A method for assessing available phosphorus content in arable topsoils over large spatial scales

Human activities, and more specifically agricultural activities, have modified the way the ecosystem functions through the use of fertilizers. When these inputs are not being utilized reasonably, they can lead to a degradation in both soil and water quality. As a result of the subsequent environmental problems and socio-economic constraints, the concept of sustainable fertilization was born. The main goal behind sustainable fertilization is to reduce the amount of fertilizers used while maintaining soil fertility and farmer revenues. We have designed herein a methodology for assessing the phosphorus bioavailability in arable topsoils on France's national scale, on the basis of this sustainable fertilization concept. We will consider the sustainability of a cropping system that requires balancing nutrient removal from the soil by fertilization, in order to avoid environmental risks. The originality of this method lies in quantitative results stemming from the French Soil Test Database (BDAT) and their evaluation using the RegiFert software, which incorporates soil characteristics and crop sensitivity to nutrient availability. According to the proposed procedure, we determined the phosphorus bioavailability class of French arable soils at the cantonal scale. Cantons are areas of average surface of 140 km2. This methodology constitutes an attempt to gather all analytical results into a common diagnostic framework on the national scale (for France), while taking into account the local pedological context and crop production requirements. With this procedure, we are able to conclude that 77% of cantons are situated below the critical level regarding crop needs. Furthermore, this diagnosis suggests that the available phosphorus content in soils is considered insufficient with respect to any crop potential requirement. From an environmental standpoint, however, this situation could be considered as generating the lowest pollution risk. Nevertheless, 23% of the cantons exhibit a soil phosphorus accumulation that is not necessary with regard to agricultural uses. In this case, no fertilization is required and the pollution risk is increased. On the national scale, bioavailability class frequencies are structured spatially, with gradients correlated to soil parameter spatial distribution, and tend to reflect land use patterns.