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Research Roundup n d u s t r y & I h c r a e s e R

and climateCARBON change

4 CSA News V54 N06 June 2009 R e s e

by Daniel Hillel and Cynthia Rosenzweig dioxide, from about 270 ppm to more than 380 ppm. Concurrently, there has been an increase in the a content of other radiatively active gases (the so-called r he earth’s constitutes a bio-thermo- c greenhouse gases), such as and nitrous oxide.

dynamic machine that is driven by solar energy h The effect, so far, appears to be a rise of more than 0.6°C and the exchanges of water, oxygen, carbon di- & I T in the global average temperature. This warming trend oxide, and other components in the –hydro- is expected to increase markedly in the coming decades, sphere– continuum. Green plants in the ter- unless strong measures are taken to mitigate it. restrial domain perform by absorbing y r t s u d n atmospheric CO and reducing it to forms of organic 2 Carbon Exchange in the Terrestrial Domain carbon in combination with soil-derived water, while utilizing the energy of sunlight. In the process, radi- The of the world, with the biota they support, ant energy is transformed into chemical energy that is are major absorbers, depositories, and releasers of or- stored in the molecular bonds of organic compounds ganic carbon. Soils altogether contain an estimated produced by the plants. This in turn provides the basis 1,700 Gt (billion metric tons) to a depth of 1 m and as for the food chain, which sustains all forms of animal much as 2,400 Gt to a depth of 2 m (Fig. 1). An estimated life. additional 560 Gt is contained in terrestrial biota (plants Roughly 50% of the carbon photosynthesized by and animals). In contrast, the carbon in the atmosphere is estimated to total 750 Gt. Thus, the amount of organic plants is returned to the atmosphere as CO2 in the pro- cess of plant respiration. The rest, being the carbon as- carbon in soils is more than four times the amount of similated and incorporated in leaves, stems, and roots, is deposited on or within the soil. There, organic com- pounds are ingested by a diverse biotic community, including primary decomposers ( and fungi) and an array of mesofauna and macrofauna (nema- todes, insects, , rodents, etc.). The ultimate product of organic matter decay in the soil is a complex of relatively stable compounds known collectively as . It generally accounts for some 60 to 80% of the total organic matter present, the balance consisting of recent organic debris of partially decomposed litter, dead roots, and the waste products of soil fauna. Since the beginning of the Industrial Revolution in the late 1800s, the expansion of , the clearing of , and especially the burning of fossil fuels have led to a dramatic increase in the atmospheric content of

D. Hillel ([email protected]) and C. Rosenzweig (crosenzweig@ Fig. 1. Carbon reserves and exchange in the land–ocean– giss.nasa.gov), Goddard Institute for Space Studies, New York, NY. atmosphere continuum. Both authors are ASA Fellows, and Dr. Hillel is also an SSSA, AGU, *Quantitative estimates regarding fossil fuels in ocean and AAAS Fellow. vary widely.

