and sequestration. Sequestration could be in organic or inorganic forms, and places where carbon is stored are called carbon sinks (Jones 2007).
Bass et al. (2000) reviewed two carbon management strategies as being (1) carbonsequestration and (2) carbon conservation. Sequestration is increasing the rate of carbon accumulation into a sink, while conservation is reducing or minimizing the already fixed carbon in a sink. For this paper, soil organic carbon (SOC) sequestration has been defined as the accumulation of carbon through sustainable land management practices. In determination of SOC sequestration, the resistant forms of soil organic carbon, such as charcoal, are not taken into account (Batjes 1996); however, sequestration focuses on resistant forms of soilcarbon. A soilcarbon pool comprises both of the soil organic carbon (SOC) and the soil inorganic carbon (SIC),cf. Lal (2004). Soil has the potential to store carbon and contribute to mitigating global climatechange. The soil is the largest terrestrial pool of organic carbon (Batjes 1996, Lal 2004). In total, soil contains about 3 times more carbon than the atmosphere, and 4.5 times more carbon than in living beings. Hence, a relatively small increase in the proportion of soilcarbon could make a significant contribution to reducing atmospheric carbon (Walcott et al. 2009, Lal 2004). A review on soilcarbonsequestration in Ethiopia is required to identify key gaps and know major priorities in research and development to increase soilcarbon stocks while increasing agricultural productivity. In the past, the most visible problem, landdegradation, has taken attention of the government and its development partners. There are several research projects focusing on the visible problem of soils and associated issues, yet no attention was given to the invisible problem, depletion of the soil organic pool. This difference in focus has necessitated review on soilcarbonsequestration in order to direct research and development projects focusing on soils in a bid to mitigatelanddegradation and climatechange.
Corral‐Nuñez et al. (2014) evaluated the status of soil organic matter (SOM) in croplands and exclo- sures (protected areas) in northern Ethiopia. The SOM was converted to SOC for ease of reporting in conformity with other reported studies. Appreciable depletion of SOC content in farmlands (1.2% to 1.7%) with significant increase in SOC content (1.5% to 3.2%) was observed in exclosures following 20-year protection and recovery period. In assessing the role of forests and soilcarbonsequestration on climatechange mitigation, Alemu (2014) asserted that more than 80% of the total terrestrial above- ground C and 70% of the total belowground SOC is sequestered by the forest ecosystems. The study em- phasized the need for adequate protection of the existing remnant forests, which serves as major ter- restrial carbon sink. The above scenario (protection of the forest remnants) is pivotal to restoration of SOC in degraded landscapes of northern Ethiopia. In investigating the SOC and N stocks of different land use systems along a climatic gradient in northwest Ethiopia, Assefa et al. (2017) reported that 60% of the total SOC stocks were found in the topsoil (0– 10 cm). More so, clear vertical gradient in SOC stock were observed down the soil profile in forests, considering the following soil depths: 0–10 cm, 10– 20 cm, 20–30 cm, and 30–50 cm. With emphasis on Desa’a dry Afromontane forest in northern Ethiopia, Berihu et al. (2017) investigated the impact of changes in land use-land cover on concentrations of SOC and TN sequestration. The study revealed sig- nificant difference in SOC distribution among dense forest (2.3%), open forest (1.7%), open grazing land (1.6%), and cropland (1.2%). More so, higher SOC (44.9 t ha −1 ) was sequestered in the top soil (0– 20 cm) compared to the subsoil layer (20–40 cm). Their findings indicated that the current manage- ment practice at the Desa’a forest area is not sus- tainable and advocates for more sustainable practices to avoid further degradation of the remnant of the forest. They further indicated that conversion of for- estland to other land use might result in huge loss of SOC and other essential soil nutrients.
Among adaptation options, Climate Smart Agriculture invites researchers, practitioners and policymakers to explore solutions combining food security, climate adaptation and mitigation, underpinning sustainable landscapes and food systems (5) .
