Executive summary
The European Commission has recently adopted the Thematic Strategy for soil protection
(COM(2006)231 final), with the objective to ensure that Europe’s soils remain healthy
and capable of supporting human activities and ecosystems. Climate change is identified
as a common element in many soil threats. Therefore the Commission intends to assess
the actual contribution of the protection of soil to climate change mitigation and the
effects of climate change on soil productivity and the possible depletion of soil organic
matter as result of climate change. The objective of this study is to provide a state of the
art and more robust understanding of interactions between soil under different land uses
and climate change than is available now, through a comprehensive literature review and
expert judgment.
1 Carbon stock in EU soils
The amount of carbon in European soils is estimated to be equal to 73 to 79 billion
tonnes. These estimates are based on applying a common methodology across Europe,
the larger estimate was based on a method developed by the Joint Research Centre of the
European Commission and the smaller estimate on a soil organic carbon (SOC) map of
the United States Department of Agriculture. These two methodologies gave similar
estimates for most of the European countries. The estimates were of the same order of
magnitude as national estimates based on national methodologies and are therefore
deemed reliable.
Carbon in EU27 soils is concentrated in specific regions: roughly 50% of the total carbon
stock is located in Sweden, Finland and the United Kingdom (because of the vast area of
peatlands in these countries) and approximately 20% of the carbon stock is in peatlands
mainly in the northern parts of Europe. The rest of soil C is in mineral soils, again the
higher amount being in northern Europe.
2 Soils sink or source for CO2 in the EU
Uptake of carbon dioxide (CO2) through photosynthesis and plant growth and loss
(decomposition) of organic matter from terrestrial ecosystems are both significant fluxes
in Europe. Yet, the net terrestrial carbon fluxes (uptake of CO2 minus respiration by
vegetation and soils) are typically smaller relative to the emissions from use of fossil fuel.
The current changes in the carbon pool of the European soils were estimated from
different studies using different methods, by land use category using models that simulate
carbon cycling in soil. The results of the different studies deviated considerably from
each other, and all results were accompanied with wide uncertainty ranges. Some studies
on the basis of measurements in UK, Belgium and France on soil carbon over longer
periods show losses of carbon especially from cropland; other studies from the UK and
from the Netherlands show no change or increases in soil carbon stocks over time.
Grassland soils were found in all studies to generally accumulate carbon. However, the
studies differ on the amount of carbon accumulated. In one study, the sink estimate
ranged from 1 to 45 million tonnes of carbon per year and, in another study, the mean
estimate was 101 million tonnes per year, although with a high uncertainty.
Cropland generally acts as a carbon source, although existing estimates vary highly. In
one study, the carbon balance estimates of croplands ranged from a carbon sink equal to
10 million tonnes of carbon per year to a carbon source equal to 39 million tonnes per
year. In another study, croplands in Europe were estimated to be losing carbon up to 300
million tonnes per year. The latter is now perceived as a gross overestimation.
Forest soils generally accumulate carbon in each European country. Estimates range
from 17 to 39 million tonnes of carbon per year with an average of 26 million tonnes per
year in 1990 and to an average of 38 million tons of carbon per year in 2005.
It would seem that on a net basis, soils in Europe are on average most likely accumulating
carbon. However, given the very high uncertainties in the estimates for cropland and
grassland, it would not seem accurate and sound to try to use them to aggregate the data
and produce an estimate of the carbon accumulation and total carbon balance in European
soils.
3 Peat and organic soils
The current area of peat occurrence in the EU Member States and Candidate Countries is
over 318 000 km2. More than 50% of this surface is in just a few northern European
countries (Norway, Finland, Sweden, United Kingdom); the remainder in Ireland, Poland
and Baltic states. Of that area, approximately 50% has already been drained, while most
of the undrained areas are in Finland and Sweden.
Although there are gaps in information on land use in peatlands, it can be estimated that
water saturated organic rich soil (peatland) have been drained for:
- agriculture – more than 65 000 km2 (20% of the total European peatland area);
- forestry – almost 90 000 km2 (28%);
- peat extraction – only 2 273 km2 (0.7%).
