potential for carbon sequestration and mitigation of climate change by irrigation of grasslands

Potential for carbon sequestration and mitigation of climate change byirrigation of grasslands

Alexander Olsson a,, Pietro Elia Campana b, Mrten Lind c, Jinyue Yan a,ba School of Chemical Engineering and Technology, Royal Institute of Technology, Teknikringen 42, SE-100 44 Stockholm, Swedenb School of Sustainable Development of Society and Technology, Mlardalen University, SE-721 23 Vsters, Swedenc ZeroMission, Stora Nygatan 45, SE-111 27 Stockholm, Sweden

h i g h l i g h t s

A generic method for climate changemitigation feasibility of PVWPS isdeveloped.

Restoration of degraded lands inChina has large climate changemitigation potential.

PV produces excess electricityincluded in the mitigation potential ofthe system.

The benefit is higher than if the PVwere to produce electricity for thegrid only.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 October 2013Received in revised form 14 July 2014Accepted 6 August 2014Available online 28 August 2014

Keywords:Grassland conservationIrrigationSoil organic carbonCO2 emission reductionCarbon sequestration

a b s t r a c t

The climate change mitigation potential of irrigation powered by a photovoltaic water pumping system(PVWPS) to restore degraded grasslands has been investigated using the Intergovernmental Panel on Cli-mate Change (IPCC) 2006 Guidelines for National Greenhouse Gas Inventories for Agriculture, Forestryand Other Land Use. The purpose of this study is to develop a generic and simple method to estimatethe climate change mitigation benefit of a PVWPS. The possibility to develop carbon credits for the carbonoffset markets has also been studied comparing carbon sequestration in grasslands to other carbonsequestration projects. The soil carbon sequestration following irrigation of the grassland is calculatedas an annual increase in the soil organic carbon pool.The PVWPS can also generate an excess of electricity when irrigation is not needed and the emissions

reductions due to substitution of grid electricity give additional climate change mitigation potential.The results from this study show that the carbon sequestration and emissions reductions benefits per

land area using a PVWPS for irrigating grasslands are comparable to other carbon sequestration optionssuch as switching to no-till practice. Soil carbon in irrigated grasslands is increased with over 60% relativeto severely degraded grasslands and if nitrogen fixing species are introduced the increase in soil organiccarbon can be almost 80%. Renewable electricity generation by the PVWPS will further increase the mit-igation benefit of the system with 7090%. When applying the methodology developed in this paper to acase in Qinghai, China, we conclude that using a PVWPS to restore degraded grasslands for increasedgrass production and desertification control has a climate change mitigation benefit of 148 Mg(1 Mg = 1 metric ton) CO2-equivalents (CO2-eq) per hectare in a cold temperate, dry climate during a20 year process of soil organic carbon sequestration and emissions reductions. Leakage due to an increase

http://dx.doi.org/10.1016/j.apenergy.2014.08.0250306-2619/ 2014 Elsevier Ltd. All rights reserved.

Corresponding author.

Applied Energy 136 (2014) 11451154

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in N2O emissions from the additional biomass production and introduction of nitrogen fixing species isincluded in this result. The most important conclusion from our case is that if soil carbon sequestrationis lower than 24 Mg CO2-eq per hectare including leakage, then the climate change mitigation benefit islarger if PV is used to produce electricity for the grid.

2014 Elsevier Ltd. All rights reserved.

1146 A. Olsson et al. / Applied Energy 136 (2014) 114511541. Introduction

Water scarcity imposes the principle limitation to biomassgrowth in arid and semi-arid grassland regions [1]. Irrigation ofsoils suffering from occasional drought can enhance biomass pro-duction and thus increase the amount of carbon returned by theroots and by dead organic material to the soil and increase the car-bon sequestered as soil organic carbon (SOC) [2]. Desertificationhas several adverse effects on soil quality, among these depletionof carbon, and this is a major environmental problem [3].

China suffers from desertification of grasslands at a rate ofabout 0.25 Mha yr1, resulting in an estimated direct economicloss of 54 billion RMB yr1 [4] or about 8 billion USD. The Chinesegrasslands cover an area of 290400 Mha [4] and comprise an areaof around 10% of the total grasslands on earth. The global rate ofdesertification is 5.8 Mha yr1 [3] and grasslands occupy 3460 Mha around the world [2] or about 26% of the ice-free landarea [5].

The possible rate of restoring carbon content in soil through soilcarbon sequestration (SCS) due to desertification control and soilrestoration in dry lands worldwide is estimated to 0.20.6 Pg car-bon yr1 (1 Pg = 1 billion metric tons) for 2550 yrs [2,3]. Restoringthe SOC pools of the earth is not only a key factor in sustainablemeat and milk production from grazing animals [6] it also consti-tutes a climate change mitigation potential. This potential shouldbe compared to the total CO2 emissions in 2010, which were9.1 Pg carbon from fossil fuels and 0.9 Pg carbon from land-usechange [7], so desertification control and soil restoration in drylands have the potential to offset about 26% of the 2010 emissionsfor a period of time.

