carbon cycling in earth systems—a soil science perspective

Agriculture, Ecosystems and Environment 104 (2004) 399–417

Review

Carbon cycling in earth systems—a soil science perspective

H.H. Janzen∗Agriculture and Agri-Food Canada, Box 3000, Lethbridge, Alta., Canada T1J 4B1

Received 26 August 2003; received in revised form 16 January 2004; accepted 26 January 2004

Abstract

The carbon cycle binds together earth’s ecosystems and their inhabitants. My intent is to review the global carbon cycle,examine how humans have modified it, and contemplate (from a soil science bias) the new questions that await us on achanging earth. These thoughts are proffered, not to propose a way forward, but to invite conversation about opportunitiesthat await us.

Terrestrial ecosystems hold a lot of carbon—about 500 Pg C in plant biomass, and 2000 Pg C in soil organic matter. Oceanscontain even more. And the atmosphere, now with about 785 Pg C, connects all of these pools. The flows of carbon betweenthe pools, and their feedbacks, have kept atmospheric CO2 reasonably constant for millennia. But humans have increasinglydistorted the balance, by changing land use and by injecting fossil C back into the cycle. Consequently, atmospheric CO2 hasincreased recently by more than 3 Pg C per year and, by century’s end, its concentration may be twice pre-industrial levels,or more.

The changing carbon cycle poses new questions for scientists. Now we will be asked, not how things are, but how they willbe. For example: How will changes in CO2 alter flows of carbon through biological carbon stocks? Can we manage ecosystemsto hold more carbon? Are current carbon stores vulnerable should the earth warm, or water cycles shift, or nitrogen flows bealtered? What will the C cycle look like a century from now; and will it then still provide all that we expect from it? These andother new questions may elicit from us fresh insights and approaches. We may learn to look more broadly at the C cycle, seeingall the ‘ecosystem services’ (not just C sequestration). We may insist on studies yielding deeper understanding of the C cycle,relevant beyond current issues. We may further emphasize ‘time’ in our studies, looking more at flows and changes than atdescribing what is—and looking long enough to see even subtle shifts. We may learn to follow C beyond the usual boundariesset by arbitrary disciplines. And we may come to see, more than before, how the carbon cycle weaves through our fields andskies and forests—and find new ways to reveal its grandeur to those who have not yet seen it. And then, it may happen thatour successors, a century from now, will look back, almost in envy, at the urgent, enticing questions we were given to solve.Crown Copyright © 2004 Published by Elsevier B.V. All rights reserved.

Keywords:Carbon cycling; Greenhouse gases; Climate change; Global change; Carbon; Nitrogen; Carbon sequestration; Mitigation; Land use;Fossil fuel

1. Introduction

In a recent book on environmental problem solving,Harte (2001)asks: “How likely is it that at least onenitrogen molecule exhaled by Caesar in his last breath

∗ Tel.: +1-403-317-2223; fax:+1-403-382-3156.E-mail address:[email protected] (H.H. Janzen).

will be in the next breath you take?”. After some sim-plifying assumptions and mathematical gymnastics, heconcludes that you are, indeed, almost certain (P =0.995) to inhale nitrogen from Caesar’s expiring ex-halation.

This quaint example illustrates how the nitrogen cy-cle ties biota together—it binds them across space andacross time. And what is true for nitrogen, is true also

0167-8809/$ – see front matter. Crown Copyright © 2004 Published by Elsevier B.V. All rights reserved.doi:10.1016/j.agee.2004.01.040

400 H.H. Janzen / Agriculture, Ecosystems and Environment 104 (2004) 399–417

for carbon. If we could follow a single carbon atomnow in the air, we might find that it enters a pine tree byphotosynthesis, returns to the air when the pine needledecays, then is fixed into a grain of rice, before escap-ing back into the air in a child’s breath. And so thecarbon atom flits from place to place, pausing here andthere for seconds or millennia, but ever passed along.

The meanderings of carbon atoms have long fasci-nated us—but now their wanderings are of more ur-gent interest. Because the carbon cycle is changing—changing abruptly—as evident by sudden increases inatmospheric CO2 within a geological eye-blink.

In this paper, I examine the global carbon cycle bypondering four questions: What are the key pools and

Fig. 1. (a) An overview of the global carbon cycle, as it was in the 1990s (based on values fromIPCC (2000, 2001a)and Amundson(2001)). All C stocks are in units of Pg C, and flows are in units of Pg C per year. (b) Global C stocks and flows, expressed as averagesper ha of continental area. Values are expressed in units of Mg C ha−1 or Mg C ha−1 per year.

flows of carbon? How have they been influenced byhuman activity? What new questions arise because ofthese effects? And: What opportunities await us as weaddress these challenges? A review like this is usuallyfrom a specific vantage point or bias; mine is from theperspective of carbon cycling in agroecosystems, withemphasis on soil organic matter.

2. The global C cycle

The wheel of life is made up of two processes—growthand decay. The one is the counterpart of the other.Howard, 1940

H.H. Janzen / Agriculture, Ecosystems and Environment 104 (2004) 399–417 401

In its life, its death, and its decomposition an or-ganism circulates its atoms through the biosphereover and over again. . . A considerable portion ofthe atoms in the earth’s surface are united in life,and these are in perpetual motion.Vernadsky, 1926

The main pools of actively cycling carbon are at-mospheric CO2, biota (mostly vegetation), soil or-ganic matter (including detritus), and the ocean (Fig. 1;IPCC, 2000, 2001a). Of these, the oceans contain thelargest reserves of C—about 39,000 Pg C—thoughmost of this (all but about 1000 Pg C) is in deep oceanlayers and not in active circulation, at least in timesmeasured in human generations.

