Agroforestry; the soilution for increasing carbon stocks?
A meta-analysis on the global carbon sequestration potential in agroforestry systems
Annemarie van Rossum
02/07/2018
Supervisor: Boris Jansen
Co-Supervisor: John Parsons
Earth Sciences
UvA
2
Abstract
Agroforestry as a sustainable land management practice has raised interest over the years because of its potential to sequester and store atmospheric carbon, host habitat, decrease soil erosion and maintain food production.
In this meta-analysis the average carbon sequestration potential is calculated for agroforestry systems in semi-arid, temperate and tropical climate zones (2.42, 3.63, 4.13 Mg C ha-1 y-1 ) and aims to give a global estimation on the carbon storage potential in agroforestry systems, using the 4p1000 initiative (increase global carbon stocks with 0.4% annually) as a directive and measure.
15 case studies from 13 countries were collected and carbon sequestration rates averaged and compared. High variability in rates showed climatic influences on carbon sequestration rates, ranging between 0.22 – 9.40 Mg C ha-1 y-1, with highest carbon sequestration potential found in the tropics, storing 1982.5 Mg C y-1, compared to 1256 Mg C y-1 in the temperate zone and 895.4 Mg C y-1 for semi-arid environments. An average global sequestration rate of 4.13 Mg C ha-1 y-1 was calculated with the potential to increase carbon stocks with 4054.4 Mg C y-1, compared to a necessary increase of 0.4%, at 30 cm, 100 cm and 200 cm soil depth, respectively 2816, 6020 and 9600 Mg C y-1. Only at a 30cm soil depth, agroforestry can increase global carbon stocks with 0.4% annually.
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Table of contents
I.
Introduction
pg.
4
II.
Theoretical Framework
II.I 4p1000
pg.
6
II.II Carbon sequestration
pg.
9
II.III Agroforestry
pg.
10
III.
Methods
pg.
12
IV.
Results
pg.
13
V.
Discussion
pg.
16
VI.
Conclusion
pg.
20
VII.
References
pg.
21
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I.
Introduction
Rapid population growth is leading to an increased global carbon footprint and required doubled food production within the next 30 years (Verchot et al., 2007). Through sustainably increasing carbon stocks in soils worldwide with 0.4% per year, the 4p1000 initiative, launched during the COP21 in Paris, tries to tackle these challenges by encouraging sustainable agro-ecological land management practices (4p1000.org, 2015).
A sustainable strategy that reduces atmospheric carbon emissions (CO2) and contributes to climate mitigation is agroforestry (Griscom et al., 2017). Implementing trees in agricultural land could store carbon on the long term while maintaining food production (Verchot et al., 2007). The process behind carbon storage is carbon sequestration, which involves the uptake of CO2 from the atmosphere through photosynthesis and storage of carbon as part of soil organic matter (SOM) through decomposition (Fujisaki et al., 2017). Soils rich in soil organic matter are more fertile and able to cope better with the effects of climate change (4p1000.org, 2015). However, estimating the potential of carbon sequestration is complex.
A recent study on the feasibility of the 4p1000 initiative by Minasny et al., (2017) concluded that increasing carbon stocks in the top soil (30 cm) in managed agricultural soils can offset 20-35% of global anthropogenic carbon emissions, but disagreement in the soil scientific community arose around the representativeness of these estimations and the used methodological approach, resulting in a discussion between soil scientists (Baveye et al., 2018; de Vries, 2018; VandenBygaart, 2018). An overview of the key arguments in this discussion is provided to gain insight in the complexity of measuring carbon sequestration potentials on a global scale. The critique points were used as a suggestive directive in this meta-analysis on the carbon sequestration potential for agroforestry systems.
Besides carbon sequestration, agroforestry has the environmental benefits of conserving biodiversity, soil enrichment and maintaining air and water quality (Jose, 2009). Agroforestry has just recently been recognized as a carbon sequestration strategy but has been practiced for many centuries, mainly in developing and tropical countries (Zomer et al., 2017; Nair et al., 2009). There is a numerous amount of modern and traditional agroforestry systems designed and adapted to various social, economic and environmental conditions, demonstrating the implementation possibility (Young, 1989). Of which forest farming, riparian forest buffers, windbreaks, alley cropping and silvopastoralism are the five main forms (Feliciano et al., 2018).
