Garbon sequestration in urylana tcosystems RATTAN LAL Carbon Management and Sequestration Center The Ohio State University 2021 Coffey Road Columbus, Ohio, 43210, USA

ABSTRACT / Drylands occupy 6 15 billion hectares (Bha) or 47 2% of the world's land area Of this, 3 5 to 4 0 Bha (57%- 65%) are e~ther deserl~fied or prone to deserlificat~on Desp~te the low soil organic carbon (SOC) concentratlon, total SOC pool of soils of the drylands IS 241 Pg (1 Pg = petagram =

10" g = I billion metrlc ton) or 15 5% of rhe world's rota1 of

1550 Pg to 1 -meter depth Desertification has caused historic C loss of 20 to 30 Pg Assuming that two-th~rds of the hlstoric loss can be resequestered, the total potent~al of SOC seques- tration is 12 to 20 Pg C over a 50-year period Land use and management practices to sequester SOC include afforestation

Dryland ecosystems are defined as regions in which the ratio of total annnal precipitation to potelltial evapotranspiration (P:E'I' or the Aridity Index, AI) ranges from 0.05 to 0.65, and include dry sub-humitl regions (A1 = 0.50-0.65) covering 9.9%; semi-arid re- gions (A1 = 0.20-0.50) covering 17.7%; arid regions (A1 = 0.05-0.20) covering 12.0%; and hyperarid re- gions (A1 = < 0.05) covering 7.5% of the earth's la1-1~1 area (Dregne 1983, Glenn and others 1993, lieyllolds and Snlith 2002). 'I'hese regions cover about 47.2% ol' the earth's land area or al,out 6.15 billion hectares (1Sha) ('Fable 1 ) , predomiiiantly in northern and south- western Africa, southwestern anti central Asia, north- western India and l'akistai~, southwestern U~lited States ,it-td Mexlco, westeln South America, and Australia (F11llc.l antl Koscnr.\~etg 2002, WKI 2000, Middleton and I horllas 1992, N a n and Clarke 1097).

I'he wor lcl's dryland 5011s corltaln 241 Pg ol sot1 organlc cat bon (SCIC) (I.,\wnr an and others 2000), wll~ctl 1s r ~ b ~ j ~ ~ t 40 ~1111~s 1 1 1 0 1 ~ ' thnl-t what war adtied mto the ' i t~~~ospt- te~ e through nnthropogcnic. acttvlttes, esti- m'ttecl at 6.3 I'g C/v tlurlng the 1990s (Schiniel nlrd othets 2001, IPCC 2001) 111 adtlltlot-t, dlyland soils cot1talrl at least a\ much as ol more so11 inolganlc c,~lhon (SIC:) than SO(: pool (B'itjcs 1!)98, I.,\walat-t anti other \ 2000). '11,inagernent ol both SOC and SIC pool5 111 tl~?lnlld ecosystems can plav n nrnjol tole in tetluc~ng

with appropriate species, soil management on cropland, pas-

ture management on grazing land, and restoration of de-

graded soils and ecosystems through afforestation and con-

version to other restorative land uses Tree specles suitable for

afforestation in dryland ecosystems include Mesqu~te, Acacia,

Neem and others Recommended soil management practices

include applicat~on of biosol~ds (e g , manure, sludge), which

enhance activity of soil macrofauna (e g , termites), use of veg-

etative mulches, water harvesting, and judicious ~rr~gation sys-

tems Recommended practices of managing grazing lands

~nclude controlled grazing at an optimal stocking rate, fire

management, and growing Improved species The estimated

potentla1 of SOC sequestration is about 1 Pg C/y for the world and 50 Tg C/y for the U S. This potent~al of dryland so~ls is

relevant to both the Kyoto Protocol under UNFCCC and the

U S. Farm Bill2002.

the rate of enrichment of' atmospheric CO, (La1 2002). Because of the vast areas arid the importance of these soil C pools, drylarlds have a strong impact on the global C: cycle. However, land degradation anti deserti- fication are pervasive in these regions, often resulting in emissioll of CO, into the atmosphere as well as other environmental degradation. The objective of this paper is to tlescribc the poterltial of soils of the dryland eco- systems to sequester C and mitigate the accelerated greenhouse effect, and to outline land use and soil/ vegetation management options to achieve this poten- tial. Results are discussect in terms of the U.S. Farm Bill 2002 and the Kyoto Protocol.

Soils and Soil Characteristics

Principal soils of dryland ecosystenls are Aridisols, Entisols, Alfisols, and Vertisols (Dregne 1976, Table 2). Whereas drought stress is the principal constraint to biomass pl-octt~ction, tieficiency of N and low SOC con- centration arc other important causes of low net pri- nlaly productivity (NPP). 1,ivestock production and ranching, basecl on extensive grazing, is the predomi- nant land use. Arable land use is restricted o~ily to semi-arid and dly sub-humid regions or where irriga- tion is available.

I'hysical and chemical properties of these soils also vary widely in accord with the major soil types ('Table 2). Most soils are coal-se-texturecl, 1 0 ~ 7 in SOC concen- tration, and h;~vc low watc?r and nutrient retention c a

Environmental Management Vol. 33, No. 4, pp. 528-544 o 2004 Springer-Verlag New York, LLC

Carbon Sequestration in Dryland Ecosystems 529

Table 1. Global distribution of drylands of the world (modified from Middleton and Thomas 1992, Noin and Clarke 1997, Reynolds and Smith 2002).

Hyper-arid Arid Scmi-arid L)iy sul>l~umitl 9) of' Earth's Region (< 0.05)" (0.05-0.20) (0.20-0.50) (0.50-0.65) Total lartd area

Afi-ica 67 Asia 0.28 Australia 0 Europe 0 N. America 0.003 S . America 0.03 Total 0.98 % of Earth's lancl area 7.5

Billion hectares 0.51 0.27 0.60 0.35 0.31 0.05 0.1 1 0.18 0.42 0.23 0.27 0.21 2.31 1.29 17.7 9.9

Table 2. Principal soils of the dryland ecosystems (modified from Dregne 1976, Noin and Clarke 1997).

Region Alfisols Aridisols Entisols Mollisols 'irertisols

Billion hectares Africa 0.235 0.5485 1.1362 0.0196 0.0169 Asia - 0.7991 0.6627 0.3898 0.0975 Australia 0.0464 0.2917 0.2453 - 0.0796 N.America 0.0294 0.3312 0.0589 0.3018 0.0147 S.America 0.0706 0.1520 0.2281 0.0923 - Total 0.3814 2.1225 2.3312 0.8035 0.2087

pacities and low inherent soil fertility. Entisols and Aridisols comprise 75% of all soils. Desert pavement, predominance of coarse fragnlents on the soil surface, is a common feature of most soils of hyper~arid and arict regions. In some soils, coarse fragments are abundant both in the surface and sub-soil horizons. Some gravelly soils also have concretionary materials (carbonates), and lower sides of pebbles may be coated with seconcl- ary carbonates. Because of the low SOC conce~~tration and pr-edominantly coarse texture, the ii hot-izons of' most soils of at-id regions arc light-coloretl and often less than I0 cnr thick. Winds are penritsivc in dryland ecoi.egions. 'l'ilns, cllyland soils ase also st[-ongly all fected by continuous ~iirltl sitiing and sorting of soil materials. 'There are vasyirrg clegr-ees of expression of' this, the extr-eme case being dune fields. This results in a variation of SOC in the soils ancl also entrapment by surface vegetation. 'I'he wind action, irregularity of rain- fall, anti variation of surface temperatures are the coin- mon denominators of dsyland soils.

