Enrichment Planting and Soil Amendments Enhance Carbon Sequestration and Reduce Greenhouse Gas Emissions in Agroforestry Systems : A Review

Agroforestry practices that intentionally integrate trees with crops and/or livestock in an agricultural production system could enhance carbon (C) sequestration and reduce greenhouse gas (GHG) emissions from terrestrial ecosystems, thereby mitigating global climate change. Beneficial management practices such as enrichment planting and the application of soil amendments can affect C sequestration and GHG emissions in agroforestry systems; however, such effects are not well understood. A literature review was conducted to synthesize information on the prospects for enhancing C sequestration and reducing GHG emissions through enrichment (i.e., in-fill) tree planting, a common practice in improving stand density within existing forests, and the application of organic amendments to soils. Our review indicates that in agroforests only a few studies have examined the effect of enrichment planting, which has been reported to increase C storage in plant biomass. The effect of adding organic amendments such as biochar, compost and manure to soil on enhancing C sequestration and reducing GHG emissions is well documented, but primarily in conventional crop production systems. Within croplands, application of biochar derived from various feedstocks, has been shown to increase soil organic C content, reduce CO2 and N2O emissions, and increase CH4 uptake, as compared to no application of biochar. Depending on the feedstock used to produce biochar, biochar application can reduce N2O emission by 3% to 84% as compared to no addition of biochars. On the other hand, application of compost emits less CO2 and N2O as compared to the application of manure, while the application of pelleted manure leads to more N2O emission compared to the application of raw manure. In summary, enrichment planting and application of organic soil amendments such as compost and biochar will be better options than the application of raw manure for enhancing C sequestration and reducing GHG emissions. However, there is a shortage of data to support these practices in the field, and thus further research on the effect of these two areas of management intervention on C cycling will be imperative to developing best management practices to enhance C sequestration and minimize GHG emissions from agroforestry systems.


Introduction
Agriculture is the second largest emitter of greenhouse gases (GHG) after the energy sector, and is responsible for about 30% of global GHG emissions [1].Agroforestry, the intentional integration of trees and/or shrubs with herbaceous crops and/or livestock in a production system, is a popular beneficial management practice (BMP) that can mitigate climate change by sequestering carbon (C) and reducing greenhouse gas (GHG) emissions [2][3][4][5][6][7][8].The Intergovernmental Panel on Climate Change (IPCC) has recognized both afforestation and reforestation as important activities supporting C sequestration [9].Agroforestry systems include many different permutations such as alley cropping, silvopasture, riparian buffers, savanna, forest farming, home-gardens, and woodlots, as well as other similar integrated land-use systems [10].In all cases, agroforestry systems are recognized as a land use management framework that simultaneously integrates the dual goals of ecological conservation and socio-economic development [9][10][11][12].
The environmental service of sequestering C and reducing GHG emissions provided by agroforestry systems are relatively well-documented globally at various management systems as summarized in Section 3 below.However, data quantifying the specific role of management interventions to improve such benefits are rare, with only a few studies reporting on the potential benefits of enrichment planting [13,14] and organic amendment of soils [2,15].Enrichment planting is commonly used for increasing the density of desired tree species in degraded (secondary) forests, particularly where these forests are low in density or occupied by less-desirable (i.e., non-productive) tree species.On the other hand, the addition of organic amendments to soils, including mulch, manure, or the application of other organic by-products from a feedlot or modified organic materials (such as biochar, composts, and manure pellets), is widely practiced in sole cropping systems but rare in agroforestry systems.
Manure is a widely available by-product from livestock production systems, particularly those involving the confined feeding of animals in large-scale livestock operations (poultry, swine, beef, dairy, etc.).Due to its high nutrient and C content, manure management and its application to soil plays a critical role in GHG emissions, including CH 4 and N 2 O [16].The Food and Agriculture Organization of the United Nations (FAO) estimated the CH 4 and N 2 O emissions associated with manure storage and processing contribute 4.3% and 5.2% respectively, and N 2 O emissions from the field applied and deposited manure contribute 16.4% of the GHG emissions in the global livestock supply chains [17].Composting, despite emitting GHGs during storage, and dried pelleting, are two methods of conserving nutrients in manure and facilitating their slow release into the soil [18].In the process, these methods reduce GHG emissions compared with raw manure application if applied to the field in an appropriate time such as avoiding wet conditions of soils [19].Biochar is pyrolysed biomass consisting of around 50% or more recalcitrant organic material [20].It is a promising soil amendment that improves physical and chemical properties of soils [21], as well as provides better environmental services such as improved nutrient cycling, increased C sequestration and reduce GHG emissions.However, studies on the effects of biochars in agroforestry systems are limited [22].
This paper reviews the current state of knowledge regarding opportunities to enhance C sequestration and reduce GHG emissions through two potential management interventions within agroforestry systems.The first is enrichment tree planting, and the second is the use of organic soil amendments.It starts with an overview of the impact of agroforestry in C sequestration and GHG emissions as documented by previous different empirical studies, and reviews reports at different temporal and spatial scales.Subsequent sections then review the role of enrichment planting and organic soil amendments and discuss the prospect of applying these interventions to agroforestry systems in order to enhance C sequestration and reduce GHG emissions.Finally, conclusions are drawn and areas of further research needs are identified on these two practices in order to further mitigate GHG emissions and promote climate change adaptation using agroforestry systems.