Such solutions already exist and can be made reality provided favorable policies and conditions. Agro- ecology is particularly relevant to drylands to increase soil fertility and water efficiency in cropping systems. Pastoral stock farming is a traditional way of valorizing dryland ecosystems while coping with the scarcity of resources. Sustainable land management can rebuild soil organic matter stocks, thus increasing soil fertil- ity and biodiversity, while sequestering significant amounts of carbon, thereby helping to mitigateclimate
About 75% of Eastern Africa is dominated by managed grassland systems. However, at the regional level little is known about the types of grassland managements and their impacts on SOC, with the existing studies focussing on small localized areas. Given the large land size of grasslands in East Africa, there is a yet untapped potential for C storage in the grasslands as an option to mitigateclimatechange. However, there is a need to review studies under- taken in the past to better understand the current and future po- tential effects of grassland management practices. Therefore, the aim of this study was review observation-based studies on grass- land systems in East African countries (Burundi, Ethiopia, Kenya, Rwanda, Tanzania and Uganda (Figure 1)) in order to: (a) assess the SOC sequestration potential of existing management practices; (b) assess the key drivers affecting SOC sequestration in grasslands in the region; and (c) identify the knowledge gaps regarding SOC se- questration potential in East African grasslands and provide recom- mendations for further studies.
SSDS and CDs also played an important role in trapping SOM. The SOM contents sediments trapped by SSDs and CDS ranged from 24 to 147 g kg − 1 and 35–531 g kg − 1 of sediment, respectively. The SOM content of sediments was higher than its SOC content. A kilogram of sediment con- tains 47–59% more SOM compared with SOC, which agreed well with the findings of FAO and ITPS, (2015) that is SOM contains roughly 55–60% C by mass. The mass SOC and SOM contained in a kilogram of sediment also showed direct correlation with R 2 = 0.99. The evaluated eight SSDs trapped ~ 56.6*10 5 SOM together with 61*10 6 kg sediment and six CDs trapped 29.2*10 5 SOM together with 78*10 5 kg sediment. This shows that SSDs and CDs are also important C sinks. In general, the studied SSDs and CDs totally sequestered 85.8*10 5 kg SOM together with ~ 68.8*10 6 kg of sediment, which is a great contribution in reducing the GHGs concentration in the atmosphere. It also helps to have a good knowledge of the current SOC and SOM sinks, its spatial distribution and its sinking mechanisms to inform various stakeholders (e.g. farmers, policy makers, land users) to provide the best opportunities to mitigateclimatechange. After carbon enters the soil in the form of organic material from soil fauna and flora, it can persist in the soil for decades, centuries or even millen- nia. When the soilcarbon is released into the atmosphere it becomes an important source GHGs. Soils are major car- bon reservoirs/sinks containing more carbon than the at- mosphere and terrestrial vegetation (Lal, 2003; FAO and ITPS, 2015; FAO, 2017).
Abstract. The Near East North Africa (NENA) region spans over 14 % of the total surface of the Earth and hosts 10 % of its population. Soils of the NENA region are mostly highly vulnerable to degradation, and future food security will much depend on sustainable agricultural measures. Weather variability, drought and depleting vegetation are dominant causes of the decline in soil organic carbon (SOC). In this work the status of SOC was studied, using a land capability model and soil mapping. The land capability model showed that most NENA countries and territories (17 out of 20) suffer from low productive lands (> 80 %). Stocks of SOC were mapped (1 : 5 000 000) in topsoils (0–0.30 m) and subsoils (0.30–1 m). The maps showed that 69 % of soil resources are shown to have a stock of SOC below the threshold of 30 tons ha −1 . The stocks varied between ≈ 10 tons ha −1 in shrublands and 60 tons ha −1 for evergreen forests. Highest stocks were found in forests, irrigated crops, mixed orchards and saline flooded vegetation. The stocks of soil inorganic carbon (SIC) were higher than those of SOC. In subsoils, the SIC ranged between 25 and 450 tons ha −1 , against 20 to 45 tons ha −1 for SOC. Results highlight the contribution of the NENA region to global SOC stock in the topsoil (4.1 %). The paper also discusses agricul- tural practices that are favorable to carbonsequestration such as organic amendment, no till or minimum tillage, crop rotation and mulching and the constraints caused by geomorphological and climatic conditions. The effects of crop rotations on SOC are related to the amounts of above and belowground biomass produced and retained in the system. Some knowledge gaps exist, especially in aspects related to the impact of climatechange and effect of irrigation on SOC, and on SIC at the level of the soil profile and soil landscape. Still, major constraints facing soilcarbonsequestration are policy-relevant and socioeconomic in nature, rather than scientific.