The SOC concentration in dryland soils is usually less than O..5% by weight. I'erkins and 'I'honlas (1993) reported that SO(: co~rcentration of the soils of the Kalahari Desert of Botswana ranges fro111 0.2 to 0.6%. For Vertisols in Central India, Kalpande ancl others (1996) observed that SOC concentration ranges from

0.1 to 0.40/0, but this generally increases in proportion to the amount of clay. The distribution of SOC with clepth and the total SO<: density (kg/m2) al-e affected by vegetation, soil texture, lanclscape position, soil trun- cation, and the effect of runon and runoff or wind erosion/deposition. C;ile and Grossman (1979) re- ported that SOC density to I-meter depth ranged from 0.9 to 11 kg/m2 from 86 pedons from the southwestern U.S. (Table 3). They also obsesved a linear relationship between SOC and clay concentrations (SOC in kg/ni2 to I-m clepth = -0.06 + 0.15% clay, r = 0.73). In China, Feng and others (2002) reported SOC density to I-meter depth from 0.02 to 12.52 kg/n12 ti-on1 340 profiles in 17 locations in tlifkrent types or clesel-t. In Africa, Branchti ant1 others (1993) reporteel SOC den- sity of 0 to 13 kg/n12.

The vegetation cover in drylancl ecosystems is highly variable with large patches of' bare ground between small shrubs, resulting in large variations in SO(: den- sity vis-5-vis barren soil, grass strips, or under a shnlb. 111 addition to clrought stress, the vegetative coves is influ- cncccl by a range of terrrperatures tkiroughot~t the year as well as during the growing season. Variations in tempe~-atures across ttlyland regions make a considel.- able tlifferer~ce in water use efficiency. Evapotranspira- tion is lower in cooler I-egioris, ;tnd soil water use clfi- cierlcy is higher even though the segion may get the same amount of precipitation. Differences in soil water use el'ficiency strongly irnpact biomass procluction and level of SOC. 'There are also witle vai-iations in thc length of the growing season, which nor-nlally varies with thc availability of soil water, except in boreal and cooler regions and sometimes in temperate regions. The length of the growing season controls the antottnt and kind of hionlass, and has sigliificarit inrpact on SO(: concentration.

('. ,lass .. strips, . I-nnging fi-om a k w to several rnetCrs wide, normally occur in soils with fine- tcxttlrecl hosi-

R. Lal

Table 3. Texture and soil organic carbon contents of two soil profiles in southwestern U.S. (adapted from Gile and Grossman 1979).

Sand Silk (:lay SOC Dcptll ( t rn) (%I ] (%) (76) (%I)

zons (e.g., loam, clay loam, and silt loam). In Sahelian soils in Niger, Guillaume and others (1999) observed that tigerbnsh soils had high capacity to store SOC clespite moderate primary production. Herrick and oth- ers (1999) observed clifferences in structural svability and infiltration rate in relation to vegetative cover in drylands of Ncw Mexico, USA. The SOC concentration under the tigerbrish vegetation was 0.93% compared with 0.45% within the bare arca. Vegetation cover dras- tically afkcted both SO(; clcnsity and its vertical clistri- bution (Jobbagy aiicl Jackson 2000).

In ;~dtlition to SOC, dtylancl soils are also ch;tractet-- ized by a relatively high concentration of SIC, namely, primary ancl seconclaty or pedogeriic SIC (La1 and Kimble 2000). A predominant carbonate accunlulation, calletl "caliche," is essentially a precipitate froni aque- ous solutions rising fi-oln below or frorn carbonate-rich moisture that moveel from the surface down.c\~artl. Most soils of tlryland regions contain some forms of pedo- genic carbonates, but not all co~~vain caliche.

I~rylantls, especially in hyper.-arid and arid regions, are also characterized by sand durics, and these strongly affect variability in soil properties. Lei (1998) reported that SOC, N, and P concentrations were significantly lower on tlunes relalive to acljacent non-dune areas in the Mqjave Desert of the U.S. Southwest.

Soil-Related Constraints to Biomass Production

Whereas lack of actequate watcr or drought stress (luring the growing season is the most obvious Pactor affecting nct primary productivity (NPP), deficiency of N :mil other essential nutrients, as well as low SO(: concentration niay also limit biomass pr-otluction (Rre- rnan and ICessler 1997, Felkrr- 1998, (kcsing ant1 others

2000). In addition, an adequate level of SOC concen- tration is crucial to sustaining soil fertility, but it can also be a significant factor to increasing soil moisture storage and mitigate drought (Tiessen and others 1!)!14). Over and above the elemental balance, there are severe problems due to extremes of soil and arnbient temperatures. Therefore, enhancing biomass produc- tion in these ecosystems is vely difficult and must first be approached through restoration of degraded soils and through sequestering SOC and SIC. 'l'hese are inlportant strategies with far-reaching economic anel environmental impacu;.

Desertification

Desertification has traditiotlally been defined as land degradation in arid, semi-arid, and dry sub-humid areas resulting from climatic variations and human activities (Le Hoilerou 1975, Warren 1996, UNEP 1992), but it has also been observed in cool, humid climates such as iceland (Arnalds 2000). The land area prone to deser- tiftcation has been estimated at 3.5-4.0 Bha or 57%- 65% of the total land area of clryland ecosystems (UNEP 1991). Of this, the land area affected by soil degradation alone (excluding vegetation degradation) ranges from I .02 (UNEP 1991) to 1.14 Bha (Olcfeman and Van I ,ynden 1998). The estimates of current rate of deserttfication also vary widely. Mainpet (1991) esti- mated the annual rate of desertification at about 5.8 million hectares (Mha), with 55% occurril~g in range- land and 45% on rainfed cropland.

In accord u~ith the low potential for NPI' and the pr evalerlt land degradation, desertification also leads to I eductioll in the ecosystem C pool and attendant de- cl~nt, in SOC. L,al and other5 (1999) estimated that soil eroslon in tlrylandc leads to emiislon of 0.21-0.26 Pg C/y, with an additional 0.02-0.03 Pg C/y due to expo- sure of carbonaceous niaterial to climatic elements caused by surface soil erosion. Therefore, total annual emission of C due to erosion- induced land degradation 111 drylancl ecosystems may be 0.23-0.29 Pg C/y (La1 and others 1999).

