# Assessment of Soil Organic Carbon in Tropical Agroforests in the Churiya Range of Makawanpur, Nepal

Authors: Lilu Kumari Magar; Gandhiv Kafle; Pradeep Aryal
Journal: International Journal of Forestry Research
 (2020)

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## Abstract

This paper reports the findings of a research study conducted in three tropical agroforestry systems in the Makawanpur district of Nepal, to quantify the spatial and vertical distribution of soil organic carbon in 30 cm soil profile depth in agrisilviculture, home garden, and silvopasture. The three agroforestry systems represent tropical agroforests of Nepal. It was found that the soil had 24.91 t/ha soil organic carbon in 30 cm soil profile in 2018, with 2.1% soil organic matter concentration in average. Bulk density was found increasing with an increase in soil depth. The soil organic carbon was not found significantly different across different agroforestry systems. Looking into the values of stocks of soil organic carbon, it is concluded that the tropical agroforests have played a role in global climate change mitigation by storing considerable amounts of soil organic carbon and the storage capacity can further be increased. Involvement of farmers in the management of tropical agroforests cannot be ignored in the process of climate change mitigation.


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## Body

## 1. Introduction

The interaction between climate change and the global carbon cycle is an important aspect of the global environment changes [1]. Soil is the largest pool of terrestrial organic carbon in the biosphere, storing more carbon than contained in plants and the atmosphere combined [2] and a relatively stable pool of various organic and inorganic carbon fractions. The soil organic carbon stock has a unique function in mitigating climate change as a key component of the biosphere carbon cycle. Changes in soil organic stock can have a considerable effect on atmospheric carbon dioxide concentration, contributing to global warming. There is a worldwide consensus that climate change is a spreading threat in this century. Nepal is among the most vulnerable countries to climate change [3]. Trees on the farm have a vital role in mitigating the effects of climate change. Both soils and trees on the farmlands are significant media for carbon storage [4]. The area currently under agroforestry worldwide is 1,023 million hectares [5]. A recent assessment by the Government of Nepal shows 40.36% forest and 4.38% other woodlands of the total area of the country. The other wooded land represents the trees on farmland and fallow land with trees [6].The land use of Nepal is changing day by day [7]. The implementation of agroforestry is less costly and more effective than other approaches [8]. Agroforestry is an agroecological praxis that contributes to the sustainable intensification of food production while providing a number of additional benefits to society. Agroforestry can make significant contributions to provision of soil-mediated ecosystem services in the humid and subhumid tropics [9]. Agroforestry systems have indirect positive effects on carbon sequestration because they reduce harvesting pressure on natural forests. Incorporating trees and shrubs in food crop systems help in addressing food insecurity, increasing carbon dioxide sequestration, and reducing the vulnerability of agricultural systems. Agroecosystems also contribute to the mitigation of climate change and are being an adaptation strategy for the farmers [10]. Agroforestry is one of the strategies of REDD+ to reduce carbon emissions, enhance forest carbon stocks, and improve the supply of forest products by promoting agroforestry [11]. The potential of agroforestry systems to accumulate carbon is estimated to be 0.29–15.21 Mg ha−1 year−1 [12]. Field data on soil carbon pools are needed for the estimation of sources and sinks of greenhouse gases at the national level to be reported to the UNFCCC, to predict the carbon sequestration potential of the nation’s forest resources and to promote nutrient balance for enhanced productivity of the ecosystems. In this context, this research was conducted to investigate how soil organic carbon in the soil profile varied under different agroforestry systems in the Churiya Range, Makawanpur, Nepal.


