The literature suggests that biochar increases the fertility of degraded, nutrient-poor tropical soils, and helps to close nutrient cycles.
In this study, we hypothesized that the addition of biochar (a) increases tree growth, (b) reduces litterfall, (c) improves the benefit-cost ratio, (d) increases total N, bioavailable nutrients (NH4-N, P, K, Ca, Mg, Fe, Mn, Zn, Cu) in soil, (e) increases nutrient retention, (f) nutrient fluxes, and (g) reduces N losses reflected by lower δ15N values of litterfall and soils.
We grew the native leguminous Schizolobium sp. and the exotic Gmelina sp. in a complete factorial split-split-plot design at each of the two sites. The treatments included the amendment of mineral fertilizer, 3 and 6 tha–1 biochar, and a control. Tree height (TH), basal diameter (BD), and diameter at breast height (DBH) were measured during 51 mo, litterfall during 12 mo (March 2012–February 2013), and soil was sampled from the 0-to-0.25 and 0.25-to-0.50 m depth layers.
Site and the mineral fertilizer treatment had significant effects on TH, BD, DBH, and most studied soil properties. Mineral fertilizer furthermore increased the modified Olsen-P, K, Ca, Mg, and Zn concentrations in the topsoil, and the mean annual P, Ca, Zn concentrations, δ 15N values, and the fluxes of all nutrients but reduced olsen- Fe concentrations and δ 15N values in soil, and Mn concentrations in litterfall. The amendment of biochar increased the modified Olsen-Ca and Zn and total N concentrations, Ca and Zn retention in mineral fertilized topsoils, and decreased total N retention in unfertilized topsoils. Biochar decreased the mean N concentrations and K fluxes with litterfall during the dry season and increased mean Ca and Zn concentrations and fluxes with litterfall.
The results demonstrate that the assumption that biochar can be used to improve the fertility of degraded Amazon soils cannot be generalized.