In fact, very few and scanty published works and grey literature existing make claims aboutdiverse socio-economic and environmental benefits of the carbonsequestration programmes (Brown, et al. 2007; Maereg, et al. 2013). The most valuable outcome categories were: 1) increased assets in the form of tree stocks could serve as a ‘carbon sink’ absorbing and storing greenhouse gases from the atmosphere to help mitigateclimatechange.; 2) increased wild resources (especially wild foods like fruits and seeds, apiculture and construction inputs) for household consumption and sale, and associated dietary health benefits; 3) improved psycho-social wellbeing as a result of a more aesthetically pleasing and comfortable community and work environment, enhanced leadership capacity of FMNR group members, and a more positive outlook; 4) improved soil fertility and crop yields, and 5) regeneration of the native forests provide important habitat for many species of wild life and enhances biodiversity, which in turn could be an attraction for ecotourism.
The global extent of conservation agriculture (CA) increased to 157 million hectares (M ha) at the rate of more than 10 M ha year -1 (equivalent to almost 47 % change) since 2008/2009. Among the 13.5 M ha of land growing rice-upland crops in double or triple cropping rotations in the Indo- Gangetic Plains (Gupta and Seth, 2007), CA covers over 3.2 M ha (Derpsch and Friedrich, 2009). Fitting CA in rice-based cropping systems remains a challenge because almost three fourth of the global rice (77 %) is produced under wetland condition with rigorous tillage followed by puddling (done by several wet tillage operations and land levelling) and transplanting (Rao et al., 2007; Rao et al., 2017). Farmers puddle soils to make them soft for transplanting (De Datta, 1981), to reduce water losses (Tuong et al., 1994), to control weeds and to reduce percolation losses of nutrients (De Datta, 1981). On the other hand, puddling for rice and then intensive tillage for dryland arable non-rice crops over a long period causes degradation of the soil structure (Sharma et al., 2002; Dalal and Mayer, 1986a), consumes a large quantity of water for rice establishment and crop growth (Sharma et al., 2002) and accelerates loss of soil organic carbon (SOC) (Six et al., 2004a; Shibu et al., 2010) and nitrogen (N) content (Dalal and Mayer, 1986b).
CO 2 e in 2030. Realizing the threat of global warming, reducing Emissions from Deforestation and Degradation
(REDD+) both the Kyoto Protocol in 1992 and Paris climatechange agreement in 2012 were built. To upon these commitments Ethiopia has adopted a new, sustainable development model by initiating the Climate- Resilient Green Economy (CRGE) for achieving the following four pillars: i) Improving crop and livestock production while reducing emissions; ii) Protecting and reestablishing forests; iii) Expanding electricity generation from renewable sources v) Use of modern and energy-efficient technologies. Among many climate smart Agriculture practice in Ethiopia, Agroforestry is the one which is inclusive others agricultural practicing. The aims of this paper were to reviews the possible opportunities to raise the potential environmental role of agroforestry in supporting the climate smart agriculture in Ethiopia.Studies on indigenous agroforestry systems in southeastern Ethiopia indicated that, the average total biomass carbon stock were, Coffee accounted for 11 % and Enset 9% of total biomass C on average of which trees accounting for 39–93 % of the total biomass carbon stock. On the other hands, SOC stocks (0–60cm) were 109–253 Mg ha −1 in the indigenous agroforestry systems.