The atmosphere, now with a CO2 concentration of370 ppmv (Keeling and Whorf, 2002), contains about785 Pg C as CO2; that amounts to about 15 Mg Cabove each ha of the earth’s surface (Fig. 1). Carbonstocks in biota are somewhat less certain, but arealmost comparable to the atmospheric pool: about400–600 Pg C (IPCC, 2000, 2001a; Smil, 2002).Most of this, about 75%, occurs in forests (Table 1).Oceanic plants, mainly algae, account for less than1% of global biomass C (Falkowski, 2002; Körner,2000; Smil, 2002).

The largest pool of actively cycling C in terrestrialecosystems is the soil. To a depth of 1 m, it containsabout 1500–2000 Pg C in various organic forms, fromrecent plant litter to charcoal to very old, humified

Table 1Summary of C stocks: plants, soil, atmosphere

Biome Areaa (109 ha) Global carbon stocksa (Pg C) NPPb (Pg C per year)

Plants Soil Total

Tropical forests 1.76 212 216 428 13.7Temperate forests 1.04 59 100 159 6.5Boreal forests 1.37 88d 471 559 3.2Tropical savannas and grasslands 2.25 66 264 330 17.7Temperate grasslands and shrublands 1.25 9 295 304 5.3Deserts and semi-deserts 4.55c 8 191 199 1.4Tundra 0.95 6 121 127 1.0Croplands 1.60 3 128 131 6.8Wetlands 0.35 15 225 240 4.3

Total 15.12 466 2011 2477 59.9

a Source:WBGU (1998), as presented inIPCC (2001a).b Source:Ajtay et al. (1979), as presented inIPCC (2001a).c Includes ice covered areas.d Estimate is likely high, due to high Russian forest density estimates including standing dead biomass.

compounds (Amundson, 2001; Table 1). (More C ex-ists as carbonates and in organic forms below the 1 mdepth, but these are often assumed, perhaps naively,not to be in active circulation.) About a third of thesoil organic C occurs in forests, another third is ingrasslands and savannas, and the rest is in wetlands,croplands, and other biomes.

All of these C pools—the atmosphere, vegetation,soil, and ocean—are connected. Atmospheric CO2 en-ters terrestrial biomass via photosynthesis, at a rateof about 120 Pg C per year (gross primary productiv-ity). But about half of that is soon released as CO2 byplant respiration, so that net primary production (NPP)is about 60 Pg C per year. This amount is stored atleast temporarily in vegetative tissue, but most even-tually enters soil upon senescence. At the same time,heterotrophic respiration (largely by soil microorgan-isms) and fire return an amount roughly equivalent toNPP (∼60 Pg C per year) back to atmospheric CO2,closing the loop. Averaged over the total area of con-tinents, these C inputs and losses amount to about4 Mg C ha−1 per year (Fig. 1).

Exchange of CO2 between atmosphere and oceanare even larger than those between air and landecosystems—about 90 Pg C per year, both ways.Some of this occurs by physical processes, involvingthe CO2—carbonate equilibria. But a surprisinglylarge exchange also occurs via biological processes;though ocean biomass is a mere 3 Pg C (Smil, 2002;


p. 195), its NPP almost equals that of all land plants,with a mass of about 600 Pg C (del Giorgio andDuarte, 2002; Falkowski, 2002).

The C flows among the various pools, and theirfeedbacks, have kept the global C cycle quite stable formillennia—at least, until recent decades when humanactivity began exerting increasing stresses on the Ccycle.

3. Human influences on the global C cycle

Man, under our very eyes, is becoming a mightyand ever-growing geological force.Vernadsky, 1945(citing Pavlov (1854–1929))

It seems appropriate to assign the term ‘Anthropo-cene’ to the present. . . geological epoch.Crutzen,2002

In the past two centuries, the human population hasincreased about six-fold—from one billion to six bil-lion. And as we have proliferated, we have increas-ingly distorted the C cycle. Our two main influenceshave been: (1) changes in land use, and (2) combus-tion of fossil C.

Fig. 2. Expansion of area devoted to pasture and croplands, since 1700. Estimates of areas prior to disturbance, in millions of km2,are: cropland: 0; pasture: 0; Tundra/desert: 31.4; steppe/savanna/grassland/shrubland: 44.1; forest/woodland: 58.6 (plotted from values inGoldewijk (2001)).

3.1. Land use change

There is one steady movement of organic material totowns and great cities and industrial centers—thereto be consumed or disposed of as waste but neverto go back to the land of origin.Osborn, 1948

Our changing land: nearly 50% of the land sur-face has been significantly transformed by our ac-tions and only∼5% remains pristine.Canadell andNoble, 2001

As our numbers have multiplied, we have used moreand more of the global net primary productivity for ourown ends. According toVitousek et al. (1986), “nearly40% of potential terrestrial [NPP] is used directly,co-opted, or foregone because of human activities.”Rojstaczer et al. (2001), similarly, estimated “that hu-mans appropriate 10–55% of terrestrial photosynthe-sis products”. In effect, humans have commandeeredthe C cycle, re-routing it, sometimes even squelchingit entirely.