Carbon exchange in the terrestrial domain and the CARBON role of agriculture

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CSA News S world’s the in soils. carbon of mass Table Estimated 1. (both CO conditions anaerobic under methane condi- and aerobic tions under dioxide carbon matter, releases organic soil which of decomposition the to due are outputs The animals. and plants of residues the by soil process of photosynthesis and its incorporation into the the in ab- atmosphere the from dioxide the carbon of sorption to due are inputs The outputs. versus inputs of balance the on depending variable, temporally and mosphere. carbon in terrestrial biota and three times that in the at- ie s rai cro, ol nrai cro tns to tends carbon inorganic , organic la- as so bile nearly not Though Gt. 748 to 695 some total to estimated are reserves carbon These carbonates. sium of inorganic carbon in the forms of calcium and magne- reserves large contain also regions) semiarid and arid of those (mainly soils their some carbon, to organic of addition content In 1). (Table as such soils soils () to 50% or more in waterlogged organic arid-zone some in mass by 1% than less greatly,from However,). so-called (the cm varies content that 40 to 20 upper the in mainly concentrated generally is constitutes less than 5% of the mass of soil material and . powerful another is which oxide, nitrous of release the cause also may matter organic of decomposition tions, TOTALS soils Other Spodosols Histosols Aridisols r u o Soil orders Soil h qatt o ognc abn n ol i spatially is soils in carbon organic of quantity The The content of organic carbon in soils in most cases most in soils in carbon organic of content The 2 c and CH e : USDA. 4 being greenhouse gases). In certain condi- 130,667 10 10,550 15,464 11,869 23,432 19,854 13,159 4,596 Area 1,526 9,811 3,160 9,161 7,110 3 km 975 2 Organic C Organic 1,699.6 232.0 120.0 312.1 323.6 237.5 90.8 18.3 54.1 98.1 29.8 99.1 67.1 17.1 Gt r hn ol b te msin ae f CO of rate emission the be would than er USDA. and contain an estimated 70 Pg of carbon in deposits in carbon of Pg 70 estimated an contain and lowlands tropical the in occur peatlands global of 10% warming. global exacerbate further thus and peatlands drained from gases greenhouse more still of release secondary the cause may emissions gas greenhouse thropogenic an- to due warming global the which by feedback, tive gases from thawing permafrost is an example of a posi- warming , the enhanced emission of greenhouse take place, and the will release . In a will decomposition aerobic aerated, and water excess of drained when Later, methane. emit saturated, still while and, out thaw to tend will they warming, to ed subject- are permafrost peat-rich mat- of areas organic large ter.As undecomposed of stocks huge contain They 2). (Fig. Alaska and Canada of parts and Siberia in abundant are which Gelisols), (termed regions cold catch andburnuncontrollably. . They tend to temperate shrink and subside unevenly and in can even amount that half roughly and i. . lbl itiuin f rnia si orders. soil principal of distribution Global 2. Fig. sion of CO of sion consequent aeration accelerates decomposition and spurs the emis- the and drained, generally are soils such use, agricultural to converted When aerated. werewell the form of CH of form the generally—wetlands. These soils tend to emit carbon in or—more marshes, swamps, fens, bogs, termed ously vari- are areas waterlogged Such peat. called matter, promotes organic decomposed and incompletely of accumulation the decomposition inhibits turn in which oxygen, of deficiency a resultsin water with saturation known as organic soils, typically form where prolonged . to subject is and conditions acidic under solubilized be 20 Mg C ha C Mg 20 pr fo te etad o cl rgos about regions, cold of peatlands the from Apart of wetlands permafrost the are concern special Of ol wt a ih otn o croaeu matter, carbonaceous of content high a with Soils 2 . Cultivated peat soils may lose as much as much as lose may soils peat Cultivated . –1 yr 4 (marsh gas), but at a rate much low- much rate a at but gas), (marsh –1 in tropical and subtropical climates subtropical and tropical in V54 N06 June2009 2 f h soil the if S r u o c e :

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as deep as 20 m. Tropical peatlands are abundant in in pastoral, agricultural, industrial, and urban areas such regions as Southeast Asia (Indonesia, Malaysia, (Fig. 3). Cultivation spurs the microbial decomposition a Brunei, and Thailand) as well as in parts of the Ama- of while depriving it of replenish- r zon Basin. Some of these deposits appear to have been ment, especially if the cropping program involves re- c destabilized by agricultural as well as by the moval of plant matter (leaving little organic residues in h occurrence of more intense droughts that seem to be the field) and if the soil is fallowed (kept bare) during & I associated with El Niño periods. Such dry spells may considerable periods. Organic carbon is lost from soils result in the spontaneous burning of peat and vegeta- both by oxidation and by of topsoil. Some cul- tion that may cause the rapid emissions of great quanti- tivated soils may, over time, lose as much as one-third y r t s u d n ties of carbon dioxide. As more tropical swamp forests to two-thirds of their original organic matter content. and peatlands are drained and converted to agriculture, Consequently, these soils degrade in quality, as their

they will likely contribute still greater emissions of CO2 fertility diminishes and their structure is destabilized. to the atmosphere, especially if El Niño events become Such soils are therefore important targets for mitigating more intense or frequent in a warming climate. the greenhouse effect by reducing and even reversing Histosols and Aridisols are two other groups of soils their tendency to emit greenhouse gases. likely to be strongly affected by . Histo- Though agricultural soils acted in the past as signifi-

