SOC trends at the national scale

Our high-resolution simulation of land use and climate change impacts on SOC stocks indicates that France will lose between 774 and 1221 Mton of SOC by 2100 (i.e. 20–30% of 1990 stock or from ca. 4,000 to 3,000 Mton C) (Fig. 1 and Table 1). More precisely, the mean SOC decrease is 1,036 ± 164 Mton C or 25.7 ± 3.9%, which corresponds with an annual decrease of circa 10 Mton C y−1 or approximately 12% of the total annual C emission of France, estimated in 2013 to be 82 Mton C y−1 43. The most important finding of this study is that future climate change will contribute approximately 10 times more to this total SOC decrease than land use change, i.e. 966 ± 168 Mton C or 23.9 ± 4.0% for climate change versus 97.4 ± 3.7 Mton C or 2.4 ± 0.1% for land use change, respectively. We will focus in this paper on outputs of the ARPEGE model SRES scenarios B1, A1B and A2, characterized by relative SOC losses of 19.6%, 26.8% and 30.1% between 1990 and 2100 (indicated in colour in Fig. 1). Since recent research has outlined that the potential role of increased NPP forms a major source of uncertainty on large scale SOC stock trend predictions and since the associated net effect remains rather unclear33,34,35,36,37,38,39,40,41, this factor wasn’t considered in the present analysis. If the latter factor seems to act nevertheless as a significant negative feedback mechanism on SOC storage losses, the 21th century’s SOC losses across France may be smaller than the figures predicted in this study.

Table 1 Total SOC stock change for entire France (Mton C) as a consequence of (i) only land use change, (ii) only climate change and (iii) the combined effect of both land use change and climate change, considering 8 different climate models/scenarios. Full size table

Figure 1 Temporal evolution of total SOC stock for entire France (Mton C) as a consequence of the combined effect of land use change and climate change considering 8 different downscaled climate models/scenarios. Full size image

Spatial-temporal trends in climate and land use

The land use and ARPEGE-model derived climate variable distributions used for the SOC predictions are shown in Fig. 2 (a,b,c–j, respectively) for 1990 (baseline) and 2100. This shows that the urban sprawl will be the most important land use change process (Fig. 2a,b). The ARPEGE-model predicted temperature maps for 1990 shows that most areas have annual average temperatures below 11 °C while by 2100 most areas will have annual average temperatures above 11 °C under the SRES B1 scenario, above 12 °C under the SRES A1B scenario and above 13 °C for A2 scenario (Fig. 2c–f). Annual precipitation maps indicate a general trend of drying with most areas witnessing a reduction of 100 mm year−1 under SRES scenarios B1 and almost 200 mm y−1 under SRES scenarios A1B and A2. The predicted precipitation changes are much more pronounced in the mountainous regions than elsewhere, with absolute difference in rainfall up to 500 mm y−1 or more for the western part of the Pyrenees (Fig. 2g–j).

Figure 2 Climate and land use change maps (1990 versus 2100). Land use (a,b) and climate (i.e. temperature and precipitation according to ARPEGE model IPCC scenarios B1, A1B and A2) (c–j) input maps used for the SOC predictions of baseline year 1990 (a,c,g) and 2100 (b,d–f,h–j). The maps were generated using ArcGIS 10.1 (ESRI, Redlands, CA, USA: http://www.esri.com/software/arcgis). Full size image

Spatial heterogeneity of SOC change and interactions with controlling factors at the landscape scale

The baseline SOC map for 1990 shows that precipitation dominates the spatial pattern of SOC, with high stocks in wet regions such as in the south-west and most westerly part of France (Britany) and in various Mountainous regions in the east and south of France (Figs 2g and 3a). In addition this map illustrates that besides climate, land use has an important influence on the spatial distribution of SOC, with low SOC stocks in cropland areas (mostly centrally located) or vineyards (Mediterranean region) and high SOC stocks in grassland or forested areas42.