The land use change with largest influence on the Ccycle was the “expansion of agriculture” (Houghton,1999). In the last four centuries, the land area


Fig. 3. Conceptual view of C cycle in an agroecosystem.

devoted to pasture and croplands increased from neg-ligible values to about 30% of global land area, largelyat the expense of forest and grasslands (Fig. 2). Ac-cording toRamankutty and Foley (1999), the net lossof forest/woodland has been about 11.4 million km2

and net loss of savannas/grasslands/steppes was about6.7 million km2. Almost invariably, this change ofland use has depleted the reserves of stored C.

Why does agriculture often reduce C stocks? Anexample—the grasslands of western Canada—may

Fig. 4. CO2 emissions from land-use change (source:Houghton and Hackler, 2002).

help illustrate. Carbon stocks in an ecosystem are afunction of inputs from photosynthesis (net primaryproduction) and losses from heterotrophic decompo-sition (Fig. 3). When photosynthetic inputs exceedlosses, C accumulates; when they fall short of losses,stored C wanes. During early stages of ecosystemdevelopment, C inputs predominate, so that C grad-ually accumulates until decomposition eventuallyapproaches inputs (Odum, 1969). Thus, in the 8000years since their last glaciation, the grasslands ofwestern Canada accumulated large reserves of organicC, sometimes in amounts exceeding 200 Mg C ha−1,mostly in soil organic matter. But about 100 yearsago, settlers began cultivating these lands for cropproduction. This conversion had at least two effectson stored C (Janzen et al., 1997). First, it stimulatedthe rate of decomposition, because the physical dis-turbance exposed previously protected organic matterto biological activity and also increased soil water byperiodically suppressing plant growth. Second—andthis may be the more important (and sometimes over-looked) effect—farming sent away large proportionsof the C acquired by photosynthesis. That, after all,is the point of farming—to capture, by photosyn-thesis, C in some marketable form and export it forprofit. Indeed, many farming systems, and the plantsused within them, have been specifically designed tomaximize the amount of saleable product, and hence,


the amount of C removed. Thus, it is not surprisingthat the grassland soils quickly lose a large fractionof their stored C—typically about 20–30% of C insurface layers—soon after initial cultivation (Janzenet al., 1998).

In the grasslands, only soil C is lost upon conversionto agriculture. But in forests, where much of the C isin vegetative biomass, losses can be even higher. In-deed, world-wide, losses of C from plant biomass havebeen about twice those from soils alone (Houghton,1995).

Globally, losses of C from land use change havebeen steadily increasing over the last one-and-halfcenturies, approaching rates of about 2 Pg C per year(Fig. 4), now mostly from tropical deforestation.Net cumulative emissions of CO2 from land usechange, from 1850 to 2000, amount to about 156 Pg C(Houghton, 2003). Additional amounts may have beenlost before 1850 (DeFries et al., 1999; IPCC, 2000,p. 44), so that historical losses may be close to 200 PgC (Canadell and Pataki, 2002; House et al., 2002).Consequently, current stocks of C in soil and, espe-cially, in phytomass, are much smaller than they were

Fig. 5. Global C cycle showing fossil C stock, CO2 emissions, and fate of CO2 in the 1990s. Carbon stocks are in units of Pg C; annualflows and changes in atmospheric CO2 are in Pg C per year. Net annual absorption by terrestrial and ocean sinks is only roughly known(House et al., 2003; Houghton, 2003); values shown are fromIPCC (2001a). Other sources include:IPCC (2000), Sundquist (1993)andRogner (2000).

in pre-agriculture times; according toSmil (2002,p. 194), “the most likely phytomass stores are nowperhaps as little as one-half, and almost certainly nomore than two-thirds, of their preagricultural total.”Clearly, human activity has profoundly distorted theglobal C cycle by re-directing the flows of C in ter-restrial ecosystems, and re-allocating the C amongits pools. Land use change was our first influenceon the global C cycle—but it would not remain thebiggest.

3.2. Fossil fuel burning

Man. . . has now plunged heavily into [Nature’s]slow-moving carbon cycle by throwing some9,000 tons of carbon dioxide into the air eachminute. . . from the combustion of fossil carbon. . .

Callendar, 1939

The fossil fuels burned in 1997 were created fromorganic matter containing [44,000 Pg C], which is>400 times the [NPP] of the planet’s current biota.Dukes, 2003


Fig. 6. CO2 emissions from fossil fuel burning and cement production (drawn from values inMarland et al. (2002)).

Before the last century, human intervention in theglobal C cycle was dominated by land use change.But since then, it has been superseded by a distor-tion more severe—the combustion of fossil fuels. Be-neath the earth’s living mantle are vast pools of fos-sil C, containing thousands of Pg C (Rogner, 2000;Sundquist, 1993), which have lain isolated from theactive C cycle for millions of years. In effect, we havebored down into an old C pool, long dormant, and havere-connected it to the active C cycle (Fig. 5), inject-ing CO2 at increasing rates as the global economy hasbecome ever more industrialized. Before 1900, emis-sions were well below 1 Pg C per year (Fig. 6). Butsince then emissions have increased steadily and an-nual emission rates now approach 7 Pg C, several-foldthose from land use change. As pointed out byMarlandet al. (2001), “since 1751, roughly 270 Pg C have beenreleased. . . Half of these. . . have occurred since themid-1970s.”