Agroforestry seems a promising sustainable land management practice and could possibly play a role in mitigating climate change and enhancing food security. Therefore this paper aims to give a global estimation on the carbon sequestration potential of agroforestry and questioned whether agroforestry as a sustainable land management practice can increase global carbon stocks with 0.4% annually. 15 case studies from three agricultural dense climate zones (semi-arid, temperate and tropical) were reviewed and average carbon sequestration rates calculated.
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Given the broad scope of information available on the subjects, this review and meta-analysis has 3 primary objectives; (i) providing an overview of key arguments in the discussion on the 4p1000 initiative, (ii) providing a meta-analysis on carbon sequestration potential of agroforestry using quantitative data, (iii) assessing whether agroforestry could increase global carbon stocks with 0.4% annually.
In the theoretical background the 4p1000 initiative, carbon sequestration and agroforestry will be elaborated on, followed by a meta-analysis on carbon sequestration potential of agroforestry. Consequently the results are discussed leading to the conclusion of the research.
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II.
Theoretical Framework
II.I The 4p1000 discussion
The 4p1000 initiative was used as a guideline in assessing the feasibility of agroforestry and isvisualized in Fig. 1, presenting its objectives.
The calculation of the 0.4% aspiration is based on annual anthropogenic carbon emissions of 8.9 Gt (8.9 x 1015 g) divided by 2400 Gt, the estimated total carbon stock for a 2 m soil depth (Minasny et al., 2017).
The 4p1000 initiative has multiple aims, and - as the website of the initiative states- ‘’The ambition of the initiative is to encourage stakeholders to transition towards a productive, highly resilient agriculture, based on the appropriate management of lands and soils, creating jobs and incomes hence
ensuring sustainable development (4p1000.org, 2015).’’
This all sounds very promising, however the initiative received a lot of critique from the soil scientific community, especially after a feasibility studies done by Minasny et al., (2017) on reaching the quota of the 4p1000 initiative. Quantitative data on carbon sequestration from 20 regions was analysed to examine whether increasing global carbon stocks with 0.4% is feasible. The numbers by Minasny et al., (2017) by many were considered an overestimation, while important factors were considered lacking from the assessment. A consequence is that stakeholders and policy makers are being provided with incorrect information on the feasibility of the initiative (Baveye et al., 2018; de Vries, 2018; VandenBygaart, 2018; White et al., 2017).
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Multiple arguments Minasny et al., (2017) received (Table 1) were regarding the fact that their analysis was solely based on carbon as emitted greenhouse gas, not including other greenhouse gases such as methane (CH4) and N2O (Baveye et al., 2018). VandenBygaart (2018) calculated that carbon represents 60-65% of the total greenhouse gas emissions and including those would reduce the estimated offset of emissions of 20-35% calculated by Minasny et al., (2017) with 14-23%. White et al., (2017) also stated; ‘’We believe that a global GHG offset of 20-35% through soil C sequestration is a gross overestimate.’’ Furthermore, global temperatures will riseresulting in increased microbial activity due to more ambient temperatures, decreasing microbial activity and simultaneously declining sequestration rates (Baveye et al., 2018), a significant factor not included in the feasibility analysis of Minasny et al., (2017). De Vries (2017) even states that ‘’Increasing the SOC content up to 2 m is totally impossible within decades. Only top-soil organic matter responds to land management changes within a few decades. ‘’
In a rejoinder, Minasny et al., (2018) commented that the 4p1000 is ‘more an aspirational target that will help in the promotion of sustainable soil management.’ In response to Baveye et al., (2018) on the fact that other greenhouse gases were not included in the analysis, they stated that the 4p1000 initiative focuses solely on anthropogenic carbon emissions (CO2). Regarding the statement of de Vries (2018) saying that only 2100 Mha is technically suitable for increasing carbon stocks with 0.4%, Minasny et al., (2018) explained that considering only managed agricultural soils, the calculations are indeed optimistic but believe a large carbon sequestration potential is attainable in unmanaged soils. Even though the initiative, and especially Minasny et al., (2017), were commented for their discrepancies in their study and overestimation of numbers, the potential of carbon sequestration for climate mitigation should not be underestimated. The following section will elaborate on this complex process including the benefits of carbon sequestration.
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Table 1.
Keypoints in discussion between Minasny et al., (2017) including referenced critique from the soil scientific community. Subjects 4p1000 Minasny et al., (2017) on
feasibility 4p1000 initiative
Critique References
GHG emissions 8.9 Gt of C emissions per year from fossil fuels.