The soil carbon pool is the earths largest terrestrial carbon poolwith a total content of approximately 2344 Pg SOC (down to 3 msoil depth) and even small changes in the terrestrial carbon poolmight have a severe impact on global warming [8]. In China, thepotential for sequestering carbon from the atmosphere back tothe biosphere is around 11 Pg carbon, at a rate of 224 Tg C yr1

[9], compared to the 2011 total GHG emissions in China equivalentto 2.6 Pg of carbon, or 9.7 Pg CO2 [10]. This would mean a potentialto offset about 8% of the 2011 emissions per year through SCS,including SOC and soil inorganic carbon [9].

The SOC stocks increase through biomass input to the soil (e.g.crop residues) and decrease through organic matter decomposition[11]. An increased soil fertility or water availability (if water is lim-ited) leads to increased biomass production and increased carboninputs, but also increased microbial activity [12]. Overgrazing is amajor cause of soil carbon depletion. SOC pools are reported todecrease at a rate of 0.19 Mg C ha1 yr1 when grasslands are over-grazed compared to moderately grazed lands [6]. Overgrazingcauses a decline in net primary production and this decreases car-bon input to the soil [13]. Organic matter decomposition isdecreased by decreased soil disturbance [11]. Reducing the pres-sure from grazing animals will decrease soil disturbance anddecomposition of organic matter. Increased SOC could alsoincrease biodiversity [14,15] and soil quality [3].

When carbon is sequestered as SOC, or when CO2 emissionsreductions are achieved under certain circumstances, carboncredits, each representing 1 Mg CO2-equivalents (CO2-eq), can becreated. This credit could be used to offset emissions through theclean development mechanism (CDM). The credits created throughCDM projects, Certified Emission Reductions (CER:s), can be soldand used on the compliance market. However, afforestation andreforestation are the only land-use change mitigation alternativesallowed within the CDM for the first commitment period (20082012) [16], hence excluding carbon sequestration methods suchas no-till farming and restoration of degraded grasslands (exceptby afforestation) from the CDM. There is also a voluntary offsetmarket, primarily driven by companies/organizations that wantto work proactively with decreasing GHG emissions andstrengthen their environmental profile, and on this market severalopportunities to use SCS is opened up [e.g. 17]. Voluntary carboncredits due to these offsets, Verified/Voluntary Emission Reduc-tions (VER:s), can be traded on the voluntary carbon markets. In2012 a volume of 95 Tg CO2-eq at a value of 576 million USDwas traded and the markets are predicted to grow further [18].The price of a carbon credit on the voluntary market (1 Mg CO2-eq) varies from 0.1100 USD and the credits are certified by a rangeof different standards [18].

A PVWPS is a system consisting of a PV array, a power-condi-tioning system, a pump and irrigation equipment. The water ispumped from a well or a surface water catchment source usingelectricity from the PV panels. A tank for storage of the water forlater use could be an option.

Irrigation using groundwater reservoirs or surface water is pos-sible in remote areas or other areas where the pressure on ground-water is low, the annual precipitation lies between 300600 mm yr1 and the ground slope is

Wang et al. [13] pointed out that exclusion of grazing animalsmight have a high opportunity cost for pastoralists and that culti-vated pastures could be a way to improve grass production perarea and at the same time restore degraded grasslands. They syn-thesized 133 papers from China on different grassland manage-ments impact on SOC stocks. They found that compared with noor light grazing average SOC concentration decreased with heavygrazing by 30%, with moderate grazing by 17% and they found aloss of SOC due to cultivation of grassland by 36%. Zhang et al.[25] could also observe an increase in both SOC and soil nitrogenwhen alfalfa was cultivated in reed meadow soils in Gansu prov-ince in China and thus cultivation of alfalfa increased the produc-tivity of the soil. Smith et al. [26] made an estimation of thesequestration rate due to restoration of degraded lands andreported a value of 3.454.45 Mg CO2-eq ha1 yr1 for different cli-mate regimes. Su et al. [27] reported increases in the 015 cm topsoil layer of 0.4 Mg C ha1 yr1 when cultivating desert soils withalfalfa. It is also important to point out that high biomass removalfrom grasslands without return of manure and residues limits thecarbon sequestration potential in pastures [28]. Synthesizing 8studies Conant et al. [6] reported that sowing legumes increasedSOC with 2% annually.

A PVWPS provides a mature technology [29,30] and manyadvantages when compared to a diesel electrical generator system,such as lower life-cycle cost, lower operation and maintenancecosts, no fuel cost and no transportation of fuel to the system,and this can explain the increasing attention PVWPS has gainedlately [31].

The objective of this paper is to analyze the possibility to userestoration of degraded grasslands with irrigation using a PVWPSas a carbon offset project and to give some guidelines for projectdevelopers in degraded land restoration. To our knowledge, thereis no study investigating the climate change mitigation benefit1

of restoring degraded grasslands employing irrigation with a PVWPS.Specifically we will try to answer the question if it is more beneficialfrom a climate change mitigation perspective to use PV to pumpwater for irrigation or feed in electricity to the grid.

2. Method

This study uses a statistical database to gather the required cli-matic data for both the solar energy and water demand assessment[32] and CROPWAT to estimate the water requirement [33]. It thencalculates the pumping power need to supply the water demandand sizes the PV system using PVsyst [34]. This part of the methodis described in detail in [35].

