NPK in European soils
Using the ca. 22,000 soil samples from LUCAS topsoil, the JRC analysed a full range of soil properties – including Nitrogen (N), Phosphorus (P), Potassium (K) – and their influencing factors. This new research activity (Ballabio et al., 2019) provides a comprehensive overview of the distribution of chemical properties (including pH, cation exchange capacity, calcium carbonate, C/N ratio, N, P and K) in the soils of 26 EU Member States (Croatia was not included because soil samples were not taken in 2009/12; Cyprus was not included because we were missing input layers for developing the soil nutrient maps), an area covering more than 4.5 million km2.
The sample analysis was performed by a single laboratory, contributing to data comparability by avoiding uncertainties due to analysis based on different methods or different calibrations in multiple laboratories. In a first phase, LUCAS topsoil samples were analysed for their physical and chemical properties following ISO standard procedures (Orgiazzi et al., 2018). In order to assess the relation between environmental features and soil chemical properties distribution, Gaussian Process Regression (GPR)(Rasmussen and Williams, 2006) was utilised for inference and mapping. A detailed description of the spatial interpolation model can be found in the publication of Ballabio et al. (2019).
The distribution of topsoil nitrogen (Figure) is highly correlated with SOC, given that nitrogen is a major component of soil organic matter. While the C/N ratio can vary, some carbon-rich soils are also nitrogen rich, at least in terms of absolute quantities. Given this relation, it is quite clear that vegetation cover and climate are the main drivers in the distribution of nitrogen. As the nitrogen map shows, forests and grassland areas tend to have higher nitrogen content. Forests in Scandinavia and in mountain areas are clearly outlined by the map (Ballabio et al., 2019). Climate also acts as a main driving force influencing nitrogen content along the Atlantic area; in particular, Ireland and the United Kingdom show higher N concentrations due to a fresh and humid climate, which favours organic matter accumulation. Soil texture also plays a role in stabilising organic matter and thus nitrogen. Areas with coarser soils, such as most of Poland, tend to have less nitrogen even if other conditions are favourable (e.g. vegetation, climate).
While the nitrogen concentration is relevant to assessing stocks and potential N2O emissions, the ratio between carbon and nitrogen can better represent the differences in the organic matter composition. Where higher rates correspond to more oligotrophic soils, typical of coniferous forests, or to peatland soils, lower rates are typical of more balanced nutrient-rich soils.
Topsoil nitrogen is highly correlated with SOC (Ballabio et al., 2019). In addition to this, vegetation (higher values in forests and grassland), climate (higher values in humid climates) and soil texture play an important role in nitrogen distribution.
Phosphorus is mainly derived from the weathering of minerals in parent rock material. It is usually the second most limiting nutrient for terrestrial primary production (Cordell et al., 2009). In agricultural areas fertilisation can result in higher levels of P, especially in highly productive areas where high input of P fertilisers is reported (Tóth et al., 2014). Modern agriculture is highly dependent on P fertilisers, and P supply is strategically critical at global level.
The map of soil phosphorus shows a clear trend in which land use appears to have a strong influence. In particular, most of the agricultural areas have higher levels of P. This is quite evident in areas such as the River Po plain (Italy) where levels of P diverge from the national average. In general, areas with natural land cover and those with a prevalence of permanent crops correspond to lower levels of P (Ballabio et al., 2019). The geological background seems to have a quite small influence, whereas climate is much more relevant; this is probably because of higher fertilisation rates in wetter climates.The P map produced in this study also confirms models of P fertilisation load (Potter et al., 2010).
The soil phosphorus map shows the strong influence of land use. In particular the agricultural land has higher levels of P than natural areas or forests.
Potassium has different functions for plant life; it is a constituent of enzymes and acts as a regulator of drought tolerance and water use (M. Wang et al., 2013). In the soil, the principal sources of potassium are feldspars and micas, which release K during weathering (Hillel, 2008). In the soil itself, potassium appears in three forms: in the circulating solution; as an exchangeable ion adsorbed to the surface of clay particles; and in organic matter. Given the wide distribution of K-containing minerals and the fact that it is prevented from leaching by cation exchange, its depletion from the soil is quite uncommon.
Soil potassium distribution is mostly driven by parent material chemistry and climate. In particular, lower than average K concentrations are typical of the sandy soils of north-eastern Europe, and of the relatively young soils of Scandinavia. Moreover, Portugal and north-western Spain also exhibit lower levels of potassium, probably due to leaching. In general, soils with higher clay content are better able to retain K, so the two variables show similar spatial distributions (Ballabio et al., 2016).
Soil potassium distribution is driven by parent material (low K in sandy soils; higher K in clay soils) and climate.
Data are available in the European Soil Data Centre.
Reference: Ballabio, C., Lugato, E., Fernández-Ugalde, O., Orgiazzi, A., Jones, A., Borrelli, P., Montanarella, L. and Panagos, P., 2019. Mapping LUCAS topsoil chemical properties at European scale using Gaussian process regression. Geoderma, 355: 113912.