Focus is solely on carbon emissions. Other greenhouse gases are excluded, CH4 or N2O.
Net changes in all GHG emissions need to be calculated to evaluate true abatement of a management practice.
This number only represents about 65% of the total greenhouse gas emissions from anthropogenic sources.
Baveye et al., (2018)
White et al., (2017)
VandenBygaart, (2018)
C/N ratio Increasing SOC by adding nutrients.
Unrealistically large amounts of nitrogen are needed to increase SOC in soils.
Baveye et al., (2018) Van Groenigen et al., (2017)
Offset emissions Global anthropogenic greenhouse gas emissions could be offset with 20-35%.
Carbon sequestration has a realistic negative carbon emission potential of 0.7 Gt Ceq. Yr-1. Mitigation potential decreases to 14-23% when including CH4, N2O and fluorinated gases.
Baveye et al., (2018) Smith, (2016) VandenBygaart, (2018) Temperature rise due to climate change
Not mentioned in analysis. An increase in ambient temperature will stimulate microbial activity, decreasing the amount of soil organic matter
Baveye et al., (2018)
Soil is a sink for CO2
Soil acts as a sink for CO2. Soil can also act as a source for CO2, plants
release CO2 into the atmosphere.
Baveye et al., (2018). Carbon sequestration over time SOC sequestration is between 2-3 Gt C year-1.
Carbon sequestration is time restrained; ‘’sink saturation.’’
Baveye et al., (2018). Smith (2016)
‘’Priming’’ effect Not considered in Minasny et al., (2017).
Added organic matter results in higher CO2
emissions plus ‘’old’’ more stable soil organic matter can get degraded in the process which may have deleterious consequences on the long-term persistence of the architecture of the soils and on their capacity to resist erosion.
Baveye et al., (2018)
Financial incentives
Not considered in Minasny et al., (2017)
Financial costs are not considered when changing from farming system.
White et al., (2017)
Soil depth 2m soil depth can store 2400 Gt C globally.
Only 20-30 cm of soil can be effectively managed, therefore only top 30cm of existing agricultural soils should be considered.
When 40cm soil depth is considered, carbon storage can be up to 860 Gt C.
VandenBygaart, (2018)
De Vries, (2018)
Potential area 4900 Mha Cropland and managed grassland constitutes for approximately 2100 Mha of managed land.
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II.II Carbon sequestration
Soils play a fundamental key role in the balancing systems on earth. Besides regulating nutrient, water and carbon cycles, soils host biodiversity and provide the base for our food systems (Fujisaki et al., 2017). Important is therefore maintaining soil fertility, a benefit of carbon sequestration additional to improving nutrient and water retention capacity, reducing risks of erosion, enhancing input efficiency use, and above all reducing anthropogenic carbon emissions (Lal et al., 2004). Carbon sequestration is a balancing mechanism in the carbon cycle and regulates carbon fluxes between atmosphere, oceans and the terrestrial system with large flux rates, consequently, changes in these fluxes can have a large impact on the carbon cycle and climate (Nair et al., 2009; Post et al., 1999).
The 4p1000 initiative aims at increasing global carbon stocks by stimulating carbon sequestration, a process defined as ‘’Any increase in carbon content of the soil resulting from a change in land management´´ by Powlson et al., (2011). Carbon from the atmosphere (CO2) is sequestered and stored in carbon stocks via the processes of photosynthesis, respiration and decomposition (Montagnini et al., 2004).
There are two ways carbon is sequestered; below and aboveground (Fig. 3). Atmospheric CO2 is assimilated and partly stored in vegetative biomass aboveground and partly transported belowground via litter deposition, root exudates, root growth and turnover. Via microbial decomposition, carbon belowground is added to the carbon stock as part of soil organic matter (SOM) (Nair et al., 2009). Approximately two-thirds of the total terrestrial
carbon is stored in soil organic matter and is determined by decomposition rate and the in-and output of carbon. The carbon sequestration potential in soils is besides influenced by environmental factors, soil properties and the availability of microbes present in the soil and reflects the decomposition rate (Krull et al., 2003; Lützow et al., 2007; Freibauer et al., 2004). Fig. 2 shows the relationship between mean annual temperature (MAT) and litter decomposition according to the average k value of litter decomposition and indicates that higher temperatures coincide with higher decomposition rates (Prescott, 2010).
