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

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. ... mehrThe 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.