Methods of Literature Collection
A wide range of published literature was collected through searches using Google Scholar and ISI Web of Science with a Boolean defined by logical strings containing "and/or" with keywords "agroforestry", "environmental service", "enrichment planting", "greenhouse gas emission", "carbon sequestration", "biochar", "manure", "manure pellet", "composting", and "secondary forest".More than 200 publications, both referred and non-reviewed, were found; they were further sorted with criteria of "carbon sequestration and agroforestry" and "greenhouse gas emission and agroforestry", "enrichment planting and secondary forest or agroforestry", "composting and greenhouse gas emission", "biochar and soil C sequestration", "raw manure and composted manure" and "manure pellet and soil carbon".With these criteria, the number of publications selected was reduced to 94.Among them, five publications were on enrichment planting, six on manure pelleting, 33 on manure, 10 on compost and management, and 40 on biochar.Key results found from these studies were compared focusing on C sequestration in vegetation and soils, as well as GHG emissions.A total of 82 publications closely related to the subject matter were used in this paper.

Role of Agroforestry in C Sequestration and Reducing Greenhouse Gas Emissions
Several studies have documented that the C sequestration potential of agroforestry systems varies depending on environment and specific management systems (Table 1).Estimated global C sequestration potential of agroforestry ranges between 12 and 228 Mg C ha −1 , leading to a net C sequestration potential of 1.1 to 2.2 Pg (1 Pg = 10 15 g) over 50 years [23].It is also estimated that improved management alone within existing agroforestry systems could sequester an additional 0.3 Mg C ha −1 y −1 , while undertaking land use changes from conventional cropland to agroforestry (crops combined with forests) could sequester an additional 3.1 Mg C ha −1 y −1 [24].The SOC sequestration rate varies among agroforestry systems across different regions, ranging from 0.1 to 4.2 Mg C ha −1 y −1 depending upon the age of agroforests, and soil depths considered in the estimation [25].In the early stage of practicing agroforestry, soil C can be lost from top soils; for example, a multi-strata agroforestry system in Ghana lost 0.4 Mg C ha −1 y −1 until 15 years after establishment, after which a small amount of SOC was stored (0.06 Mg C ha −1 y −1 ) through 25 years age within the 0-15 cm soil layer (cited in [25]).Furthermore, the vegetation C sequestration rate differs by forest types.As an example, within windbreak systems, broadleaved trees demonstrated an almost double C storage capacity (4.39 ± 1.74 Mg C ha −1 y −1 ) than conifer trees (2.45 ± 0.42 Mg C ha −1 y −1 ) in a study involving nine ecoregions across the USA [26].Levels of C sequestration in vegetation and soils are known to vary among ecoregions.In the humid tropics, 70 and 25 Mg C ha -1 can be sequestered within vegetation and the top 20 cm of soil, respectively [28].In Mediterranean regions, total C sequestration rates in vegetation and soils of different agroforestry systems can be up to 1.3 Mg C ha −1 y −1 [35].In temperate climates, the potential C sequestration by aboveground vegetation of agroforestry systems could be as large as 2.1 × 10 9 Mg C y −1 , while in tropical regions, it could be 1.9 × 10 9 Mg C y −1 [36].Collectively, these examples show that C sequestration rates and resulting C stocks vary widely, reflecting marked variation in climatic conditions, soil properties, vegetation types, and the ongoing management of agroforests.However, observed variation in estimates of C might also be due to the use of different methods for estimating soil C sequestration potential under contrasting conditions, coupled with the inherently high natural variability of soil C stocks within agroforestry systems associated with divergent agro-ecological zones [37].
