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iving organisms use oxy-gen and produce carbondioxide in cellular respira-

tion. Additionally, plants useCO2, water and sunlight to pro-duce carbohydrates and oxygenin photosynthesis. In otherwords, unlike animals, plantsconsume CO2 which plays a rolein keeping the global level ofCO2 in balance. Changes in thisbalance can be understood bystudying the carbon cycles of theecosystems of the world.

Since most of the carbon isstored below ground, soil has a

central role in the carbon cycle.It is therefore important tomeasure how the carbon ex-change of the soil varies in theshort term, and to estimate itsimpact in the longer term. Theconcept of soil respirationrefers to the efflux of carbondioxide at the soil surface, quan-tified as the amount of carbondioxide given off by living or-ganisms and roots in the soil.

All-in-all, the soil respirationrate is the result of a combina-tion of biotic, chemical andphysical processes that takeplace in the soil.

The biotic processes are re-lated to the respiration of plantroots, micro-organisms and larg-er fauna living in the soil. Plant

roots contribute directly to soilrespiration by releasing lots ofeasily decomposable carbohy-drates. The soils microbiologic-al properties, on the other hand,determine its potential to de-compose organic matter andcontribute to soil respiration.

The chemical processes thattake place in the soil include thechemical oxidation of minerals.Although this is normally a rela-tively small source of CO2 com-pared to others, it increases athigher temperatures. The physi-cal processes consist of soil CO2

degassing and the transport ofCO2 through the soil to the sur-face.

Continuous research is be-ing conducted to find out howthe soil respiration rate respondsto changing temperatures andchanging states of wetness ordryness. We know that the rateof CO2 efflux varies seasonallyand even during each day, due tochanging environmental factors.In addition, the soil respirationrate is affected by the textureand quality of the soil, the vege-tation and biome type, the to-pography and the geographicalcoordinates. Studies on how dif-ferent factors affect the rate atwhich different soil types, underdifferent conditions, store and

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Diffusion-Based Soil RespirationSoils form the largest reservoir of carbon in our terrestrial ecosystems,containing more than two-thirds of the total carbon on our planet.Soil respiration, or surface CO2 efflux, is one of the two main mecha-nisms by which carbon is transferred from the soil to the atmosphere.The other is the human-contributed burning of fossil fuels. A smallchange in the soil respiration rate can thus have a major impact onthe whole atmospheric CO2 budget and on the global ecosystem. Notsurprisingly, ecologists, meteorologists, physiologists and scientists inresearch institutes throughout the world are keenly interested in

measuring soil respiration. For these researshers, diffusion-based CO2measurement can provide advantages over traditional pump-aspira-tion systems.

Maria Uusimaa, M. Sc. (Eng.)Application ManagerVaisala HelsinkiFinland

In soil respiration measurements on the ground surface, the CO2 efflux ismeasured inside a box that is placed on the ground at the site underexamination.

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release carbon are helping re-searchers to understand how car-bon storage might change in thefuture.

Soil temperature andmoisture are significant

One of the most dominant fac-tors affecting the soil respirationrate is soil temperature. Typical-ly, when soil temperature startsincreasing, the soil respirationrate also increases. However, itmust be pointed out that totallyopposite results have also beenrecorded at certain conditions.

One objective of scientists is tobuild a comprehensive modelthat predicts how increases in at-mospheric temperatures willchange the soil respiration rateand affect the carbon balance.

Soil moisture is another im-portant factor influencing soilrespiration. In dry conditions,root and micro-organism activityis typically low, resulting in lowsoil CO2 efflux. Increasing thesoil moisture normally increasesthe bio-activity in the soil. But ifthere is very high soil moisture,total soil CO2 efflux is reduced,because of limited diffusion ofoxygen and subsequent suppres-sion of CO2 emissions.

The soil respiration rate ismostly controlled by the result

of the interaction between soiltemperature and moisture. It isoften difficult to separate be-tween these two influences. Theinfluence of soil temperatureand moisture on soil respirationis so strong that they must beconsidered together in order topredict the level of respiration.One of the reasons is that theyaffect soil microbial activity.

Usually soil respiration re-sponds most to whichever of thetwo factors temperature ormoisture is the more limiting.