Carbon Sequestration in Soils of Dryland Ecosystems

I,al and others (1999) ectimated 111sto11c loss of eco- syitent C due to ctese~ ttficntlot~ at 9-1 4 Pg of SOC pool, with losses from the biotlc/vegetation pool at 10-15 Pg. Slmllarly, Ojlma nncl othels (1995) estlrllated that grass- lnnd5 and dtylandi of the world have lost 1W24 Pg C clue to dece~ t~ficat~on. Although all estimates of ecosys-

Carbon Sequestration in Dryland Ecosystems

Soil management in dryland ecosystems

favorablelprime soils

4 Preventionireduction of soil degradation degraded soils desertified lands

I

Enhancing WUE Erosion management

prevention

INM and organic Reducing decline in matter input soil structure

Improved cultivars Preventing nutrient and cropping systems

PI Precision farming

and improved pastures

Prevention of grazing L Water harvesting 7

Figure 1. Soil management optior~s for enhancing terrestrial carbon pool in drylantl ecosystems.

tem (: loss due to desertification are speculative, the numbers are large (20-30 Pg). If only 2 /3 of this would be sequestered (Cole 1995) through desertification control and adoption of recommended land t ~ s e and soil management practices, this would amount to 12-20 Pg over a .50-year period.

The schematics in Fig. 1 outline management op- tions f ix enhancing SOC concentration. Technological options difier among soil type ant1 land ttse systems (Hinrnan and Hinman 1992).

Agricultural Intensification and Soil Management

Atloption 01' recontmcnded management practices (KMPs) 011 f'avorattle soils with good soil moisture re- gime and the possibility of supplemental irrigation can increase SOC concentration. Enhancing water use effi- ciency (WUE), by I-educing losses title to surLxce r~tnol'f' and evaporation and decreasing soil temperature by residue nrulching, is important. An objective of soil management practices is to increase the input of' bio- solids to the soil while enhancing soil quality including soil structr~r-e, and improving the soil moisture regirne and the overall protluctivity.

C~onsmc~ttor~ ftllug~ and T P F L ~ U P martcxg~rn~r~t. Conber- s o n from plow 1111 to no-t~ll o r c onrervatlon ttllnge cnn Increase WUE, reduce 11sk5 oi so11 degr'idatlon, and ovel tirrre Implove so11 qunl~ty and SO(; concentratloll. In Bushland, Texar, sweep-tilletl delayetl fallow con- w ~ n e d 11.8 g/kg SOC cornpaled wlth 9.3 g/kg 11-1 a plowed clean inllow treatntent Atel 24 y e a r (1943 to 196(i) of culttvat~on (Jane\ arlti o t t re~ s 10!)7). I trc dat'i I e p o ~ ted by k,tk ancl o t h e ~ c (1967) ~nthcntcd the po- tentla1 ol (: sequcstratlon on \ral~ous so115 In couther 11

GI eat Platnc. Ungcr ( 191) 1 ) measu~ed SO(: cctnt entl'i- tlon to 20-~1x1 depth 8 years 'tltc~ no-t~ll nnd ctubble m ~ i l ~ h tre'itntentc W C I ~ 1111t1ated on palreti w'ite~cheds crtlt~\,lted to wheat-sol ghum-fallow I otatlon Ave~ ,ige SO(: concent1,ttlon to 10-ern depth wac 16 3 nnct I5 8 g/kg f o ~ no-1111 ,inti \trlbble mulch t~en tmc~l t s , ~rrcllcnt- lng a t ~ e n d towartls pin 111 SOL In no-t~ll tlentmcnt A slgn~ficnnt increase 111 SOC concentl~ition o c c u ~ r e d 111

the top 2-crn depth. S tewa~t and l iob~ncon (2000) \tated that one of the gzatlfytng con5cqttcncc~ of the no-1111 system 15 tnclease in SOC concentlatloll In soll, wlr~ch mdy range from 60 to over bOO kg (:/hn/v I he h~ghes t rates ale ~ ~ s u a l l v nccoclatcd w ~ t h legumtnortc

R. Lal

winter cover crops where I-esidues are left on the soil sul-face. In northern Colorado, Potter and others (1997) reported 560 kg <:/ha/y accunlulation during 10 years of no-till contini~ous cropping wheat system. In (2uec1lsl;lnd, Australia, 1)alal (1989) observetl higher SO(; concc.ntration in Vcr-tisols under no-till with crop ~-esidue retc.ntiorr tlxtn under conventional till. Signifi- cant irlcreases in SOC were obse t~~ed in the top 23- to .i.O-cm layers after 18 years of'no- till practice (1)alal and others 1901, 1995, Dalal ant1 Carter 2000). In Inclia, Kihani ;tnd More (1984) contlr~cted soil analyses on a 4.5-year old tillage expe~.inicnt antl reported that incor- poration of hiosolids i~nprovcd S<>C concentration. 111 the West Afi-ican Sahel, Rationo and others (2000) observeti that an annual addition of 4 Mg/ha of crop residne resulted in the maintenance of'SO(: at the same level as in the Fallow in the top 20-cm layer. In southern Spain, Murillo and others (1998) reported that SOC concentration in 0- to 5-cm depth was 0.84% in tradi- tional tillage and 1.1% in conservation tillage after 2 years, and 0.89% in traditional tillage conipared with 1.34% in consewation tillage after- 4 years. 'The increase in SO<: co~lccntratiorl increased the overall soil quality. Indeed, widespread adoption of no-till farming can lead to SOC sequestration in d r y l a ~ ~ d ecosystems.

1iotation.r and cover cr0fi.r: The beneficial effects of using conse~vation tillage for enhancing SOC concen- tration are accentuateti when used in conjunction with rotations b;tsed on appropriate cover crops or pastures (liyan and others 1997). In Saudi Arabia, Shahin and others (1998) observed that introducing alfalfa in rota- tion with wheat g~-own on a sandy soil increased SOC; concentration threefold as conlpal-ed with continuous wheat. 111 ~Zlgcria, AI-abi and Roosc (1989) ;r~lrl Koose (1906) observed in~provenlents in soil quality with lc- gume-based rovations and sylvo-pastoral systems. In Svria, liyan (1998) reported that incorporation of M ~ d i - cclgo in rotation increased S O ( : coricentration to I -n~ depth. Jenkinson and others (1999) assessed the SOC pool tunclc~ d~ffererrt rot'it1011s 011 '1 calcnreous 5011 in Syrr,~ I he SOC pool In wlleat-mcadow rotatlon In- c~casetl by 1 6 illg/ha nt 'I rneitn late of 0 17 Mg C/ha/y 111 coriipari\on w ~ t h wheat-wheat rotat~on, and by 3.8 M g / h ~ at the ~ncan late of 0 38 Mg (:/ha/v In compar- 15011 with wheat-f'illow rotatlon In Australia, White- house ,mtl I ,~ t t l e~ (1984) obselvctl nn Incre'rse In SOC concent~,tt~on f~ om 1 18% to 1 57% in 0- to 1.5-cm tleptll nftc~ 2-4 yen1 s of lucc~ ne + ~ I ~ I I I ~ grass pastnrc l),llal nnd ottlcls ( 1995) I cpol tcd that the 1 dte of in- ( I ease In 'iOC contentlation wns h50 kg C/ha/y 111 n Vcl tlsol rtr~tlet gmss + legilnlc pasntlc Skjemstncl nntl olhc15 (1904) I cported incIcasc of 550 kg C,/I.ln/y rn n VCI tlsol urrde~ Rhodcs g~ nss Holford ( 1990) and

Chan (1997) observed similar effects of pasture on SOC concentrations in a Vertisol in New South Wales. In a Vertisol in central India, Mathan and others (1978) rcportecl that continuous cropping and nlanuring in- creased SO(, concentration by 20%-40% over 3 years. In northern India, Singh and othcrs (1996) observed that incotpot-ation of legumes in a rice-wheat rotation increased SOC concentratioll. Growing crops with a deep and prolific root system generally has a f'avorable impact on SOC concentration in the sub-soil. In semi- arid regio~ls of Argentina, Galantini and Iiosell (1997) reported that rotations of mixed pastures (5 1 /2 years) arid annual crops (4 1 /2 years) maintained the SO(; pool at 17.3 Mg C/ha conlpared with 11.2 Mg (:/ha in continuous cultivation with a wheat-sunflower rotation. Miglierina and others (1993, 1996) also reported that SOC concentration was high in wheat-grassland antl wheat-alfalfa rotations, especially with conservation tillage.

