Livestock Research for Rural Development 32 (10) 2020 LRRD Search LRRD Misssion Guide for preparation of papers LRRD Newsletter

Citation of this paper

Carbon sequestration potentials of rangelands under traditional management practices in the Rift Valley of Ethiopia

Ahmed Hassen

Department of Animal Sciences, Debre Berhan University, P O Box 445, Debre Berhan, Ethiopia
hassenahmed211@gmail.com

Abstract

Rangelands have greater potential to sequester carbon in soil system and management practices significantly limit the soil carbon stock. An experiment was conducted in Mieso district of eastern Ethiopia to evaluate soil carbon stock potentials in three traditional rangeland management practices (Communal grazing land, prescribed fire and grazing enclosure). Soil samples were collected from three soil depths (0-20 cm, 20-40 cm and 40-60 cm) and below ground carbon were estimated in all treatments. The soil attribute was analyzed using analysis of variance GLM procedure. The result showed that soil carbon was markedly higher (p< 0.01) in stock exclusion (226.2 t C ha-1) area followed by prescribed fire grazing land (156.2 t C ha-1) and communal grazing lands (115.3 t C ha-1) in the study district. Moreover, the soil organic carbon composition was higher in 0-20cm depths which were dramatically reduced with their depth.

In general, potential of carbon sequestration in grazing land is highly focused on the effective management practice. Hence, enclosed rangelands for dry season grazing had higher soil carbon content than the other traditional grazing practices. Therefore, traditional grazing systems can be an important practice to use rangelands for economic and ecological gains in pastoral areas of eastern Ethiopia.

Key words: carbon sequestration, pastoralists, rangeland management, soil carbon


Introduction

Rangelands cover about 62% of the total landmass in Ethiopia and contribute as the primary feed sources for livestock and wild animals in arid and semiarid areas, in addition to reducing atmospheric carbon dioxide (CO2) (IPCC 2007). Land use changes in these areas, such as overgrazing, can exacerbate the climate change problem as they leave fewer plants to reabsorb the carbon. In addition, human activities, especially inappropriate and unsustainable use of rangelands are one of the causes for sharp increase in CO2 emissions in the atmosphere. Management of rangelands can aid in the mitigation of rising atmospheric CO2 concentrations via carbon storage (Derner and Schuman 2007). Vashum and Jayakumar (2012) reported that more than 30% of the world carbon stock is found in the soil of rangelands. A higher carbon stock was stored in the soils than in the aboveground vegetation (Bikila et al 2016), indicating the importance of vegetation cover in preventing the release of CO2 that could have great influence on climate variability and change. If the effects of global warming are to be kept to a minimum, carbon already emitted to the atmosphere as a result of human activities must be sequestered (Fynn et al 2009). However, under traditional land management system, overgrazing is one of the causes of loss of soil carbon (Tessema et al 2011).

Different rangeland management systems such as grazing enclosure and conservation of rangelands are used to increase forage production and soil carbon stock (Homann et al 2008). According to Food and Agriculture Organization (FAO) (2010), different rangeland management practices believed to increase soil carbon by an average of 0.35-3 t C ha-1. Hence, traditional rangeland management practices such as enclosed grazing and prescribed fire could increase soil carbon (Bradd Wilt et al 2011). Though, many studies have been carried out on rangeland productivity (Kassahun et al 2008; Tessema et al 2010; Tessema et al 2011), information on carbon sequestration under different management practices are lacking. Today, the study areas pastoralists are facing the consequences of rapid deforestation and degradation of rangeland resources. Thus, it is critical to assess the soil carbon content for enhanced carbon sequestration under different management uses. Such information would be useful in bringing to light in making building of green economy more successful. Therefore, the objective of this study was to investigate the soil carbon stocks potential through different traditional rangeland management and soil depth in eastern Ethiopia.


Materials and methods

Description of the study sites

The study was undertaken at Mieso (848 - 919’ N and 4056 - 409’E), district of eastern Ethiopia (Figure 1). The study district was selected based on their rainfall distribution, variability of rainfall in the short and long rainy seasons, accessibility of the rangelands, fluctuation of livestock feed availability, frequency of drought, livestock potentials and rapid degradation of grazing lands. Food insecurity is a major challenge in the study areas. Average annual rainfall and temperature for Mieso, is 721 mm with a coefficient variation (CV) of 24.2% (1984-2018) and 22.5C, respectively (1984-2018). Under normal condition, Mieso receives its highest rainfall amount during long rainy season (June to September) while the short rains prevail from March to April. In Mieso, the dry season sometimes extends from October to May or June when the short rainy season fails. Rain falls over most of the study district only for few months, where dry conditions with highly variable rainfall prevail during substantial parts of the year. As a result, recurrent drought is a major problem in the study area. Topographic orientation of the study area varies from typical lowlands to midlands. For instance, the altitude of Mieso district ranges between 823 to 2475 m above sea levels (Aklilu et al 2014). Though, pastoral production system is practiced in the study district, agro-pastoralism is the dominant agricultural practice. In both pastoral and agro-pastoral areas, rangeland is the primary feed resources for livestock.

