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

Highlights

•	Biochar addition slightly increased NH4+-N and PO43−-P sorption, reduced NO3−-N release but triggered K+ release.
•	Clay addition enhanced sorption of NH4+-N, K+, and PO43−-P better than biochar but had no effect on NO3−-N.
•	Both substrates can contribute to fertility of Arenosols by increasing their nutrient content and sorption capacity.

Abstract

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.

Funding

Bundesministerium für Bildung und Forschung(01 LL 0904 A-E)