Topography and climate dictate soil NPK stoichiometry across Africa

Optimal soil nitrogen-to-phosphorus-to-potassium (NPK) stoichiometry is critical for agricultural production in Africa because it presents the appropriation of input materials to avoid limited or excessive fertilizer applications. Furthermore, an optimum nutrient supply to the plants is crucial to mitigate or eliminating Africa’s food crisis. However, what drivers influence its levels, distribution and variability across different landscapes and scales? Insights regarding these aspects are necessary for (1) the derivation of robust policies associated with crop production and food security, (2) monitoring of changes associated with hotspot areas between NPK stoichiometry and designated drivers, (3) instigation of suitable and well-targeted efforts to ensure that areas with optimal soil NPK levels are maintained since these eventually affect and influence crop yield output, and (4) identification of areas that are imbalanced in NPK content of the soils and thus may need fertilization. Freely accessible major soil nutrient data [i.e., nitrogen (N), phosphorus (P) and potassium (K)] for Africa were obtained from the iSDAsoil platform, aggregated to 250 m, and used to compute the NPK stoichiometry estimate. In addition, similar NPK stoichiometry estimates were derived for national-scale major food crop exporters, including South Africa, Ethiopia, and Malawi. All these across-scale NPK stoichiometry estimates coupled with different driver estimates (e.g., human activities/agricultural activities/cropping systems, soil texture, soil pH, etc.) provided the data used to assess pairwise mechanistic and explainable model insights using structural equation models [SEM(s)] plus partial dependence plots (PDPs) respectively. Climate-related factors along with topography were the main direct drivers of NPK stoichiometry connected to the topsoil of Africa (i.e., the entire continent including some selected nations).  Human-related activities contributed less to soil NPK stoichiometry. Interestingly, aboveground biomass was discovered to be interdependent with NPK stoichiometry. This cross-scale benchmark alludes to the variations in NPK stoichiometry under both changing climatic conditions and topography in Africa.

Keywords: NPK Stoichiometry, Soil Nutrients, Climate Change, Food Security, Structural Equation Modeling, Topographic Effects, Nitrogen, Phosphorus, Potassium, Africa