Does biochar improve nutrient availability in Ultisols of tree plantations in the Ecuadorian Amazonia?

The application of biochar to strongly weathered soils is thought to supply nutrients and improve nutrient retention. We hypothesized that biochar increases (a) total N, bioavailable macronutrient (NH$_{4}$–N, P, K, Ca, Mg), micronutrient (Fe, Mn, Zn, Cu), and plant-beneficial Na concentrations; and (b) nutrient retention in the topsoil. We grew the native leguminous Brazilian firetree [Schizolobium parahyba var. amazonicum (Ducke) Barneby] and the exotic beechwood (Gmelina arborea Roxb.) in a full factorial split-split-plot design at La Victoria and Los Zapotes, Ecuadorian Amazonia. The treatments included amendment of mineral fertilizer plus lime, 3 and 6 t ha−1 biochar (locally produced charcoal), and a control. We sampled the 0-to-0.25- and 0.25-to-0.50-m soil depth layers before the start of the experiment in 2009 and six times until 2013. The site at Los Zapotes was more fertile as reflected by a significant site effect on most studied soil properties in both depth layers. Biochar increased modified Olsen (NaHCO3+EDTA)-extractable Ca (p < .05) and Zn concentrations (p < .1) and total N concentrations (p < .05) in topsoil. Mineral fertilizer plus lime increased Olsen-extractable P, K, Ca, Mg, and Zn concentrations (all p < .05) but reduced Olsen-extractable Fe concentrations (p < .05) in topsoil. Biochar increased Ca (p < 0.1) and Zn (p < .05) retention in mineral fertilized topsoils but decreased total N retention (p < .05) in unfertilized topsoils. The amendment of up to 6 t ha$^{-1}$ biochar did not increase the fertility of the studied degraded Amazonian Ultisols sufficiently to enhance tree growth