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

Beusch, Christine; Melzer, Dennis; Cierjacks, Arne; Kaupenjohann, Martin

FG Bodenkunde

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 (NH+4-N), nitrate-N (NO−3-N), potassium (K+), and phosphate-P (PO3−4-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 NH+4-N maximum sorption capacity (qmax) of the Arenosol, by 34% and 130%, respectively. The qmax of PO3−4-P slightly increased with ascending biochar shares (1–10%) by 14%, 30%, 26%, and 42%, whereas the undiluted biochar released PO3−4-P. Biochar addition slightly reduced NO−3-N release from the Arenosol but strongly induced K+ release. On the other hand, clay addition of 10% and clay itself augmented qmax of NH+4-N by 30% and 162%; ascending clay rates (1–100%) increased qmax for PO3−4-P by 78%, 130%, 180%, 268%, and 712%. Clay rates above 5% improved K+ sorption; however, no qmax values could be derived. Sorption of NO−3-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.