Intepated nutrient management: Soil fertility enhance- ment is crucial to improving SOC concentration. A p plication of nitrogen fertilizer is important to obtaining high yields, but nlay have little impact on SOC concen- tration unless used in conjullction with no-till and res- idue management (Russell 1981, Dalal 1992, Skje~nstad and others 1994, Dalal and others 1995). In semi-arid conditions, the SOC sequestration is limited by the input of biomass carbon. Although, crop yields are sufficiently increased by N application, the residue in- put is not stlficient enough to balance the mineraliza- tion rate. In Syria, Ryan (1998) reported a significant increase in SOC concentratiorl by application of rec- ommcndctl rates of fertilizers. Dalal (1 989) obsen~ed a positive el'fect o n SOC concentration after 13 years of no-till, residue retainetl anct N application (34.5 Mg C/ha vs. 35.8 Mg C/ha). In India, Gupta and Ven- kateswat-lu (1 994) observed that application of inanrlre at 10 Mg/ha increasetl SOC: concentration. Applica- tions of Farmyard manure, green nlanure, compost and other biosolids have a positive impact on SOC concen- tration. Use of high-lignin amendments, recalcitrant to tiecomposition, increases SOC concentration.

Irrigation manc~grnlunt: Irrigation management is cnl- cia1 to enhancing biomass production in clryland eco- systems. Some 40% of the world's food now comes from 17% of the crop1;ind that is irrigated (I'ostel 1999). 'There is a large scope of enhancing SO(; concentration in ir-rigatecl soils. In Mexico, Follctt antl others (2003) repor-ted that adoption of consewation tillage on irri- yated Vertisols sequestered soil C at the rate of 1.8 Mg/ha/y. Furthermore, the SOC sequestration effi- ciency was 8-1096, silnilar to that observed in the north- ern Great Plains (Follctt and others 1997). This is an

Carbon Sequestration in Dryland Ecosystems

Table 4. Strategies of soil management in dryland ecosystems for carbon sequestration

Strateby Practice I,ocation/regior!

1. Erosion a) Netill Farntirig Bushland, TX, USA

b) Mtilrhirig -storre cover -residue mulch -mulch

2. Crop diversification a) Rotations

3. Integrated nutrient a) Manuring managentent anti recycling

b) Orgarlic by-products c) Soil fauna d) Sewage sludge

4. Water management a) Irrigation and conservatior~ tillage b) Irrigation with sewage c) Irrigation with silt-laden water d ) Saline aquacitlture e) Water harvesting

Northern (:O, USA Q~~ee~islantf, Australia West Africa S;ahrl Southern Spain

Negev Desert (:ltih~rahuan Ilesert Surinarne Sa~rdi Arabia, West Asia Algeria, Nortti Africa Syria, M'est Asia i2ust1-alia Northern 111tli;l Argentina Maitiuguri, Nigeria

Spain Chihuahuan Desert Spain Mexico Israel Ningxia, China Dlvlands

Reference

Jones arid others 1997

Potter and others 1997 Dalal anti otl~c,~ s 2000 Batlono nntl othe~s 2U00 Mt~rtlio nritl others 1!)!18

I.ahav anti Ste~nberger 2001 Kostagno 'rntl Sosebal 2001 llreenlan ,ind Prot/ 1988 Shah~n ant1 otlte~ 5 1998 At a b ~ and Koose 1989 Jenh~~~,ort atrti o the~ \ 1!)!111 M'h~tel.io~~\e nntl 1.1ttlet 1084 Singti ant1 otlters 1'396 Galantiri~ a~itl Kosell 1997 Aweto ,inti Ayub 1993

I'ascual and others 1998 Nash and Whitford 1995 Pedreno and otliers 1996 Follett and others 2003 Hiilel 1998 Fullen and others 1995 Glenn and others 1993

exceprionally high rate of SOC sequestration and indic- ative of the potential of irrigated soils in carbon seques- tr-atioil. A judicious use of irrigatioll water is crucial to sustainable trse of soil arrd water- I-esor11-ces in dty areas. Rather than flood irrigation tising drip, f~lrrow or srth- irrigation can enhance WUE and inlprove soil quality i r r irrigated croplands.

Use of wastewater for irrigation req~tircs a special mention, especially in dryland ecosystems. 1,and appli- cation of sewage lnay be an important stratqgy to nlin- irnize pollution of' rivers and irnprove soil quality. In Israel, well over t~7o-thirds of tlornestic sewage is cur- rently being recycled for use it1 agriculture, and the plat) is to reuse u p to 80% (Hillel 1998). M'astewater irrigation elrhailces soil fertility 2nd iml>roves SOC: con- centration. However, there are health hazards with re- gards to heavy metals and contarnillation of groundwa tei-, which must be addressecl. In Ningxia, China, Fullen and others (1995) obsetvcd that irrigatirig desert soils with silt-laden irrigation water fi-om the Yellow River enhanced soil quality anti biolllass production. Increase irr biomass returr~ed can irnprovc SO(: concentratiotl of irl-iga~erl soils.

111 addition to cr-opland, irrigation with saline water may enhance productiotl of halomorphic plants (LA and others 1999). Halomo~.phic plants, grown by irri- gatiolr with brackish/seawater., can produce 17-35 Mg/ ha/y of biomass (Glenn and others 1993). Growing suitable species with saline-water irrigation can lead to 1>1-odl1ction of high-grade fodt1e1-, fhragc, oil, food, and it-rdrtstrial I.aw mater-ial.

Soine examples of' soil m;tlragcment p~tc t iccs that may leatl to SO(: scquestratior~ arc listed ill l 'ahle 4. S~irl'acc. applicatiorr of hiosolicls, manuring, use of'se\%i- age sludge ant1 organic by-products call cnhaince tire SO(; pool. Activity of' soil fauna, especially tcrmitcs, ilnpi-ovcs soil stl-rtcturc allti enhances the SO(: pool in tile long rrun. An appropriate use of stone cover and gravel mulch can also irnprove soil moisture rth' ' r1111e and cnhance the SO(: pool.