A diverse undergrowth can produce litter exposed to decomposers following establishment of a new rotation and hence the 10 year old stand had the highest C and N concentrations at a time when all pioneer species are gone which could be attributed to incorporation of F and H material into mineral soil. This high accumulation of C could be associated with litter input from the non- tree vegetation (Black et al., 2009). In this way the process of succession and decomposition of pioneer species enhanced more fine roots and litter in the upper soil thereby increasing C concentrations in the early years of establishment (Hoosbeek et al., 2011). There was also an increases in C inputs into upper soil layers despite the burning for land preparation where fine roots are burnt and charcoal is added into the soil. Black et al. (2009) studied Picea sitchensis, Bong. Carr. and found highest sequestration rates at 10 years, which subsequently declined after canopy closure in older and thinned stands. There could also be other factors affecting C and N dynamics including management activities such as pruning, thinning and other tending operations. In this study, the C and N were lowest soon after establishment but recovered rapidly by the age of 10 years, after which it declined possibly as a result of silvicultural operations such as thinning and pruning after which was a recovery again by the age of 25 years. This shows that plantations can be used efficiently to create C sinks because of the rapid growth rates soon after establishment and additions that come as the trees develop and grow into older age classes.
Soil organic carbon (SOC) stocks and fluxes in forest ecosystems are influenced by natural and human disturbances. In the tropical regions the highest impacts on disturbance in forest C cycles are related to human activities such as conversion of natural lands to cropland and pasture areas and to forest plantations. The disturbances in the forest C cycles will release CO 2 emissions to the atmosphere triggering global warming. In this study the focus was set in subtropical soils in Brazil, south extreme region of Bahia. The aim of the study was to investigate whether reforestation of Eucalyptus plantations under former pasture areas will help mitigateclimatechange through carbonsequestration. Field measurements were made on the total SOC and nitrogen amount, along with soil physical and chemical attributes, between different land use systems , also to analyze if there will be any positive effect on soil chemical and physical properties with the reforestation. The study areas included the intact rainforest Mata Atlântica called Native Forest, as a reference, pasture areas, which have been settled in the past from deforestation of Mata Atlântica, and Eucalyptus plantations recently reforested under former pasture areas aimed for paper and pulp production. With the field measurements and simulated amounts of SOC using the CO-Fix V.3.2 programme it could be compared the effects on SOC sequestration in short and long term ( max. 50 years) under the Eucalyptus reforestation. Our results show significant differences with lower SOC, higher pH and soil compaction under pasture areas after deforestation of the rain forest. Meanwhile reforestation with eucalypt plantations on former pasture areas did not lead to any significant total nitrogen and total SOC accumulations in short term. However, the simulated results showed that Eucalyptus reforestation will play a role on carbonsequestration in soils with time. After 20 years of production the Eucalyptus forests will gain higher SOC accumulations than in pasture systems. After 50 years the simulated SOC accumulation showed closer values to the amounts measured on field under the Native Forest areas. These results indicate that the Eucalyptus plantations are efficient at sequester carbon in the soil in the long term. However, the comparison with the Native Forest field measurements should be carefully interpret since the measurements on field were made within a certain depth while the program shows the total amount with no limited soil depth. For a complete comparison it remains to take deeper soil samples in the field measurements.