After the initial sizing procedure, this study uses a simplifiedmethod for investigating the possible increase of SOC when usinga PVWPS. Intergovernmental Panel on Climate Changes report2006 IPCC Guidelines for National Greenhouse Gas Inventoriesfor Agriculture, Forestry and Other Land Use (here called IPCCGuidelines) gives default values for Tier 1 reporting - the simplestandmost easily applicable level of reporting that are non-countryspecific and can provide an estimate of the annual changes in car-bon stocks in grasslands remaining grasslands for any soil type andclimate region. The method developed here is based on IPCCGuidelines Tier 1 methodology and a complete reference list fromwhich the default values are collected is provided in the report[36]. The method is simplified but it can be applied as a first feasi-bility analysis to any climate regime. The approach in IPCC Guide-lines [36] is equivalent with the one given in the earlier guidelinesfrom IPCC [37].

In Section 2.1 of this article a general equation that can be usedfor calculating the SCS in any grassland that undergoes a manage-ment change is derived. The equation can be applied to any areawhere a PVWPS is used to increase SOC in the soil, with input val-ues from IPCC Guidelines [36]. In addition to SCS, the emissionsreductions due to grid electricity substitution by the PV systemare calculated in Section 2.5.

2.1. Derivation of soil organic carbon change equation

Carbon stocks in grasslands consist of carbon in living above-ground biomass, living belowground biomass, dead organic matterand SOC. The rate of SCS is highly dependent on climate, biometype and management improvement method [6]. Experimentaldata is required to model the increase in SOC over time and whenthese data are not available IPCC has developed default valuesbased on data to be used for Tier 1 reporting [36]. These data serveas the basis for a first estimate of the SCS potential in the methodused here.

Any dead organic matter is assumed to be oxidized or decom-posed to form SOC during the year of removal. This is in line withthe IPCC default assumptions and hence

DCGG DCGG;LB DCGG;Soils 1where:

DCGG = annual change in carbon stocks in grasslands remaininggrasslands, Mg C yr1

DCGG,LB = annual change in carbon stocks in living biomass ingrassland remaining grassland, Mg C yr1

DCGG,Soils = annual change in carbon stocks in soils in grasslandremaining grassland, Mg C yr1

Aboveground biomass production can be large in grasslands buthas rapid turnover and rarely exceeds a few Mg biomass per hect-are. The annual change in aboveground biomass is thereforeassumed to be 0. This is an obvious underestimate since theincreased grass production following irrigation will lead to anincrease in the constant vegetative cover. However, the increasein aboveground biomass is highly variable and depending on grassspecies used etc. and to keep the estimation conservative this car-bon pool is excluded.

The belowground biomass can be estimated with the root-to-shoot ratio in grasslands. This ratio is high and the belowgroundbiomass can be up to 4 times as large as the aboveground biomass.Despite that, there is not enough scientific information for estab-lishing default values for the belowground biomass pool in theabsence of aboveground biomass data and carbon in belowgroundbiomass is therefore neglected. These assumptions are in line withthe Tier 1 method in IPCC Guidelines [36]. This means that equa-tion (1) simplifies to

DCGG DCGG;Soils 2If organic soil, i.e. peat or muck soil, is converted into mineral

soil pastures, the release of carbon to the atmosphere must betaken into consideration. Likewise, if lime is applied as fertilizerto the grassland, the connected GHG emissions must be subtractedfrom the SCS and hence

DCGG;Soils DCGG;Mineral DCGG;Organic DCGG;Liming 3where:

DCGG,Mineral = annual change in carbon stocks in mineral soils ingrassland remaining grassland, Mg C yr1

DCGG,Organic = annual change in carbon stocks in organic soils ingrassland remaining grassland, Mg C yr1

DCGG,Liming = annual carbon emissions from lime application tograssland, Mg C yr1

1 Climate change mitigation benefit and potential here means the net reduction inradiative forcing in the atmosphere measured in CO2-eq.

A. Olsson et al. / Applied Energy 136 (2014) 11451154 1147

For pasture on mineral soils, without addition of lime, equations(2) and (3) simplify

DCGG DCGG;Mineral 4The carbon stock change in a mineral soil is given as

DCGG;Mineral SOC0 SOC0TA

=T 5where:

DCGG,Mineral = annual change in carbon stocks in mineral soils,Mg C yr1

SOC0 = soil organic carbon stock in the inventory year,Mg C ha1

SOC(0-T) = soil organic carbon stock T years prior to the inven-tory, Mg C ha1

T = inventory time period, yr (default is 20 yr)A = land area of each parcel, ha

SOC0 and SOC(0-T) are calculated using:

SOC SOCREF FLU FMG FI 6where:

SOC = Mg C ha1 for 030 cm depthSOCREF = the reference carbon stock, Mg C ha1 for 030 cmdepthFLU = stock change factor for land use or land-use change type,dimensionlessFMG = stock change factor for management regime,dimensionlessFI = stock change factor for input of organic matter (applied onlyto improved grassland), dimensionless

IPCC Guidelines gives default reference (under native vegeta-tion) SOC stocks (SOCREF) for a given soil type and climate region.The management factors in Eq. (6) are then multiplied to this ref-erence value to obtain the SOC of the soil.

For a given climate regime, grasslands often have higher SOCcontent than other types of vegetation [37]. Unlike in forests whereaboveground biomass is the main carbon sink, the SOC is usuallyby far the largest sink of carbon in grasslands.

