Comparison of soil organic carbon and carbon density between two forest types in Bhutan


Bachelor Thesis, 2020

23 Pages


Excerpt

Table of contents

Introduction
Background
Problem statement
Research objectives

Literature Review
Soil Carbon sequestration
Soil Carbon stocks in different slope and aspect
Soil organic carbon stock along the altitudinal gradient
Vertical variation of carbon stock
Influence of succession of soil organic carbon storage
Soil Bulk density
Method for measuring Carbon
Loss on Ignition method
Walkley-Black method
Non-Destructive Techniques

Materials and Methods
Study area
Soil sampling procedure
Determination of soil organic matter and organic carbon
Determination of soil bulk density
3.2.3. Calculation of Carbon Density
Statistical data analysis

Results and Discussion
Checking Normality of data
Soil bulk density of different forest types
Soil organic carbon concentration in two types of forest
Organic carbon density at different aspect
Organic carbon at different slope
Soil organic carbon and carbon density along altitudinal gradient
Variation of soil organic carbon density with depth

Conclusion

References

Abstract - The soil being one of the carbon pools hosts the largest terrestrial carbon and plays a crucial role in the global carbon balance by regulating the exchange of greenhouse gases with the atmosphere. The Earth is facing increased pressure from emission of Greenhouse gases which leads to change in natural climate system from good to worse. Bhutan is considered as carbon neutral country and the country have l arger mass of forest soil which have huge potential for carbon absorption than to release into atmosphere which would otherwise attributed to Global warming. This study investigated the total Soil Organic Carbon (SOC) density and its distribution under two different forest type considering the change in carbon with elevation in two different depth of the soil in Nahi Gewog under Wangdue Dzongkhag. The soil sample destined for carbon analysis were collected at two different soil depth and soil organic carbon was measured by loss on ignition method. Different types of statistical test such as Kruskal Wallis, Independent sample t test and Pearson correlation tests were performed to compute the result. The result showed that the Pearson correlation test for elevation and bulk density was significant (P < 0.0, r = -0.83); Pearson correlation for organic carbon and elevation (P < 0.05, r = 0.494); Pearson correlation for slope and organic carbon was significant (p < 0.05, r = 0.334). The independent sample t test between aspect and organic carbon was t(38) = -0.05, (P > 0.05, r = 0.033). The Kruskal Wallis test between carbon density and depth of the soil show significant difference between the variables (P < 0.05). The organic carbon density was found to be more in the first 0-25cm depth of the soil.

Key words: Aspect, Carbon density, Elevation, Slope, Soil depth, Soil organic carbon.

Introduction

Background

Carbon is considered as building block of life and is present in every living being. Anything that act as a storage house for large amount of carbon is termed as carbon pool, or sometimes called as carbon reservoir (Izaurralde et al., 2000). There are five carbon pools namely, above-ground biomass, belowground biomass, litter, dead wood and soil carbon that need to be monitored and reported as part of greenhouse gas inventories (Makaipaa et al., 2012). The soil being one of the carbon pools hosts the largest terrestrial carbon and plays a crucial role in the global carbon balance by regulating the exchange of greenhouse gases with the atmosphere. It is said that, soil stores about 1,500 Gt of Soil organic carbon (SOC) in the upper one-metre depth (FAO, 2001), of which according to Schlesinger (1997), is twice the amount of Carbon in the atmosphere and thrice in the terrestrial vegetation.

Soil carbon content is the result of the net balance between carbon inputs and outputs. These biologically regulated fluxes mainly depend on primary production and organic matter decomposition (Pauses, 2007). Therefore, Soil can be source or sink of atmospheric carbon and the amount of carbon contained in the soil varies with change in elevation, forest types and slope aspect depending on the land use types, climate, management practices, and CO2 level in atmosphere (Jobbagy & Jackson, 2000; Kirschbaum, 2000). SOC plays an important role in exchange of CO2 between atmosphere and biosphere and hence, a minor change in SOC stock could result in a significant change in atmospheric CO2 level (Stockmann et al., 2013).