Unlike the current result, the soil is the largest carbon reservoir in the terrestrial ecosystem (Chinasho et al., 2015; IPCC, 2003; Lal and Bruce, 1999). But forest management and the existing condition of the forest greatly affect soil organic carbon. SOC is influenced through land use and management activities that affect the litter input (for example how much-harvested biomass is left as residue and SOM output rates, tillage intensity affecting microbial survival) and the estimates depth to which carbon is accounted, commonly 30 cm (IPCC, 2003; Lal and Bruce, 1999). Hence, in the current study, the above-ground biomass has two-fold greater than SOC. This might be due to the presence of mature Syzygium guineense (with better DBH and H value) in good density. The use of litter as fuel also affects the SOC (IPCC, 2003).
plantations to low carbon degraded land. Oil palm permit swapping provides a pathway for furthering agricultural expansion without the loss of additional tropical forests (Venter et al. 2012). It involves retiring existing permits on carbon dense land and taking-out new permits on highly degraded land that has suitable climatic and edaphic conditions for cultivating oil palm, by undertaking spatial-targeting and community surveys of candidate sites (Gingold et al. 2012). The benefits of permit swapping are manifold; reducing emissions from the oil palm sector whilst also finding productive uses for abandoned land. The costs incurred from this process include purchasing new permits, negotiating with affected permit holders, communities and governments (Venter et al. 2012) and can include substantial legal costs. Financial compensation would need to be provided to concessionaires to harness support, such as through a compensation fund or by offering discounted credit to those willing to participate, as timber revenues from clearing forests are used to defray plantation set-up costs (Irawan et al. 2013). Ideally, the restrictions would be integrated into a spatial-planning reform, whereby high taxes are imposed for plantations planned on carbon-rich forests (Van Paddenburg et al. 2012). Of critical importance is ensuring that the interests of those using abandoned or degraded land are actively involved in any decision-making regarding land planning (McGregor 2015).
been excluded for six years, followed by two-year exclusion, and presently grazed area. These consequences on soil quality have indirect effects on SOM, further reducing its content on overgrazed and degraded pastures.
There are also inherent landscape factors that influence soilcarbon and bulk density within a pasture. A study by Sigua and Coleman (2010) discovered that soil organic carbon on a rotational pasture in a tropical climate was significantly affected by slope aspect, slope position, and soil depth. Carbon content was suspected to be greatest at the top slope position followed by the middle position and the bottom position, a potential outcome of the preference of cattle to congregate downslope (Sigua & Coleman 2006). This reduces vegetation and limits the input of carbon into the soil at this slope position. Because cattle are herded animals, they tend to graze in close proximity to one another. Sigua and Coleman (2009) suggest that soil compaction is greatest near cattle congregation sites, such as near water or in shaded areas. Congregation sites can also be associated with decreased soil moisture and reduced vegetation, influencing both soil fertility (Sigua & Coleman 2006) and soil organic carbon (Franzluebbers et al. 2000). Others suggest that in eroded landscapes, the removal of topsoil could lead to the
As negotiations on REDD continued to evolve, other activities were included such as forest conservation, sustainable management of forests, and enhance- ment of forest carbon stocks. While these concepts have not been fully defined yet, forest conservation generally refers to the protection of standing forests that have not historically been under threat of deforestation. Sustainable manage- ment of forests refers to activities which increase carbon stocks and/or reduce carbon emissions from forests by changing the way in which they are managed. Management changes may include implementing harvest methods that result in less damage to remaining trees, extending harvest rotations thereby leaving more carbon stored on the land, increasing the stocking of poorly stocked forests by encouraging growth of denser/healthier trees and converting previously har- vested forests to no-cut protected areas. Enhancement of forest carbon stocks may include forest restoration, reforestation, and/or forestation. These activities, when added to REDD are referred to as REDD+. Finally, some countries would like to include carbon emissions from all land-use types, including agricultural land, wetlands, and others. When these activities are included, it is often referred to as AFOLU (agriculture, forestry, and other land uses) .