Only a fraction of the fossil C has been burned todate—accessible reserves are about 1000 Pg C and to-tal resources include thousands of Pg C more (Rogner,2000; Sundquist, 1993). Consequently, the influenceof fossil fuel burning on the global C cycle will notsoon be halted by depleting reserves; if all the world’ssupplies of coal were burned, the atmospheric CO2concentration might reach 2000 ppmv (Kump, 2002).

What has been the fate of the CO2 injected into theatmosphere from land use change and fossil fuel burn-ing? A summary of the C budget for the 1990s mayhelp illustrate (Table 2). During that decade, averageannual emission of CO2 was more than 8 Pg C—about6.3 Pg C from fossil fuels and about 2 Pg C from landuse change. But less than half that amount—3.2 PgC—was measured as increased atmospheric CO2; theremaining C—about 5 Pg C—must be absorbed else-where. The oceans can account for about 2 Pg of that.But that still leaves another∼3 Pg C unaccountedfor. And, by default, that is assumed to be enteringterrestrial ecosystems—the “residual terrestrial sink”(Houghton, 2003; Schimel et al., 2001) (once calledthe ‘missing sink’).

Scientists do not yet agree where and why thisC is accumulating; possible processes include: CO2fertilization and increased growth of forests fromN deposition, though increasing evidence pointsto recovery from past disturbances, notably there-expansion of forests in the northern hemisphere(Adams and Piovesan, 2002; Houghton, 2003).Regardless of the mechanism, it seems that ter-restrial ecosystems annually absorb about 1 Pg Cmore than they emit; the emissions from land usechange are more than offset by accumulations else-where.


Table 2Estimates of the amount and fate of human-induced CO2-C emissions in the 1990s, expressed in Pg C per year. Also shown are estimatesof cumulative historical emissions, removals, and atmospheric C increase, up to the year 2000, expressed in Pg C

Annual C flux or increase in the 1990s (Pg C per year) Historical, up to 2000 (Pg C)

EmissionsFossil fuel 6.4± 0.4 6.3± 0.4 280Land-use change 1.7± 0.8 2.2± 0.8 200

RemovalsOceanic sink −1.7 ± 0.5 −2.4 ± 0.7 −124Terrestrial sinka −2.9 ± 1.1 −166

Temperate/boreal −1.3 ± 0.9Tropical −1.9 ± 1.3

Atmospheric C increase 3.2 (±0.1) 3.2 (±0.2) 190

Source The Royal Society (2001)b Houghton (2003) House et al. (2002)

a Houghton (2003)refers to this flux as the “Residual ‘terrestrial’ flux”. Some earlier publications refer to it as the ‘missing sink’.b Based onIPCC (2001a); see also values inSchimel et al. (2001, Table 1), and also a recent update byHouse et al. (2003).

3.3. Coming changes

Can we control the carbon dioxide in the atmo-sphere?Dyson, 1977(Title of paper)

The coming 50 years are likely to be the final periodof rapidly expanding, global human environmentalimpacts.Tilman et al., 2002

Fig. 7. Atmospheric CO2 concentrations. Early measurements, compiled byCallendar (1958), show high variability, but suggest gradualincreases in CO2 concentration. More recent measurements, like those byKeeling and Whorf (2002), exhibit unmistakable, progressiveincreases. (Values fromCallendar (1958)are taken from Table I and Table B—Appendix. Some of the values are averages from severalyears; if so, the concentrations were plotted at the approximate mid-point of the time span. Values fromKeeling and Whorf (2002)areannual means.).

The global C cycle, clearly, is remarkably well-buff-ered—much of the CO2 humans discharge into theatmosphere is absorbed by oceans and terrestrialecosystems. But not enough. Despite these sinks,atmospheric C is increasing, every year, by morethan 3 Pg C. Fifty years ago, scientists first noticedthat concentrations were deviating upward from thepre-industrial baseline of about 280 ppm (Fig. 7). And


Fig. 8. Long-term trends in atmospheric CO2 based on ice-core analysis (Barnola et al., 2003) and recent atmospheric measurements(Keeling and Whorf, 2002) (1 kyr = 1000 years).

now concentrations have surpassed 370 ppm (Keelingand Whorf, 2002), and the rate of increase (slope ofthe line) in recent years is about 1.5 ppmv per year(Fig. 7).

As the result of human activity, the CO2 concen-tration has now reached levels apparently unprece-dented in thousands of years, based on analyses ofice cores (Fig. 8). As observed byFalkowski et al.(2000), “we have left the domain that defined theEarth system for the 420,000 years before the Indus-trial Revolution”. And increases in CO2 concentrationfrom our emissions still to come may far exceed thosealready witnessed (IPCC, 2001a). The consequencesof this abrupt departure, now and into the future, re-main obscure (and, hence, controversial). But the sud-den upheaval in atmospheric CO2 is no longer debated,and has provided impetus to find ways of curbing fur-ther changes.