The Earth is facing increased pressure from emission of Greenhouse gases which leads to change in natural climate system from good to worse. Anthropogenic greenhouse gas emissions have increased since the pre-industrial era, driven largely by economic and population growth, and are now higher than ever (Intergovernmental Panel on Climate Change (IPCC, 2014). The atmospheric concentration of CO2 has increased from 280 ppm in 1750 to in 1999 and is currently increasing at the rate of 1.5 ppm/year (IPCC, 2001). Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. Limiting climate change would require substantial and sustained reductions in greenhouse gas emissions which, together with adaptation, can limit climate change risks (IPCC, 2014). Soil Carbon sequestration according to Bhandari and Bam (2013), is considered as one of the mitigating strategies of climate change, if sequestration of atmospheric CO2 in the soil, terrestrial biomass, geologic formation, and ocean is increased.

Bhutan is considered as one of carbon sink country. National environment commission’s (NEC) Climate Change division head Thinley Namgyel (2015), during COP21 said, “Bhutan today emits 2.2 million tonnes of carbon dioxide equivalent against the sequestration by forests, which is about 6.3 million tonnes of carbon dioxide.” And in 2011, National Environmental Commission has estimates that Bhutan emits about 1.5 million tonnes of Carbon annually and absorbed 6.3 million tonnes of Carbon annually which means that unlike other countries, Bhutan serves as a carbon sink rather than as a source for atmospheric CO2. During the fifteenth Conference of Parties of United Nations Framework Convention on Climate Change (UNFCCC, 2009), Bhutan has committed to remain Carbon neutral. Further, during the 21st Conference of Parties at Paris, Bhutan committed not only to be Carbon neutral, but to remain Carbon negative.

Problem statement

Bhutan is considered as carbon neutral country mainly due to larger areas we have for carbon sink. Larger mass of forest soil has huge potential for carbon absorption than to release into atmosphere which would attribute to Global warming. According to FAO (2015), during the carbon cycle, the carbon gets exchange among the atmosphere, ocean, terrestrial biosphere and geological deposits. Carbon sequestration occurs when carbon from the atmosphere is absorbed and stored in the soil. This is an important function because the more carbon that is stored in the soil, the less carbon dioxide there will be in the atmosphere thereby mitigating climate change. In order to enhance Carbon sequestration, a proper understanding about Carbon dynamics is crucial, which is not adequately studied in Bhutan and in the Himalayan region at large. This is largely attributed to limited studies done in the region. As such, complex mechanisms and processes regulating Carbon sequestration in the soil are least studied (Phuntsho, 2016). Till date no study on carbon density along different elevation, slope and aspect have done in Bhutan apart from few studies done on aboveground Carbon stock and sequestration (Dorji, 2015). Therefore, the study assesses the relationship between carbon density in a soil along different elevation, in two forest types and different slope aspect considering soil depth up to 50cm in Nahi Gewog and this paper will also serve as baseline data for further research on soil carbon density in future.

Research objectives

The study investigated the soil organic carbon and carbon denity variations along the altitudinal gradients at different topographic aspects in the Nahi gewog with an objectives below:

1. To investigate the relationship between organic carbon content in the soil at different slope aspect and elevation from in Nahi gewog.
2. Compare soil organic carbon and carbon density between two forest type (Chirpine forest and mixed broad leaf forest) at 0-25cm and 25-50cm soil depth.

Literature Review

Soil Carbon sequestration

Soil Carbon sequestration is the process of capturing and storing atmospheric CO2 into soil through crop residues and organic solids in an ephemeral form (Izaurralde et al., 2000), or it is a transferring of atmospheric CO2 into long-lived pools and storing it securely so it is not immediately reemitted. Soil is crucial in the dynamics of carbon pool cycle because it links atmosphere, vegetation, and oceanic pool (Agriculture and Rural Development (ARD), 2012). Soil is the biggest reservoir of terrestrial Carbon (Lal and Kimble, 2000) which is about 1,500-2,000 PgC for the top one metre (Batjes 1996). According to ARD, (2012), the soil C pool is 3.3 times the size of the atmospheric pool (760 Gt) and 4.5 times the size of biotic pool (560 Gt). Soil Carbon sequestration is an important measure to combat climate change by reducing CO2 concentration in the atmosphere.

Soil Carbon stocks in different slope and aspect

SOC stock is different in different aspect slope. According to Maggi (2005), very steep slope areas contain little vegetation cover compared to low slope areas. This means that there is less organic carbon in a soil as low carbon is associated with less organic matter and less vegetation would mean less decomposition. According to Barthes (2002), steep slope increase potential of material to move down and cause erosion and this idea was also supported by Castillo-Santiago (2003), soils that are found in steep slope are more vulnerable to erosion.