sols are organic soils containing large concentrations cant sources of atmospheric CO2 enrichment, their pres- of peat. As they tend to dry out in a warmer and drier ent carbon deficits offer an opportunity to absorb sub-

climate, enhanced oxidation could result in accelerated stantial amounts of CO2 from the atmosphere and store oxidation and the release of large quantities of carbon it as added organic matter in the coming decades. The dioxide to the atmosphere. Aridisols cover about 12% historical loss of carbon in the world’s agricultural soils of the land surface. They are particularly vulnerable has been estimated to total some 42 to 78 Gt. Ideally, to processes of , salination, and desertifica- we might hope for complete restoration of that loss, i.e., tion. Higher temperatures can be expected to increase the intensity of evaporation, and hence seasonal water shortages. Climate change is likely to affect soil erosion through its impact on rainfall intensity and amount, vegetative cover, and patterns of . Whereas wetter condi- tions may exacerbate the hazard of water erosion, drier conditions may intensify wind erosion. Desertification can occur when the climate becomes drier and/or the vegetative cover of an area is so degraded that the de- nuded landscape comes to resemble a .

Human Management of Soils The balance of soil carbon is greatly influenced by human management, including the clearing or restora- Fig. 3. Changes in soil carbon content after / tion of natural and the patterns of land use cultivation and reforestation.

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CSA News National Laboratory, Richland,WA. inl ilg, rtcs ol gis eoin ad eut in results increased storage of carbon in the soil. and erosion, against soil protects conven - , with tional obtained those to similar yields crop duces pro- practice The dioxide. carbon atmospheric in increases down slowing of way one offers agriculture No-till 4. Fig. by mechanical tillage, is prone to deterio- structureproneto soil is tillage, mechanical by organic matter. Whenever soil aggregates are disrupted occluded of decay the resist further and re- aeration strict which of interiors the aggregates, water-resistant to decay.further Moreover, resistanttight form to tend soils clayey become thus which humus, macro- of organic molecules the and particles the of surfaces active the between bonds strongphysicochemical form hand, other the on soils, Clayey matter. organic little retain capacity,generally adsorptive little have and ed aerat- well be to tend which soils, Sandy fraction. clay their of nature and content the with associated mately original “virgin” levels. the above raised be soils of content carbon organic the resistant to decay and then applying it to the soil) might anaerobically charring organic matter or so that it is vegetation highly high-residue fertilizing intensively and irrigating (e.g., circumstances special certain in Only significant. very be can amount that Still, loss. C toric may management, be on the order of one-half to two-thirds soil of the his- of strategies recommended of tion adop- the assuming practice), in content carbon their of restoration potential the (i.e., soils carbon-sink many of capacity actual The feasible. economically be not may it possible, is restoration such where Even state. original their recover fully to soils of capacity the ishes processes)other and/or salination, compaction, dimin- cover,vegetative of erosion,tillage, pollution, leaching, residue retention. and production plant maximize to supply water and nutrient optimizing while disturbance soil minimizing saturation.” The way to “carbon restore soil organic matter of is by state pre-agricultural a to soils of return a h ptnil f ol t sqetr abn s inti- is carbon sequester to soils of potential The burning from (resulting degradation soil reality, In S r u o c e : Pacific Northwest CO of absorption increased promoting by and operations) cultural operations (avoiding unnecessary fuel-burning agri- of efficiency the improving by done be can This sequestration. carbon of enhancement through as well house gas mitigation through reduction of emissions, as green- for opportunity significant a present soils tural on theorder ofsome1.6Gt ofcarbonperyear. in the tropics apparently results in additional emissions agriculture for clearing land Continuing atmosphere. the in accumulated has which of some and ocean the in absorbed been has which of much carbon, of Gt 170 about total to estimated been have centuries three past and soils due to deforestation and cultivation during the rapidly. more decompose to tends matter organic soil and rate atr s wrh ts i isl, eod t potential its beyond itself, in task worthy a is matter organic soil of management the improving that is ciple prin- important The continue. can fuels, fossil for tutes substi- as biofuels) (e.g., energy renewable of duction pro- the and use energyreduced as such management, by outbreaks offire. or methods tillage inappropriate to temporarily even reverting by reversed be may practices conservation of decades or years over achieved carbon soil of gains the that danger even is there fact, attains In saturation. content effective carbon organic its as or state equi- librium an approaches location each in soil the as cacy, effi- in diminish to likely are soils in carbon sequester to designed practices time, Over adjustments. specific sal terms, their application to different sites will require univer in stated be can principles basic the While fect. ef- greenhouse the mitigate help to as so soils manage no simple universal prescriptions regarding practices to and from one period to another. Therefore, there can be another to location one from greatly differ conditions promote foodsecurity. quality,ter ,enhance cropand boost yields, fect, reduce soil erosion, improve soil structure and wa- ef- greenhouse the mitigate scale, to help a large can practices such on consistently and efficiently mented imple- and adopted If ergyfuels. replacecropsfossil to en- of production the and conservation, water , soil of amendments (e.g., lime application to neutralize acidity), improved sludge), and , composts, (by , nutrients soil of augmentation crops, cover of planting 4), (Fig. farming agroforestry,no-till 900 Mtperyearoveraperiodofseveraldecades. and 600 between total to estimated variously been has practices management in changes through soils tural agricul- global in carbon of sequestration potential The Taking a positive view, we may surmise that agricul- The combined losses from the earth’s native However, other advantages of carbon conservation carbon of advantages other However, A necessary caveat is that climate, soil, and economic reforestation, include practices recommended The 2 by green plants and its stable storage in the soil. the in storage stable its and plants green by V54 N06 June2009 - R e s e