Trees are also known to help reduce CH 4 and N 2 O emissions, particularly in relation to neighboring cropland [5].In the sub-tropics, agroforestry systems combining trees and inter-cropped shrubs store more C in vegetation and soils compared to systems with only trees or trees grown with legume or cereals as inter-cropped systems [38,39].Similarly, windbreak and riparian forest buffers store significant amounts of C, in addition to providing other valuable ecosystem services such as improved water quality, biodiversity, and biomass feedstock availability [40].Sequestered C in low-till croplands with adjacent treed windbreaks was 75% greater than in low-till lands without adjacent windbreaks [40].Compared to sole herbland pastures, the presence of trees in the former leads to greater topsoil and subsoil C content, and larger litter inputs result in higher free and occluded organic matter (OM) fractions, and ultimately higher levels of stabilized SOM fractions [41].A study in central Alberta, Canada showed that silvopastoral systems had higher SOC and lower GHG emissions compared to agroforestry systems containing either hedgerows or shelterbelts combined with annual cropland [32,34].Mean SOC in the bulk soil at 0-10 cm depth was 81, 48 and 63 g kg −1 in the silvopasture, shelterbelt and hedgerow systems, respectively.Soil C in the more stable fine fraction (<53 µm) of the soil was higher in hedgerow systems (34 g kg −1 ) compared to both shelterbelt and silvopasture systems (29 and 29 g kg −1 , respectively).Within each agroforestry system, total SOC and the SOC concentration within each size fraction was consistently greater in the forested land-use compared to the adjacent agricultural herbland [32].The SOC stock in both the 0-10 cm and 10-30 cm soil layers were greater within the forested land cover type than in the adjacent herbland [42].In terms of GHG emissions, the silvopasture system had 15% greater CH 4 uptake and 44% lower N 2 O emission compared with the shelterbelt and silvopasture systems [34].
Despite their potential to mitigate GHG emissions, agroforestry systems can be a significant sink or source of GHGs depending upon management practices.In the humid tropics, agroforestry mitigated N 2 O and CO 2 emissions from soils, and increased CH 4 uptake, compared to sole cropping systems [28].While N 2 O emission in the agroforestry system was as low as 3%, CO 2 emissions were 70% of the high input cropping systems, and CH 4 uptake was almost double that of the low input cropping system [33].In contrast, management practices that disturbed soil and vegetation, such as tillage, burning of biomass, fertilization, and manuring, lead to net emissions of GHGs from soils and vegetation to the atmosphere [43].Among different agroforestry systems, multi-strata systems reduce N 2 O emissions and CH 4 oxidation, but emit similar CO 2 compared to shifting cultivation, crop/rubber agroforestry and short fallow systems [28].

