72 between 1981 and 1991, and 1.82 between 1991 and 2001.2 The waste generated in the city reaches the central sector of the Indian Sundarbans through Dry Weather Flow (dwf) and Storm Weather Flow (swf) canals. The present study reveals that Kolkata city has a higher average soc (2.06%) compared to the Indian Sundarbans (1.25%).
In Kolkata, the trend is Stn. 11 (3.25%) > Stn. 5 (3.10%) = Stn. 9 (3.10%) > Stn. 10 (2.87%) >Stn. 8 (2.47%) > Stn. 7 (2.32%) > Stn. 12 (1.93%) > Stn. 6 (1.78%) > Stn. 2 (1.26%) > Stn. 3 (1.23%) > Stn. 4 (0.97%) > Stn. 1 (0.48%) as seen in Figure 1.
The overall trend shows comparatively more soc in the southern part of the city (p = 0.02; Table 3). The sampling stations in Kolkata have been selected on the basis of magnitude of anthropogenic pressures, which are higher in the southern part of the city (for example Stn. 11, Stn. 5, Stn. 9, Stn. 10, Stn. 8, Stn. 7) compared to stations of the northern part of the city (for example Stn. 1, Stn. 4, Stn. 3, Stn. 2, Stn. 6). The sampling stations are urban hotspots and thus selected to assess the anthropogenic stress caused by major infrastructural developments such as concrete roads, bridges, shopping malls, roadside extensions, business complexes, and huge apartment spaces. This may be attributed to the influx of large populations from neighboring countries as well as neighboring states after the Indian partition in 1947, and further of displaced persons in search of livelihood and social securities. The southern part of the city is still in a growing stage unlike the north. A lack of proper drainage facilities and municipal waste disposal systems unlike the northern part of the city has led to the gradual accumulation of organic waste on the topsoil. Moreover, the Tolly Nallah (Nallah is a local word for canal), which receives the garbage situated in south of Kolkata has been choked over the course of the last few decades due to massive siltation and absence of dredging. The development of high raised concrete platforms over the canal soil bed for metro railways has further added to the problem. This resulted in the gradual accumulation of organic matter and subsequently organic carbon in the surface soil. The northern portion of Kolkata city is much older than the southern part that flourished in the 18th century. Here, the drainage system is well developed, which carries the sewage and rain water and runoff through the Bagjola khal and Kestopur canal that finally drain into the dwf canal. The accumulation of organic matter is therefore low in the soil. The present townscape of Kolkata is predominantly developed over landfill sites and the soil thus historically faced major relocations and disturbances. The construction of metro railways in the city in the 1970s and 1980s has also contributed heavily to the soil disturbances across the city resulting in the creation of new spaces with landfills dug out from underground railway channels. Spatio-temporal analysis of different land use types in Kolkata is urgently needed to understand the spatial distribution of the city over a considerable time period.
In the Sundarbans, the trend is Stn. 4 (1.63%) > Stn. 5 (1.55%) > Stn. 1 (1.54%) > Stn. 6 (1.48%) >Stn. 7 (1.43%) > Stn. 10 (1.32%) > Stn. 2 (1.31%) > Stn. 12 (1.15%) > Stn. 3 (1.04%) > Stn. 8 (0.91%) > Stn. 9 (0.83%) > Stn. 11 (0.81%) as observed in Figure 2.
In this ecosystem, more soc is observed in those stations where anthropogenic activities like fish landing, fish processing, loading and unloading of cargo and passengers occur. Stations 4 and 5 are the areas where fish landing and processing take place. These areas also sustain busy markets with poor drainage systems. Hence maximum organic matter is deposited into the soil. Station 1 harbors a busy passenger jetty and is the only way to reach the biggest island of the Sundarbans, Sagar Island. This station also lacks a proper drainage system and garbage disposal site. Mangroves are found in patches in these stations which also contribute to soc. Stations 6 to 12 are located within the protected forest area where all human activities are prohibited. The soc in these stations therefore arises from the decomposed litter of mangroves. However, station 12, Jharkhali, is a tourist spot and the entry point to Sundarban Forest. Excessive tourism-related activities in this station particularly in the post-monsoon season have resulted in a high organic carbon load in the intertidal mudflat of this station. The dominant mangrove floral species in these stations are Avicennia spp., Excoecaria agallocha and Sonneratia apetala. It is interesting to note that although station 3 harbors pilgrims, a busy market and a dense population along with mangroves, the soc is low (1.04%). This station is an erosion prone zone because of its location at the confluence of Bay of Bengal where the average tidal amplitude is around 3.5 m. The continuous tidal action washes away the top soil of this station resulting in a low soc value. The significant spatial variation of soc (p < 0.001) may be attributed to a large extent to mangrove diversity, anthropogenic activity, accretion, and erosion processes.
soc is a good indicator of biological and anthropogenic activities. It also reflects the magnitude of natural phenomena like erosion, accretion, and microbial activities. In the deltaic Sundarbans, soc is a balance between input of surface litter (fallen leaves and dead organisms) and the rate at which microbes break down organic compounds. Leaching through erosion also reduces the soc in the upper layer of intertidal mudflats in the Sundarbans. However, in the highly urbanized city of Kolkata, the input is replaced by anthropogenic activities like the dumping of garbage from domestic, municipal and hospital wastes. Our study reveals that there is significant variation in soc between the soil of Kolkata and that of the Sundarbans (p = 0.0273; Table 3).
Energy and Carbon Emissions Profiles of 53 South Asian Cities, prepared on the basis of figures for 2007–08, point out that Kolkata accounts for a carbon dioxide emission equivalent to 9.33 million tons annually, while the per capita annual emission has been estimated at 1.83 tons.3 The Kolkata soil can be a potential sink for these considerable carbon dioxide emissions. Kolkata was geographically a tidal mudflat, an extension of the Sundarbans’ tidal mangrove ecosystem and lowlands. Both Kolkata and the Sundarbans are predominantly located in the lower Gangetic delta. Thus, the basic natural characteristic of the soil is similar in most characteristics. This paper hence points to the fact that urban cities and townscapes have the potentialities to store carbon and act as a potential sink, thus resulting in carbon sequestration and climate change mitigation.
The financial support (Grant Number moes/11-mrdf/1/34/P/08) provided by Ministry of Earth Science, Government of India is gratefully acknowledged. The authors are grateful to Dr. Kakoli Banerjee for assisting in collecting data from the remote islands of Sundarbans. The authors are also thankful to Mr. Abhinav Mehta for his technical assistance in preparation of maps of the sampling stations.
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