When more carbon is stored belowground than emitted, the net carbon exchange is positive and soil acts as a sink. A negative carbon exchange implies that carbon is emitted back into the atmosphere through respiration, and therefore acts as a source of atmospheric carbon. This is also dependent on environmental factors and used land management practice (Powlson et al., 2011). Loss in carbon stocks results in decreased soil quality and soil degradation (Kirkby et al., 2013).
The following section will elaborate on carbon sequestration potential in agroforestry systems.
Fig. 2. Relation between mean annual temperature and litter
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II.III Agroforestry
Agroforestry gained more attention because of its potential for carbon sequestration while maintaining agricultural production (Upson & Burgess, 2003). When speaking of agroforestry, one speaks of ‘’the combination between forestry and agriculture creating integrated sustainable land-use systems,’’ citing Schoeneberger (2009). And Young (1989) offers a formal definition of agroforestry; ‘’Agroforestry is a collective name for land-use systems in which woody perennials (trees, shrubs, etc.) are grown in association with herbaceous plants (crops, pastures) and/or livestock in a spatial arrangement, a rotation or both, and in which there are both ecological and economic interactions between the tree and non-tree components of the system.’’
Trees are implemented in agricultural systems in many different ways, forest farming, riparian forest buffers, windbreaks, alley cropping and silvopastoralism are the five main forms (Feliciano et al., 2017).
Table 2 provides an overview of these agroforestry practices, including additional benefits, derived from Schoeneberger (2009).
In this research, agroforestry includes all forms of agroforestry practices.
Table 2.
Description of five main agroforestry practices (Schoeneberger, 2009).
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Agroforestry systems have multiple benefits, including carbon sequestration, conserving biodiversity and increasing food production (Feliciano et al., 2018). But implementing trees in agricultural systems also reduces run-off, increases soil porosity and increases infiltration and retention of water in soils. Furthermore, evapotranspiration rates are high in agroforestry systems, maintaining an aerated soil condition. And the value of crops produced by trees is usually higher than row crops (Verchot et al., 2007).
Carbon sequestration potential in agroforestry systems
Compared to crop and pasture land, agroforestry systems have potential to sequester more carbon because of the incorporation of trees in the system, resulting in a higher above- and belowground sequestration potential (Nair et al., 2009). Carbon is stored in aboveground and below ground woody biomass; trees, crops and root system (Fig. 3) (Schoeneberger, 2009). The biomass of trees alone already consists for 46-51% of carbon, of which about 70% of the carbon is derived through above ground carbon sequestration and 30% through soil carbon sequestration (Kim et al., 2016).
Additionally, Muñoz et al., (2007) did a studies on the influence of canopy cover from leguminous trees (Acacia Caven) in Espinal agroforestry systems in semi-arid Chile. Soils without canopy cover contained 28-40% less carbon than soils when covered and they concluded that increased canopy cover can result
in a carbon stock increase of approximately 50%.
Because of tree growth, carbon sequestration rates in agroforestry systems are high. However, after some years (dependent on tree species) sequestration rates stabilize when trees are fully grown (Feliciano et al., 2018).
Although agroforestry increases carbon sequestration rates, climatic factors (temperature and precipitation) influence the decomposition rate and consequently carbon sequestration. Moist conditions and high temperatures affect sequestration positively, resulting in high decomposition and respiration rates in tropical soils, but these are also highly weathered, therefore, in the tropics more carbon is stored in aboveground biomass. Whereas lower temperatures tend to accumulate more soil organic carbon (Freibauer et al., 2004; Fujisaki et al., 2017).
Fig. 3. Process of carbon sequestration in trees illustrated (USDA
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III.
Methods
III.I Study covering area’s
37% of the global land use area is used for agricultural purposes and has potential shifting from conventional agricultural practices to more sustainable land management practices, in this case agroforestry (FAOSTAT, 2016). As has been mentioned, carbon sequestration is influenced by climatic factors, therefore three agricultural dense climate zones are taken as variable in this study to give a global estimation on the potential of agroforestry. The case studies in the different climate zones are selected according to climate classification of IPCC and FAO.
Semi-arid environments: Mean annual temperatures between 20-45°C. Annual rainfall varies between 0-800 mm (FAO, 1989).
Temperate: Areas where mean annual temperature is between 0-20°C. Rainfall; variable (IPCC, 2002). Tropics: Areas where mean annual temperature (MAT) is more than 20°C. and driest month
precipitation is larger than 60mm (IPCC , 2002).