Impacts of Enrichment Planting on C Sequestration and GHG Emissions
Enrichment planting, also known as in-fill or gap-planting, is commonly practiced to increase the density of desired tree species in degraded (secondary) forests [13,44], including those found in shelterbelts or silvopastoral plantations of agroforestry systems.Enrichment planting enables newly establishing trees to utilize available resources, including light, moisture and nutrients [45].Agricultural systems that include trees generally store more C in deeper soil layers compared to treeless systems, and higher SOC content in the former has been associated with greater species richness of trees and tree density [4,46].Enrichment planting in old fallow fields is beneficial in sequestering C, improving over-story tree diversity, and enhancing social, cultural, and ecosystem services [13].The improved C storage observed after enrichment planting in eastern Panama was around 113 Mg C ha −1 , which is comparable to that in industrial teak plantations and primary forests [13].
The choice of tree species within plantations affects C storage in phytomass, necromass, and underlying soils.For example, after 40 years of growth in plantations, conifers had higher biomass and litter C, while broad-leaved forests had considerably more soil C [47].The greater decomposition rate of broadleaf litter contributed favorably to soil C sequestration compared to that from conifer litter, but due to relatively steady photosynthetic rates throughout the year and high drought tolerance, conifers had more stable live biomass [47].Understory vegetation biomass was also negatively correlated to tree-biomass, with conifer stands leading to less understory C mass due to increased canopy closure and associated light limitations [47].
Natural regeneration of vegetation in abandoned pasture land is known to sequester C in a manner similar to planted vegetation, although the rate of sequestration can be slower [48].The aboveground C accumulation rate in 12-14 year-old forests was 5.6 Mg C ha −1 y −1 , and SOC accumulation rates were 1.49 Mg ha −1 y −1 [48].These results indicate that natural regeneration of tree species can mimic enrichment planting after pasture abandonment.

Impact of Organic Soil Amendment on C Sequestration and Greenhouse Gas Emissions
Soil amendment with organic input is a common practice in conventional agricultural practices (e.g., on annual cropland or forage land), and involves manuring, mulching, green manuring and biochar addition.Agroforestry systems can provide various types of feedstock for bulking agents such as residues from annual crops, small woody biomass from pastures, leaf litter, as well as twigs, branches and woody biomass from trees for use in composting, pelleting or biochar production.In this section, we review relevant literature and summarize potential impacts of these amendments on C sequestration and GHG emissions in agroforestry.