Figure 3 SOC change maps (1990 versus 2100). SOC stock maps (kg C m−2) for 1990 and 2100 as a result of ARPEGE model IPCC scenarios B1, A1B and A2 as well as land use change predictions (a–d), including absolute (e–g) and relative difference (h–j) maps. The maps were generated using ArcGIS 10.1 (ESRI, Redlands, CA, USA: http://www.esri.com/software/arcgis). Full size image

The future SOC maps (based on ARPEGE model) for 2100, show that SOC stocks across the entire French territory, are remarkably lower as compared to the baseline SOC map of 1990 (Fig. 3a–d). When considering SRES scenario B1, SOC losses exceeds 1 kg C m−2 in most areas, with a nationwide average loss of 1.43 ± 1.25 kg C m−2 (Fig. 3e). These losses become gradually higher under the A1B and A2 scenarios, with in most areas SOC losses exceeding 2 kg C m−2 and nationwide average losses of respectively, 1.97 ± 1.40 kg C m−2 and 2.21 ± 1.51 kg C m−2 (Fig. 3f,g). Nevertheless, note that there is a very large spatial heterogeneity on these nationwide SOC trends. Absolute differences are generally highest in the more wet regions (i.e. in the west and southwest of France as well in Mountainous regions in the east and south of France). Declining SOC stocks in these environments will most probably be the result of the combined effect of significantly (i) higher temperatures (Fig. 2c–f), causing elevated rates of microbial decomposition of organic matter and hence increased C-respiration44,45,46 and (ii) drier conditions (Fig. 2g–j), causing lower soil moisture conditions, which hampers plant growth and hence reduces C input33,47. Despite the fact that on the other hand drier conditions may results as well in a lower degree of microbial activity and thus a lower degree of decomposition, it is most probable that the decreased C input will be the dominant factor resulting in an overall reduction of SOC. This can also be observed nowadays in most Mediterranean dryland regions which are commonly characterized by very low SOC concentrations48.

Furthermore, urban expansion is causing remarkably large SOC decreases near the edges of almost all existing cities (Fig. 2e–j). As urbanization causes SOC to reduce significantly over time, a decline to 0 kg C m−2 is assumed. Consequently, these SOC stock changes are similar for each SRES climate change scenario and as large as −4.74 ± 1.65 kg C m−2, −7.46 ± 2.42 kg C m−2, −7.83 ± 1.80 kg C m−2 and −2.44 ± 1.60 kg C m−2 for conversions from cropland, grassland, forest and vineyard into urban land use (Fig. 4). This corresponds with a total SOC stock change at the national scale under SRES scenario A1B of −45.77 Mton C, −19.47 Mton C, −15.52 Mton C and −1.71 Mton C, respectively (Fig. 5). Despite the fact that comparable SOC stock gains can be made with de-urbanisation, these are uncommon land use changes under the business-as-usual land use change scenario, and hence associate total SOC stock gains at the national scale are rather low, i.e. varying between +3.14Mton C for urban to forest conversions and +0.04 Mton C for urban to Vineyard/Orchard conversions under SRES A1B scenario (Fig. 5). Although land use change on the urban periphery is the most common and the most significant contributor to land use change related SOC change; national SOC stock change is dominated by changes in SOC stocks in areas unaffected by land use change. These SOC stock changes per unit area range from ca. −1 kg C m−2 for unchanged vineyard/orchard, ca. −1.5 kg C m−2 for unchanged cropland to ca. −2 kg C m−2 for unchanged grassland and forest (Fig. 4), which corresponds to relative net changes of ca. −40% for vineyards and orchards, of ca. −30% for cropland and ca. −25% for grassland and forest. Associated net contribution to the total SOC stock loss at the national scale is very large due to the vast area they occupy. Using SRES A1B scenario as an example, unchanged cropland, grassland and forest are predicted to lose 357, 222, 336 Mton C, respectively; even unchanged vineyard/orchard are predicted to lose 14 Mton C (Fig. 5). Cumulatively, these represent 86% of the predicted national total SOC stock loss due to land use and climate change, indicating that future SOC stock changes are foremost affected by climate change.