This is important as the largest emissions of CO2 from soils are resulting from land use
change and related drainage of organic soils and amount to 20-40 tonnes of CO2 per
hectare per year. The emission from cultivated and drained organic soils in EU27 is
approximately 100 Mt CO2 per year. Peat layer have been lost by oxidation during land
use, but the estimate derivable from the published data, ca. 18 000 km2, is very probably
an underestimate.
4 Land use and soil carbon
Monitoring programs, long term experiments and modelling studies all show that land use
significantly affects soil carbon stocks. Soil carbon losses occur when grasslands,
managed forest lands or native ecosystems are converted to croplands. Vice versa soil
carbon stocks are restored when croplands are either converted to grasslands, forest lands or natural ecosystems. Conversion of forest lands into grasslands does not affect soil carbon in all cases, but does reduce total ecosystem carbon due to the removal of
aboveground biomass.
The more carbon is present on the soil, the higher the potential for losing it. Therefore the
potential losses of unfavourable land use changes on highly organic peat soils are a major risk. The most effective strategy to prevent global soil carbon loss would be to halt land conversion to cropland, but this may conflict with growing global food demand unless
per-area productivity of the cropland continues to grow.
5 Soil management and soil carbon
Soil management practices are an important tool to affect the soil carbon stocks. Suitable
soil management strategies have been identified within all different land use categories
and are available and feasible to implement. These are:
- On cropland, soil carbon stocks can be increased by
(i) agronomic measures that increase the return of biomass carbon to the soil,
(ii) tillage and residue management,
(iii) water management,
(iv) agro-forestry.
- On grassland, soil carbon stocks are affected by
(i) grazing intensity
(ii) grassland productivity,
(iii) fire management and
(iv) species management.
- On forest lands, soil carbon stocks can be increased by
(i) species selection,
(ii) stand management,
(iii) minimal site preparation,
(iv) tending and weed control,
(v) increased productivity,
(vi) protection against disturbances and
(vii) prevention of harvest residue removal.
- On cultivated peat soils the loss of soil carbon can be reduced by
(i) higher ground water tables.
- On less intensively / un-managed heathlands and peatlands, soil carbon stocks
are affected by
(i) water table (drainage),
(ii) pH (liming), fertilisation,
(iii) burning
(iv) grazing.
- On degraded lands, carbon stocks can be increased after restoration to a
productive situation.
Given that land use change is often driven by demand and short term economic revenues,
the most realistic option to improve soil carbon stocks is to a) protect the carbon stocks in
highly organic soils such as peats mostly in northern Europe, and b) to improve the way
in which the land is managed to maximise carbon returns to the soil and minimise carbon
losses. Increased nitrogen fertilizer use has made a large contribution to the growth in
productivity, but further increased use will lead to greater emissions of nitrous oxide
(N2O). Hence future emphasis should be concentrated on the other main driver of
productivity, i.e. improved crop varieties.
6 Carbon sequestration
Soils contain about three times the amount of carbon globally as vegetation, and about
twice that in the atmosphere. There is a significant and large uncertainty associated with
the response of soil carbon (and other pools of biospheric carbon) to future climate
changes. Most response are calculated with simulation models with some models
predicting large releases of additional carbon from soils and vegetation under climate
change, and others suggesting only small feedback. The maximum possible amount of
carbon that soil sequestration could achieve is about one third of the current yearly
increase in atmospheric carbon (as carbon dioxide) stocks. This is about one seventh of
yearly anthropogenic carbon emissions of 7500 Mt C. In Europe emissions of greenhouse
gases amount to approximately 4100 Mt CO2 (or 1000 Mt C) per year.
Today, soils in Europe are most likely a sink and the best estimate is that they sequester
up to 100 Mton C per year. Higher sequestration is possible with adequate soil
management. Soil C-sequestration alone is surely no ‘golden bullet’ to fight climate
change but is it realistic to link climate change with soil carbon conservation, as soil
carbon sequestration is cost competitive, of immediate availability, does not require the
development of new and unproven technologies, and provides comparable mitigation
potential to that available in other sectors.