## 2. Materials and Methods

### 2.1. Research Site

The research was carried out in Laisiran and Bangdirang of Raksirang Rural Municipality of Makawanpur District in the midhill region of Nepal (Figure1). Makawanpur district lies between 27°21′ to 27°40′ N latitude and 84°41′ to 84°35′E longitude and is 34 km south of Kathmandu. The meteorological data from Hetauda and Manahari stations of Makawanpur district during 1985–2015 periods (31 years) show that the mean annual temperature for thirty-one years is found to be 23.0°C. The rainfall shows the decreasing trend of total annual precipitation in Hetauda, Makawanpur, at the rate of 5.6607 mm per year. Annual rainfall fluctuates regularly in last 31 years. The highest rainfall recorded is about 3323.1 mm in the year 2002 and the least amount rainfall, i.e., 1626.2 mm, was recorded in the year 2012. Even though it fluctuated regularly throughout 31 years, the rainfall has decreased a bit [13]. Three major agroforestry systems: agrisilviculture system, home garden, and silvopasture system, were selected for data collection. Nepalese soils are most often developed on micaceous parent materials such as phyllites, schists, quartzite, and granites, and the texture is normally loam and sandy loam in the hilly region [14]. The textural class of agricultural soil of Makawanpur district ranges from sandy loam to silt loam [15].Figure 1
Map showing the agroforestry system-applied area in Raksirang Rural Municipality, Makawanpur, Nepal.


### 2.2. Data Collection

Ten pits were randomly dug in three major agroforestry systems: agrisilviculture system, home garden, and silvopasture system. Soil samples were collected from 0 to 10 cm, 11 to 20 cm, and 21 to 30 cm from each pit using a cylindrical soil corer of 10 cm length and 3.45 cm radius. So, a total of 30 soil samples were collected from each agroforestry system, with total 120 soil samples in all agroforestry systems. One additional soil sample (undisturbed soil sample) from each soil depth was collected using the soil core sampler to determine the bulk density. Each sample was bagged, labelled, and transported to laboratory for analysis. The soil sample collection and laboratory analysis processes were performed as in [16].


### 2.3. Laboratory Analysis of Soil Samples

Soil bulk density was determined using the soil core sampler having a diameter of 5.7 cm [17]. The soil organic carbon concentration was determined by dry combustion of oven-dry soil samples at 900°C [18]. The percentage of soil organic matter was converted to soil organic carbon by multiplying with 0.58, as soil organic matter is assumed to contain 58% of organic carbon. Total nitrogen (TN) was determined by the Kjeldahl digestion-distillation method [19]. Soil pH of a composite sample (of all soil layers and soil pits) from each agroforest was determined using a pH probe with a glass-calomel electrode (Suntronics pH electrode, Suntronics, India), keeping 1 : 1 soil : water ratio [20]:Bulk density of soil = oven-dry weight of soil in grams/volume of soil in cubic centimetres.Soil organic carbon in ton per hectare = organic carbon content (%) × soil bulk density × depth of soil layer.The significanct difference of soil organic carbon was tested using ANOVA at a 5% level of significance. SPSS software (IBM SPSS Statistics, IBM Corporation, version 20) was used for statistical analysis of the data.


## 2.1. Research Site

The research was carried out in Laisiran and Bangdirang of Raksirang Rural Municipality of Makawanpur District in the midhill region of Nepal (Figure1). Makawanpur district lies between 27°21′ to 27°40′ N latitude and 84°41′ to 84°35′E longitude and is 34 km south of Kathmandu. The meteorological data from Hetauda and Manahari stations of Makawanpur district during 1985–2015 periods (31 years) show that the mean annual temperature for thirty-one years is found to be 23.0°C. The rainfall shows the decreasing trend of total annual precipitation in Hetauda, Makawanpur, at the rate of 5.6607 mm per year. Annual rainfall fluctuates regularly in last 31 years. The highest rainfall recorded is about 3323.1 mm in the year 2002 and the least amount rainfall, i.e., 1626.2 mm, was recorded in the year 2012. Even though it fluctuated regularly throughout 31 years, the rainfall has decreased a bit [13]. Three major agroforestry systems: agrisilviculture system, home garden, and silvopasture system, were selected for data collection. Nepalese soils are most often developed on micaceous parent materials such as phyllites, schists, quartzite, and granites, and the texture is normally loam and sandy loam in the hilly region [14]. The textural class of agricultural soil of Makawanpur district ranges from sandy loam to silt loam [15].Figure 1
Map showing the agroforestry system-applied area in Raksirang Rural Municipality, Makawanpur, Nepal.