Aspect can have a strong influence on temperature. This is because aspect affects the angle of the sun rays. Due to the position of sun radiation, the seasonal cycle of climate differs between N and S facing slopes and between steep and gentle slopes (Bryn 1948 and Bayat 2011). In the study conducted by Yohannes, Soromessa, and Argaw (2015), reported that higher values of carbon stocks were found on North aspect and mostly South aspect is hotter and dryer. North facing slope has less sunlight and in turn has increased moisture levels and greater tree vegetation growth and cover resulting in more organic matter. The probability for high soil carbon stock in East aspect might be due to climatic condition, litter fall accumulation, rate of decomposition and density of stems. Griffiths, Madritch, & Swanson (2009), found that high litter fall biomass carbon in East and North-West aspect and lower amount was in South facing. This might be related to the presence of tree species that produce more leaf foliage, favorable temperature and precipitation and low disturbances. West and South facing slopes are generally warmer and East and North facing slope has lower temperature. This could have an effect on organic matter turnover, photosynthesis and species composition. Since decomposition depends on temperature and moisture it results in variation of soil organic carbon stock.

Soil organic carbon stock along the altitudinal gradient

Altitude is known to have greater impacts on diversity, biomass, and carbon stock in forest ecosystems (Alves et al., 2010). Soil organic carbon stock trend along the altitudinal gradients is mostly controlled by temperature and precipitation. As pointed out by Jobbagy and Jackson (2000), soil organic carbon density increases with precipitation and decreases with temperature. These changes in climatic factors along the altitudinal gradient influence vegetation composition, consequently affecting the quantity and turnover of soil organic matter (Quideau et al., 2001). Therefore, increasing trend of soil organic carbon stock with altitude was reported in forest land (Wolde et al., 2014). Ganuza and Almendros (2003), studied that the total soil organic carbon increased with elevation which is due to its closeness to human settlements and disturbances in the lower altitude and this clearly shows that rate of soil carbon is maximum in higher altitude where disturbance to soil were less. However, Avilés et al. (2009), found that total soil organic carbon from forest soils decreased with elevation in a toposequence in Mexico due to variations in the organic matter decomposition rate, and similar finding was documented by Lozano and Parras (2014), in a traditional Mediterranean olive grove which was due to erosion. Parras et al. (2004), explained that the difference in soil organic carbon was mainly due to high soil erosion rates, caused by high erosivity of rainfall, high erosionability, steep slopes, low vegetation cover and the lack of conservation practices in the area.

Vertical variation of carbon stock

Globally, Jobbagy and Jackson (2000), described vertical distributions of soil organic carbon for different ecosystems and demonstrated that vegetation type determined vertical distributions of SOC through its root-shoot allocation and root distributions along soil profile. According to Gill et al. (1999), the vertical patterns of soil organic carbon are determined by a dynamic balance between carbon inputs from plant production and outputs through microbial decomposition. As a major source of carbon inputs in soil, vertical distributions of roots play an important role in shaping vertical distributions of soil organic carbon (Jobbagy, & Jackson, 2000). Soil organic matter appears to be concentrated in the first 25 cm, where the mineralization and immobilization Carbon processes are slightly active (Alcantara, Garcia, & Espejo, 2015). In addition to that, Weaver et al., (1935),has reported the depth-dependent decomposition rates as an another potential mechanism for explaining vertical patterns of soil organic carbon because a higher proportion of total root biomass occurred in surface soil than that of deeper soil profile. In general, Sevgi and Tecimen (2009), concluded that the soil organic carbon decreases with the increase in depth of soil.

Influence of succession of soil organic carbon storage

The soil organic carbon storage in the soil is affected by various factors like land use management practices, climatic conditions, vegetation, organisms, and parental rocks present in the soil. Land use types and vegetation cover affects the carbon dynamics by influencing soil respiration, Carbon flux, and Carbon fixation within the soil and substrate (Batjes, 1996). In agriculture land, types of land management practices like dry land, wetland, use of farmyard manure and nitrogen fertilizers had greater impact on soil organic carbon stock and eventually on carbon sequestration. Sitaula et al. (2004), reported that intensive usage of mineral fertilizers in agricultural field would reduce soil carbon, though it will increase crop productivity in short run. Practice of non-tillage practices will enhance soil organic carbon in agricultural field. Overall, the soil organic carbon storage in the soil can be increased by practicing sustainable land management practices. As the predominant source of soil organic carbon, the vegetation cover was regarded as important factor on the variation of soil organic carbon in the soil. Different plant species shows significant difference in carbon input in the soil (Dar & Sundarapandian, 2013). Tree species could potentially cause bridging effect between soil organic carbon and cation chemistry of soil, thereby alter carbon decomposition and affect soil organic carbon stock. Moreover, types of vegetation cover would alter the rate of leaf litter deposition, temperature, moisture, and soil microbial, which changes the amount of soil organic carbon stock in the soil.