III.II Data collection
For the meta-analysis 15 peer reviewed studies on the subject of carbon sequestration in agroforestry systems were collected, of which most were published after 2000, only one study was published in 1999. Literature was selected using web search engines Google Scholar and Web of Knowledge. Used keywords were soil carbon sequestration, agroforestry, temperate zone, tropics, Mediterranean, Europe, semi-arid, carbon stocks. These were put in separately or in combination with each other. Data was collected and analyzed in three major categories; semi-arid, temperate and tropical. Study areas were restricted to the climate zones according to FAO and IPCC climate classification based on rainfall and mean annual temperature (FAO, 1989; IPCC, 2006).
Different agroforestry practices in the case studies are furthermore addressed under the same term of agroforestry.
III.III Descriptive meta- analysis
For each climate zone, the average carbon sequestration is calculated by adding up the rates from each case study and dividing the total with the number of case studies used per climate zone, 5. The average carbon sequestration rates in Mg C ha-1 y-1 are multiplied with potential area, in Mha, suitable for agroforestry, calculating the annual carbon stock increase in Mg C y-1 per climate zone.Adding up the sequestration rates of each climate zone and dividing the total with three resulted in the average global carbon sequestration rate in Mg C ha-1 y-1, multiplied by global potential area gives the global estimation of carbon stock increase. The necessary increase ( in Mg y-1 ) in carbon stocks is calculated by multiplying the value for global carbon stocks at 30cm, 100 cm and 200cm soil depth with 0.004.
For some case studies, the carbon sequestration rates had to be adjusted, usually from petagram (x 1015) or gigagram (x 109) to megagram (x 106). Furthermore, some studies only measured the carbon stocks at the beginning and end of experiment in Mg C ha-1, carbon sequestration rate was then calculated as (SOCend - SOCbeginning) / t = ∆ SOC in Mg C ha-1 y-1. In which t is expressed in years as duration of the experiment.
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IV.
Results meta-analysis
Data from 15 studies were used in this analysis, covering various soil and climate conditions from 13 different countries. Results show that carbon sequestration rates for different agroforestry practices in semi-arid environments are lowest, ranging from 0.22 – 9.40 Mg C ha-1 y-1 , with an average rate of 2.42 Mg C ha-1 y-1 (Table 3).However, the sequestration rate in Spain (9.40 Mg C ha-1 y-1 ) being an outlier for the semi-arid climate zone and compared to rates of the other climate zones. For the temperate climate zone, an average sequestration rate of 3.63 Mg C ha-1 y-1 was calculated. The results also show high variety between sequestration rates, two rates being smaller than 1 Mg C ha-1 y-1 for the case studies in the United Kingdom (0.7 Mg C ha-1 y-1 ) and Canada (0.69 Mg C ha-1 y-1) and the other three case studies each showing values above 4 Mg C ha-1 y-1 with the highest sequestration rate coming from the US (7.15 Mg C ha-1 y-1) (Table 3).In the tropics, all sequestration rates are above 3 Mg C ha-1 y-1, ranging from 3.3 – 5.48 Mg C ha-1 y-1, with the highest sequestration rate found in India for an agrisilviculture system. The carbon sequestration rate in the tropics is 4.13 Mg C ha-1 y-1 and highest compared to the average sequestration rates in the semi-arid and temperate climate zones.
Multiplying the sequestration rates with the potential area for agroforestry per climate zone, the potential increase in carbon stocks was calculated in Mg y-1 (Table 4). The tropical climate zone, with the largest estimated area suitable for agroforestry of 480 Mha and the highest average sequestration rate of 4.13 Mg C ha-1 y-1, has the potential of increasing carbon stocks annually with 1982.5 Mg y-1, almost half of the global potential. In the temperate climate zone 1256 Mg y-1 carbon increase is possible per year, with an estimated suitable area of 346 Mha, compared to Africa where 370 Mha is technically suitable but increase in carbon stocks is 360.6 Mg y-1 less compared to the temperate zone. To calculate the possible carbon increase in carbon stocks on a global scale, the sequestration rates for each climate zone were averaged, 3.39 Mg C ha-1 y-1, resulting in a potential global to increase carbon stocks with 4054.4 Mg y-1 (Table 4).
