Research Challenges

The discipline of biogeochemistry confronts a wide array of scientific and methodological challenges, as is evident in the balance of this book. These are not limited to the cycles of carbon and nitrogen, but include the role of phosphorus, iron, calcium, aluminum, and acidity, to name just a few. In this section, I will identify four cross-cutting challenges that illustrate aspects of the science.

1.1 Large-Scale Carbon Sinks: Detection and Attribution

The problem of the terrestrial missing sink remains. Where and why is there net uptake in terrestrial systems? The two questions,

TABLE 1 Chemical Composition of the Atmosphere

Constituent

Chemical formula

Volume mixing ratio in dry air

Major sources and remarks

Nitrogen

N:

78.084%

Biological

Oxygen

O,

20.948%

Biological

Argon

Ai-

0.934%

Inert

Carbon dioxide

CO,

360 ppmv

Combustion, ocean, biosphere

Neon

Ne

18.18 ppmv

Inert

Helium

He

5.24 ppmv

Inert

Methane

CHj

1.7 ppmv

Biogenic and anthropogenic

Hydrogen

H,

0.55 ppmv

Biogenic, anthropogenic, and photochemical

Nitrous oxide

N,0

0.31 ppmv

Biogenic and anthropogenic

Carbon monoxide

CO

50-200 ppbv

Photochemical and anthropogenic

Ozone (troposphere)

o,

10-500 ppbv

Photochemical

Ozone (stratosphere)

o3

0.5-10 ppm

Photochemical

Noninethane hydrocarbons

5-20 ppbv

Biogenic and anthropogenic

Halocarbons (as chlorine)

3.8 ppbv

85% anthropogenic

Nitrogen species

NO,

10 ppt- i ppm

Soils, lightning, anthropogenic

Ammonia

NH,

10 ppl-1 ppb

Biogenic

Particulate nitrate

NO;r

1 ppt - i 0 ppb

Photochemical, anthropogenic

Particulate ammonium

NHj+

10 ppt-10 ppb

Photochemical, anthropogenic

Hydroxyl

OH

0.1-10 ppt

Photochemical

Peroxvl

HO,

0.1-10 ppt

Photochemical

Hydrogen peroxide

H,0,

0.1-10 ppb

Photochemical

Formaldehyde

CH,O

0.1-1 ppb

Photochemical

Sulfur dioxide

SO;

10 ppl -1 ppb

Photochemical, volcanic, anthropogenic

Dimethyl sulfide

CII3SCII3

10-100 ppt

Biogenic

Carbon disulfide

CS,

1 -300 ppt

Biogenic, anthropogenic

Carbonyl sulfide

oes

500 pptv

Biogenic, volcanic, anthropogenic

Hydrogen sulfide

H,S

5-500 ppt

Biogenic, volcanic

Particulate sulfate

so.,--

10 ppt-10 ppb

Photochemical, anthropogenic

where and why, cannot be separated. Different parts of the world and differing ecosystem types are influenced by differing nitrogen additions, disturbance, and pollution. Answering the question "why is there a sink" requires explaining the differences between climate zones, management, and disturbance regimes and chemical climate. This is a practical problem because, in the future, there will be increasing pressure to manage carbon sinks. How can sinks best be induced and sustained? How can the effects of intentional measures be quantified and verified? What impacts does managing ecosystems for carbon storage have on other ecosystem goods and services, including diversity? Without scientific understanding, no intelligent design of management systems can emerge. Equally important is the fact that without scientific consensus there can be no political will to implement expensive management systems. Carbon science must integrate a basic understanding of process with powerful measurement techniques. Models are also required that have the credibility to be used in what-if exercises to aid in designing new management systems. Local models are crucial because sinks must be long lasting and management systems to store carbon must aim at a decadal lo centennial timescale. The agronomic paradigm of "test plots" is needed but limited in utility because of timescale. Large-scale models are needed to test the global effect of an international regime, including the stability of induced ecosystem sinks to potential changes in the chemical, physical, and human environment.

1.2 New Methods for Measurement

Measurement capability has been a continual challenge to the carbon research community in accomplishing the ambitious goals. The foundation of carbon cycle research lies in stable absolute calibration, an initial priority of the Keeling Mauna Loa effort and a persistent feature of the community. New measurements, such as of stable and radio-isotopes, the 02/N2 ratio, and remote sensing have been developed and adopted by the community. Techniques for dealing with spatial heterogeneity are also in rapid evolution: these include ecosystems studies, local eddy covariance flux measures, mesoscale aircraft and tall-tower techniques, and continental to global inverse modeling techniques (Valentini et al., 2000). As the scope of large-scale biogeochemical research expands beyond a carbon cycle and greenhouse gas focus, techniques will need to be developed for the spatial-temporal integration of a range of processes. It is likely that new techniques will be needed for the study of airborne and waterborne nutrient transport as in gas, suspended and dissolved water-borne and aerosol phases. Techniques for spatial integration of belowground processes are crucial—there still exist only rudimentary measures for root growth and soil C and N turnover at or above the plot scale (Valentini et al, 2000). Continuing adoption and endogenous development of measurement and data analytical techniques is a priority for biogeochemistry.