If these mitigation efforts are effective, they area third way that humans are altering the global Ccycle—indeed, they areintendedto alter the global Ccycle, at least relative to a baseline scenario. In theory,there are at least three ways of slowing the buildup ofCO2 in the atmosphere (Fig. 9a–c):

(a) Reduce the CO2 release from energy use: Effortsto dampen atmospheric CO2 may depend most onfinding ways to quell emissions of CO2 from fossil

fuels (House et al., 2002). These emissions can bereduced by: using less energy; using alternative,non-CO2 emitting energy sources, such as wind,solar, or nuclear energy; or capturing and storingthe CO2 generated from fossil fuel, preventing itsrelease to the atmosphere.

(b) Replace fossil fuels with bio-fuels: Burning ofwood, ethanol, and other fuels from vegetationstill generates CO2. But now that C is from re-cently photosynthesized atmospheric CO2, ratherthan from fossil C. In effect, it re-cycles CO2,rather than introducing new, previously dormantC into active cycling. The benefits, of course, arediminished or negated if excessive fossil C is usedto produce the bio-fuel, or if removal of moreNPP reduces the amount of C stored in terrestrialecosystems (Sauerbeck, 2001).

(c) Increase the amount of C stored in vegetation andsoil (C sequestration): Any practice that increasesnet primary productivity or reduces the rate ofheterotrophic respiration will increase C storage.Planting more trees, for example, or reducing theintensity of tillage on cropland, or restoring grass-lands on degraded lands will all increase C storagein plants, soil, or both.

The last of these options, first proposed in 1977(Dyson, 1977), has attracted particular attention


Fig. 9. The accumulation of atmospheric CO2 can be suppressed in one of three ways, by: (a) reducing emissions from fossil fuels; (b)using recently photosynthesized C as a fuel (bio-fuel); or (c) increasing the amounts of C stored in soils and phytomass.

among ecologists and others probing the global Ccycle. It has several alluring features:

(i) because of past losses of C from terrestrial stocks(about 200 Pg C), terrestrial ecosystems may havesignificant capacity to mop up excess CO2; in the-ory, this capacity seems large compared to annualatmospheric CO2 accrual (3.2 Pg C per year);

(ii) replenishing previously depleted C stocks oftenhas benefits aside from mitigating CO2 increases:restored soil fertility, enhanced biodiversity, andmore (IPCC, 2001b; Smith, in press);

(iii) often it may be quite cost effective, compared toother options (Marland et al., 2001).

But despite its appeals, stockpiling excess CO2 intrees and soils is not without uncertainties and po-tential pitfalls. At the very least, it places more de-mands on our understanding of the global C cycle.Now we will be asked—we are already asked—newquestions: How much more C can we shift into terres-trial ecosystems? And at what rates? What are the mostcost-effective ways to achieve and promote C gains?With the massive amounts of C already stored in theseecosystems (roughly 2500 Pg C), do we know how to

measure the gains of, say, 1 Pg C per year when theydo occur? And can we show that these gains are ourdoing, or merely the ‘natural’ intrinsic buffering of thebiosphere? How are these C gains linked to flows ofother nutrients and water? And, perhaps most urgent:How secure is the newly stored C? What is the risk ofbuilding great stockpiles of biological C which, uponrelease in a changing world, only compound futureproblems?

These questions will likely vex and perplex us forsome time. But beneath them is a deeper questionwhich, if unresolved, threatens to undermine any an-swers to the previous questions. It is this: What willthe C cycle look like 50 years from now? 100 yearsfrom now? The global C cycle has changed profoundlyin the last century. And the change will almost cer-tainly continue; regardless of the success of our miti-gation efforts, the global C cycle will look different acentury from now. Many of the stresses that inducedpast changes still persist, and may intensify. Theseinclude:

(a) Climate change: Global temperatures seem al-ready to have edged upward, by about 0.6◦C in


Fig. 10. An estimate of global temperatures in the last 144 years (Jones et al., 2001).

the past century (Fig. 10, IPCC, 2001a). But pastchanges are small compared to possible changesto come. Though we cannot predict future cli-mates with any confidence, the IPCC projectsthat averagetemperature increases from 1990 to2100 will be 1.4–5.8◦C. How might that alter theflows and storage of C? But climate change, atleast in the near term, may not be the most impor-tant aspect of global change (IGBP, 2001; Walkerand Steffan, 1999); there are more pressing andvigorous changes underway.

(b) Atmospheric CO2 concentration: The concen-tration of CO2 in the atmosphere, now about370 ppmv, has already increased by more than30%, compared to pre-industrial levels. But thisincrease is only the beginning. By 2100 the con-centration may be about 500 ppm (IPCC, 2001a).Or it may approach 1000 ppmv, depending onemission and mitigation scenario. Though scien-tists still debate how climate might change, thereis no longer much doubt about atmospheric CO2:it has already changed profoundly; and muchlarger changes are coming. Carbon dioxide isthe raw material for photosynthesis, so increasedconcentrations affect the growth of plants and thecomposition of their tissues. How will that affectthe global C cycle?

(c) Enhanced nutrient injection: The cycles of N andC are inextricably linked and interwoven; changes

in one inevitably alter the other. Recent decadeshave seen massive increases in supplementalplant-available N, largely to support burgeon-ing crop yields. The amounts of N fixed glob-ally from human activities now roughly equalsthat from natural sources (Schuur and Matson,2000; Smil, 2002; Vitousek et al., 1997) (Fig. 11).And, according toTilman et al. (2002), they mayincrease several-fold in coming decades. Howwill that affect future C cycles?

