Verification of soil carbon sequestration : uncertainties of assessment methods

Abstract (Swedish)

Populärvetenskaplig sammanfattning: Kunskap om mängden kol som växter tar upp i från atmosfären är i dag en viktig
frågeställning inom naturvetenskaplig forskning. Resultaten av sådan forskning kan bl.a.
användas vid internationella konventioner som t.ex. Kyoto avtalet. I denna studie har
jord prover från semi-arida agro-ekosystem i Sudan statistiskt analyserats för att se om
skillnaden i organiskt kol över tid och rum kan detekteras (“minimum detectable
difference” (MDD)). Proverna kom från tre olika fält. ”MDD” är den minsta
detekterade skillnaden mellan medelvärden när variansen, signifikansnivån, statistiska
styrkan och antalet prover är kända. Inom de tre fälten varierade variansen mellan 51 000
och 212 000. Den höga variabiliteten gör en verifiering av mängden kol som jorden kan
ta upp svår. Över 1000 jord prov skulle behövas för att observera en 5 % förändring i
organiskt kol (motsvarande 25 g m-2). En fördubbling av variansen innebär också ett
fördubblat behov av antalet jordprover för att få samma “MDD”. En s. k. ”jackknife
routine” användes för att simulera delar av det ursprungliga datasetet (de observerade
jordproverna). Delarna representerade antalet jordprover och varierade mellan 2 och 99
samt simulerades 500 gånger var. Från resultaten beräknades medelvärden, variansen
och deras 95 % konfidensintervall. Användning av det översta konfidensintervallet av
variansen i ”MDD” beräkningar kan leda till fördubblat behov av antalet jordprover. En
sensitivitets analys samt framtida scenarier utfördes med ekosystem modellen
CENTURY. Målet av denna analys/scenarier var att observera effekten på organiskt kol
vid varierande klimat (nederbörd, temperatur), markanvändning, jordens
kornstorleksfördelning, andel efterlämnad biomassa efter skörd och kväve (N) fixering.
Modellen visade sig vara mest känslig för mängden N fixering av träd. Effekten av
klimatförändringar var mest framträdande då mängden N fixering var hög. Möjliga
framtida klimatförändringar visade ingen ökning i organiskt kol. Olika typer av odlings
metoder och grödor leder till signifikanta skillnader i mängden organiskt kol i marken.
Hundra år kontinuerlig odling av hirs (Pennisetum typhoideum) leder till en förlust av 0,3
g C m-2 år-1
medan odling:träda ratio 5:10 och endast betning leder till en ökning av 0.4 g
C m-2 år-1 respektive 1.5 g C m-2 år-1
. De presenterade resultaten är simulerade helt utan
N fixering. Vid närvaro av N fixerande vegetation ökar samtliga presenterade resultat.

Abstract

Knowing the amounts of atmospheric carbon soils can potentially sequester is gaining
great interest among the scientific community. International conventions, such as the
Kyoto Protocol, help promote such research. In this study, soils samples from a semiarid
agro-ecosystem of the Sudan were statistically analyzed to evaluate if changes in soil
organic carbon (SOC) over time or space were cost-effectively detectable or not, given a
sample size (with minimum detectable difference (MDD) method). One hundred (100)
samples were taken from each of three fields. The MDD is the smallest detectable
difference between treatment means once the variation, significance level, statistical
power and sample size are specified. The variances of the SOC contents varied between
51 000 and 212 000. This makes short-term verification of carbon sequestration difficult
as over 1000 soil samples would be required to observe a 5% change (25 g m-2) in SOC
over time or space. A doubling of the variance also doubles the required amount of soil
samples to achieve the same MDD. A “jackknife routine” was setup to simulate
alternative data sets from the available data (observed soil samples). These datasets
represented sample sizes ranging from 2 to 99 and were extracted 500 times for each
sample size. For the results, the mean, the variance and their 95% confidence interval
were calculated. Using the upper 95% confidence interval for the variance estimates in
the MDD calculations can lead to as much as a twofold increase in the sample size
requirement compared with the average variance of all samples. A sensitivity analysis
and future scenarios were done with the CENTURY ecosystem model to evaluate the
effects of varying climates (precipitation and temperature), land management and harvest
practices, soil texture and nitrogen (N) fixation by trees on SOC. The model showed to
be most sensitive to N fixation by trees, and the effects of variations in climate were
amplified when N fixation was allowed in the system. Probable future climate scenarios
did not indicate a significant increase in SOC, whereas different land management
practices did. Long fallow periods (crop:fallow ratio 5:10) and grazing showed the
potential of this semi-arid area to sequester up to 0.4 g C m-2 yr-1 and 1.5 g C m-2 yr-1
respectively, while soils under continuous cultivation of millet (Pennisetum typhoideum)
acted as a C source, losing 0.3 g C m-2 yr-1 over the next century. These numbers result
in simulations without N fixation allowed from trees. The presence of N fixing legumes
will increase the sequestration rates.

