Biochar amendment did not influence the growth of two tree plantations on nutrient‐depleted Ultisols in the south Ecuadorian Amazon region

Abstract

The literature suggests that biochar increases the fertility of degraded, nutrient-poor tropical soils. We hypothesized that the addition of biochar (a) increases tree growth in two plantations on Ultisols in the south Ecuadorian Amazon region, (b) reduces litterfall during the dry season because the soil remains moister, and (c) improves the benefit-cost ratio of the plantation. We grew two tree species-the native leguminous Schizolobium parahyba var. amazonicum (Ducke) Barneby and the exotic Gmelina arborea Roxb-and used a full factorial split-split-plot design of all treatments for both tree species at each of two sites. The treatments included the amendment of mineral fertilizer plus lime, 3 and 6 t ha -1 biochar, and a control. The plots were replicated three or four times. Tree height (TH), basal diameter (BD), and diameter at breast height (DBH) were measured several times during 51 mo after planting in September 2009 and litterfall during 12 mo (March 2012-February 2013. The site and the mineral fertilizer plus lime treatment had significant effects on TH, BD, and DBH. The amendment of mineral fertilizer plus lime increased TH, BD, and DBH by 47, 43, and 58%, respectively, relative to the control. The litterfall of G. arborea was on average 84% higher than that of S. parahyba. The amendment of biochar did not significantly influence TH, BD, DBH, or litterfall. The benefit-cost ratio of wood production was >1 in the mineral fertilizer plus lime treatment and controls but <1 in the biochar treatments and decreased with increasing addition of biochar. Our results demonstrate that the assumption that biochar can be used to improve the fertility of degraded Amazon soils cannot be generalized.

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

“…N. Xu et al (2016) attributed this increase in part to reduced N leaching losses because of the improved N retention after amendment of biochar. However, the increase in total N concentrations in topsoil at our two study sites did not result in improved growth in the treatments with only biochar but no mineral fertilizer plus lime (Gonzalez Sarango et al, 2021). Most of the additional N must have been stored in nonbioavailable forms, which is in line with the finding that the Olsen-NH 4 + -N concentration did not respond significantly to the biochar amendment (Supplemental Table S2).…”

“…This short-term increase might have been sufficient to precipitate Fe(OH) 3 , which then is not easily redissolved. Although the biochar amendment did increase the pH values, particularly at La Victoria (Gonzalez Sarango et al, 2021), the effect was less pronounced, and we suspect that the soil solution pH values did not change similarly strongly as in response to liming. As a consequence, our biochar amendment did not have a significant effect on the Fe availability in the topsoil, which is similar to findings of Abedin and Unc (2020) who reported that an amendment of 15 t ha −1 biochar did not significantly change Fe availability in the topsoil.…”

“…Our ANOVA revealed significant effects of the site on total N and Olsen-extractable P, Ca, Mg, and NH 4 OAc-extractable Na concentrations and marginally significant effects (p < .1) on Olsen-K concentrations in the topsoil (Supplemental Table S2) reflecting the fact that the site at Los Zapotes was inherently more fertile than that at La Victoria (Gonzalez Sarango et al, 2021). As expected, the mineral fertilizer plus lime amendment had significant positive effects on Olsen-P, -K, -Ca, and -Mg concentrations but not on total N, Olsen-NH 4 + -N, and NH 4 OAc-Na concentrations in topsoil.…”

“…On the other hand, the significant decrease in Fe availability in the topsoil in response to the amendment of mineral fertilizer plus lime (Figure 3a-c; Supplemental Table S3) was likely attributable to the significant increase of the pH value of the topsoils at La Victoria (Gonzalez Sarango et al, 2021) favoring the precipitation of Fe(OH) 3 . At Los Zapotes, however, there was only a small and nonsignificant increase in soil pH, which was at the start of the experiment already higher than at La Victoria.…”

“…Wang et al, 2016). Biochar can be produced from locally available feedstocks like fermentation residue (Maroušek, 2014), sewage sludge (Yue et al, 2017), and wood chips (Gonzalez Sarango et al, 2021).…”