Year

FIGURE 1 Global average atmospheric carbon dioxide mixing ratios and long-term trend determined using measurements from the NOAA CMDL cooperative ail" sampling network. Also shown is the global average growth rate for carbon dioxide; the variability in this is diagnostic of changes in biospheric and oceanic exchange. Data from National Oceanic and Atmospheric Administration's (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL), Carbon Cycle-Greenhouse Gas Group.

Year

FIGURE 1 Global average atmospheric carbon dioxide mixing ratios and long-term trend determined using measurements from the NOAA CMDL cooperative ail" sampling network. Also shown is the global average growth rate for carbon dioxide; the variability in this is diagnostic of changes in biospheric and oceanic exchange. Data from National Oceanic and Atmospheric Administration's (NOAA) Climate Monitoring and Diagnostics Laboratory (CMDL), Carbon Cycle-Greenhouse Gas Group.

1.3 Biological Diversity and Evolution

The roots of biogeochemistry are in geochemistry and ecosystem ecology. Most work in biogeochemistry has followed chemical fluxes and treated ecosystems as series of linked compartments rather than as associations of species. In a sense, this always represented an operational convenience more than a hypothesis that species characteristics were irrelevant. The global loss of species diversity raises the concerns that critical thresholds of diversity may exist below which the functioning of ecosystems or their reliable delivery of ecosystem goods and services will be impaired. All but the most aggregated ecosystem models recognize the role of different functional types of plants, and some recognize at least implicitly the distinction between major microbial functional types (bacteria and fungi). Several questions remain open and contentious. The hypothesis that organisms in particular ecosystems are optimally adapted to local conditions (or statistics of those conditions) is often used as an operating rule in biogeochemistry. This is a defensible assumption in steady-state conditions: can it be assumed during changing times? Second, what role does the diversity of organisms in a given ecosystem play in system function? Can all variation be explained based on losses or gains of particular functional types, or does diversity itself play a role? These topics remain controversial to say the least (Hector et ah, 1999; Huston, 1997), and it is not yet clear whether the community has even formulated the right questions to ask about diversity and ecosystem function. It is clear that this area, the role of genetic, phenotypic, and taxonomic diversity in biogeochemistry, requires investigation. It poses an immense scale and measurement challenge because, while diversity varies on the smallest scales, ecosystem function (productivity, hydrology, nutrient cycling) intrinsically occurs in the aggregate. Measuring the rela-

0 500 1000 1500 2000 2500 2750 3000 3200 3300

FIGURE 2

core (Petit

50000 100000 150000 200000 250000 300000 350000 400000 Age [yr BP]

Climate and atmospheric composition over the past 420,000 years from the Vostok et al., 1999).

FIGURE 2

core (Petit

50000 100000 150000 200000 250000 300000 350000 400000 Age [yr BP]

Climate and atmospheric composition over the past 420,000 years from the Vostok et al., 1999).

0 500 1000 1500 2000 2500 2750 3000 3200 3300

C> BRW (Obs.)

10 -

Q.

-

^^\ ^

Q.

o -

O o

-

>

-10 -

. i , i . i

i . i , i

O STM (Obs.)

i ! i 1 i 1 i

O CBA (Obs.)

.i.i.

FAJAODJMM

FIGURE 3 Comparison between the observed seasonal cycle of CO, and the simulated seasonal cycle produced by coupling the monthly estimates of net ecosystem production estimated by the Century model and fossil fuel emissions with the Hamburg ocean and atmospheric transport models for each of the seven high-latitude monitoring stations. The first six months of each cycle are displayed twice to reveal the annual variation more clearly. Mean and standard deviation are shown for the observed data (McGuire et id., 2000).

tionships for testing diversity-function theory based on models and model ecosystems will require another quantum advance in the field's ability to make integrative measurements.

1.4 Belowground Processes

Our understanding of processes occurring above-ground is far more complete than our knowledge of soil processes. Soils contain 2-3 times more carbon than vegetation does. They are the primary reservoir of long-lived organic matter in the terrestrial biosphere and provide critical resources needed for photosynthesis. They remain persistently difficult to study, being impenetrable to remote sensing, locally variable such that meter-to-meter changes in organic matter or microbial acivity can approximate the mean changes across continents, and of secondary interest to many academic researchers. New techniques involving isotopes, especially 1 'C, 1 'C, and LlN are beginning to open the black box of soil turnover times, as are new techniques for chemical analysis of soil organic matter (Schulze et al, 2000). No theory of ecosystem behavior will be complete without a far better understanding of soil biology than we currently have. While current models of soil processes have significant predictive skill, many crucial processes are represented empirically with no real understanding of the underlying biology and chemistry. This is an important step in integrating the largest reserve of biodiversity—soils—and the longestTived-ecosystem organic-matter reserves (also in soils) into ecosystem theory.

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