Some SOC is going to be stored as mineralized carbon (seeFig. 1). This pool of carbon is neglected in the calculations to keepthe estimates conservative and since IPCC Guidelines does notinclude changes in inorganic carbon pools under the Tier 1 and Tier2 reporting [36].

All the assumptions together can be summarized as Fig. 1. Thesystem boundary for the SCS must reflect all the assumptionsand is consequently put around a hypothetical SOC pool separatedfrom the belowground biomass and mineralized carbon reachingdown to a depth of 30 cm.

Finally, combining Eqs. (4) and (5) gives

DCGG SOC0 SOC0T A

=T 7

Eq. (7) in combination with Eq. (6) is used to calculate the annualSCS due to irrigation of grasslands.

For Tier 1 reporting, IPCC Guidelines uses the default assump-tion that a management change leads to a SCS during 20 yr beforea new equilibrium has been reached. This suggests that the averageannual change in carbon stocks for grasslands remaining grass-lands is the difference between the new equilibrium SOC contentgiven by the new management method, and the old SOC contentgiven by the old management method, divided by 20 yr [36]. Eq.(7) then becomes

DCGG SOC0 SOC020 A=20SCS rates are usually not constant over time [38]. Despite this,

IPCC Guidelines assumes a linear SCS for Tier 1 reporting, and con-stant SCS is assumed to be a good approximation when lackingother data, and the path does not matter for the accumulated resultof carbon sequestered.

2.2. Project and baseline scenarios

When a project activity decreases emissions of GHG or seques-ters carbon in a carbon sink, this is measured relative to a baselinescenario. The decrease relative to the baseline scenario is the cli-mate change mitigation potential of the project activity. Dependingon what the baseline is, the climate change mitigation potentialwill be different. Ould-Amrouche et al. [39] used a baseline sce-nario where PVWPS replace diesel driven generators and calcu-lated the reduction in diesel consumption achieved with therenewable system. To our knowledge, there is no study lookingat the baseline where farmland or grassland without irrigation willcontinue in this, degraded, state. Nor are there any studies lookingat the climate change mitigation potential of the excess electricityfrom the PVWPS.

Severely degraded grasslands have undergone major long-termloss of productivity and vegetation cover, due to severe mechanicaldamage to the vegetation and/or severe soil erosion [36]. The base-line scenario is that the degraded grassland does not improve ordegrade further, but the SOC in the grassland soil is kept at a con-stant level.

2.3. Additionality

Additionality may be proven if it is clearly shown that the pro-ject scenario differs from the baseline scenario, where commonpractice today is to not restore the degraded grassland. The CDMrulebook [40] lists the following barriers that may proveadditionality:

(1) Investment barrier(2) Access-to-finance barrier(3) Technological barrier(4) Barrier due to prevailing practice(5) Other barriers

At this point it is only possible to discuss additionality in qual-itative terms. However, restoration of degraded grasslands facesseveral barriers and a PVWPS is characterized by a high initial

Fig. 1. The carbon cycle with system boundary around the SOC pool marked withthe dashed line.

1148 A. Olsson et al. / Applied Energy 136 (2014) 11451154

investment cost and a long term commitment due to the long life-time of the PV panels. Barriers No. 14 are all likely to be applicablebut remain to be examined more closely.

2.4. Leakage

According to the CDM a leakage is emissions of GHG that can bedirectly attributed to the project but occur outside the systemboundary of the project [41]. Leakage for the PVWPS might includefossil fuel combustion arising during the construction of the well,burning of biomass and emissions of N2O due to reseeding withnitrogen fixing species.

Since residue burning is not common practice and residuesshould be left in the field to prevent wind and rain erosion [42],emissions of GHG due to burning of biomass in grasslands andagricultural residues are assumed to be zero. This would have tobe monitored and if the project is operating in an area where bio-mass burning is common practice, this must be assessed.

At harvest the plant residues are left in the field. IPCC Guide-lines provide an estimation of how much of the alfalfa plant thatis left after harvest considering both aboveground biomass andbelowground biomass [36]. To calculate the leakage of N2O fromthe plant residues the amount of nitrogen in the specific foragecrop being produced is needed. The amount of nitrogen is thenmultiplied with an emission factor (EF), also given by [36] to getthe amount of the available nitrogen that is emitted as N2O tothe atmosphere.

N2ODirect-N FN EF 8where:

N2ODirect-N = the direct N2O emissions arising from the nitrogenfixing plants (Mg N2O-N)FN = Amount of nitrogen fixed by nitrogen fixing crops culti-vated annually (Mg N)EF = Emission factor for emissions from nitrogen inputs (MgN2O-N/Mg N input)

N2O has a global warming potential (GWP) of 310 times that ofCO2 [43] and could therefore contribute with significant leakageemissions.

It is assumed that the degraded state does not produce any N2Oemissions due to the low production of biomass, which is a conser-vative approach.

Trost et al. [23] found several examples where irrigation led toan increase in N2O emissions, but in wetter climates. The two stud-ies reported in Trost et al.s where increased N2O emissions werenot observed after irrigation were conducted in semiarid climates,just like the case area in this paper. Liu et al. [44] compared irri-gated pastures with pastures only receiving precipitation waterin Inner Mongolia, China. They found no increase of N2O emissionsdue to irrigation. Wulf et al. [45] found that the cumulative N2Oemissions where the same for an irrigated plot and a plot onlyreceiving rain water. This was probably due to the lack of microbialactivity in the dry plots and a large availability of reactive nitrogencompounds. In the irrigated plots, the mineralization was morestable. So it is not possible to say that irrigation will increaseN2O emissions in semiarid grasslands. Also, in the CDM tool Esti-mation of direct nitrous oxide emission from nitrogen fertilizationdirect emissions of N2O from nitrogen fertilizers is only consideredif projects are implemented in wetlands or if flooding irrigation ispracticed, which is not the case since the water scarcity in the arearequires water saving equipment.

