Amending a tropical Arenosol: increasing shares of biochar and clay improve the nutrient sorption capacity

Tropical Arenosols may be challenging for agricultural use, particularly in semi-arid regions. The aim of this study was to evaluate the impact of the addition of increasing shares of biochar and clay on the nutrient sorption capacity of a tropical Arenosol. In batch equilibrium experiments, the sorption of ammonium-N (\(\hbox NH_4^+\text-N\)), nitrate-N (\(\text NO_3^-\text-N\)), potassium (\(\text K^+\)), and phosphate-P (\(\text PO_4^3-\text-P\)) was quantified for mixtures of an Arenosol with increasing shares of biochar and clay (1%, 2.5%, 5%, 10%, 100%) and the unmixed Arenosol, biochar, and clay. The mid-temperature biochar was produced from Prosopis juliflora feedstock; the clayey material was taken from the sedimentary parent material of a temporarily dry lake. Only the Arenosol–biochar mixture with 10% biochar addition and the biochar increased the \(\text NH_4^+\text-N\) maximum sorption capacity (\(q_max\)) of the Arenosol, by 34% and 130%, respectively. The \(q_max\) of \(\text PO_4^3-\text-P\) slightly increased with ascending biochar shares (1–10%) by 14%, 30%, 26%, and 42%, whereas the undiluted biochar released \(\text PO_4^3-\text-P\). Biochar addition slightly reduced \(\text NO_3^-\text-N\) release from the Arenosol but strongly induced \(\text K^+\) release. On the other hand, clay addition of 10% and clay itself augmented \(q_max\) of \(\text NH_4^+\text-N\) by 30% and 162%; ascending clay rates (1–100%) increased \(q_max\) for \(\text PO_4^3-\text-P\) by 78%, 130%, 180%, 268%, and 712%. Clay rates above 5% improved \(\text K^+\) sorption; however, no \(q_max\) values could be derived. Sorption of \(\text NO_3^-\text-N\) remained unaffected by clay amendment. Overall, clay addition proved to enhance the nutrient sorption capacity of the Arenosol more effectively than biochar; nonetheless, both materials may be promising amendments to meliorate sandy soils for agricultural use in the semi-arid tropics.