Pasture and Rangeland Management

(;ra/ing 1s the piet lo~n~n,tnt 1,irrd use in drylaird eco%ysterns, allti adop t~on of lrnprovecl gr'iring plat- tlce4 call Imp1 ovc (: \equestl ation thl ough consen atloll .~rld bettc.1 mnir,igc.nrcll t of ~ I I I face I es~tl~tc.. ( , o ~ t \ c ~ ston

R. Lal

Table 5. Strategies of pasture and range land management for soil carbon sequestration

Strategy Practice Location/l-egion

1. Improvetl species a) Sowing legtines Vertisols, Allstralia Norther11 Colorado Sadore, Niger

1,) Agrofbrestry West Afi-ican Saltel 2. Fire management I'rescl-i1)ctl burning Wyoming, LJSA

a) Stocking rate Negev, Israel 3. (;razing rn;ut;tgcntent h) (:ontrolletl grazing Kawas, USA

Reference

Chan and others 1997 IIavlin and others 1990 Hiernaux and others 1999 Breeman anct Kessler 1997 Schuman and others 2002 Zaady and others 2001 Rice and Owensby 2001

4. Irnprovirtg grasslands I~~tegratetl nlanagernent M'orlti's drylands Conant and others 2001 5. Erosiott inart;t~er~terrt Intc~gratctl rnanagernent World's clrvlat~tis 1,al 200 1

of clegiacled cropland ,oils to pngtrtre and improvtng the pasture managerilent enhance SOC concentlation in diyland ecosystems. Some examples of improved practice4 with positive impact on the SOC pool are lirted In 'Table 5 Important among these are graling managenrent through controlled stocking and rota- tional grazing, fire management, ancl agroforestry prac- tices involving legume species (Conant and other5 2001)

In Arlstralian semi-artd tropics, Chan and others (1997) reported that SOC concentration in degradecl Vertisols increased Iron1 1.3% to 1.6% in 0- to 5-crn clepth in 4 years by restoration of' pasture w~th barrel mcd~c (~Vledzcngo trunmtuln) and Mitchell grass (Astr~bla lupj~ac~n). Increase in SOC concentration by incorpora- tion of legumes is partly due to b~ological iiltrogen fixation (RNF) . Thus, application of nttrogenous fertil- i / e ~ s to degraded pastures ni~ly also impro\e SOC con- ~entr'~tiori. In .tdtlitton t o gr owlng tmproved p,istul e \pecies, lrlcor poratlon of pel cnnial woody Icgumc~s Ento tllc gia/ing systcnl may also enhance SOC concerrtra- ti011 hv tian5ferl ing the cai bon to sub-\oil at lowet clepths Stewart aild Robiri\olr (2000) compared the SOC pool for foul \itc\ on Pttllrnan qilty clay loan1 in west Icxns, USA. Otie site wa, cr~lt~vatecl to tlryland wheat for mole than 50 ye,u-5, nnd the other t h e e \ite\ were grasslantl-one native grassland, one previous cropland convertetl to grasslancl 37 years prior tc-I sam- pling, and one fieltl i.eturned to gl-assland 7 years prior to sariiplirig. The clata showecl a substantial gain in SOC concellti-ation even under semi-arid conclitioi~s. Sinrilar ~.csults have becn reported from northeastern Coloratlo by 1t)ori and othcrs (1 095), Havlin ant1 others (1990), anct Potter and others (1997).

Prescribed burning and controlled stocking rate arc also important to maintaining and improving SOC con- sen tration. Modification of' fire rcgimeil anti grazing at the right illtensity may influence SOC concentrations both ctix-ectly and indirectly. I11 addition to the impact on chai-coal carbon, redttction in fire frequency may

lead t o a shift in favor of woody vegetation, which produces lignified litter.

Preventing Soil Degradation and Restoring Degraded Soils

Preventing or minimi~ing risks of soil degradation is important to decreasing losses of terrestrial C from the ecosystem. The global rate of soil degradation may be as much as 10 Mha/y, most of which is occurring in dryland ecosystenis. Reducing risks of environmental degradation implies controlling soil erosion by water and wind, decreasing risks of secondary salinization, and reducing decline in soil structure and nutrient pool. There are several options of soil quality restora- tion by combating desertification (Squires and others 1995,I,al2001). Erosion control through establishment of vegetation cover and afforestation, soil fertility man- agement thro~igh BNF and application of biosolids, anrl reclamation of \aft-aiTected soils are important strategies (Fig. I ) . There are also numerous practices of water harvesting ancl runoff management, which are esential to enhancing biomass productioi~. There are .tpproximateIy 3.6 Bha of degradetl and desertified lane15 In dryland ecosystenis. A coordinated effort at their restoration would be a win-win stratek7.

l~nllowrng: Taking land out of agricultural uses and permitting natural vegetation to grow prevents ciegra- datiorl and restores degraded soils. Bush fallowiiig, per- mitting natural vegetation to grow without grazing or biomasg rernoval, can restore soil quality and enhance SOC concentration. In northern Nigeria, Abubakar (1996) monitored the impact of duration of fallowing oil changes in SOC concentration. 'The meall SOC concentration of the surface soil Mia\ 0.94% for 2-year fallow, 1.13% for .',-year fallow, 1.42% for 10-year fallow, and 1.44% for 1 .?-year fallow. The data froni a 30-year fallowing experiment conclucted in eaaern Spain showed that SOC concentration in the top 10-cm layer irrcrcasecl after 20 years of fallowing (Ruecker and oth- ers 1998).

Carbon Sequestration in Dryland Ecosystems 535

Table 6. Soil management options for C sequestration in soils of dryland ecosystems.

Strategy/techniqne L,ocation/region Reference

Stirface application of biosolids Chihuahua~t Desert Rostagno and Sosebal 2001 . . Stone cover Negev Desert I.ahav and Steinbrt.ger- 2001 Enhancing ter~itites ac:tivity C:ltihuahuan Desert Sash ant1 M'hitfosti 1995 Manuring hclaitiuguri, Nigc.1-ia Aveto ant1 Aytrt) 1993 Desert soil macrofarina (termites/ants) (:ltihnah~i;in Desel-t M'llitfortl 1996 Sewage sludge Spirt IJeclreno and others 19\16 Organic by-products Spairt I'ascrial and others 1998

Afforestation: Afforestation is an important strategy of restoring degraded soils and ecosystems. With 47.2% of the earth's land area covered by dryland ecosystems, conversion to an appropriate land use, restoration of desertified and degraded soils, and adoption of RMPs can enhance the terrestrial C pool leading to increase in both biomass C and SOC pools. Establishing a vig- orous vegetation cover is an effective strategy for C sequestration in soil and vegetation. However, not all trees can be grown everywhere. Soil water, tempera- ture, and tolerance to frost differ among species. For arid and semi-arid regions with rainfall of 400-800 mm/y, there are numerous seasonal, perennial, and xerophytic plants, which are naturally adapted to d~y - land ecosystems. In regions with low rainfall (200-400 mm), valleys that receive run-on and have a favorable soil moisture regimen need to be cleveloped for affor- estation with appropriate tree species. Some promising species are listed in Table 6