Impacts of Biochar Applications
Biochar has a slow decomposition rate and its application to soils can sequester SOC compared to non-amended soils [49].For example, 1.4 times higher total soil C was found in hardwood biochar amended soils compared to non-amended soils [49].Biochar has been tested in different cropping systems to assess its impact on enhancing C sequestration and reducing GHG emissions.A review of a wide range of agro-ecosystems with biochar application showed that despite using a variety of feedstocks and crops, the resultant impact of biochar application was a decrease in GHG emissions by up to 66% in CO 2 and up to 50% in N 2 O emissions [2].In addition, biochar addition led to reduced leaching of plant nutrients and contamination of downstream water sources [2].However, some biochar amended soils increased CO 2 emissions, which was attributed to increased soil porosity, lowered bulk density and higher pH, all of which may favor microorganism activity [2].Biochar from wood and herbaceous feedstocks performed the best in reducing emissions (ca.−60%), while manure-based biochar was less effective, the latter of which altered N 2 O emissions by −46% to +39% [50].Biochar feedstock, pyrolysis conditions, and C/N ratios were key factors influencing the emissions of N 2 O [50].
A previous meta-analysis of published data obtained from laboratory and field experiments to explore the effects of biochar on N 2 O and CH 4 emissions reported nearly 50% less N 2 O emissions across different soil types [51].This same study indicated a potential to increase the uptake of CH 4 due to enhanced methanothrophy following biochar addition [51].Effects of biochar on N 2 O emissions also varied among feedstocks, with both woody and crop residue biochars decreasing emissions, while biochars derived from other feedstocks (e.g., manures, bio-solids, paper mill residues) had no significant effects [51].A laboratory incubation study conducted across ten different soils in the USA and receiving the same hardwood biochar found no significant differences in the emissions of CO 2 and CH 4.However, this same study reported a decrease in N 2 O emission (up to 63% less) across all soils after biochar applications [52].Similarly, biochar produced from pine sawdust at 500 • C with or without steam activation decreased CO 2 and N 2 O emission (up to 32% in forest soils), though no differences in CH 4 uptake were detected [53].Pine biochar reduces GHG emissions by decreasing microbial and enzyme activities [53].Moreover, by changing the physical (gas diffusivity, aggregation, water retention), chemical (e.g., pH, redox potential, availability of organic and mineral N and dissolved organic C, organo-mineral interactions), and biological properties (e.g., microbial community structure, microbial biomass and activity, macro faunal activity, N cycling enzymes) of soils, biochar influences N mineralization-immobilization, turnover, and nitrification or denitrification processes, all of which ultimately affect N 2 O emissions [51].
The impact of biochar on GHG emissions within amended soils is dependent on both biochar and soil properties [20].Relationships between the biochar and soil N dynamics revealed that adsorption of NH 4 + and NO 3 − in biochar during the pyrolysis process decreased N loss during composting and after manure application, thus offering a mechanism for the slow release of fertilizer in the field [54].Higher pyrolysis temperatures during the manufacture of biochar from manure and bio-solids also result in biochars with decreased hydrolysable organic N and increased aromatic N [54].Short-term N 2 O emissions are therefore likely to decrease following biochar application, though no clear information exists on the long-term effects of this practice.In summary, biochar input to agroecosystems represents a potential mitigation strategy for environmentally detrimental N losses, specifically as N 2 O [54].Biochar can also enhance the process of composting manure and reduce GHG emissions during composting and subsequent field applications.The impact of biochar addition in conjunction with composting, including their application to soils with manure and manure pellets, on GHG emissions and C sequestration, are summarized in Table 2. Application of biochar during the composting of chicken manure increased peak CO 2 emission, while emissions of both CH 4 and N 2 O decreased [55,56].Composting of cattle manure with added biochar increased aeration, and hence the activity of methanogens, which reduced CH 4 emission [15].Biochar reduced N 2 O and CH 4 emissions during field applications due to a change in the microenvironment for the microbial population, including soil water content, and availability of oxygen, N, and C [57].In calcareous soils, biochar application alone increased total organic C stocks by 1.4 fold, while the application of biochar mixed with manure increased C levels by 1.7 fold [49].
Impacts of biochar on GHG emissions have shown mixed results depending on soil type, feedstock type and season of the application [58,59].Biochar addition to upland soil increased CH 4 emissions by 37% during the summer, but had no effect in winter, while decreasing N 2 O emissions up to 54% and 53% during the summer and winter seasons, respectively [58].In Chernozemic soils amended with straw, and its biochar reduced N 2 O emission but there were no significant effects on CH 4 or CO 2 emission compared with the unamended soils [59].A soil-column experiment using non-treated soils and those amended with biochar prepared by pyrolysis of pig manure and spruce sawdust at 600 • C found no differences in N 2 O, CH 4 and CO 2 emissions between the two treatments until they received an application of fresh pig manure during the 10th week [60].After 10 weeks, cumulative GHG emissions were higher from soils amended with biochars and manure for up to four weeks compared to the non-treated soil.However, this same study found NO 3 − N leaching was 51% and 43% lower in pig manure biochar amended soils and wood biochar amended soils, respectively, compared to the pig manure only-amended soils [60].Life cycle analysis is an emerging tool to link the full C footprint of products from their origin via different stages of the product supply chain [61][62][63].A life cycle analysis of biochar systems comparing biochar produced from three feedstocks, namely corn stover, yard waste (waste from industrial-scale composting) and switchgrass energy crops, found that the net energy provided by corn stover and yard waste was negative (−864 and −885 kg CO 2 e per Mg dry feedstock, respectively) while switchgrass was a net emitter (+36 kg CO 2 e per Mg dry feedstock) [63].These findings indicate that careful selection of biochar feedstock is required to avoid unintended environmental consequences, such as indirect increases in GHG emissions elsewhere in the global C and N cycles.% CO2 emissions in fertilized area compared to control plots.Annual CO2 emissions from control plots were 5.1 and 4.2 Mg C ha −1 y −1 in silty and sandy soils, respectively, in year 1, and 5.0 and 4.0 in year 2.