Table 5 shows the necessary increase in global carbon stocks according the 0.4% guideline. This accounts for an increase of 9600 Mg annually for a 2 m soil depth, 6020 Mg at 1m soil depth and 2816 Mg for the top layer, 30 cm soil depth.
14 Table 3.
Calculated average carbon sequestration rates for three climate zones.
Region Soil sequestration potential in Mg C ha-1 y-1
Remarks References
Semi – arid regions
France 0.25 Alley cropping Cardinael et al., (2015)
Senegal 0.22 Faidherbia albida tree plantation Tschakert (2004)
Kenya 0.75 Average sequestration rate
different agroforestry systems
Batjes (2004)
Spain 9.40 Agrosilvopasture agroforestry practice with Radiata Don tree
Mosquera-Losada et al., from Kumar&Nair (2011)
Argentina 1.5 Average of agroforestry plantation, trees species; Pinus Ponderosa
Nosetto et al., (2006) Range: 0.22 – 9.40 Average rate: 2.42 Mg C ha-1 y-1 Temperate North America, US and Canada
4.03 Average sequestration rate for 3 major agroforestry practices; riparian buffers, alley cropping and silvopasture
Udawatta & Jose, from Kumar&Nair (2011)
Belgium 5.3 Boundary and alley cropping Pardon et al., (2017)
Ontario, USA 7.15 Average sequestration rate of two intercropping systems
Peichl et al., (2006)
United Kingdom 0.7 Intercropping Upson & Burgess (2003)
Canada 0.69 Alley cropping Oelbermann et al., (2005)
Range: 0.69 – 7.15 Average rate: 3.63 Mg C ha-1 y-1
Tropics
Cameroon 3.55 Rotational agroforest with cacao Palm et al., (1999)
Indonesia 3.57 Rotational agroforest with rubber Palm et al., (1999)
Togo 4.76 Coffee and Albizia tree plantation Dossa et al., (2008)
India 5.48 Agrisilviculture system Swamy & Puri (2005)
Brazil 3.3 Conversion cropland to
agroforestry after slash and burn
Mutuo et al., 2005 Range: 3.3 – 5.48
Average rate: 4.13 Mg C ha-1 y-1
15 Table 4.
Calculated carbon storage potential per climate zone.
* numbers are rounded.
1 Dixon et al., (1995). Estimation of land technically suitable for agroforestry, averaged. 2 Vleeshouwers et al., (2002); Estimation of agricultural land use in Europe.
1 Global estimated carbon stocks used in studies of Minasny et al., (2017) from Batjes (1996);
2 Annual anthropogenic carbon emissions 8.9 giga tonnes / 2400 Pg global C stock in 2m soil depth = 0.4%.
Average
sequestration rate for the climate zones
Average potential area technically suitable for agroforestry practices in Mha ( x106 ha )
Annual increase C stock in Mg (x106)
Semi-arid 2.42 Mg C ha-1 y-1 Africa: 370 Mha1 895.4 Mg y-1
Temperate 3.63 Mg C ha-1 y-1 North America: 115 Mha1
Europe: 231 Mha2
417.45 Mg y-1 838.53 Mg y-1
Total: 1256 Mg y-1
Tropics 4.13 Mg C ha-1 y-1 South Asia: 257.5 Mha1
South America: 222.5 Mha1 1063.5 Mg y-1 919 Mg y-1 Total: 1982.5 Mg y-1 Global 3.39 Mg C ha-1 y-1 1.196 Mha 4054.4 Mg y-1 * Table 5.
Necessary carbon stock increase according to the 4p1000 initiative.
Soil depth Global
carbon stocks in Pg1 Carbon stock increase in Pg of 0.4% 2 Necessary increase in Mg y-1 (x 106) 30 cm soil depth 704 2.816 2816 Mg C 100 cm soil depth 1505 6.02 6020 Mg C 200 cm soil depth 2400 9.6 9600 Mg C
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V.
Discussion
V.I Potential of agroforestry in increasing global carbon stocks with 0.4% annually
This study aimed to assess the potential of agroforestry in reaching the 4p1000 initiative and confirmed that agroforestry can increase carbon stocks with 0.4% up to a soil depth of 30cm with the potential to increase carbon stocks with 4054.4 Mg a year (Table 4). Comparing the results of this studies with the necessary increase according to the 0.4%, an approximate of 2000 Mg C y-1 falls short at a 1m soil depth and about 5.550 Mg C y-1 at a 2m soil depth (Table 5). On the other hand, only the top layer of the soil can be effectively managed (VandenBygaart, 2018).