Establishing appropriate tree species enhances soil fertility and sequesters C in soil and biomass. 'There are numerous tree species adapted to d~yland ecosystenrs (Stiles 1988). Important arnong these are Tamarisk ( Zhman'x spp.) , gurn tree (L<ILM&I~~LS spp.) , leucaena (Leucaenu spp.) , cypsess (C~~pr~ssus spp.) , casual-inas (C~(zsucln'na spp.) , ~nesquite (Prosof)is spp.) , neem (ilza- rlirachtn spp) , acacia (ilcrtcin spp.) , teak ( Tl~ctone gr(tn- dis), cassia (Chsia spp.) and other-s. In Sahel and s u l ~ Saharan Africa, Acacia nlbidrc enhances the SOC pool (Chat-reau and Vidal 1965, Dancette and Poulain 1969, Pieri 1991 ). Sitnilarly, Prosopis sp has been usefill i r i SOC sequestration, and especially in reclamation of salt-affected soils (Singh and others 1994, Gupta 199.5, Garg 1998, Felker and others 1981, Rhodes and Felker 1988, Baker and others 1995, Ahrnad anci others 1994). Some 1'rosopi.s species are multipurpose 11-ees producing food and valuable timber (Felker 2000). Other tree species suitable fbr salt-affected soils are Acacias (Craig and others 1990), C;asurinc~s (El-1,akany anci 1,uasd 1982, Ng 1987), and other halomorphic species (I'atil and others 1996). The usefulness of' establishing ilccccic~

and Prosopis sp in SOC sequestration has also been demonstrated in the western U.S. (Connin and others 1997, East anti Felker 1993). Geesing and others (2000) estimated that SOC sequestration potential of growing Prosopis and Acacia spp in 3.12 Bha of drylancls is 6.2 Pg C/yr. In eastern Spain, Iiuecker ancl others (1998) observed that tree establishment on degraded soils sig- nificantly increased SO(: concentration in 0-10 cm layer after 30 years of growttt. In northeastern Sudan, Alstad and Vecass (1994) reported improvements in soil qwality following afforestation with Acacia tortillis. Whereas these species also store C in the above-ground biomass, which may be prone to leakage when har- vested, the buildup of SOC pool is a long-term increase provided that subsequent soil clegradation is prevented (Swisher 1997, Batjes 1998). Further, the residence time of C in drylancls is nluch longer because of the slower decomposition sate than that in the hrtnlicl en- vironments ((;ifford and othel-s 1992).

Secondary Carbonates

In addition to SOC, there is potential ofSI(: seques- tration in soils of' ci~ylarid ccosystenls through forma- tion of seconda~y carbonates, which deperlds on latrtl use and soil/crop managtmcnt systcrris (Fig. '2). Upon dissolution, CO, i l l soil air k,rrns carbonic acid that precipitates as carbonate of (:aiS or Mg " (Eqs. 1, 2 ant1 3).

(20, + H 2 0 G H + + HCO, Ft H2(;01 ......... ( I )

( ;a ' ' + 2 HCO, irt Cn(:O, + f-I,CO,. .. . . . . .. . . . (2)

Ca ' ' + HCO, irt Ca(;O, + H... . . . . .. . . . . (3 )

The SIC scqrlestl-ation happens if c;ttions (<:a ' ' ant1 MStS) arc supplietl fi-om ou~side the ecosystem (Nosdt and others 2000). Some external sources of' cations incl~lde eolian dust, (:ai' dissolved in rainwater, sc ;~ salts and sprays, use of' [el-tilizers, application of biosol- ids ancl ;imcntllnertts. 'Ihus, formation ol' secondaly carbonates ciepends upon the tlyitanlics of several pe-

R. Lal

Figure 2. 'I*cclinological options Icading to formation of'seconciary carbonates in soils of dryland ecosystems.

Land use and soil management options for secondary carbonates

formation

dospheric processes irlcluding soluhilizatiol~, leaching, and precipitation of carbonates, and availability of cat- ions ( ~ a ' " h4g" &', Na + etc.).

l'he partial pressure of CO, in soil air is governecl by the availability of biosolids, microbial activity, and soil temperature and moisture regimens. There is a strollg synergistic interaction between SOC and secondary car- bonates; the latter increases with an increase in SOC concentration or increase in rate of application of bio- solids to the soil. 'l'he acldition of biornass, both above- anti below-grouncl, is critical to the formation of sec- ondaty carbotlates. 1)ecomposition of biomass in- creases the partial pressure of CO, in the soil air. Rreetnari and Protz (1988) reported that the rate of f'ormation of sc~condal-y carbonates Mias 6.8 g (;/rrr2/y with biomass addition ctC5 Mg/ha/y and 13.9 g c/n12/y with biornass addition of'20 Mg/h;l/y. l'hc adtlition of l~iomass also enhances biotic activity of soil fauna (e.g., rertnites). Secondary cal-bonatcs are formecl in the vi- cinity of' the foci ol' such activities (Monger ancl Gal- Icgos 2000). Hurn~~s-c.ati~lg termites (13nlne ant1 Kuhl 11196) and those that generate CH, and CO, (Sander- son 1996) may enhance fi)rmation of biogenic carbon- atcs. Root acriviry is also irnportallt to carbonate dynam- ics. liclativcly high concentration of carbonates in the vicinity ol ' plant roots may be due to excretion or (:a ' ', HC:Q, atitl some acitl exudates. Khizospheric processes enhance precipitation of' (:;1(:0, both within and out- sitlc tlrc roots. Along with biosolids, techniques that enhance soil fixtilily may also leatl to formation of'

Rainfed ecosystems

seconclary carbonates. Iniproverrrents in soil ki-tility in- crcasc production of shoot and root biomass, which npon tleconiposition accentuate the p~.oduction of CO, in soil air. Further, the acltlitioli of (:ai' with fertilizers

Irrigated lands

and othcr soil amendments inc~cnw\ the formation of secondnr)~ rarhonates.

1 1 1

biodiversity management conservation

Water management is crucial to enhancing biomass. Water harvesting and trapping of sediments can bring in the cations (Caf 5 ,gt' etc.) from outside the eco- systems. Adoption of conservation effective measures also enhances the bioma5s production. Irrigation leads to a drastic increase in biomass production. lrrigation with good quality water (not saturated with carbonates) can dissolve carbonates and leach these into the ground water (Nordt and others 2000). Thus, quality of irrigation water, with regards to concentration and na- ture of salts, plays an important role in the dynamics of carbonates.

Some researchers argue that the rate of SIC seques- tration is low at 0.03-0.05 Mg C/ha/y (Schlesinger 1990, 1997). In contrast, others have shown that the rate of formation of secondary carbonates may be 0.1 1- 0.1.3 Mg (:/ha/y (Monger and Gallegos 2000). The flux oi secondary carbonates with the atmosphere is esti- nrnted at nhout 0.023 Pg C/y (Schlesirlger 1997). Thus, the turnover tirne of C in secondary carbonates may be 30,000 to 90,000 years. With adoption of IiMl's, i t is evtdent that the formation orsecontlary carbonates and leaching of carboriates in ground water are important methanisms of SIC seqnestration in cltyland soils.