Figure 4 Absolute difference in SOC stock (kg C m−2) as a function of land use dynamics using ARPEGE model IPCC scenarios B1, A1B and A2 (standard deviations represents variability at the national scale). Full size image

Figure 5 Absolute difference in total SOC stock for entire France (Mton C) as a function of land use dynamics using ARPEGE model IPCC scenarios A1B predictions (lower error bars represents ARPEGE model IPCC scenario A2 predictions, higher error bars represents IPCC scenario B1 predictions). Full size image

Amongst non-urban land use changes, most change combinations will results in net SOC stock losses. Largest losses are recorded for conversions from grassland and forest into cropland, estimated under SRES scenario A1B at 44 Mton C (or 4.4 ± 1.6 kg C m−2) and 10 Mton C (or 5.4 ± 1.4 kg C m−2), respectively (Figs 4 and 5). Most other land use change conversions are characterized by relatively small SOC changes with limited total stock losses (<2 Mton C). The only land use changes that result in a net gain in total SOC stock are those associated with conversions of cropland or vineyard/orchard into forest or grassland. Under the SRES A1B scenario, cropland to grassland conversion results in a net SOC stock gain of 3.4 Mton C (Fig. 5), which corresponds to a mean SOC gain of 0.7 ± 1.6 kg C m−2 (Fig. 4), indicating that not all areas subject to this particular land use conversion demonstrate a net gain. The SOC change maps (Fig. 3e–j) indicate that cropland to grassland conversion is most effective in storing C in the valleys in the central and northwest of France. These regions are characterized by relative low temperatures and have soils with relatively high clay contents (e.g. valley of the Seine river, Fig. 3e–j). This illustrates the important interaction between soil type and climate change in determining C sequestration potential at the national scale.

In general, sandy regions (e.g. Vosges, Landes and Sologne) are typically characterized by rather high SOC losses. The important influence of soil type on the spatial heterogeneity in SOC changes becomes also very clear in the cropland dominated region in the south of France (i.e. north of the most central part of the Pyrenees) where high SOC losses occur in soils with low clay contents and low SOC losses are predicted for soils with high clay contents. Furthermore, areas with limited SOC stock gains are stony forested soils in the Mediterranean region (Fig. 3e–j).

Implications for land use change management impacts on SOC storage

Previous research has suggested that land use change within Europe could make a significant contribution to climate change mitigation by creating an important net SOC sink30,31. Nevertheless, the latter studies have explored the effect of widespread land use change unconstrained by local decision-making. Here we model high-resolution business-as-usual land use change based on local decisions made in the recent past and, under these conditions, we observe contrasting results. We find that the land use change conversions result in a small net soil C loss, while climate change results in a large C loss and hence will have a dominant effect on SOC stock dynamics in mineral, mid-latitude soils in the 21st century. Although the conversion of urban areas into other land uses seems to be the most effective strategy to sequester C in the soil (i.e. resulting in net SOC stock gains between 3 and 8 kg C m−2, Fig. 4) it is not expected that there is much scope for large soil C storage here, in particular because this study shows that under a business-as-usual land use scenario urban sprawl will be the most frequent land use change during the 21st century. Moreover, as a consequence of the lack of organic matter, nutrients and stable aggregates or the potential to be polluted, it most likely will take a rather long period before soils subject to de-urbanisation will act as sustainable sinks of CO 2 . Besides de-urbanisation, only the conversions of cropland or vineyards into forest or grassland are predicted to result in a net soil C stock gain. Their potential to act as important soil C sinks is, however, strongly limited due to the small net SOC stock gains per unit area and/or the restricted land surface area that can be subject to these land use conversions.

Due to the small SOC stock gain per unit area when converting cropland to grassland or forest (i.e. only 0.5–1 kg C m−2), conversions of 100,000 to 200,000 km2 of cropland into grassland or forest would be required to offset 10% of the climate change induced loss of SOC from areas of unchanged land use. This is unrealistic as the total area of cropland in France in 1990 was only 228,000 km2 and maintaining cropland will be crucial to fulfil the predicted future food demand49, especially given the recently stagnating crop yields40 and potential future nutrient limitation39. The potential SOC stock gains related to conversions from vineyard into grassland or forest are higher (i.e. around 1.5 kg C m−2 and 4 kg C m−2, respectively (Fig. 4)), but the net total SOC stock storage potential at the national scale is small due to the restricted land surface area of vineyard/orchards. So even converting all the existing vineyards/orchards into forest, will only lead to a net sequestration of 50 Mton C, which is only 5% of the total estimated SOC stock loss. Such a change is even less probable, given the economic and cultural importance of the vineyards in France.

Global Policy implications