Soil Bulk density

Soil bulk density describes the spatial arrangement of the solid particles that compose soil matrix, providing an indication of basic soil quality index (Chan, 2005). As a key state variable, bulk density provides valuable information relating to porosity, compaction, and penetration resistance of soil (Horn et al., 2003). In addition to physical and biological roles, it is also used to convert soil organic carbon and other nutrients form content into stock at any specified depth. However, the measurement technique used may have dramatic implications for calculating carbon mass in soils (Throop et al., 2012).

Bulk density is not an intrinsic soil property but depends on external conditions, with changes associated with a variety of factors and with various natural and anthropogenic processes (Zeng et al., 2013). It can also change as a consequence of root growth, rainfall or normal traffic (Drewry, 2006). Both wetting drying and freezing-thawing cycles after tillage may also cause the bulk density to increase because of natural soil reconsolidation (Assouline, 2011).

Method for measuring Carbon

Both destructive and non-destructive techniques are available for the determination of total organic carbon in soils. The destructive techniques are by far the most common techniques in use today and generally involve some form of sample preparation followed by sample extraction and quantitation (Tiessen & Moir, 1993). An innovative nondestructive technique using non-elastic neutron scattering is also being developed for total organic carbon determination (Wielopolski et al., 2000).

Carbon content in the soil can be determined either by dry combustion, loss on ignition (LOI), or by Walkley-Black method as described in bellow.

Loss on Ignition method

According to Santisteban et al., (2004); Wang et al., (2012), Loss on Ignition method measures soil carbon and to estimate organic matter, carbonates, and mineral content in sediments. It is simple and easy use in laboratory (Beaudoin, 2003; Boyle, 2004). Loss on ignition method requires muffle furnace to compute soil organic matter. The method is based on differential thermal effects of the soil samples. Loss on ignition measures the weight loss by the soil sample after drying the soil under 360–450 °C (Goldin, 1987), 550 °C (Santisteban et al., 2004), for couple of hours. Then the loss on ignition calculates organic matter (OM percent) by measuring the difference between weight before and after drying of soil samples (Robertson, 2011).

Despite being widely used, LOI method has various biases. According to Goldins (1987), ignition volatizes the soil components other than organic matters like water in crystalline clay lattices, hydroxyl groups, and carbonates. Santisteban et al. (2004), stated that this method is strongly influenced by the amount of sediment present. The method is also rarely used in less fertile soil (Wang et al., 2012), despite its higher accuracy in estimating soil inorganic Carbon (Wang et al., 2011). With higher variability and less precision in analysis, the method is crude for routine soil analysis (Jackson, 1958; Goldins, 1987). Moreover, the method results in error like sample spillage and incomplete drying (Hoskins, 2002).

Walkley-Black method

Walkley-Black is a wet chemistry method done using Potassium dichromate (Donovan, 2013). This method is as accurate as Elementary method except for soils with low organic Carbon. Further, Wang et al. (2011), supports that, soil sample shows 100 % recovery of soil organic carbon in arid soils with external heating. It is cheap, easy method compared to Elementary method, but may exhibit the recovery variables, therefore, requires correction factor to determine soil Carbon (Phuntsho, 2016).

Non-Destructive Techniques

Non-destructive techniques do not rely on quantification of organic matter as a proxy for soil organic carbon but can provide direct estimations without the extraction of organic matter. Nuclear Magnetic Resonance spectroscopy had, until the development of in-situ techniques, been the most popular non-destructive method (Preston et al., 1994). NMR spectroscopy distinguishes the chemical structures that are characteristic of newly formed organic matter in samples to give an idea of the amount of organic soil carbon (Schumann, 2002). Advancements in the field of soil organic carbon measurement have led to the development of Inelastic Neutron Scattering (INS) which can provide non-invasive soil organic carbon measurement in situ (Johnston et al.¸ 2000). INS works on the principle of gamma-ray spectroscopy which can process information on multiple elements at once (Wielopolski et al., 2005). This technique is still under development, though marks important progress for the field as it will allow quantification and monitoring of soil carbon pools (Wielopolski et al., 2005).