benefits in mitigating the atmospheric greenhouse ef- nign cycle of productivity gain. Enrichment of the top- fect. It can not only turn the soil from a net source to a soil with organic matter makes it less prone to compac- a net sink for greenhouse gases, but can also boost soil tion, crust formation, and erosion, which in turn affects r productivity and reduce environmental damage due to the quality of the environment downstream. It also im- c erosion. proves the quality of the soil with respect to , h Various feedback mechanisms are involved in the aeration, seed germination, and plant nutrition. The ag- & I interactions between climate change and the carbon ricultural sector can thus contribute to the mitigation of

cycle. Increasing concentrations of CO2 in the atmo- global warming in three principal ways: y r t s u d n sphere can stimulate greater rates of photosynthesis, an 1. reducing its emissions by adopting such practices effect called CO2 fertilization. In principle, a portion of as no-till plantings; the extra products of photosynthesis (plant biomass) is 2. absorbing CO from the atmosphere by enhanced transferred to the soil via surface litter and the root sys- 2 photosynthesis and storing a sizable fraction of the tem, and a fraction of that is stabilized therein as soil carbon in the soil; and humus. Moreover, rising temperatures tend to hasten plant growth and prolong the growing season in regions 3. producing renewable sources of energy, known as where growth is normally inhibited by cold weather. biofuels, derived from agriculturally grown bio- Such processes tend to moderate the greenhouse effect. mass that can be converted to ethanol and biodie- On the other hand, rising temperatures may exceed sel.

optimal levels for some plants in some regions, thus re- Conventional tillage is defined as the mechanical stricting carbon assimilation, and also hasten decompo- manipulation (pulverization, mixing, and inversion) of sition of organic matter and the emission of carbon di- the topsoil that leaves no more than 15% of the ground oxide (as well as, perhaps, methane and nitrous oxide), surface covered with crop residues. Such tillage tends thus tending to exacerbate the greenhouse effect. Ris- to disrupt soil structure, accelerate the decomposition ing temperatures may also favor infestations of insect of soil organic matter, and render the bared topsoil vul- pests and fungal diseases of crops. Whether positive nerable to erosion by rain and wind. In contrast, no-till feedbacks are likely to outweigh the negative feedbacks management is defined as the avoidance of all unnec- or vice versa will depend on site-specific conditions as essary mechanical manipulation of the topsoil so as to well as on human intervention and management of the leave it largely undisturbed and covered with surface ecosystem. In any case, the change in the soil temper- residues throughout the sequence from harvesting of ature regime, which generally entails a change in the the prior crop to the planting and establishment of the regime, is certain to affect the content and new crop. Such vegetative residues constitute a pro- turnover rate of soil organic matter. tective mulch, which shields the soil against the direct impact of raindrops during the wet season as well as Agricultural Practices Affecting Soil Organic against extreme desiccation and deflation by wind dur- Matter ing subsequent dry periods. Depletion of organic matter in soils initiates a vicious The best agricultural practices are those that result in cycle of degradation, affecting food security and envi- augmentation of soil carbon and enhanced productivity ronmental quality, often on a regional scale. Reversing due to better soil structure and soil moisture conserva- that depletion via can induce a be- tion. The relevant practices include precise and timely applications of fertilizers, use of slow-release fertilizers