(d) Changing farming practices: Expanding agricul-ture, as we have seen, has helped re-shape theglobal C cycle. And those changes may not yet

Fig. 11. World production of fertilizer N (drawn from data inFAOSTAT database (2003)).


Fig. 12. Past and projected population increases (sources:Goldewijk, 2001for 1700–1950;United Nations, 2001for 1950–2000;Lutzet al., 2001for 2000–2100).

be finished. Indeed, sayTilman et al. (2001): ‘thenext 50 years may be the final episode of rapidglobal agricultural expansion.’ But it may not behow much land we farm, but theway we farmit that changes most. For example, plant breed-ers now have unprecedented power to re-fashionplants—the way they look, the way they grow,the way they accumulate and apportion theircarbon. Will that capability lead to altered Cflows?

(e) Population growth: Perhaps the biggest pressureon the C cycle is exerted, simply, by the numberof us. The global population, at three billion just40 years ago, now exceeds six billion (Fig. 12).And while the rates of increase may be slowingsomewhat, total population may reach nine bil-lion or more before peaking (Lutz et al., 2001;Cohen, 2003). These increases may drive furtherland use change, as the demand for food growsand the space to build homes and roads and in-dustry expands; they may stimulate further use ofenergy to fuel our factories and automobiles andamusements. If so, how will that alter tomorrow’sC cycle?

The global C cycle has changed in the past cen-tury. And it will certainly change in the next. Howdo ecological scientists respond to that changingC cycle?

4. Challenges and opportunities on achanging earth

Humans are forcing the Earth’s environmental sys-tems to change at a rate that is more advanced thantheir knowledge of the consequences.Schneider,2000

We have made the natural world our laboratory,but the experiment is inadvertent and thus not de-signed to yield easily decipherable results.Kump,2002

Past and pending changes to the earth’s C cycleneed not engender only gloom and despair. Indeed,for scientists, these changes pose perplexing, press-ing questions that give our work new vigor and stim-ulus. How do we best wrestle with these questions?Let me contemplate a few thoughts, partly random,on ways to make our grappling more efficient andinstructive. I offer them, not as exhaustive, sage ad-vice, but as fledgling notions, destined to be correctedor expunged, and maybe even, I dare hope, to incitethought.

4.1. Consider all ecosystem functions

Land is used to raise crops, graze animals, harvesttimber and fuel, collect and store water, create theby-ways of travel and the foundations of commerce,


mine minerals and materials, dispose of our wastes,recreate people’s bodies and souls, house the mon-uments of history and culture, and provide habitatfor humans and the other occupants of earth. Canland. . . also be managed to retain more carbon,and thereby mitigate the increasing concentrationof atmospheric carbon dioxide?IPCC, 2001b

To me the greenhouse is not the main issue. Themain issue is the management of the earth’s ecology,with primary emphasis on trees and topsoil ratherthan on the atmosphere.Dyson, 1992

There is a tendency, in light of mitigation efforts, toview ecosystems as storehouses of C, and to becomepreoccupied with looking for ways to store more C.Clearly, they may have a role as C sinks. But ecosys-tems are much more than tanks for excess CO2. Theyprovide us with a livelihood; they feed us and clotheus, and give us timber and fuel; they filter our air andpurify our water; they protect and sustain diverse an-imal and plant life; and they offer us aesthetic respiteand recreation (Daily, 1997). Yes, our ecosystems canbe a C sink; but they provide many more ‘services’(Westman, 1977), and CO2 mitigation may not be fore-most among them. Indeed, C sequestration may be,at best, a limited, short-term response to rising CO2levels (Scholes and Noble, 2001; House et al., 2002;Smith, in press). Decades hence, we may no longerthink much about how well our ecosystems store ex-cess CO2, but we will still want them to nourish us,to house us, and to uplift us. So the way in which weend up managing our ecosystems is dictated not byhow much C is stored, but by these other, higher andenduring priorities.

Fortunately, how well an ecosystem performs otherfunctions is often correlated to the amount of C itstores; ecosystems that are conserving C may also begrowing more productive, more resilient, and more bi-ologically diverse. For example, an agricultural soilthat is accumulating organic matter (carbon) may begrowing more fertile and less prone to erosion (Lal,2002). And if that is so, then maybe C storage (or atleastchangesin C storage) are best viewed as indica-tors of ecosystem performance. From this perspective,sequestered C becomes, not the goal, but a way ofkeeping score, to help us see how well the ecosystemsare able to perform theirother, more vital, functions.

Fig. 13. The changing rationale of C cycling research in agroe-cosystems.

