Carbon concentrations and stocks in forest soils of Europe

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Abstract

This study presents the results of a series of evaluations of a continent-wide soil database (EU/UN-ECE Level I) with the aim to estimate baseline soil carbon concentrations and stocks. The methodology included the biogeographic stratification of soil carbon measurements throughout Europe using climatic zones derived from the Soil Regions Map of Europe. The presented stock estimates range from 1.3 to 70.8 t C/ha for the O-layer, and from 11.3 to 126.3 t C/ha for the mineral soil 0–20 cm (Germany: 0–30 cm) (5 and 95 percentiles). Histosols were excluded because of methodological differences and data gaps. When looking at the median values of the strata investigated, relationships were found. For example, carbon stocks in the O-layer of sandy soils are distinctly higher than those of fine-textured soils. However, the variability is so high that some of these relationships disappear. For example in western and central Europe, the level of carbon stocks in the mineral soil between shallow soils (Leptosols) and more deeply developed soils (Podzols and Cambisols) do not differ very much. It was also found that just the investigation of topsoils is not sufficient to understand the regional pattern of organic matter in forest soils – unless the subsoil becomes included as well. It is hypothesized that for Europe, the impact of site factors such as climate, texture and relief are difficult to extract from such a database if the data are only stratified according to macro-climatic areas. It has to be considered that the effect of systematic error in the database is quite large (but cannot be identified on the level of the current data availability). In order to receive a first impression of the landscape-level distribution of carbon, a map of carbon concentrations in the topsoil was generated. The results support the relationships found between carbon stocks and site factors, such as climatic zones and soil type. Compared to the much lower carbon concentrations of agricultural soils, the results demonstrate clearly the importance of forest soils for the terrestrial carbon cycling in Europe.

Introduction

Forest soils in the Northern hemisphere undoubtedly play an important role in the terrestrial greenhouse gas balance (Janssens et al., 2003, IPCC, 2003, IPCC, 2006). For example, forest soils in Europe store roughly 1.5 times more carbon than trees (EC/UN-ECE, 2003). It is expected, that the importance of forest soils for the European carbon cycle increases even more in the future (Liski et al., 2002, Smith et al., 2006). However, an accurate continent-wide assessment of the role of soils to store carbon is still difficult considering the great range of available model and inventory results. For example, Cannell et al. (1992) have presented a first rough estimate of 3–4 Gt carbon stored in European forest soils. Liski et al. (2002) gave a model-based estimate of 5 Gt C. Goodale et al. (2002) have estimated a forest soil C stock of 13.7 Gt including the O-layer. Jones et al., 2004, Jones et al., 2005 estimated a pool size of 79 Gt C for all European soils including peat. Of course, each estimate refers to a different area extent, soil depth, and methodology, and deviations to country-specific results can be quite substantial. Schils et al. (2008) conclude that existing regional data are difficult to integrate into a single harmonized continent-wide assessment.

These frame conditions also affect the estimation of current and future carbon sinks and sources. Such trend estimates are still mostly model-based because data from repeated soil inventories or monitoring are still the exception (Bellamy et al., 2005). According to Liski et al. (2002), the carbon content of forest soils in western Europe has been increasing at the rate of 26 Mt C a−1 in 1990. De Vries et al. (2000) estimated a sequestration of 76 Mt a−1 in forest soils (EU 15; see also Table 1). Janssens et al. (2003) have published an estimate of 110 g C m−2 a−1 (which equals 151 Mt C a−1 after applying the same size of forested area calculated by Liski et al. (2002)). Schils et al. (2008) cite various national studies mostly based on modelling, and mostly indicating sink rather than source behaviour of forest soils: values differ from study to study, ranging from 0.01 to 0.8 t ha−1 a−1 for managed forest land, and from 0.02 to 0.45 t ha−1 a−1 after conversion of cropland to forest. The reliability of these model results depends on reliable baseline and/or calibration data (Chertov et al., 2002, Faubert et al., 2006). According to Lal et al. (2003), the development of reliable baselines is still a knowledge gap in the search for reliable information about carbon sequestration in forest soils.

