# Remote Sensing Data for Digital Soil Mapping in French Research--A Review
## 3 Developments Related to the Extraction of Soil Properties from Remote Sensing Data
### Trends in French Remote Sensing Research in Relation to Threats to Soil  
In this section, we focus on articles that explicitly mention soil threats in our corpus of publications (Figure 1). Soil threats are classified according to the proposals made by the EU Soil Thematic Strategy [9; 211] and the EU soil strategy for 2030 [10]. The relative importance of soil threats (Figure 3) is related both to their importance in the political agenda and to the feasibility of using RS products to monitor these threats. The strong dominance of SOC is clearly related to the feasibility of reporting, monitoring, and verifying soil carbon stocks according to article 3.4. of the Kyoto Protocol (e.g., [212; 213]). It also links to the COP21 Paris Agreement and the four per Mille initiative [177; 214; 215]. In addition to advances in temporal frequency, high spatial and spectral resolution RS imagery offers an unprecedented opportunity to study and monitor the spatiotemporal dynamics of SOC changes [111; 181]. The second threat is the sealing of soils by urbanization, industrialization, road construction, etc. It corresponds to an almost irreversible loss of many soil functions and is particularly widespread in some regions of France (mainly peri-urban and coastal areas). The fine spatial resolution of the new RS sensors enables them to monitor the expansion of sealed areas over time. In France, the Land Cover Scientific Expertise Center (CES OSO, [[https://www.theia-land.fr/](https://www.theia-land.fr/)]([https://www.theia-land.fr/](https://www.theia-land.fr/)) accessed on 29 May 2023) automatically produces land-cover maps for metropolitan France from Sentinel-2A and Sentinel-2B data. These maps have a ground sampling distance (GSD) of 10 m. They are used to monitor the rate of soil sealing and/or to take into account the services rendered by soils in land-use planning (e.g., [216; 217; 218]). The third threat is erosion, which is a serious problem in French agricultural soils (e.g., [219; 220; 221]). Soil crusting and slaking have been studied using RS for a long time (e.g., [98; 222]). High-resolution imagery is an increasingly used technology for mapping erosion [223]. Soil surface changes can also be captured by DEM series from UAVs [224]. Figure 2: Evolution of the cumulative number of articles per soil property. Examples of soil contamination are less numerous, often related to the impact of mining or industrial activities on trace elements (e.g., [225; 226]), hydrocarbon and oil contamination (e.g., [227; 228; 229]), and some radionuclides [230]. Many studies are still based on hyperspectral, fluorescence, or photoluminescence measurements performed in the laboratory or by proximal RS (e.g., [226; 231]). There are, however, a few studies using airborne hyperspectral or gamma-ray spectrometry data [227; 228; 230; 232; 233; 234; 235]. The fifth most important threat is soil salinization, which is a very serious problem for soil fertility. It mainly concerns arid and semi-arid regions and is often linked to the quality of irrigation water [4]. Mainland France is not a salinization hot spot, except for a few local areas such as the Rhone delta (Camargue). However, French research is very active in the field of soil salinization RS. This is partly due to the history of France and its relationships with certain countries. This is also related to the fact that salinization crusts are quite easily detectable, even in the visible, and that studies on soil salinity mapping and monitoring have been developing for a long time and are still ongoing (e.g., [99; 101; 236; 237]). Other soil threats are more rarely studied using RS. Biodiversity, compaction, and acidification are not directly related to RS imagery. Indicators of soil biodiversity or acidification can, however, be captured by RS through vegetation or land use. In this case, they are often combined with other covariates in DSM tools. Figure 4 shows the evolution of the cumulative number of articles per soil threat.