June 2009 V54 N06 CSA News 9 Re s e a r c h & In d u s t r y 10 CSA News entail increased use of energy. Among those practices those energy.Among of use increased entail production agricultural intensifying at aimed practices function andbiodiversity. ecosystem maintain and deforestation, stop land, ginal productivity. That, in turn, can relieve pressure on mar soil enhance also but energy conserve only not which tillage, zero and tillage conservation of adoption the is important Especially soils. in carbon of creasedstorage in- and fuels), fossil than (rather energy renewable on based on lowered energy consumption, greater reliance gas–efficient farming and in general, become to substantial carbonsinks. as so afforested or perennial to productivityseverelyconverted be impaired,may they agricultural their and past the in degraded badly been have soils additional the Where cause gases. greenhouse thus of and emissions fuel of consumption the raise herbicides of application and transport, facturing, manu- The herbicides. of use increased require hence no-till and weeds of infestation greater of in result may farming practice contrary the weeds, of eradication decades. its prior equilibrium (or C saturation) state within a few approach to tend will soil depleted historically any as indefinitely continue to expected be cannot increments ha C Mg 0.7 to 0.1 from varying rates at soils been in storage carbon has boost to found farming no-till to Conversion content. bon car organic soil’s the restore even and protect to help can practices Such soil. the of disturbance mechanical of minimization and crops, cover high-residue of periods, use fallow of elimination or shortening erosion, of prevention volatilization), or leaching minimize (to take soilsampleswithstudents. Rosenzweig Cynthia Studies Dr.Space for Institute Goddard the of left) from (third and left) (far Hillel Daniel Dr. There are, however, necessary caveats. Some of the of Some caveats. necessary however, are, There Needed altogether is anewparadigmofgreenhouse the is tillage of functions classical the of one Since –1 yr –1 . However, such positive such However, . - - this article have taken the initiative to convene a day- a convene to initiative the taken have article this of authors the change, context climate the regional and in global agriculture of of role the to related sues Announcing aForthcomingSymposium CO atmospheric of sion emis- net to absorption net from to (i.e., negative vulnerable turning and labile case any in is soil the in bon car of balance The sequestration. C inhibit hence and global to organicaccelerate may warming decomposition matter due temperatures Higher decades. several in attained be may equilibrium) dynamic in are cesses (wheresaturation carbon proemission and absorption - of organic matter in soils is generally finite. Soil organic sequestration for potential The time. in diminish may farming conservation of benefits Some transportation. and control, weed and pest fertilization, irrigation, are flooding, orfire. drought, as such perturbation some of occurrence the interruptedis by it if or maintained not is management gas emissionsorabsorption. greenhouse influence greatly can farming dryland as well as irrigated in management moisture soil of mode Finally, the help. can bacteria) -fixing sociated as- their and green(legumes reliancemanureon plants eutrophicationfreshwaterIncreasedas of bodies). such pollution environmental cause may (which losses ent to maximize nutrient use efficiency and minimize nutri- as so rates calibrated precisely at and needed most are preferentially fertilizers applies field, wherethe they in soils of heterogeneity the recognizing agriculture, sion preci- Modern sensing. remote preferably,or,by pling to develop appropriate methods of monitoring by sam- moval or burning, and fallowing. Research is necessary re- residue tillage, traditional to reversion by rapidly very lost be residueretentioncan crops,and cover age, takingly achieved by such practices as conservation till- pains- gains the since basis continuing a on results the however, be based on an effective system of monitoring practices. Schemes to reward carbon sequestration must, invested infarmingoperationsandtransportation). (i.e., that the energy produced is greater than the energy positive indeed is production such of equation energy the that ensure to needed is accounting careful though al- benefit continuing a be also can fuels fossil replace to crops energy of production sustainable and efficient erosion.controlsoil the The of and fertility soil of ment the improvement of , including the enhance- with true is same The maintained. is management soil tillage, can continue as long as that form of conservation efficient operations, especially with the adoption of zero sist indefinitely. Reduction of fuel use brought about by To further clarify the fundamental and practical is- practical and fundamental the clarify further To Some benefits of conservation management can per can management conservation of benefits Some Policies are needed to promote and guide C-efficient 2 i te carbon-augmenting the if ) V54 N06 June2009 - - R e s e

