Agriculture increases the bioavailability of silicon, a beneficial element for crop, in temperate soils

Abstract

Crops may take benefits from silicon (Si) uptake in soil. Plant available Si (PAS) can be affected by natural weathering processes or by anthropogenic forces such as agriculture. The soil parameters that control the pool of PAS are still poorly documented, particularly in temperate climates. In this study, we documented PAS in France, based on statistical analysis of Si extracted by CaCl₂ (Si_CaCl2) and topsoil characteristics from an extensive dataset. We showed that cultivation increased Si_CaCl2 for soils developed on sediments, that cover 73% of France. This increase is due to liming for non-carbonated soils on sediments that are slightly acidic to acidic when non-cultivated. The analysis performed on non-cultivated soils confirmed that Si_CaCl2 increased with the < 2 µm fraction and pH but only for soils with a < 2 µm fraction ranging from 50 to 325 g kg^-1. This increase may be explained by the < 2 µm fraction mineralogy, i.e. nature of the clay minerals and iron oxide content. Finally, we suggest that 4% of French soils used for wheat cultivation could be deficient in Si_CaCl2.

Figure 1

Importance of variables in the Random Forest model (in % of increasing Mean Squared Error, %IncMSE).

Figure 2

Regression Kriging predictions of Si_CaCl2 concentrations in French topsoils at 90 m resolution. (a) continuous colour gradient representation; (b) categorical representation in 3 classes of Si_CaCl2 concentrations based on critical values defined for sugarcane and rice^,, for soils cropped in wheat, obtained by crossing the ‘arable land’ pixels, ‘permanent crop’ and ‘heterogeneous agricultural’ areas with the exception of ‘agro-forestry’ of Corine Land Cover classes with municipalities for which type of farming is primarily cereal crop, according to French OTEX classification (Orientation Technico-Economique des Exploitations). We removed the municipalities from 7 departments (Ain, Haut-Rhin, Bas-Rhin, Gironde, Pyrénées-Atlantiques, Hautes Pyrénées, Landes) where corn is the main cereal under production. Maps were generated using ArcGIS software version 10.7.1 (ESRI: https://www.esri.com/en-us/home).

Figure 3

Boxplots of Si_CaCl2 concentration as a function of (a) < 2 µm fraction content classes, (b) pH classes , (c) Fe oxides concentration (estimated by the Mehra Jackson method) classes (d) soil organic carbon content classes for non-cultivated soils only. Classes have been created so that uncertainty around the central value does not overlap with the uncertainty around the central value of the surrounding classes. Classes with less than 40 individuals were merged. Groups of individuals sharing a letter are not significantly different according to pairwise comparisons of Wilcoxon test with Tukey adjustment for multiple comparisons.

Figure 4

Boxplots by parent materials and land uses of: (a) Si_CaCl2, (b) soil pH, (c) < 2 µm fraction content, and (d) < 2 µm CEC. Groups of individuals sharing a letter are not significantly different according to pairwise comparisons of Wilcoxon test with Tukey adjustment for multiple comparisons.

Figure 5

Relationship between topsoil Si_CaCl2 and pH for the different < 2 µm content classes defined in Fig. 3, for non-cultivated soils. The classes of < 2 µm contents are expressed in g kg⁻¹ and reported in the grey box at the top of the corresponding graph. The colours identify the classes of < 2 µm CEC corresponding to the classes of clay mineral CEC values provided by Goldberg et al.. Observations with < 2 µm CEC values larger than 100 cmol⁺ kg⁻¹ were eliminated (20 observations). Associated R² values and levels of significance are reported: ≤ 0.001 ‘***’, [0.001;0.01] ‘**’, [0.01, 0.05] ‘*’, > 0.05 ‘ ’.