4.2. Aim for fundamental understanding, pertinentbeyond the ‘problem of the day’

In the early stages of agricultural experimenta-tion . . . , the work was naturally elementary, basedlargely on observations, comparative trials andsimple experiments which did not attempt to deter-mine the underlying conditions or establish definiterelationships. These types of work have given re-sults which although largely empirical have beenextremely useful.. . . [But] they need to be replacedby more rigorous methods and by investigationwhich goes to the heart of the problems.Allen, 1922

Scientific knowledge is part of the endowment weleave to future generations.Carpenter, 2002

Scientists have been studying facets of the C cy-cle for a long time. For example (re-visiting an ear-lier illustration), they have been measuring C in thegrasslands of western Canada since their initial culti-vation, now more than 100 years ago (e.g.,Lawes andGilbert, 1885). Early analyses showed an abrupt andsubstantive loss of soil C soon after cultivation (Alwayand Trumbull, 1910; Shutt, 1910), prompting decadesof research on ways to prevent further losses and re-store C already lost (Janzen, 2001). Viewed in hind-sight, therefore, research on C cycling has spannedmany decades, but its rationale has shifted (Fig. 13).Initially, the question was: How can we prevent im-mediate collapse of productivity? Then: How can weensure these agroecosystems stay productive for gen-


erations to come? And now: How can we increase Cstorage to extract excess atmospheric CO2? And soon,if not already: what will be the influence of globalchange on C cycling? And beyond that, undoubtedly,lie questions our prescience is still too dim to see.

Despite the shifting focus, the insights from onegeneration of scientists can often be applied to thequestions of the next. The scientists of generations pastwho studied C cycling to reverse dwindling produc-tivity could not have guessed that what they learnedwould one day be applied to abate fears of a warm-ing earth. And if we get it right, today’s science willprovide insights relevant to tomorrow’s questions, stillbeyond our view. But only if we get it right, onlyif we furnish fundamental understanding of how Cflows through our ecosystems and of how it restswithin them. Though our data may be aimed at today’squeries, the understanding that emerges, if it is deepenough, can offer clues to tomorrow’s questions stillunseen.

4.3. Consider time—explicitly

History matters in ecology, so context also involvestime.Carpenter, 2002

The carbon cycle has a very long equilibrium time.The consequences of actions taken now will persistfor many centuries.Scholes, 1999(cited byPearce(1999))

The C cycle, it seems, is changing. If so, then it isno longer enough to say how things are; but also howthey were, and how they will yet be. And if the intentis to look at change, then time becomes a variable; wewill measure dC/dt rather than only C, flows ratherthan only pools, changes rather than only amounts.

Measuring change may span time periods of variouslengths, depending on the response time of the sys-tems or components we are studying (Fig. 14). For ex-ample, changes in enzymes or microbial populationsmay occur over days or weeks, changes in soil organicmatter over decades to centuries, and changes in car-bonate storage over millennia. In all cases, however,if change is the urgent question, then time becomesthe prominent variable.

Ecosystem responses can often be seen with cer-tainty only after many years or decades. For example,

Fig. 14. Time scales of ecosystem response (based onShaveret al. (2000)).

much of what we know about past changes in soil Ccame from long-running plots (e.g.,Paul et al., 1997).The question arises then: will we have the sites andthe studies in place to measure coming changes whenthey do happen, perhaps decades from now? And dowe have the patience and fiscal stamina to wait and towatch?

4.4. Follow C beyond usual boundaries

Until we understand. . . the interconnectedness ofthe many Earth systems, conscious human modifi-cations of the planet will be rather analogous toa five-year-old trying to repair a Swiss timepiece.Ernst, 2000

[The student] should be led to grasp the importantfact that. . . living organisms. . . do not recognizethe artificial subdivisions of science.Brittain, 1928

Historically, studies of C cycling have focused ontightly defined plots or fields or ecosystems. In effect,we have imposed a border around the sites and tac-itly ignored the flows of C across that border. Nowthat we seek to understand the global C cycle, thatfocus can no longer be justified. The C cycle on theland we study will affect C flows beyond its fence;and flows beyond the fence will alter the C cyclewithin.

To ponder these far-reaching interactions, considerthe findings ofRidgwell et al. (2002)who lookedat how C storage on land might influence oceanic


Fig. 15. Simplified view of how, in theory, soil carbon conservationmight conceivably reduce oceanic uptake of atmospheric CO2

(Ridgwell et al., 2002). Widespread efforts to conserve soil carbonmight reduce atmospheric dust, decreasing the deposition of ironto oceans, thereby reducing phytoplankton growth and CO2 uptakein the oceans because of iron deficiency.

CO2 uptake (Fig. 15). Widespread C sequestration interrestrial ecosystems, their models say, would stabi-lize soils, reducing dust in the atmosphere and, thus,the amount of iron deposited on oceans. Since someocean waters are iron deficient, this would diminishphytoplankton growth, and reduce the amount of CO2absorbed by oceans. Thus, increased C storage in ter-restrial ecosystems could, in theory, be partly offset belower CO2 uptake in oceans. This scenario may neverhappen, but it does illustrate that we can no longerdraw borders around ecosystems, however large, andignore consequences for C cycling beyond thoseborders.

But adopting a wider perspective presents a prob-lem. Earlier, we said we need also a more fundamen-tal understanding of the mechanisms and processesthat constitute the C cycle. Often, that means detailedstudies on small scales: in small plots or even testtubes. Tying these results directly back to the globalpicture may not be easy—or possible. But at the veryleast, maybe I should ask myself, at the outset of anexperiment: where in the global picture might my re-sults fit?Platt (1964), building on a fable byForscher(1963), comments wryly that “We speak piously oftaking measurements and making small studies thatwill ‘add another brick to the temple of science.’ Mostsuch bricks just lie around the brickyard.”

Looking beyond usual borders has advantages, no-tably that it disperses fresh insights farther afield.