Emissions arising from production of PV panels and other com-ponents are lower than 50 g CO2-eq kW h1 [46] but tracking theseemissions back through production is cumbersome and associatedwith several uncertainties. Given the low lifecycle emissions this

part is neglected in the analysis, which is in-line with the CDMmethodology AMS-I.D.: Grid connected renewable electricity genera-tion, version 17.0 (AMS is short for Approved methodology for smallscale projects) [47]. GHG emissions could arise from constructionof the water well or production and installation of the PVWPS.However, also these emissions are neglected due to the samereason.

Furthermore, there could be an increase in grazing animals witha connected increase in GHG emissions. But the increased produc-tion of forage could very well lead to a decrease grazed area andmore land being set aside. This could vary from project to projectand an analysis of the situation should be a part of a real case studybut is not considered in this generic model.

2.5. Substitution of grid electricity

The possibility to use the excess electricity produced by the PVarray, especially during the seven months when no irrigation isneeded could lead to increased operation time of the system andincreased reduction of GHG emissions. This could be done by con-necting the PV system to a local mini grid, which means that theentire surplus from the system can be utilized. The PV systemcould also be made mobilized, which allows for connection tothe grid during the seven months the system is not used for irriga-tion. In the latter configuration, some of the excess electricity willbe lost. A combination of the two approaches could also be possi-ble, depending on the feasibility for a certain project. The GHGemission reduction from grid electricity substitution (the gridemission factor) is calculated using the combined margin of theGHG emissions from Chinese grid electricity. This is a conservativemanner according to the CDM methodology AMS-I.D.: Grid con-nected renewable electricity generation, version 17.0 [47].

2.6. Case Grassland in Qinghai, China

Qinghai province is located just north of Tibet and covers theeast-northern part of the Tibetan plateau. The province has vastgrasslands and around 10% of the total pastures in China arelocated in the province. 22.3% of the total grassland in Qinghaican be considered feasible for PV-irrigation [19].

A grassland case-site in Dulan county (simplified Chinese: ), Qinghai province, China, (geographical coordinates: 36300N,98080E) was chosen and the Tier 1 method was applied to this site.The climate is semi-arid and the data obtained from the weatherstation in Meteonorm (v7) gives the mean annual temperature tomaximum 9.7 C and minimum 2.7 C, the mean annual precipi-tation as 316 mm rain yr1 and the reference evapotranspiration as1113 mm yr1. The altitude is 3194 m above sea level and the soiltype is cambisols [48], which falls within the IPCC definition ofhigh activity clay (HAC) and the IPCC climate zone is cold temper-ate, dry [36], Kppen climate classification BSkw [49]. The highaltitude might put using the IPCC method in question due to chan-ged biogeochemical reaction rates. Many studies of conversionfrom cultivated land to grassland in China have focused on lowaltitude rangeland areas [50], but Li et al. [51] and the recent studyby Shang et al. [50] supports the use of the methodology at highaltitudes.

The grassland area can be restored using non-nitrogen fixingforage or a mix of grasses including alfalfa which is a nitrogen fix-ing species (legume). The introduction of a nitrogen fixing specieswill increase the SCS [52] and potentially help combat the locustplague [53], which could be a huge problem to farmers in grasslandareas [54]. Calculations are made on a project scenario where aPVWPS is used to irrigate severely degraded grasslands duringthe grass growth season from May to September. When irrigated,the grasslands are assumed to be fenced to protect the irrigation


equipment. The grass is later harvested and fed to grazing animals.SOC is assumed to be sequestered at a constant rate for 20 yr.

Since the method developed here is simplified it can be seen asa starting point for studying the feasibility of PVWPS to restoredegraded lands in an area. If possible it should be adapted to thelocal circumstances or verified using local studies. In the case ofthe alpine and temperate grasslands of China we recommend thesynthesis made by Wang et al. [13], and in lack of local studies,the global synthesis made by Conant et al. [6] can be used. The Har-monized World Soil Database (HWSD) [48] can be used to verifythe SOC content. However, since loss of SOC from soils takes placewithin a few years [11], using HWSD could be misleading. GlobalLand Degradation Information System (GLADIS) [55] is a globaldataset with a combination of qualitative and quantitative mea-sures developed to inform decision makers about land degradationstatus and trends. GLADIS illustrates soil health trends and couldgive an indication of and justify the baseline.

3. Results

Eqs. (6) and (7) can be used to calculate the increase in SOCwhen using a PVWPS in any grassland climate with any soil type.

The result is here applied using the Dulan, Qinghai case-site andits specific climate conditions and soil type. The land use factorsused are presented in Table 1 and the default values used for theQinghai case calculations are presented in Table 2. The methodcan be applied to all IPCC climate zones where native grasslandsexist by using the default values provided by IPCC Guidelines[36], but local circumstances will affect the leakage.