Abstract

Summary: It is well documented that atmospheric carbon dioxide (CO2) is increasing globally.
Many scientific studies also show that increased CO2 in the atmosphere may raise the
mean global temperature and disturb climates in unforeseen ways. The Kyoto Protocol is
a framework aiming for an international collaboration for the reduction of atmospheric
greenhouse gases (GHG), of which CO2 is one of the most important. Within the Kyoto
Protocol, there are some “mechanisms” designed to help countries reach their reduction
targets as cost-effectively and efficiently as possible. One of those mechanisms is called
the Clean Development Mechanism (CDM), which allows developed countries to invest
in sustainable development projects in developing countries. Some studies show that
improved land management practices can result in an increase of the rate at which carbon
is sequestered from the atmosphere into soils, and this is one of the basis on which the
CDM concept is designed. If significant investments are made to promote projects that
could sequester carbon, it is important to be able to quantify as precisely as possible the
changes in soil organic carbon (SOC) over time or space in order to relate the changes to
the Kyoto Protocol and to justify adequate monetary sums for projects. A reason why
soils should be considered as carbon “sinks” is that carbon in soils can have very long
residence times (up to thousands of years) if it can reach a stable state in which is
becomes protected from physical and chemical destructive processes.
SOC can be estimated though direct soil sampling. The number of soil samples over an
area will determine the accuracy of the estimates. The detection of changes in SOC over
time or space will also be relevant to the number of soil samples taken each time or place.
Three hundred soil samples (100 samples for each of 3 fields) from a semi-arid agroecosystem
of the Sudan, Africa were statistically analyzed to see if the detection of such
changes would be possible for short-term verification. The variance is a statistical
parameter explaining the variability of the samples in regards to the mean value. A
number of statistical tests are designed to evaluate if two or more groups of samples are
significantly different or not. If a statistical test results in fields not being significantly
different, then changes in SOC cannot be detected. A method known as the “minimum
detectable difference” can provide the number of soil samples required to statistically
differ sampling groups, given the variability (variance) of each group. This is helpful for
the design of efficient sampling schemes for the verification and monitoring of SOC.
Given the variability of the samples analyzed, more than 1000 samples would be required
over at least two time periods or places in order to detect a 5% increase of the SOC.
Nevertheless, as the variability decreases by half, so does the number of soil samples
required to detect the same change in SOC. Estimating the variance for soil properties
can be difficult because many factors can influence important changes over very small
ground areas. For this reason, a “jackknife routine” was used. Such a routine is used to
simulate alternative data sets from the available data (observed soil samples). These
datasets represented sample sizes ranging from 2 to 99 and were extracted 500 times for each sample size. For the results, the mean, the variance and their 95% confidence
interval were calculated. It is therefore possible to evaluate how representative are the
overall mean and variance for each sampling group.
Computer models can be used to simulate SOC from different scenarios. One of those
models, called CENTURY, was tested to evaluate its sensitivity to variations in some
input parameters concerning climate, land management and harvest practices, soil texture
and plant physiology regarding the modeling of SOC. A number of future scenarios were
also designed to have an idea of the faith of SOC in the study area for different land
management practices and climate change scenarios. Future climate scenarios did not
indicate important changes in SOC, but improved land management, such as the stop of
continuous cultivation and the return of long fallow periods (crop:fallow ratio 5:10 years)
did show important increases in SOC. More SOC has the benefit of increasing soil
fertility, crop productivity (food security), as well as helping combat other serious
problems such as desertification and the decline in biodiversity. In a world where
adequate food supplies and climate can cause rigorous problems, such considerations
should be reflected upon. It is nevertheless important to remember that scientific proof
can only be valid if socio-economic and political issued are also considered.

• author: Poussart, Jean-Nicolas
• supervisor: Uppgift saknas
• organization: Dept of Physical Geography and Ecosystem Science
• year: 2002
• type: H1 - Master's Degree (One Year)
• subject: Earth and Environmental Sciences
• keywords: geomorfologi, naturgeografi, climatology, cartography, Jackknife routine, kartografi, klimatologi, modeling, physical geography, Africa, pedology, geomorphology, marklära, sequestration, soil organic carbon (SOC), Sudan, semi-arid, minimal detectable difference (MDD), sensitivity, CENTURY
• publication/series: Lunds universitets Naturgeografiska institution - Seminarieuppsatser
• report number: 90
• language: English
• id: 1333028
• date added to LUP: 2005-10-24 00:00:00
• date last changed: 2011-12-06 08:36:31