Figure 1. Geographical location of the study area, Meiso district


Methodology

Site selection and soil sample collection for soil carbon study

The three rangeland practices studied were communal grazing areas, prescribed fire as a rangeland management tool not more than five years after fire conduction and area enclosure grazing land management practices using as dry season grazing for more than 15 years. Prior to the field layout and sampling techniques, a reconnaissance survey (discussion) was made with elder pastoralists and agro-pastoralists who had a deep knowledge of the sites starting from its past history to its present situation. In order to establish permanent plots, the area of interest was clearly identified. First walk was done around the area to be sampled to assure that the sample plots do not all fall in the area with the densest or least vegetation area (eroded areas). After detailed field observation, we followed random sampling procedure to collect the soil samples. The soil samples were collected immediately after rainy season (from September to November, 2017). Within the 30mx40m plot five soil samples per plot at three different depths 0-20cm, 20-40cm and 40-60cm were taken from the four corners and from the center of each plot by using a motorized soil sampler with a radius of 2.983 cm to determine bulk density for the three depths (Bikla et al 2016). The soil samples from a given plot were bulked together according to their depth, so that each plot had three composite samples representing each soil depths. The collected soil samples were prepared for laboratory analysis. These samples were air-dried and then crushed before storage in bulk form. Subsamples were oven dried at 105oC for 24 h, ground and passed through a 2mm sieve mesh and analyzed at Haramaya University soil laboratory center.

The bulk density was calculated as the ratio of the mass of oven dried soil sample to core volume. Walkley’s and Blacks titration method (Jackson 1967) was used to estimate the soil carbon content. Rau et al (2009) method was used to calculate the carbon contents in the collected samples.

Soil C kg ha-1= BD (kg cm-3) x (1-rock (gravel content) x d x 100,000,000 x C%

Where:

d = soil depth (cm),

BD = bulk density in kg cm-3,

C% = percentage carbon content of the sample, and

100,000,000 is the conversion factor = (kg cm-3) x (10,000 cm 2m-2) x (10,000 m2ha-1).

Statistical Analysis

The data that was generated from soil attributes were analyzed using analysis of variance to test differences in soil carbon stock with soil depth and traditional range land management practices using a General Linear Model (GLM) procedure of Statistical Analysis System (SAS) version 9.1 (SAS 2008). Tukey HSD test was employed for mean comparison.


Results

Effects of rangeland management practices on soil carbon

Soil organic carbon (SOC) stock was significantly higher in enclosure grazing system than other types of grazing systems during the study (Table 1).

Table 1. Overall soil organic carbon stock across the three rangeland management practices of Mieso

Management practice

SOC (t C ha-1)

Enclosure grazing

226.2a

Fire rangeland grazing

156.2b

Communal grazing land

115.3c

P value

0.023

Means with the same letter superscripts along columns are not different at p=0.05
t C ha-1, tonnes of carbon per hectare, SOC = soil organic carbon

Soil carbon stocks under different management

According to this result, the soil organic carbon composition was different in the three depths which were dramatically reduced with their depth (Table 2).

Table 2. Carbon content across soil depths

Soil depth

SOC (t C ha-1)

0-20

212.2a

20-40

131.5b

40-60

74.1c

p value

0.0012

Means with the same letter superscripts along columns are not different at p = 0.05
SOC = Soil organic carbon; t C ha-1 = tonnes of carbon per hectare


Discussion

Effects of rangeland management practices on soil carbon

The total soil carbon content in grazing enclosure was highest (p<0.05) than the other two traditional grazing lands management practices. This was associated with availability of higher tree and shrub densities as compared with other rangeland management practices. The soil carbon reported in the present study in communal grazed areas (115.3 t/ha) was less than that reported by Bikila et al (2016), who reported 128 t/ha. This variation in communal soil carbon stocks might be due to the differences in climatic and edaphic between the Borana rangeland and eastern Ethiopia. Similar finding was observed by Sheikh et al (2008) who indicated that SOC was higher for enclosure than the open grazing lands. Similar results also reported high SOC for protected areas (Sheikh et al 2008; Fynn et al 2009; Bikila et al 2016). In each type rangeland management practices, a higher soil carbon stock was measured in enclosure grazing system. The findings of the present study is in agreement with the previous studies (Girmay et al 2008; Bikila et al 2016), who reported the enclosure grazing contain higher soil organic carbon than other rangeland management practices. But the SOC obtained in this study was lower than Bikila et al (2016), who reported that the carbon sequestration of enclosure grazing, prescribed fire and communal grazing practices were 237, 172, and 128 t C/ha, respectively. This difference might be due to the variation of SOC which depends on frequency and intensity of grazing and frequency of fire application.

Soil carbon stocks under different management

The soil organic carbon composition was different in the three depths which were dramatically reduced with their depth. The findings were supports the reports of Abule et al 2005, and Bikila et al (2016). The highest carbon stock was observed in the first 0-20cm profile of the soil depth. This is also in line with the findings of Shiekh et al (2008) and Fynn et al (2009). Soil organic carbon reduces along the soil depths; this report was supported by Abule et al (2005) who studied the effects of woody plants and grazing intensities on grass composition and soil nutrients in the Middle Awash of Ethiopia. In addition, Bikila et al (2016) also observed similar results in Borana rangelands. Similar to this study Bikila et al (2016) and Fynn et al (2009) also observed the highest soil carbon at above layer soil profile. However, in contradict to these findings, Hiederer (2009) noted higher soil organic carbon in lower soil depth profile.


Conclusions


Acknowledgement

The authors would like to acknowledge Deutsche Gesellschaft fur Internationale Zusammenarbeit Ethiopia, Pastoral, and Agro-pastoral Research and Capacity Building Directorate and Somali Region Pastoral and Agro-pastoral Research Institute of Ethiopia for the financial and material support during the field and laboratory studies.


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