An objective of this study was to calculate a global sequestration rate for agroforestry systems according to three agricultural dense climate zones; semi-arid, temperate and tropical zone with average sequestration rates of respectively 2.42, 3.63 and 4.13 Mg C ha-1 y-1. This resulted in a global average sequestration rate of 3.39 Mg C ha-1 y-1 (Table 3). A low value compared to an average rate of 7.2 Mg ha-1 y-1 calculated in the studies of Kim et al., (2016) from 56 peer reviewed publications on agroforestry worldwide. This might possibly be related to various factors such as tree and crop species, soil type and climate conditions.
Regions with high temperatures and precipitation tend to have higher decomposition rates, explaining the high rates found in the tropics (Table 3) (Freibauer et al., 2004). Furthermore, a larger area is suitable for agroforestry practices in tropical regions in comparison to semi-arid or temperate climate regions (Table 3), resulting in an overall higher potential to increase carbon stocks in this climate zone. Besides, encouraging agroforestry practices in the tropics could help to address the problem of deforestation, converting tropical forests into cropland or pasture results in a larger carbon loss in comparison with agroforestry systems (Oelbermann et al., 2004).
Compared to the tropics the potential increase in semi-arid regions carbon stocks is almost half (Table 4). Long dry periods and high temperatures result in low productivity and decomposition of carbon (Takimoto et al., 2008). Table 3 shows a large variety in sequestration rates, ranging from 0.22 – 9.40 Mg C ha-1 y-1, which is in line Nair et al., (2009) stating that carbon sequestration rates in agroforestry systems are highly variable ranging from 0.29 to 15.21 Mg ha-1 y-1.
The outlier of 9.40 Mg C ha-1 y-1 in the semi-arid environment from Spain (Table 3) can be explained by the fact that it concerns an agrosilvopastural system where livestock is incorporated in the system, providing an organic carbon input and positively influencing the carbon sequestration rate (Mosquera-Losada et al., from Kumar&Nair, 2011).
In the temperate climate zone, lower temperatures result in a lower decomposition rate compared to the tropics (Freibauer et al., 2004). Questionable is however whether the case studies represent the global temperate zone since used studies are from the same country, two from Canada and the US (Table 3). This also accounts for the other climate zones, five case studies do not represent the potential for agroforestry therefore, caution is required when interpreting the results.
Nonetheless, the results of the present study show similarity with the results of a study by Feliciano et al., (2018). Fig. 4 shows the mean above and below carbon sequestration rates for agroforestry systems in different climates, with the highest mean rates noticeable in tropical climates and lowest in semi-arid environments. Higher variability is found in the temperate zone, which could be explained by weather fluctuations in this climate zone.
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Moreover, Minasny et al., (2017) calculated an annual carbon sequestration rate between 2000 and 3000 Mg C for a 1m soil depth compared to 4054.4 Mg C y-1 calculated in this meta-analysis, though caution is required when interpreting the results. The analysis of Minasny et al., (2017) considers only managed agricultural land to have potential for increasing carbon stocks and is estimated for 1m soil depth, even though the 0.4% value is calculated according to the current carbon stock for 2m soil depth. The percentage of 0.4% used as a guideline in this studies is a value derived from the 4p1000 initiative and concerns a suggestive value for policymakers (Minasny et al., 2018).
V.II Other greenhouse gases
In order to give a complete estimation on mitigation potential, other GHG emissions (CH4 and N2O) should be included in the calculations as well, not solely carbon sequestration potential because sustainable land management practices do not solely increase carbon stocks, but could also increase CH4 or N2O emissions, gases contributing respectively 25 and 298 times more to global warming compared to the Global Warming Potential (GWP) of CO2 (Powlson et al., 2011). In contrast to a research conducted by Kim et al., (2016) concluded that CH4 and N2O emissions remained nearly the same in agroforestry systems as for conventional agroforestry systems.
Including all greenhouse gas emissions in calculating the total mitigation potential was however out of scope for this research.
18 V.III Limitations of sequestration
Carbon accumulation in the soil is limited, once an equilibrium is reached, sink saturation occurs. This usually happens after 20-50 years (Powlson et al., 2011; Feliciano et al., 2018).
The upper figure (Fig. 5A) shows a decline in sequestration rate in time, relating to the figure below (5B) which shows carbon saturation in soils under agroforestry practices. It is therefore questionable whether how effectively carbon sequestration mitigates climate change on the long term (Kim et al., 2016).