Therefore, given that climate change needs to be tackled urgently if atmospheric carbon
dioxide concentrations are to be stabilized below levels thought to be irreversible, soil
carbon sequestration or the even more effective conservation of current carbon stocks in
soils has a key role to play in any raft of measures used to tackle climate change.
7 Effects of climate change on soil carbon pools
We have not found strong and clear evidence for either an overall combined positive or
negative impact of climate change (raised atmospheric CO2 concentration, temperature,
precipitation) on terrestrial carbon stocks. There are suggestions for enhancing soil C
stocks at higher atmospheric CO2 concentration and reducing soil C stocks when
temperatures are rising. Most studies have taken moderate changes in temperature
increases and sudden and more severe changes in temperature of precipitation have not
been considered, as the management of land and soils overrules any impact on soil carbon from climate change.
All of the factors of climate change (raised atmospheric CO2 concentration, temperature,
precipitation) affect soil C, with the effect on soils of CO2 being indirect (through
photosynthesis) and the effects of weather factors being both direct and indirect. Climate
change affects soil carbon pools by affecting each of the processes in the C-cycle:
photosynthetic C-assimilation, litter fall, decomposition, surface erosion, hydrological
transport. Due to the relatively large gross exchange of CO2 between atmosphere and
soils and the significant stocks of carbon in soils, relatively small changes in these large
but opposing fluxes of CO2 may have significant impact on our climate and on soil
quality. Therefore, managing these fluxes (through proper soil management) can help
mitigate climate change considerably.
8 Monitoring systems for changes in soil carbon
Today, monitoring and knowledge on land use and land use change in EU27 is
insufficient, yet land use and land use change are a key source of greenhouse gas
emissions in many of the EU27 member states. Soil monitoring in EU27 seems like the
Tower of Babel: countries tend to have their own systems, if any, sometimes even more
than one system, and the results are not fully compatible across EU27. The few existing
systems tend to have been set up for different purposes, often not including that of
providing evidence concerning the impact of climate change on soil carbon pools. This
19
lack of systematic and comparable data gathering and analyses seriously hampers any
attempt to provide reliable, EU-wide data on the soil carbon stock and changes therein.
Moreover, the new goal of monitoring stock-changes rather than stock-magnitudes may
necessitate significant changes to current soil sampling procedures.
Given the lack of reliable national monitoring systems and without an EU wide
harmonized system of monitoring of soil carbon in place, it would be a significant
advance if the EU were to ask for a design or initiate implementation of a harmonized
EU27 monitoring for land uses and for specific activities that affect soil carbon stocks
and emissions of CO2. Such monitoring would also allow for adequate representation of
changes in soil carbon in EU27 in reporting to the United Nations Framework
Convention to Combat Climate Change (UNFCCC).
9 EU policies and soil carbon
We have critically reviewed EU policies that are likely to have impacts on soil carbon (C)
to assess whether any of those policies might have adverse impacts on soil C in the long
term. Policies reviewed were the Common Agricultural Policy (CAP), the Nitrates
Directive, the Renewable Energy Sources Directive, the Biofuels Directive, Waste policy
and the EU Thematic Strategy for soil protection.
Legislation to encourage the production of arable crops to provide feed stocks for
renewable energy is perhaps the legislation most likely to lead to decreases in the overall
carbon content of European soils. While studies may indicate much of the demand may
be met by imports from outside the EU, and hence may have little impacts on soil C
within the EU, there may be serious implications for soil C stocks in those countries
which supply renewable energy or their substrates.
We conclude that the need to comply with environmental requirements under the Cross
Compliance requirement of CAP is an instrument that may be used to maintain SOC. The
measures required under UNFCCC are not likely to adversely impact soil C. Nor are
there any measures in the proposed Soil Framework Directive that would be expected to
lead to decreases on soil C.