If the soil is very dry, the soil res-piration is not sensitive to tem-perature. When the moisturecontent increases, the level ofrespiration becomes much moresensitive to temperature. Simi-larly, at temperatures below 5Csoil respiration is not sensitive tomoisture, but becomes increas-ingly responsive at higher tem-peratures.

Soil texture or porosityalso play a role

Soil texture and type also have astrong effect on soil respiration

because the diffusion of CO2 isinfluenced by the soil porosity.Small pores, for instance, holdwater well. Larger interconnect-ing pores allow water and air tomove freely into and out of thesoil. Many current studies are ex-amining the effect of soil textureon the respiration rate not onlyat the surface of the soil, but alsobelow it.

Basically, soil respirationmeasurements can be divided in-to two categories: below-groundCO2 and soil respiration boxmeasurements.

Below-ground CO2measurements

The aim of below-ground CO2measurements is normally to

measure the CO2 profile atchanging depths in the soil.Most measurements, particular-ly those with agricultural appli-cations, are nowadays made fair-ly close to the surface layer ofthe soil.

The CO2 concentration be-low ground increases with thedepth and varies to a great extentaccording to the soil type andenvironmental conditions. Inorder to discover the CO2 pro-file of the soil, several sensorscan be buried below ground at

different depths1. It is also im-portant to record the exactdepth to maintain the compara-bility of the measurements toother below-ground measure-ments. The challenge in below-ground CO2 measurement is toprevent the sensors from chang-ing the conditions inside thesoil. Therefore, small, diffusion-based CO2 sensors with lowpower consumption are prefer-able.

The CO2 flux in the soilchanges rapidly when it rains.After a shower, the humidity in

the soil is very high for quite along time. Sensor durability andstability are thus key issues inbelow-ground CO2 measure-ments.

Soil respiration boxes

In soil respiration measurementson the ground surface, the CO2efflux is measured inside a boxthat is placed on the ground atthe site under examination. Al-though many different setupsare possible, the measurement istypically made in a closed boxthat is pre-ventilated. During themeasurement period, the CO2concentration increases or de-creases inside the box dependingon the type of box and the localenvironmental conditions.

Most scientists build theirown respiration boxes usingsheet metal, glass or transparentplastic. The size of the box is cri-tical for many reasons. The spa-tial resolution of the respiration

measurement depends on theshape and size of the bottomarea of the box. The speed of theCO2 build-up inside the box canbe adjusted by changing the boxvolume allowing for the factthat the box must be highenough for the ground vegeta-tion growing inside it. In somecases, a small fan unit is neededin order to mix the air inside therespiration box.

For a correct measurementresult, it is essential to ensurethat the box is tight against theground. Different box sizes areneeded because of different veg-

etation types and because theCO2 efflux varies very much be-tween soil types. In some casesthe concentration increases fromambient by only 10-20 ppm,while sometimes readings ashigh as 300 ppm above ambientare recorded. The boxes varyfrom simple portable boxes tomore complex ones that auto-matically close the cover period-ically in order to make a meas-urement.

At present most soil respira-tion box measurements are con-ducted with pump-aspirated in-

struments. The pump aspirationmethod is problematic, since itinduces pressure differences thatcan affect the CO2 efflux insidethe box. Most pump-aspiratedCO2 analyzers are also bulky, ex-pensive and have a high powerconsumption.

A soil respiration box withdiffusion-aspirated CO2 meas-urement tackles the problem ofpressure differences. Diffusion-aspirated CO2 measurement isalso less expensive and moremobile because it does not re-quire a complex, bulky and pow-er hungry sampling system. Allthis enables scientists to performmore frequent measurements inthe field.

Vaisala offers CO2 measure-ment instruments for a large vari-ety of applications. The basic aspi-ration method for these instru-ments is based on diffusion. Forfurther details, please consult yourlocal Vaisala representative.G

Reference

Tang, J., D. D. Baldocchi, Y. Qi, and L.

Xu, 2003. Assessing soil CO2 efflux

using continuous measurements ofCO2 within the soil profile with small

solid-state sensors. Agricultural and

Forest Meteorology, 118, 207-220.

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Measurements Vaisala carbon dioxidemeasurement instruments,

such as the GMT220, have

been used in soil respiration

studies in which the sensors

have been buried at differ-

ent depths in the soil. G