Materials and Methods

Study area

The study was undertaken in two different forest types (Chirpine forest and mixed broadleaved forest) of Nahi Geowg under Wangduephodrang Dzongkhag located in central parts of Bhutan with (27030’0”N, 90007’59” E). The total area of Wangduephodrang dzongkhag is 4,038 km[2] ha with an elevation ranging from 800 to 5800 masl. The mean annual precipitation in the Dzongkhag is 42.77mm influenced by south-west monsoon which accounts for 90% of the annual precipitation. The mean annual temperature is 21°C with minimum of 15°C during the cooler dry season and a maximum of 27°C during the warm rainy season (RGOB, 2014).

Soil sampling procedure

The study area was stratified into two sub areas, one Chirpine forest and another mixed broadleaf forest. The soil sample was collected from both the forest along altitudinal gradient of Pangsho Goenpa in Nahi gewog. The soil samples destined for carbon analysis was taken with an auger at 80 points at 0-25cm and 25-50cm soil depth. In the sampling areas, topographic variables (slope, aspect and altitude) was recorded by Clinometer and GPS. On collecting soil samples, eroded knolls, depression, saline areas, fence lines, old roadways and yards, channels, manure piles, and field edges were avoided. The systematic sampling procedure was used to collect the soil sample. A transect line of 2000m in length was laid across each forest type and the quadrant of 10m by 10m was laid. The samples were taken from four corners of the quadrant and from the middle of the quadrant making it one composite sample. The sampling of soils was done at interval of 100m along the transect line, giving a total of 20 composite sample per forest cove type and a total of 40 composite samples.

Colleting of soil sample was initiated from November month. Prior to soil carbon analyses, all samples were air-dried at room temperature, crushed and passed through a 2 mm soil sieve to discard coarse particles. The sample was then taken to laboratory for analysis.

Determination of soil organic matter and organic carbon

There is various method soil carbon can be determined. In this particular study, Loss on Ignition Method was used to calculate the Organic matter and organic carbon of soil. The loss-on-ignition method for the determination of organic matter involves the heated destruction of all organic matter in the soil. A known weight of sample is placed in a ceramic crucible or similar vessel which is then heated to 105[0]C overnight and then it was weighted. The sample is also then kept at oven overnight which have 500[0]C and then weighted after cooling in a desiccator.

The organic matter contents was the calculated by using formula;

Abbildung in dieser Leseprobe nicht enthalten

Where, W1 is a weight of crucible plus dry soil, W2, weight of crucible and oven dry soil and W3, weight of muffer furnance and dry soil.

In order to determine soil organic carbon (SOC) content from organic matter it is necessary to apply a conversion factor (Schumacher, 2002). Though this value varies depending on soil features, the traditional 1.724 ‘Van Bemmelen’ conversion factor, that assumes SOM contains 58% organic carbon, was used as it is a standardised approximation used in several studies (Burt, 2004).

Abbildung in dieser Leseprobe nicht enthalten

Determination of soil bulk density

Analysis of soil bulk density was calculated following the methodology described by Cresswell and Hamilton (2002). Oven-proof container was first weighed before carefully pushing out the trimmed soil cores into it. The oven-proof container and the soil was again weighted and the weight was recorded. The same procedure was repeated for all the soil core samples before oven drying it in a well-ventilated oven at 105°C for 48 hours until the weights of the soil were constant. The container with the soils was removed from the oven and cooled in the desiccator before weighing the container and the oven dried soil. Soil bulk density (g cm-[3]) was calculated as depicted in equation 4 below;

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Details

Title
Comparison of soil organic carbon and carbon density between two forest types in Bhutan
College
Royal University of Bhutan  (College of Natural Resources)
Course
Environment and Climate studies
Author
Year
2020
Pages
23
Catalog Number
V520406
ISBN (eBook)
9783346126122
ISBN (Book)
9783346126139
Language
English
Tags
comparison, bhutan, carbon density, aspect, slope, elevation, soil organic carbon
Quote paper
Thukjey Nidup (Author), 2020, Comparison of soil organic carbon and carbon density between two forest types in Bhutan, Munich, GRIN Verlag, https://www.grin.com/document/520406

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