Global Potential of Soil Carbon Sequestration in Dry lands

Global potential of C sequestration in drylanct eco- systems is large and has been estimated by several re- searchers. These estimates are crude ant1 tentative at best and are merely indicative of what may be possible under icleal condirions. These potentials are calculated on the basis of the lancl area under a specific ecosystem that may be converted to a restorative land use, multi- plictl by the rate of SOC sequestratiorr under improved

Carbon Sequestration in Dryland Ecosystems

land use and management system as reported in the literature, and multiplied by the number of years the land will be maintained under the improved system or for the duration over which the soil's SOC sink capacity may be filled. La1 and others (1998) computed the SOC sequesl.ration potential, using the following eqttation: where A is the land area (ha), R is the I-ate of SOC sequestration (Mg C/ha/y), ancl 11 is the duration in years.

Squires and others (1995) estimated that a vigorous effort to control degradation in the drylands can result in a net sequestration of up to 1 Pg C/y. L'al and others (1999) reported that sequestration potential as a result of desertification control, fossil fuel offset through bio- fuel production, and erosion control is 0.9-1.9 I'g C/y. Geesing and others (2000) estimated that if an increase of 2 Mg/ha of soil C could be achieved by growing Prosopis and Acacia species, it would lead to sequestra- tion of 6.2 Pg C/y. These and other data show that the potential of soil C sequestration in the dryland ecosys- tems is about I Pg C/y. Realization of this potential, however, requires a vigorous and a coordinated effort at a global scale towards desertification control, resto- ration of degraded ecosystems, conversion to appropri- ate land uses, and adoption of RMPs on croplancl and grazing land.

Potential of Soil Carbon Sequestration in U.S. Drylands

Because of relatively low NPI' and thus low quantity of biomas., I eturned t o the soil, rate5 oi SOC \eqtre\tr,t- tion in soils of the dryland ecosystems are generally lower than in more h~lmid regions. Conlnlonly ob- served rates of C sequestration in soil and biomass range from 40 to 400 kg/ha/y for SOC and 2000 to 4000 kg/ha/y for biomass (Table 7). The rate of fiw- mation of secondary bicarbonates is 100-120 kg/ha/y (La1 and Kimble 2000). With these assuri~ptions, ant1 on the basis of the data on land use in tlryland soils of'the U.S. (Table 8 ) , the data in Table 9 show the potential of' soil C sequestration through I-estorarion and mat~age- rnent of these ecosystems. The total potential of (:

sequestratiorl in dtyland soils of the U.S. is 38-56 Tg C/y (Tg = teragranl = 10" g = 1 million metric ton). 'I'his potential is rather small compared with the soil C sequestration potential in U.S. cropland of 75-208 'I'g C/y (la1 and others 1998), grazing land of 18-91 Tg C/y (Follett and others 2001), and Sorest land soils of 49-186 Tg (:/y (Kiinble and others 2002).

Several assumptions are made in assessing the po- tential of soil (: sequestl-ation repot-teci herein (1.a1 200 1 ) .

Table 7. Rates of carbon sequestration through land use change and adoption of recommended management practices (adapted from La12001 a, Lal and others 2000, Lal 2002).

(: sequesttatiotl rate

Sotl Vrgct,+tio~l

L.nni1 use/m,+nagement (Mg <:/h't/k)

Erosion coritrol 0.04-0.06 2-3 KMPs on agrictrlttu-;ti 1;untl 0.08-0.1 0 - Manageme~~t of irrigated land 0.2-0.3 - Kestoration of' rlegratletl soils 0.04-0.06 - Rrclar~iation of' salt-affvctetl soils 0.2-0.4 .% 4 Bioiiiel productior~ Ihr tiirrct

cotnhustion - 2-3 Secondary carbonates 0.10-0.12 -

(i) Restoration of degraded soils and ecosystems irilprove C pool in terrestrial dryland ecosystems.

(ii) Adoption of RMPs on agric~lltural soils inlproves soil quality and productivity over time.

( i i i ) 'The data on soil C sequestration from a few benchmarks oils and ecoregions can be extrapolated to other soils and ecoregions of the global drylands.

(iv) 'The hiclden C costs are included in the RMPs package, whose adoption is inevitable to feed the rising world population.

(v) Carbon seqrtc~stered in the terrestrial ecosystenls has a long residence tinie and is not re- emitted into the atmosphere.

(vi) Inputs required for atloption of RMPs on agri- cultural soils are available and at affordable prices es- pecially to resource-poor farmers.

(vii) Adoption of KMPs and afforestation of' tle- graded soils improve the economy of the region and augment farm income.

(viii) Soil/ter-restrial (: sequestl.atior1 enhances pos- itive kedbacks with numerous ancillary I~enefits.

An importarlt assumption is that adoption of'liMPs is driven by the overriding need for enhancing hot i pro- duction ancl artgrrienting fBrm income while sequester- ing C and impro\,ing the environment.

Nutrient Requirements for Soil Carbon Sequestration

Carbon is only one of the several constitrlcnts of hrtnlus. Other essential constitrtenls include N, I), S, %n, Cu, ;~ncl sevet-a1 nlicron~rtt~ietrts (Jenkinson 1988). Whereas tiecomposition of hun~us leads to I-elease of these nutrient elements, conversion of C from crop resitlrres and other biosoliils into hurnus rcqilircs their

Table 8. Land use and the risk of desertification in the U.S. (recalculated from Dregne and Chou 1992).

Area afkctetl by desertification

I .and usc 'Totztl area Slight Mode~.ate Severe Very Severe Moderate + M l ~ t Irrigateti crop1;zncl 15.2 11.2 3.5 0.4 0.1 4.0 RainSed croplantl 30.1 26.5 3.5 0.09 0.02 3.6 Rangelanti 325.1 40.1 98.0 173.0 5.0 276.0 I~Iyper-arid 1 .S - - - - -

1ot;tl 371.7 86.8 105.0 173.5 5.1 283.6

Table 9. Potential of soil carbon sequestration in U.S. drylands over a 50-year period.

Str atem

Rate of C C: seqnert~ation 1,dnd area \eqtlesrrrrtiori potr~~tlnl (Tg (Mli,~) (hg/hd/y) c/y)

Desertification coritrol 284 Recorrtmendetl nlairageriit~nt pr;lcticesX - Irrigateti cropland I1 - Iiair~fed croplat.rc1 27 - Rangeland 49

Secondary carhonates 372 Total

"I.;lntl arc;+ ilscd in rhcsc c;tlritl;ltio~~s is ;~c!jttstccl for that nntlcr dcscrtific;ition co~itrol LO aroirl duplication.

a\~allabtl~ty towards forrnntlon of humic substances. Therelore, Incleasing SOC concentration or sequester- lng carbon a150 rnvolves 1mmob1li71ng other elements (e.g., N, P, S etc.). Humus contam\ nbout 58% C (Stevenson 1982). Fur tlier, the r'ttio of C to other clernents (welght bas]\) In humu5 1 5 about 10 f o ~ N (All~sorr 1973), 50 for 1' (Jcnklnson 1988), and 70 tor S (jenklnion li188). 1-Iirnes ( I 998) est~mated that \eque\- t~ a t ~ o n of 10,000 kg of carbon Into hurnilr w~ll rc-quire

