Positive effects of composted biochar on plant growth and soil fertility

Mankind is actually facing serious issues due to the overexploitation of fossil fuels, biomass, soils, nitrogen, and phosphorus. It is claimed that biochar addition to soil improves C sequestration to prevent CO2 from atmospheric cycling. Biochar addition should also increase soil fertility in a similar way as anthropogenic dark earths of Central Amazonia. Previous studies have shown that biochar stimulates plant growth and increase fertilizer efficiency, especially when biochar is combined with organic fertilizers such as compost. However, little is known about optimum addition amounts and mixture ratios of biochar and compost. Indeed most experiments to mimic Terra preta de Indio focused on biochar alone or biochar in combination with mineral fertilizers. Therefore, we studied optimum biochar and compost amounts and mixture ratios with respect to plant response and soil fertility. We tested the effect of total amount from 0 to 200 Mg/ha, and biochar proportion from 0 % to 50 % biochar, of 18 different compost mixtures on growth of oat (Avena sativa L.) and soil properties in a fully randomized greenhouse study with sandy and loamy soil substrates. We sampled soil substrates before and after plant growth and analyzed plant growth and yield, total organic carbon (TOC), total nitrogen (TN), mineralized nitrogen (Nmin), soil reaction (pH), and electrical conductivity (EC) applying standard procedures. Results show that biomass production was increased with rising biochar and compost amounts. Oat plant height and seed weight was improved only with rising biochar amounts, but not with compost amounts. This could be explained by increase of total organic C and total N but not by plant-available ammonium and nitrate. The positive influence of composted biochar on plant growth and soil properties suggests that composting is a good way to overcome biochar’s inherent nutrient deficiency, making it a suitable technique helping to refine farm-scale nutrient cycles.