90% 60% 13%
NS ** 85% 37% 9% [67] % GHG emissions in fertilized area compared to control plots.CO2 emission from control plots were 31.1 kg CO2-C and 0.005 N2O N (Mg ha −1 ).* NA = not available; ** NS = not significant.Table 3. Summary of the previously documented effects of applying raw manure, composted manure, or manure pellets on subsequent C sequestration and GHG emissions in croplands and grasslands.Swine slurry (primarily liquid), farmyard manure (primarily from large mammals), and poultry manure are common by-products of livestock production that are recycled in the field as nutrient input to agricultural plants.Inorganic N content, labile C content and the water content in manure provide essential substrates to micro-organisms that affect GHG emissions from soil.However, GHGs can be produced and emitted to the atmosphere in each step from livestock confinement, to manure storage and treatment (i.e., handling and transport), and ultimately during application to the land [74].Composting is a well-established manure management process because it utilizes livestock manure and residual biomass of livestock feed and bedding, and produces manure that has reduced pathogens and weed seeds [75].On the other hand, manure pelleting, a physical method of densification, increases manure bulk density, reduces storage space requirements, reduces subsequent transportation costs, and makes these materials easier to handle.Cattle manure with 50% moisture content, and processed at a temperature of about 40 • C and a pressure of 6 MPa, resulted in maximum pellet durability [76].
In the field, slurry and manure application methods and their forms affect GHG emissions [68,77].Effects of raw farmyard manure, compost and pelleted manure application on soil C sequestration and GHG emissions are summarized in Table 3. Conventional injected pig slurry emitted greater CH 4 compared to injection with soil aeration, while manure spread on the surface emitted higher N 2 O than both types of injection [68].Slurry application season, method and rate all affected CO 2 and N 2 O emissions in the field [69][70][71].
The inclusion of compost into soil provides better nutrient input compared to raw manure from the perspective of C sequestration [77].For example, four years after application about 36% of applied compost remained in the soil as sequestered C, as compared to only 25% of applied raw manure [77].Composting increases the aromatic bonds and reduces the soluble C/N ratio in manure compost [75], which leads to the slow release of nutrients.Composted manures are more effective in reducing N 2 O emissions than raw manures for soil amendments [73].In general, N 2 O is produced through the denitrification process of organic fertilizers [78] while nitrification is the most important process for inorganic fertilizer.From one to five percent of total N applied from organic manure was emitted with emission rates depending largely on soil nitrate levels, dissolved organic carbon (DOC) content and aeration, as well as soil temperature, moisture and pH [79].
Effects of manure from different livestock and composts also vary in GHG emissions from soils.Raw cattle manure emitted higher amounts of CO 2 (~1 g kg −1 dry matter), followed by bio-waste composts, poultry manure, and sheep waste compost, within arable soils in Germany [72].Application of cattle manure and straw mixed together as a compost enhanced C sequestration and reduced N 2 O emissions; thus, composting of manure containing high lignin, such as rice-husk or wheat straw, is beneficial [72].Such compost reduces soil pH, which slows down the nitrification process and reduces N 2 O emissions [72].
Pellets derived from a mix of manure and urea enhanced nutrient use efficiency via the slow-release of nutrients and led to increased crop yields [80].Unlike composting, pelleted manures are less effective in reducing GHG emissions than raw (i.e., untreated) manure [19].Annual cumulative emission of N 2 O from pelleted poultry manure applied in the field was almost four times higher than that from raw poultry manure.Similarly, higher CO 2 emissions were detected from soils amended with intact pelleted poultry manure compared to the application of ground pelleted poultry manure under anaerobic incubation [19].N 2 O emission was 154 mg N kg −1 dry soils from intact pelleted manure-amended soils, which was almost seven times higher than from ground pelleted manure-amended soil [19].Soils emitted significantly higher N 2 O when treated with pelletized poultry litter (6.8% of applied N) than for fine-particle litter (5.5%) at 55% of water field capacity (WFP).In contrast, at 90% of WFP, fine-particle litter treated soils emitted higher N 2 O (3.4%) than soils receiving pelletized litter (1.5%), indicating GHG responses to pellet application depended on moisture, with pellets leading to more GHG if moisture is low [73].Reported CO 2 emissions ranged from 29 to 43 g C kg −1 across moisture levels, though they were not statistically different.Results indicate that N 2 O emissions, but not CO 2 emissions, from soils treated with poultry litter depend on its physical characteristics of litter and soil water regime.Diminishing rates of N 2 O emission after the application of manure pellets to soil are attributed to the polymer chain reaction defined by the specific type of nitrite reductase encoded by the nirS gene, which fluctuated with time; however, the nirK gene remains relatively stable, making nirS responsible for the denitrification process of N in manure pellets [15].
In forest ecosystems, application of organic and inorganic fertilizer has shown different effects on GHG emissions depending upon geographic location.In Germany the application of composted household waste manure increased CO 2 emission by 24% in silty soils and by 66% in sandy soils compared to control plots [66].On the other hand the application of organic and inorganic fertilizer to tropical forest in Brazil increased CO 2 emission by 90%, and 60% in composted sewage sludge, and raw sewage sludge amended sites, respectively, compared to emissions from controlled plots [67].Surprisingly, N 2 O emissions were 85 and 37 fold higher in the composted sewage sludge amended and raw sewage sludge amended plots compared to control plots [67].