long symposium, to be held during this November’s gion of the United States. Agric. For. Meteorol. 130:59–69. ASA–CSSA–SSSA International Annual Meetings in Johnson, J.M.-F., R.R. Allmaras, and D.C. Reicosky. 2006. a Pittsburgh, PA, under the auspices of Division A3. The Estimating source carbon from crop residues, roots, and r title of the symposium is “Climate Change and Agro- rhizodeposits using the national grain-yield database. c : Impacts, Adaptation, and Mitigation.” The Agron. J. 98:622–636. h & I specific aims are to generate a vigorous and rigorous Laird, D.A. 2008. The vision: A win–win scenario interdisciplinary examination of the key factors, pro- for simultaneously producing bioenergy, permanently cesses, and challenges ahead; to stimulate cooperative sequestering carbon, while improving soil and water research; and to formulate strategies to deal effective- quality. Agron. J. 100:178–181. y r t s u d n ly with the potential consequences of climate change Lal, R. 2006. Enhancing crop yields in the developing coun- on the environment in general and on future food se- tries through restoration of the soil organic carbon pool curity in particular. More information on this session in agricultural lands. Land Degrad. Develop. 17:197–209. and on this year’s meetings will be posted to www. Lal, R., and R.F. Follett (ed.) 2009. Soil carbon sequestration acsmeetings.org in the coming months. and the greenhouse effect. Soil Sci. Soc. Am. Special Publ. 57. SSSA, Madison, WI. Further Reading Nelson, R.G., C.M. Hellwinckel, C.C. Brandt, T.O. West, D.G. Guo, Y., P. Gong, R. Amundson, and Q. Yu. 2006. Analysis of De La Torre Ugarte, and G. Marland. 2009. Energy use factors controlling soil carbon in the conterminous Unit- and carbon dioxide emissions from cropland produc- ed States. Soil Sci. Soc. Am. J. 70:601–612. tion in the United States, 1990–2004. J. Environ. Qual. Hillel, D. 2008. Soil in the environment: Crucible of terres- 38:418–425. trial life. Academic Press/Elsevier, Amsterdam. Paustian, K., and B. Babcock. 2004. Climate change and Hillel, D., and C. Rosenzweig. 1989. The greenhouse effect greenhouse gas mitigation: Challenges and opportuni- and its implications regarding global agriculture. Mas- ties for agriculture. CAST Task Force Rep. R141. CAST, sachusetts Agr. Exp. Stn. Res. Bull. 724. Massachusetts Ames, IA. Agric. Exp. Stn., Amherst. Rosenzweig, C., and D. Hillel. 1998. Climate change and the Hollinger, S.E., C.J. Bernacchi, and T.P. Meyers. 2005. Carbon global harvest: Potential impacts of the greenhouse effect budget of mature no-till ecosystem in North Central re- on agriculture. Oxford University Press, New York. Canopy SunScan SS1 Canopy Analysis System • Direct display of Leaf Area Index (LAI) • Usable in cloudy, clear and changeable conditions • Measures incident and transmitted PAR in plant canopies

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June 2009 V54 N06 CSA News 11