833 kg of N, 200 kg of P, ancl 143 kg of S. T h e ~ e l o ~ e , scq~1eit1at1o1-r of 1 l'g (:/y In the d ~ y l n ~ l d cco\ystenis \vould requllc 83 m ~ l l ~ o r l ton, of N , 20 nrillion tons of 1', nrrd 14 n r ~ l l ~ o n toll\ of K, ~imountlng to a total of 1 17 rnill~on tons of f e ~ t ~ l i r c ~ nut^ lent5 In colnpar I-

ion, the globdl fertl l l te~ use in 2000 was 136 rrllllion tons (IFDC 2002). 'Thcsc arc large rlunrbe~s, anel tIrc\e ~ l l ~ t ~ i e ~ r t s rrltlst IIC rlintlc availnble l ~ o ~ n '1 wide range of wurcc, Some nutllcnts nrc cotitained in crop r cslclitc., (1,al 199.5) Onc Mg 01 c e ~ eal st1 dw, on ,nelngc, contains 12-13 kg N, 1-4 kg P, nntl 1-2 kg 5 (Scott aticl Aldrlch 1983, Altllich ant1 others 1986) 1 herciorc, 10 W g of car 1,011, synthesl~ecl In humus, 1s contalricd 111 125 Mg of cereal 1e51due such as wheat and corrr (average C: content of 40% and hunrtfica- tion clficlency of 20%). 7 hts amount of re\ldue con- tairli 1500-1875 kg N, 125-500 kg l', ,111d 125-250 kg S Thus, nutltcnts supplied 111 clop I c51due a ~ c ntore

than adequate for conversion of C into humus. In practical terms, therefore, the process of C seques- tration is limited more by the availability of adequate amount of residue rather than that of' nutrients. Be- sicles residues, thcre are other sources of nutrients. In drylands, with limited leaching, there is sorne nitratc-nitrogen accumulated in the soil profile (Stewart and Robinson 2000), which can also be immobilized. 'Then, there is some addition of N from rainfall and BNF. -1'11ere are numerous other sources of N applied to the soil on an annual basis. Globally, 350 million tolls of N are returned to soil annually, of which 80 million tons are from fertilizer (Vitousek 1997). The fertilizer nitrogen use efficiency is gener- ally 20% to 40% (Halvorson and others 2002), and cvetl lowcr fbr P in soils of high P fixation capacity. A large fi-actiorr of urlused N and P arc pollutants of water ant1 air. 'Thus, i t may be advantageous to im- mobilize iV in humus to nrininiize volatilizatior~ (as N,O a greellhouse gas with 310 GMTP) or leaching (as NO,) into the ground water. Elemental recycling from sub-soil through establishment of deep-rooted species is also an important strategy to enhancing nutrient supply. Thus, the question of nutrient avail- ability for SOC sequestration is a challenging but doable task that niust be done through adoption of KMPs inclutli~lg precision farming.

Carbon Sequestration in Dryland Ecosystems

Relevance to the Kyoto Protocol

Following the Kyoto sumrnit of 199'7 (UWFCCC 1997), there has been a strong international interest in identifjiing and harnessing terrestrial, geologic, and oceanic sinks for long-term storage of atmospheric CO,~". The data and review presented in this and other reports indicate a large potential of soil (terrestrial) C: sequestration over the very extensive arid lands. The importance of soil C seq~~estration in cllylancl ecosys- tems catlnot be overemphasized. If 1 I'g out of 3.2 Pg of the annual increase in atmospheric concentration of CO, can be sequestered in soil, i t will be an important factor in stabilizing the concentration of greenhouse gases in the atmosphere. Furthermore, the sequestered C has economic value that can be realized through creation of certified carbon credits and traded on the international market under the Clean Development Mechanism, Joint Implementation etc. of the Kyoto Protocol. The Kyoto Protocol creates an enabling envi- ronment for creation of Certified Emission Credits (CERs), including those obtained by sequestration. There are also opportunities of trading carbon under the Bio-Carbon and Prototype Carbon Funds being developed by the World Bank. Such mechanisms also enhance opportunities to bring in corporate and public sector funds to finance C sequestration and improved land management.

Trading Certified Emission Credits necessitates "comn~odification" of carbon sequestered in soils. In other words, soil carbon must have an economic value similar to wheat, corn, milk, meat, or any other farm produce. Further, SOC must have value as a pr~blic good to mitigate climate change. 'The "com~~loclity price" and the mectlanism for promotion may be quite different. 'The price of SOC may be on the basis of cost/Mg, whereas the value of public good may be cost/ha. Both payment mechanisms can be operative within a geographic region.

'There is also a strong need for establishing criteria for determining the societal value of' soil carbon. In conlparison with the market forces, the societal value of soil carbon may be determined by on-site and off-site herlefits. The on-site benefits are due to long-tern1 im- ~x-ovenlents in soil quality, procluctivity, and land value. 'The off-site benefits are due to red~~ct ion i l l erosion and sedimentation, improvement in water quality, and decrease in net emission of greenhouse gases into the

' A l t l r o t ~ ~ h not a signatol?- ro thc K y o ~ o 1'1-otocol, I'rc\idrrrt 13usi1 ha\ also tmptr;~si~rd tlrc irnpo~.tarrcc of li)rc.st a t~t t soils i r i (: scqurstmtiur~ o n a volurttary Imsis.

atmosphere. La1 ant! others (1998) estimated the value of soil carbon at $25O/Mg, compared with the market value of $4 to $lO/Mg.

In this regard, the importance of restoring degracl- ed/desertified soils and ecosystems cannot be over- emphasized. 1.7stahlishing plantations of Acacia, I'rosopls, Weem, and other species is an important stlatcgy. A ~uclicious management of irrigated lands anti growing halomorphic plants by irrigation with saline water can also be in~plementecl. Soil-specific liMI's rrlust be adoptecl on croplands in dryland ecosystems. Intern* tiotlal agreements neecl to be implementecl to protect terrestrial/soil C sinks, initiate long-term projects to enhance and restore SOC/SIC pools, discourage bio- mass burning, and restore degraded soils and ecosys- tems. In view of its strong relevance, there is a strong need to develop simple and economic techlriques of validating/monitoring the SOC pool and its dynamics.

Conclusions

Analyses of the available information support the following conclusions:

I . World dryland soils cover a vast area with diverse land uses and a wide range of soils and ecosystems.

2. There is a severe problem of desertification or degradation of drylancls probably caused by land mis- use, soil mismanagement, ancl harsh climate.

3. Historic loss of SO(: clue to clesertification and degradation is estimated at 20-30 I'g for a total pool oS 241 Pg. Of the historic loss, 12-20 Pg (about two-thirds) can be resequestered through restoration of clesertified soils and ecosystenls.

4. 'The rnean potential of'SOC sequestration is about 1 Pg C/y, through alfbrestation with Mesquite, Acacia, Neem etc., and adoption of KMPs on croplatlcis and grazitlg lands.

5. The potential of SOC sequestration in d~yl;tnd soils is highly relevant to the Kyoto Protocol and the IJ.S. Farm Bill 2002.

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R. Lal

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