Table 1 shows that the soil carbon pool in irrigated grasslands isincreased with over 60% relative to severely degraded grasslandsand if nitrogen fixing species are introduced the increase in soilorganic carbon can be 80%.

3.1. Baseline scenario result

The baseline scenario is severely degraded grassland that doesnot improve or degrade further. FI only applies to improved grass-lands and Eq. (6) for the baseline scenario becomes

SOCBL SOCREF FLU FMG 50 1:0 0:7 35 Mg C ha1 for 030 cm depth

where:SOCBL = baseline carbon stock, Mg C ha1 for 030 cm depthIt should be noted that the amount of studies behind FMG for

severely degraded grasslands are insufficient, which leads to anerror of 40%, i.e. 95% of the studies has a deviation of 40% or less

from this factor [36]. As more studies are becoming available,the error of this approach will be reduced. The error has beenchanged in IPCC Guidelines 2006 from the previous value 50%[37].

Using FMG = 0.7 is supported by a synthesis of 133 Chineseexperiments where heavy grazing reduced SOC pools with 30%and moderate grazing reduced SOC pools with 17% [13]. Accordingto the HWSD the top soil organic carbon density in the case area is0.65% and the bulk density is 1.39 kg dm3. This equates to27 Mg C ha1 down to a depth of 30 cm. The IPCC method yields35 Mg C ha1 and thus overestimates the initial carbon content inthe soil. For the studied area GLADIS illustrates a very negativeor slightly negative process.

3.2. Soil carbon sequestration rate applied to the Qinghai case site

If the degraded grassland, in a cold temperate climate with soiltype HAC, is restored by irrigation and controlled grazing, itbecomes an improved grassland with nominal input, according toTable 1 and thus

SOCCase 50 1:0 1:14 1:0 57 Mg C ha1 for 030 cm depth

where:SOCCase = case carbon stock, Mg C ha1 for 030 cm depthThe average annual SCS rate is then calculated as

SOCCase SOCBL=T 57 35=20 1:1 Mg C ha1 year1 for 030 cm depth

After conversion from carbon to CO2 the final SCS is 4.0 MgCO2 ha1 yr1. 20 yr of SCS gives a total of 81 Mg CO2 ha1.

If nitrogen fixing species, such as alfalfa, are included in thegrass reseeding mixture, the stock change factor for input oforganic matter is considered high and Eq. (6) becomes

SOCCase 50 1:0 1:14 1:11 63 Mg C ha1 for 030 cm depth

Table 1IPCC Guidelines default land use factors used in the case.

Factor Level Climate regime IPCC guidelines revised default Error

FLU All All 1.0 NAd

FMG Nominally managed (non-degraded) All 1.0 NAd

FMG Severely degraded All 0.7 40%e

FMG Improveda grassland Temperate 1.14 11%e

FI Nominalb All 1.0 NAd

FI Highc All 1.11 7%

a Represents grassland which is sustainably managed with moderate grazing pressure and that receives at least one improvement (e.g. fertilization, species improvement,irrigation).

b Applies to improved grassland where no additional management inputs have been used.c Applies to improved grassland where one or more additional management inputs/improvements have been used (beyond that required to be classified as improved

grassland).d Not applicable since this is a reference value.e Two standard deviations, expressed as a percent of the mean. Where sufficient studies were not available for a statistical analysis a default of 40% has been used [36].

Table 2IPCC Guidelines reference value for SOC used in the case.

Region Soil type Value,SOCREF

Unit

Cold temperate,dry

High activity clay(HAC)

50a Mg C ha1 for 030 cmdepth

a The data nominal error estimate is given to 90% [36].


The average annual SCS rate is then calculated as

SOCCase SOCBL=T 6335=20 1:4 Mg C ha1 year1 for 030 cm depth

After conversion from carbon to CO2 the final SCS is 5.2 MgCO2 ha1 yr1. 20 yr of SCS gives a total of 104 Mg CO2 ha1.

Conversion of freely grazed grasslands to cultivated pasturessequestered 1.563 (0.413.02) Mg ha1 yr1 as a mean (range) inthe top 40 cm soil layer [13]. Initial SOC concentrations negativelyaffect the annual SCS rate [13] meaning that SCS rates are higherthe lower the initial SOC content. A positive relationship betweenannual change of SOC and soil nitrogen concentration has beenconfirmed [13].

3.3. PV water pumping system specifications

The specifications of the PVWPS are given in Table 3. This sys-tem is assumed to meet the water demand during the month withthe highest grass growth. Fig. 2 provides a schematic of the system.

No GHG emissions arise from the use of the PVWPS.

3.4. Substitution of grid electricity

The PV array is assumed to generate 990 kW h kWp1 in totalexcess electricity and all of this could be utilized if the PV systemis connected to a mini-grid. The combined margin for electricitygeneration in the northern Chinese grid is 931 g CO2-eq kW h1

[56]. 1 ha of grassland is irrigated with a PV array the size of3.4 kWp. Using the excess electricity from the irrigation system

would substitute 3 365 kW h ha1 of electricity from the grid. Thisgives a total reduction of 3.1 Mg CO2-eq ha1 yr1.

The PV array is assumed to deliver electricity at the specifiedrate during 20 yr, after which the GHG emissions from grid elec-tricity are uncertain. Emission reductions during 20 yr give a totalof 63 Mg CO2-eq ha1.