Conclusions
Agroforestry has emerged as a holistic land use practice creating a win-win scenario for environment and society [81].Combining woody vegetation with cropping and livestock production via agroforestry systems increases total production, enhances food and nutrition security and mitigates the effects of climate change [81,82].Carbon sequestration and GHG emissions in agroforestry systems are complex and depend on various biophysical factors such as climatic conditions, soil properties, water regime, vegetation characteristics, and the site-specific management practices undertaken, including inputs.The ability of an agroforestry system to enhance C sequestration and reduce GHG emissions depends on the region-specific biophysical condition.Several estimates showed that agroforestry systems in temperate regions have higher C pools than other climatic regions [28,36].Silvopastoral systems were found to be superior in terms of both C sequestration and reducing GHG emissions [32,34,41] compared to agroforestry systems that included annual cropland.Interventions like enrichment planting and organic amendment of soils to slow down nutrient release are also site-specific in regulating their effectiveness [13].Our review indicates that broadleaved tree species used in enrichment planting contribute towards more soil C, while conifers sequester more in their biomass over the long-term [47].Results of this literature review showed that the effects of enrichment planting in agroforestry are not studied widely.The paucity of literature on this topic limits the drawing of conclusions with respect to the type of enrichment planting that will be most effective in optimizing ecosystem goods and services from agroforestry.
Livestock manure applied to soils in the form of pellets, compost or biochar, can play a significant role in increasing C sequestration and reducing GHG emissions.Soil amendment with biochar increases soil porosity, and aids water and nutrient retention, thereby creating a favorable situation for nutrient uptake by plants.By enhancing biomass production, biochar can play an important role in sequestering C in vegetation and soils.Additionally, decreased emissions of N 2 O and CO 2 , and increased uptake of CH 4 , have been reported in the literature after biochar application.However, many of these studies were carried out in annual cropping systems, leaving a substantial knowledge gap with respect to their effectiveness in agroforestry systems.Further studies on the specific effects of organic amendments to soils within either the treed area or adjacent cropland may provide a better idea on how agroforestry systems can be collectively managed to achieve greater C sequestration and reduce GHG emissions.In general, raw manure management and field applications of manure were found to be sources of CO 2 , CH 4 , and N 2 O emissions, although the magnitude of these GHG emissions varied with application season, methods and amounts.Composting and pelleting of manures can reduce GHG emissions while making manure more convenient to store and use.A more thorough study is warranted to better understand the relationship between different types of feedstocks and their capacity to enhance C sequestration and reduce GHG emissions within agroforestry systems.
Overall, this review found that enhancement of C sequestration and reducing GHG emissions in agroforestry are possible through management interventions.Enrichment planting practiced in secondary forestry management and organic amendment of soils in conventional cropping systems should be further explored within an agroforestry management framework, as potential interventions to enhance C sequestration and reduce GHG emissions.Further studies will provide better evidence of such beneficial practices for environment, economy, and society.

Table 1 .
Carbon sequestration rate, carbon stocks and greenhouse gas (GHG) emissions in different types of agroforestry systems.A negative flux shows consumption of GHGs.

Table 2 .
Review of previous studies examining the effects of biochar and/or compost manure addition on relative changes in GHG emissions.Negative value shows reduction in GHG emissions.

Table 2 .
Review of previous studies examining the effects of biochar and/or compost manure addition on relative changes in GHG emissions.Negative value shows reduction in GHG emissions.

Table 2 .
Review of previous studies examining the effects of biochar and/or compost manure addition on relative changes in GHG emissions.Negative value shows reduction in GHG emissions.

Table 3 .
Summary of the previously documented effects of applying raw manure, composted manure, or manure pellets on subsequent C sequestration and GHG emissions in croplands and grasslands.