## 2.2. Data Collection

Ten pits were randomly dug in three major agroforestry systems: agrisilviculture system, home garden, and silvopasture system. Soil samples were collected from 0 to 10 cm, 11 to 20 cm, and 21 to 30 cm from each pit using a cylindrical soil corer of 10 cm length and 3.45 cm radius. So, a total of 30 soil samples were collected from each agroforestry system, with total 120 soil samples in all agroforestry systems. One additional soil sample (undisturbed soil sample) from each soil depth was collected using the soil core sampler to determine the bulk density. Each sample was bagged, labelled, and transported to laboratory for analysis. The soil sample collection and laboratory analysis processes were performed as in [16].


## 2.3. Laboratory Analysis of Soil Samples

Soil bulk density was determined using the soil core sampler having a diameter of 5.7 cm [17]. The soil organic carbon concentration was determined by dry combustion of oven-dry soil samples at 900°C [18]. The percentage of soil organic matter was converted to soil organic carbon by multiplying with 0.58, as soil organic matter is assumed to contain 58% of organic carbon. Total nitrogen (TN) was determined by the Kjeldahl digestion-distillation method [19]. Soil pH of a composite sample (of all soil layers and soil pits) from each agroforest was determined using a pH probe with a glass-calomel electrode (Suntronics pH electrode, Suntronics, India), keeping 1 : 1 soil : water ratio [20]:Bulk density of soil = oven-dry weight of soil in grams/volume of soil in cubic centimetres.Soil organic carbon in ton per hectare = organic carbon content (%) × soil bulk density × depth of soil layer.The significanct difference of soil organic carbon was tested using ANOVA at a 5% level of significance. SPSS software (IBM SPSS Statistics, IBM Corporation, version 20) was used for statistical analysis of the data.