3.5. Leakage

Using Eq. (8), the leakage from direct N2O emissions can be cal-culated. The alfalfa production used in the leakage estimation ishere kept high to make the estimation conservative, and 12 Mgdry matter ha1 yr1 is assumed to be produced. Table 4 givesthe values for the calculation.

N2ODirect-N FN EF 12 0:29 0:027 0:4 0:019 0:01 44=28 0:0029 MgN2ODirect-N ha1

where 44/28 comes from the conversion from elementary nitrogen(N) to N2O. This amount has to be multiplied with the GWP for N2Owhich is set to 310 [43]:

CO2-eqN2O 0:0029 310 0:90 Mg CO2-eq ha1 yr1

Over 20 yr this becomes 18 Mg CO2-eq ha1, a quite significantleakage.

Due to the increase of grass production, the potential leakagefrom non-nitrogen fixing forage must also be calculated and sub-tracted as a leakage. In the same way the value for non-nitrogenfixing forage crops:

CO2-eqN2O 0:0021 310 0:64 Mg CO2-eq ha1 yr1

Over 20 yr this becomes 12 Mg CO2-eq ha1.

3.6. Total climate change mitigation potential

Total climate change mitigation potential

carbon sequestered emissions reductions leakage

Table 3Design specifications of the PVWPS, based on study [35].

Array peak power 3.4 kWpMotor pump 2.4 kWHydraulic head 60 mIrrigated area 1 haDaily water demand 50 m3

Fig. 2. System diagram of the PVWPS.

Table 4Default factors 643 for N2O leakage estimation.

Crop Above-ground biomass cropresidue (AG)

N content of above-groundresidue

Ratio of below-groundresidues to AG

N content of below-groundresidues

Emission factor(EF)

Alfalfa 0.29a 0.027 0.4b 0.019 0.01Non-N-fixing

forages0.3b 0.015 0.54b 0.012 0.01

a Error 31%.b Error 50%.


For the Qinghai case with nitrogen fixing species we have:

Total climate change mitigation potential 104 63 18 148Mg CO2-eq ha1

numbers do notadd up due tothat they arerounded

And without nitrogen fixing species we have:

Total climate change mitigation potential 81 63 12 131 Mg CO2-eq ha1

numbers do notadd up due tothat they arerounded

Despite the very conservative estimation of increased N2Oemissions, it is still beneficial to introduce nitrogen fixing speciesin the grassland restoration process.

It is interesting to see if SCS gives a higher climate changemitiga-tion benefit than simply feeding the renewable electricity into thegrid. The combinedmargin for electricity generation in the northernChinese grid is 931 g CO2-eq kW h1 [56]. 1 ha of grassland is irri-gated with a PV array the size of 3.4 kWp and the annual electricityproduction is 4678 kW hha1. If this electricitywere to be producedfor the grid, the emissions reductions would amount to 87 MgCO2-eq ha1 during 20 yr, compared to the achieved 148/131 MgCO2-eq ha1. Thus, the mitigation benefit is increased thanks tothe restoration of grasslands. Even if the SCS potential is reducedto 30%/35% of the calculated value for irrigation + nitrogen fixingspecies/only irrigation, i.e. if only 1.2 Mg CO2 ha1 yr1 was seques-tered instead of 4.3/3.45 Mg CO2 ha1 yr1 as calculated, themitiga-tion benefit would still be higher if using the system for irrigation.

Table 5 gives the values calculate for different cases and somecomparing values for other types of projects.

4. Discussion

The increase in SCS when using a PVWPS to irrigate grasslandsis comparable to other carbon sequestration projects (see Table 5).Through irrigation of grasslands, carbon offset credits can be cre-ated and sold on the carbon offset markets and this could poten-tially make PVWPS a more attractive solution when conservinggrasslands subjected to intensive grazing. According to the IPCCTier 1 method used here SCS can go on for 20 yr and after that anew equilibrium is reached. Any management practice, such asirrigation, must not change for another 80 yr after this 20-yr periodor else the SOC will start to decrease in the grassland soil. There-fore it is important that the system has a multitude of co-benefits

(other than climate change mitigation) to keep the incentive topractice sustainable grassland management. Higher grass produc-tion and combating locust pest are some of the direct co-benefitsof irrigation. The result is somewhat higher than the one reportedby Su et al. [27], but they only included the 015 cm top soil layer.Carbon sequestration might take much longer time than thedefault value of 20 yr given by IPCC [38] and this could be anotherreason for the somewhat higher value. However, the results aresimilar to locally reported results [13] and this justifies the useof the default method when investigated the feasibility of PVWPSin the case site.

The model developed in this article is a simple and genericmodel and can be seen as a first feasibility analysis and a proposedmethodology for investigating the feasibility of the PVWPS for anygiven location. However, when applied to a project the rate of SCSand the leakage of GHG emissions might vary a lot locally and mustbe carefully assessed for each project site. If available, local studiesshould be used to justify the use of the method. To calculate thebaseline we have used GLADIS to justify that no natural restorationprocess is occurring.