Not included in this study is that carbon sequestration is a process that will be affected by climate change as well. Rise in temperatures results in an increase in decomposition rate, losing large amounts of carbon in boreal and temperate regions (Fujisaki et al., 2017).
V.IV Methodological difficulties Lack of uniformity
As has been stated, the results of this research should be interpreted with caution since the values were derived from studies with high variability in conditions.
Assessing carbon sequestration potential is difficult, soil carbon sequestration is influenced by plant and soil characteristics, agro-ecological conditions, management factors and characteristics of the whole system, i.e.; function; purpose and products (Nair et al., 2009).
The values for the meta-analysis were derived from different case studies, selected according climate zone and focusing on agroforestry systems and concerned multiple different agroforestry practices with various soil types, combinations of trees, crops and livestock and different climatic influences per study area, resulting in high variety among carbon sequestration rates and potential.
Furthermore, a lack in uniformity in methodological approach in the used studies challenge the scientific comparability. Time span of studies, sampling soil depth and measurement techniques differ, questioning if sequestration rates are representative and realistically comparable with each other (Kumar & Nair, 2011).
Fig. 5. Sequestration in agroforestry systems after
19 Estimation of potential area suitable for agroforestry
Data on potential area that could be used for agroforestry practices are usually based on global estimations. However, dissimilar numbers are found. IPCC (2002) estimates the potential area for agroforestry at 630 Mha, while Minasny et al., (2017) states that all managed agricultural lands have the potential to increase carbon stocks and consider an area of 3900-4900 Mha. Tilman et al., (2001) provided a future forecast on agricultural land use change estimating 5330 Mha are of crop and pasture land in 2020 and 5900 Mha in 2050.
The estimated values used for this meta-analysis were derived from Dixon et al., (1995), providing estimates per continent and therefore enabling to calculate the potential of carbon storage per climate zone as well as globally, however, in the light of conflicting numbers, one must interpret the estimates with caution.
Estimation of global carbon stock
In this studies the values for the carbon stocks are derived from Batjes (1996) (Table 5), the same that have been used in calculating the 0.4% and used in Minsasny’s et al., (2017) study. However, these values account for global carbon stocks, but as many studies have stated including Minasny et al., (2017), increasing carbon stocks has most potential in managed agricultural land, or arable land and permanent crops, and permanent meadows and pastures (Sommer & Bossio, 2017). To give a correct estimate for the carbon sequestration potential for agroforestry the carbon stocks present in agricultural lands should be used for a correct calculation, since these have potential of being converted to agroforestry systems.
20
VI.
Conclusion
Agroforestry has great potential as a sustainable land management practice, besides benefits for soil, climate and food production it enhances the process of carbon sequestration. Implementing agroforestry systems on a global scale could increase carbon stocks with 0.4% annually up to a soil depth of 30 cm, and therefore play a role in reaching the aims of the 4p1000 initiative. In tropical areas, the carbon sequestration potential is highest compared to semi-arid and temperate climate zones due to climatic factors and the large area susceptible for implementing agroforestry systems. However, carbon sequestration is a limited process. Once a carbon equilibrium is reached, the soil is saturated and sequestration declines after 20-50 years. Sustainable land management practices that increase carbon stocks, including agroforestry, could therefore be recognized as short term solutions in mitigating climate change. However, regarding the issue of food security, agroforestry is promising because of increased or maintained food production.
Assessing agroforestry on a global scale is a complex matter, involving many different factors influencing the potential of the system. High variability in carbon sequestration rates for agroforestry practices in the different climate zones is found, due to environmental influences, soil type and tree and crop species used in the systems. Furthermore, the type of agroforestry practiced also determines the sequestration potential, silvopastural systems have higher carbon sequestration rates due to the incorporation of livestock. Inconsistency in methodology of the researches challenges the representativeness of the global carbon sequestration potential for agroforestry systems because the average rates calculated are not necessarily applicable in every region in the designated climate zones (semi-arid, temperate and tropics). Moreover, estimations on the global carbon stock and potentially available area susceptible for agroforestry practices vary and questionable is therefore the representativeness, and asks for further research. As well as long term research on the overall mitigation potential of agroforestry is suggested, including all greenhouse gas emissions and correct estimations on carbon stocks and potentially available area.
21
VII.
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