Explaining and mapping total phosphorus content in French topsoils

Abstract

Total phosphorus (TP) build-up in agricultural soils represents both a threat to aquatic ecosystems and a valuable resource for future crop production, given the context of increasing food demand combined with the rapid depletion of the world's phosphate reserves. Therefore, it is crucially important (i) to identify the main factors controlling topsoil TP and (ii) to develop methods for mapping its spatial distribution. Multiple linear regression models were used with two distinct approaches to calculate TP and covariates linked to the P cycle. Firstly, covariates were selected from the R eseau de Mesures de la Qualit e des Sols database, the French soil monitoring network, which consists of soil samples collected from 2158 sites on a 16-km regular grid. Secondly, covariates were selected to map TP from spatially exhaustive datasets in France. The first approach explains 80% of variability in topsoil TP. The variables selected are linked to the autochthonous origin of P (parent material), to allochthonous origin (organic carbon and nitrogen contents) and to the retention capacity of soil (Al, Fe, Ca and clay contents). The predicted map obtained from the second approach provides a mean TP of 0.76 g/kg. This study demonstrates that creating national scale maps of TP, based on detailed soil sampling and many variables, is feasible and can be used to model the P cycle and P transfer processes. Such maps can be used in P erosion and transfer models over river basins, and therefore to predict P exports to surface waters.

Eutrophication: A new wine in an old bottle?

“…In groundwater, wetlands and lake sediments, nitrates can, to a certain extent, be transformed into gaseous nitrogen by denitrification (biogeochemical processes). In soils and sediments, storage of the phosphorus introduced for more than a century by human activity has resulted in there being an excess of phosphorus (Delmas et al, 2013) in relation to nitrogen, although nitrogen can also be stored for decades in soils (Sébilo et al, 2013). Differences in biogeochemical processes controlling the cycling and transfers of N and P along the land-sea continuum can result in marked changes in nutrient stoichiometry from headwater catchments to the sea (Alexander et al, 2000).…”

Soil parent material—A major driver of plant nutrient limitations in terrestrial ecosystems

“…Far more than all other factors, total soil P content drove pools of available P in soils. In turn, total P content of the soil depended on factors including topography, the sand content of the soil, soil weathering stage, and soil parent material (Castle & Neff, 2009;Delmas et al, 2015;Hahm et al, 2014;Homyak, Sickman, & Melack, 2014;Porder & Ramachandran, 2013): flat lowlands (Mage & Porder, 2013), sandy soils (Achat, Pousse, Nicolas, Br edoire, & Augusto, 2016;Buckingham, Neff, Titiz-Maybach, & Reynolds, 2010), weathered soils (Reed et al, 2012), and soils developed from nonmafic rocks (Mage & Porder, 2013) were poorer in P than other kinds of soils. Soil sand content explained soil P fertility better than soil clay content because sand content is a surrogate of quartz content (Bui & Henderson, 2013) and because rock content in quartz is negatively correlated with the P total content of siliceous rocks (Hahm et al, 2014;Vitousek et al, 2010).…”

The phosphorus legacy offers opportunities for agro-ecological transition (France 1850–2075)

“…The initial value of the total soil P pool, representing the pre-industrial P stock (before 1850), is adjusted in each region to the value required to correctly simulate the average measurements of current total P in crop-and grassland soils (Delmas et al 2015). Internal processes of P exchange between the labile and stable P pools are represented by first order rate kinetics for both processes.…”