It has been demonstrated that the role of soils cannot be accurately determined without comparable and harmonized inventory data. Such an inventory exists with the EU/ICP Forests Level I soil condition inventory 1990–1995 (UN-ECE ICP Forests, 1998). Because its design has not initially focussed on soil carbon monitoring, some improvements were recently made (UN-ECE ICP Forests, 2006). Meanwhile the inventory was repeated (2006–2008; http://biosoil.jrc.ec.europa.eu/). Until now, baseline values for the carbon content of forest soils were not derived from this database. The general difficulties involved in applying large databases for such a purpose were commented by Amichev and Galbraith (2004). In the case of the Level I inventory, national design modifications and data gaps cause difficulties to develop a harmonized soil carbon baseline. An additional challenge for evaluations of plot-level inventories in Europe is the variability of its soils (Montanarella, 2005).

This study attempts for the first time to produce a harmonized assessment of the carbon content of forest soils in Europe, using data from the EU/ICP Forests Level I inventory. The quantification of soil carbon stocks requires that carbon concentration measurements are further recaclulated to a volume to area basis. For that, additional data are needed which are often not measured in extensive surveys, such as the bulk density and stoniness of soils. This study will develop reliable estimates at the continental level. In order to assess the effects of regional variability, a map of soil carbon concentrations is to be developed. At the same time, effects of climate must be additionally considered because the study area covers a large geographic gradient, and because climate strongly affects the properties of otherwise pedo-genetically similar soils (Finke et al., 1998). Methodically, the results of this study are expected to support the further use of the Level I forest soil condition database, for example in combination with the results of repeated samplings, both nationally and across Europe. This study will produce harmonized baseline soil carbon stocks which help to assess the plausibility of diverse published inventory data. They can be directly applied to soil carbon models, and thus contribute to support the comparability of model results. Therefore, these results will indirectly also improve the reporting of soils in IPCC greenhouse gas inventories.

Section snippets

ICP Forests Level I forest soil inventory in Europe

The International Co-operative Programme on the Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) has established a forest health monitoring programme using an extensive 16 km × 16 km survey grid (Level I) and a network of stratified intensive monitoring plots (Level II) (www.ICP-Forests.org). In 1990–1995, 31 countries participated in a Level I soil condition inventory (Vanmechelen et al., 1997, UN-ECE ICP Forests, 1998). The data are stored in the ICP Forests Soil

Carbon stocks for typical soils under forest cover

Fig. 3 presents the descriptive statistics of soil carbon stocks, stratified according to climatic zone and main soil type (median and frequency distributions: quartiles (25, 75), min/max, extremes and outliers; SPSS Inc., 2007). It has to be noted that very few plots occur in the temperate continental climatic zone, so that it is mostly absent from representation in Fig. 3. Histosols were excluded because of the systematic differences in sampling and nomenclature between some countries.

Soil

Discussion

For the first time, a Europe-wide evaluation of SOC stocks has been conducted using measured carbon data from the EU-ICP Forests Level I soil inventory. Until now, the methodical differences between countries and data gaps have prevented such an evaluation at the European level. Therefore, the methodical framework of this study can be used as a reference for further applications of this database.

The behaviour of soils to store carbon seems to support known observations that coarse-textured

Conclusions

Carbon stocks in forest soils have been estimated based on the EU/ICP Forests Level I soil inventory in Europe. The results have led to hypotheses about the effects of site factors on soil carbon storage, for example climatic zone and soil type. In order to support these hypotheses, a map of typical carbon concentrations in forest soils was produced which represents the regional pattern of carbon in the very top part of the mineral soil of forests. The reliability of the map is best where a

Acknowledgements

The results presented were developed in a sequence of studies investigating the use of regional and national soil inventories for SOC baseline development. The work was initiated by the European Commission Joint Research Centre (JRC) in support of its greenhouse gas inventory quality control programme. The evaluations of the Level I database were then further pursued in the EU-Project CarboInvent (Multi-source inventory methods for quantifying carbon stocks and stock changes in European

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