stochastic carbonsequestration in the EU climate policy for mitigating carbon dioxide emissions. Minimum costs with and without carbon sequestrations are then derived with a safety-first approach in a chance-constrained framework for two different scenarios; one with the current system for emission trading in combination with national allocation plans and one with a hypothetical system where all sectors trade. The theoretical results show that i) the value of carbonsequestration approaches zero for a high enough risk discount, ii) relatively low abatement cost in the trading sector curbs supply of permits on the ETS market, and iii) large abatement costs in the trading sector create values from carbonsequestration for meeting national targets. The empirical application to the EU commitment of 20% reduction in carbon dioxide emissions shows large variation in carbonsequestration value depending on risk discount and on institutional set up. Under no uncertainty, the value can correspond to approximately 0.45% of total GDP in EU under current policy system, but it is reduced to one third if all sectors are allowed to trade. The value declines drastically under conditions of uncertainty and approaches zero for high probabilities in achieving targets. The allocation of value among countries depends on scenario; under the current system countries make gains from reduced costs of meeting national targets, under a sector-wide trading scheme buyers of permits gain from reductions in permit price and sellers make associated losses.
microorganisms and fragments into finer POM. This fine POM becomes increasingly encapsulated with minerals and microbial products, forming new microaggregates (53–250 µm) within the macroaggregates. With active root growth stabilizing macroaggregates, intense biological activity (induced by root exudation) may also cause further encrustation of microbial products and mineral particles, forming microaggregates around root-derived POM. It has been found that this microaggregate formation within macroaggregates is crucial for the long- term sequestration of C because microaggregates have a greater capacity to protect C against decomposition compared with macroaggregates. The final step of the aggregate turnover cycle (t2 ’! t3) is when the macroaggregates break down and release microaggregates and microbially processed SOM particles. The macroaggregates break up because over time with further decomposition the labile constituents of the coarse sized SOM are consumed, microbial production of binding agents diminishes and the degree of association between SOM and the soil matrix decreases. However, microaggregates are still stable enough and not as sensitive to disruptive forces as the macroaggregates, and therefore survive.
Biochar use and cover crops promote carbonsequestration for all soil texture types. Such an enhancement of SOC does not vary significantly with soil texture (Table 1). The ability of conservation tillage to enhance SOC, however, differs with soil texture (Fig. 4). Conservation tillage merely reduces soil disturbance and normally does not add extra materials to soils. It can be inferred that the effect of conservation tillage on SOC is more texture-dependent than the other two management practices. Biochar is a carbon-rich material with a charged surface, organic functional groups, and a porous structure, which can potentially increase soil aggregation and cation exchange capacity (Jien & Wang, 2013). Similarly, cover crops directly provide carbon inputs to soils, and their root development and rhizodeposition can also benefit soil structure. These benefits are embedded in the source of biochar and cover crops per se. Thus, the effectiveness of biochar and cover crops in increasing SOC may depend on their properties other than soil texture.
During the assessment of land use transition of Kaftahumera, the basic concept of net change and swap change was properly addressed in order to capture all significant transitions along the temporal gradient. Accordingly the values of gain, loss, net change, swap and total change for the period 1972 – 2014 for each LULC class are presented in previous chapters of this dissertation. Along the study period, cropland and grassland are the dominant categories that experienced the highest gains. The gain in cropland and grassland is about 54 % and over 11 % of the study area respectively. On the other hand woodland has the highest loss in over 62 % of the area, followed by grassland with about 3 % of the area. The swap levels of woodland, cropland and grassland are 1.9, 1.7, and 4.9 respectively. The three dominant land categories: woodland, cropland and grassland show a significant amount of net change over the study period respectively. The loss in woodland is attributed to expansion of subsistence and large scale farming and underlying causes like population growth which competes over the natural vegetation of the region. The growth in population, both from resettlement and immigration due to casual labor pave the way for directly exerting pressure on the woodlands. The weak approach to set and implement legal procedure for protecting the natural ecosystem is another factor which allows easy access for the exploitation of woodlands (Lemenih et al., 2014). In general, the main driving forces for land use transitions in Kaftahumera are both proximate and resulting from underlying causes of land use changes. This resulted in systematic transitions affecting mainly the woodland vegetation of the region. The socio-ecological field survey also confirmed the involvement of human activities and policy intervention (resettlement and agricultural investment) that played the major role in exposing the woodland vegetation for change.