## 3. Results and Discussion

Local people practised three major agroforestry systems, namely, agrisilviculture, home garden, and silvopasture in the study area. These systems are characterised in Table1.Table 1
Characteristics of agroforestry systems in the study area.
Agroforestry systemArrangement of componentsSpecies(1)Agrisilviculture (crops and trees including shrubs)Cereal crops and lentils grown on terraced rainfed cultivated land (bari) under widely spaced naturallygrowing fodder tree speciesCereal crops:Zea mays, Triticum aestivum, and Panicum miliaceumTrees: Dalbergia sissoo, Artocarpus lakoocha, and Bombax ceiba(2)Home garden (cereal crops, trees, fruits, house, and domestic animals)Multipurpose trees, fodder trees, fruit trees, nontimber forest productsTree species:Ficus semicordata, Melia azedarach, Bauhinia purpurea, and Artocarpus lakoochaFruit species: Citrus limon, Musa paradisiaca, Ananas comosus, Carica papaya, and Mangifera indicaNontimber forest products: Curcuma domestica and Zingiber officinale(3)Silvopasture (pasture/animals and trees)Fodder trees: forage grasses and animalsForage grasses grown on terraced bari under naturally growing fodder trees; goat, cow, and buffalo in cut and carry systemTrees/fodder species:Garuga pinnata, Schima wallichii, Madhuca indica, Bombax ceiba, Ficus hispida, Leucaena leucocephala, and Bauhinia purpureaGrasses: Thysanolaena maxima, Saccharum spontaneum, and Eulaliopsis binateAverage bulk density in agroforests up to 30 cm depths was found to be 1.19 gm/cm3 close to the value 1.28 gram per cubic centimetre [21], 1.17 gram per cubic centimetre [22], and 1.18 gram per cubic centimetre found by Ghimpre et al. [22] in two Shorea robusta-dominated tropical forests of Nepal. The average bulk density of soils in different depths 0–10 cm, 11–20 cm, and 21–30 cm were found to be 0.375 gram per cubic centimetre, 0.38 gram per cubic centimetre, and 0.39 gram per cubic centimetre, respectively (Table 2).Table 2
Average bulk density (gm/cm3) at various depths and agroforestry systems.
Agroforestry systemSoil depth (cm)Total average (gm/cm3)0–1011–2021–30Agrisilviculture1.061.161.171.13Home garden1.291.331.341.32Silvopasture1.031.131.231.13The average soil pH in 30 cm soil depth of the agroforests was found to be 6.6. The pH value was closer to the values reported by Kafle (2019) [21] (5.3), Sigdel [23] (5.90–6.42), Karki [24] (6.4–7.1), and Singh and Singh [25] (6.7-6.8). The soil pH was found near to the neutral value suitable for agricultural production.The average soil organic carbon in 30 cm soil depth of the agroforests was found to be 2.1%. There was a gradual decrease in the percentage of soil organic carbon with an increase in soil depth. The order of percentage content of average soil organic carbon in 30 cm soil depth was silvopasture (2.73%) > home garden (1.87%) > agrisilviculture (1.69%) (Table3). These values were close to the values reported by Paudel and Sah [26] (1.74% ± 0.31% soil organic carbon in the mixed Shorea robusta forest). This may be due to a similar physiographical range, climatic condition, and soil sampling. Schwab et al. [27] found 1.6% mean soil organic matter content in agroforests, Neupane and Thapa [28] found an organic matter content of 1.5–2.3%, Desbiez et al. [29] reported 2–3%, and Carson [14] assessed 0.5–3% with an average under 1%. The higher average soil organic carbon percentage in soil samples in our results may be caused by the higher organic matter content of the agroforestry systems in the study area.Table 3
Average soil organic carbon percentage at various depths and agroforestry systems.
Agroforestry systemSoil depth (cm)Total average % carbon0–1011–2021–30Agrisilviculture2.021.681.371.69Home garden2.381.691.531.87Silvopasture3.092.742.382.73The average stock of soil organic carbon in 30 cm soil depth in the agroforests was found to be 24.91 ton/ha. These values were close to the values reported by Baral et al. [30] who estimated 23.48 t/ha soil organic carbon in agricultural land. The order of stocks of average soil organic carbon in 30 cm soil depth was Silvopasture > home garden > agrisilviculture (Table 4). The amount of soil organic carbon in agroforestry systems differs with regions, agroforestry systems, and soil depths [31].Table 4
Total soil organic carbon stock (ton/ha) at various depths and agroforestry systems.
Agroforestry systemSoil depth (cm)Total soil organic carbon stock (ton/ha)0–1011–2021–30Agrisilviculture7.706.305.5419.55Home garden10.747.296.9124.94Silvopasture11.6210.388.2430.25There was a gradual decrease in the stocks of soil organic carbon with an increase in soil depth in the agroforests. The higher organic carbon percentage in the top layer may be due to rapid decomposition of litter in a favorable environment. Soils with rich organic carbon levels generally indicate high fertility, and therefore, it is important to maintain its optimum level that requires a careful land use and management practices [32]. Gautam and Mandal [33] also reported a decreasing trend of soil organic carbon with increased depths of soil in a tropical moist forest in eastern Nepal. Similar findings on the decline of stocks of soil organic carbon with the increase in soil depth were reported by Pandey et al. [34] in S. robusta-dominated forests of hills and Terai regions of Nepal; Ghimre et al. [22] in the S. robusta-dominated tropical community forest of Nepal, and Kafle [21] in a tropical community forest of Nepal. Song et al. [35] also reported that the concentrations of soil organic carbon decreased with depth, and the greatest concentration was in the 0–10 cm topsoil in selected forests of China.


## 4. Conclusion

Results of this research have shown evidence of tropical agroforests playing a role in the storage of soil organic carbon in Nepal. The average bulk density of soils in tropical agroforests was found to be 1.15 gm/cm3 in 30 cm soil profile depth. The average soil organic carbon of the soil was found to be 24.91 tons/ha. The soil organic carbon was found decreasing with increased soil depths. Bulk density of the soil was found increasing with increased soil depths. It is found that the soil of tropical agroforests had a high potentiality of storage of organic carbon and is playing a role in global climate change mitigation. The topsoil contained higher amounts of SOC, so the agroforest management practices should consider retaining organic matter on the floors of agroforests.



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*Source: 8816433-2020-10-15.xml*