Fig. 16. Relationship between uncertainty and depth of under-standing. As understanding improves with continuing research, thelevel of uncertainty may increase initially as new variables areuncovered, before eventually diminishing.

For example, the idea of controlling atmosphericCO2 by stashing it in forests and soils seems tohave first sprung, not from the musings of a for-est ecologist or a soil scientist, but from thoseof a quantum physicist—Freeman Dyson (Dyson,1977).

4.5. Admit (and quantify) uncertainty

Honest scientists will frequently have to answer:‘We don’t know’, ‘We cannot know’ or ‘These areour guesses’.Haerlin and Parr, 1999

Science is as much about clear articulation of whatwe do not know, and what we can do about it, as itis about the known.. . . In ecosystem management,what we do not know also affects choices.Carpenter,2002

The study of C cycling remains an inexact science,especially as the scale expands to include the wholeearth. And it is perhaps naı̈ve to assume that the un-certainty will soon be diminished by the tide of re-search studies now underway (Fig. 16). Indeed, asPielke (2001)avers, “advances in knowledge can addsignificant uncertainty.. . . Ignorance is bliss becauseit is accompanied by a lack of uncertainty.”

And yet, despite the uncertainty, the public (andthose who fund our work) expect answers. In fact, it isbecauseof the uncertainty that they expect answers. Inthe words ofKlemeš (2000): “Only in the face of un-certainty are decisions relevant. A patient hardly needsa diagnosis based on complete information obtainedfrom his autopsy!”


The solution, then, is not to forestall giving an-swers, but to find ways of admitting and quantifyingour uncertainty in ways that “are easily understoodby policymakers and the public.” (Pouyat, 1999).We have become quite good at assigning probabili-ties to the significance of differences—we know, forexample, how to decide whether one compound isbetter than another at killing weeds. But we are notas good at assigning uncertainty values to estimatesof global C pools, especially those projected into thefuture.

In some ways, acknowledging uncertainty is itselfalready a step forward. SaysHansen (2002): “Doubtand uncertainty are the essential ingredient in science.They drive investigation and hypotheses, leading topredictions.” And, in the words of Boorstin (cited byKlemeš (2000)), “The greatest obstacle to discoveryis not ignorance, it is the illusion of knowledge.”

4.6. Relish and reveal the C cycle’s beauty

The aesthetics of nature extend well beyond ourprimitive ability to write equations.Stewart, 1999

How many farmers know anything about the re-markable structure of the soil they till, of its fasci-nating history, of the teeming population of livingorganisms that dwell in its dark recesses;. . . ofthe wonderful wheel of life in which the plant takesup simple substances and. . . fashions them intofoods. . . and packs them with energy drawn outof the sunlight—energy which enables us to moveand work, to drive engines, motor-cars, and allthe other complex agencies of modern civilization?No one knows much about these things; but if weknew more, and could tell it as it deserves to betold, we should have a story that would make thewildest romance of human imagination seem dullby comparison. . . . Russell, 1924

Only rarely, does the word ‘beauty’ inhabit a scien-tific paper. And I hesitate to use it here, but a contem-plative look at the C cycle seems unfinished withoutacknowledging humbly its elaborate intricacy, the wayit weaves through time and space, connecting one or-ganism to another, one ecosystem to the next.Russell(1924)may be right in hinting that we who study thecycle could become more adept at showing it to the

uninitiated. He closes his paper with the admonition:

Agricultural science must be judged not only by itsmaterial achievements, but also by its success in re-vealing. . . something of the wonder and the mys-tery of the great open spaces in which [the farmer]dwells.

We might expand that thought to include, not justagricultural science, but all ecological science; andnot just farmers, but all who ‘dwell’ in this place of‘wonder’ and ‘mystery’.

5. Closing thoughts

The global C cycle is a massive, magnificent tangleof interwoven flows, connecting thousands of billionsof tonnes of restless C in land, trees, water, and air.We are a part of that cycle: our bodies are themselvestemporary repositories for a little of the C, and it isby burning organic C back to CO2 that we derive theenergy to move and to think. But we are also, increas-ingly, a part of the cycle because we have acquiredthe power, by our sheer numbers and the force of ourinventiveness, to re-shape it and re-direct it. And withthat power, comes the responsibility to steward thatcycle, or, at the very least, to knowhow we do andcan affect it.

There is no better time to be studying C cycling.The global C cycle is changing, so we have a long listof tantalizing, relevant questions to puzzle, amuse, andbemuse us. And, like never before, we have a readyaudience, waiting for our findings and insights. Thatmay make us a little nervous, but at the same timeit adds urgency and spice to the science. It may be,a century from now, when our work is done and thecurrent turbulence of the C cycle has been calmed, thatour successors will look back and say: it must havebeen an exciting time for scientists, back then, in thegolden age of C cycle science.

Acknowledgements

I thank Leslie Cramer for help in preparing themanuscript and Yvonne Bruinsma for procuring manyof the papers I have cited. I am also indebted to BenEllert for continuing carbon conversations, and to Pete


Smith and an anonymous reviewer for constructivecomments on this manuscript. An early draft of thispaper was presented in May 2002 at the joint meet-ing of the Canadian Geophysical Union and the Cana-dian Society of Soil Science in Banff, Canada. Thework reported here was funded by Agriculture andAgri-Food Canada (manuscript contribution number38703066).

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