Restoring the degraded grasslands with irrigation has a climatechange mitigation potential of 68 Mg CO2-eq ha1 over 20 yr in thecase-site in Qinghai. Introduction of nitrogen-fixing species willincrease this value to 86 Mg CO2-eq ha1, including the higherleakage of N2O emissions. The PV system generates an excess ofelectricity and if the potential climate change mitigation benefitof this is taken into consideration the value increases to 131 and148 Mg CO2-eq ha1 respectively, i.e. increasing the benefit with7090%. This proves the need to utilize the system also duringthe time it cannot be used for irrigation purposes.

Although there seems to be a high potential to mitigate climatechange using a PVWPS, no assessment of the price of the carboncredits has been made here and other factors, such as installationcost, maintenance cost and other costs that differ for different car-bon sequestration projects must be taken into consideration whenassessing the feasibility of using a PVWPS to sequester carbon.These costs might differ considerably for different types of projectsand are subject to future research. Leakage from the system mightalso vary significantly in different locations and this should beassessed for each project as part of a feasibility study.

China is covered to 40% by grassland and even small changes inthe SOC pool might lead to large emissions of GHG to the atmo-sphere. This also means that the potential to sequester carbon inthe SOC pool is large since grasslands often have higher SOC thanother vegetation types [37].

5. Conclusions

The method developed here based on IPCCs default values canserve as a feasibility study for using PVWPS to restore degradedlands. Use of the method is verified in the context of alpine and

Table 5SCS and CO2 emission reduction above baseline scenario for different biological sequestration methods.

Carbon sequestration method (including leakage) CO2 reduction (MgCO2 ha1)

Agroforestry, woodlots (two projects)a 95205Afforestation/reforestation CDM projectsb 241302 (average 357)No-till farming on long term cultivated HAC soils in cold temperate, dry/tropical moist climate (no leakage assumed)c 55/176Irrigation of severely degraded grassland in cold temperate, dry climate with HAC soils, without/with grid electricity substitutionc 68/131Irrigation and use of nitrogen fixing species to restore severely degraded grassland in cold temperate, dry climate with HAC soils, without/

with grid electricity substitutionc86/148

a Values from Watson et al. [57].b Values from 45 A/R CDM projects [58].c Values calculated using IPCC Guidelines default values (20 yr soil improvement) [36].


temperate grasslands in China by Wang et al. [13]. Using the differ-ence in SOC stocks between two states is probably more significantfor a feasibility study than looking at annual rates of SCS, since theannual rates reported in literature vary with initial SOC concentra-tion [6,13]. Using the HWSD alone to verify the SOC content isproblematic since this gives no information about the native con-ditions of the soil. Using global datasets of land degradation, likeGLADIS [55], could be a way to justify baseline and possible iden-tify areas of potential restoration.

Using irrigation powered by a PVWPS to restore degraded grass-lands for increased grass production and desertification control canachieve a climate change mitigation benefit of 131/148 Mg CO2-eq ha1 in a cold temperate, dry climate with HAC soils during a20 yr process of SOC sequestration and emission reduction with-out/with introducing nitrogen fixing species. The carbon seques-tered is comparable with that of other carbon sequestrationprojects. Further research is needed to investigate the feasibilityof using a PVWPS as carbon offset projects and should include acomparison of investment and maintenance costs, but also otherco-benefits than SCS such as biodiversity. Leakage of GHG fromthe system could be increased N2O emissions and this is includedin the analysis. However, project-specific leakage might occurand it is recommended that this is assessed for each projectindividually.

An important conclusion is that SCS can achieve a higher cli-mate change mitigation benefit than by simply producing electric-ity for the grid. Even if only 30% of the SCS value calculated here isachieved, it is still preferable to use the electricity for irrigation,from a climate change mitigation perspective. However, if theSCS, with leakage subtracted, achieved over 20 yr should be lowerthan 24 Mg CO2-eq per hectare, then the climate change mitigationbenefit is larger if the PV array is used to produce electricity for thegrid. The amount of SCS needed is sensitive to the GHG emissionsfor the national grid and this could be calculated using the com-bined margin approach [47].

The system should be used to irrigate severely degraded grass-lands to reach the full SCS potential. The system has a large poten-tial to mitigation climate change in China through SCS andemissions reductions.

Acknowledgements

The author would like to thank Sida, Swedish InternationalDevelopment Cooperation Agency and Swedish Agency for Eco-nomic and Regional Growth for financial support.

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Glossary

CO2-eq: CO2-equivalentsGHG: Greenhouse gasesGLADIS: Global Land Degradation Information SystemGWP: Global warming potentialha: HectareHAC: High activity clayHWSD: Harmonized World Soil DatabaseIPCC: Intergovernmental Panel on Climate ChangeLAC: Low activity clayPVWPS: Photovoltaic water pumping systemRMB: Ren Min BiSCS: Soil carbon sequestrationSOC: Soil organic carbonUSD: U.S. Dollaryear: Year


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Potential for carbon sequestration and mitiga1 Introduction2 Method2.1 Derivation of soil organic carbon change 2.2 Project and baseline scenarios2.3 Additionality2.4 Leakage2.5 Substitution of grid electricity2.6 Case Grassland in Qinghai, China

3 Results3.1 Baseline scenario result3.2 Soil carbon sequestration rate applied to3.3 PV water pumping system specifications3.4 Substitution of grid electricity3.5 Leakage3.6 Total climate change mitigation potential

4 Discussion5 ConclusionsAcknowledgementsReferencesGlossary

potential for carbon sequestration and mitigation of climate change by irrigation of grasslands

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