Soil carbon accrual and yield response to biochar and compost in a four-year organic field study

Agricultural practices increasingly apply biochar through co-composting with organic fertilizers rather than via direct application. However, the agronomic assessment of these compost-biochar blends in organic farming, especially under repeated applications based on compost nitrogen (N) input, has not been comprehensively investigated. In a four-year organically managed field trial in southern Germany, the effects of annual biochar carbon (C) applications (0, 500, 1,000, 2,000 kg ha −1 ) on crop yield, as well as changes in soil organic carbon (SOC) and total nitrogen (TN) stocks, were assessed. Biochar was applied either without additional fertilizers or co-composted with solid manure and grass-clover biomass (120 kg N ha −1 ). Crops studied included spelt, grass-clover ley, winter wheat, and silage maize. Manure and grass-clover compost significantly increased spelt yields (e.g., control treatment: 3.21 t ha −1 ; manure compost: 4.20 t ha −1 ), winter wheat and silage maize yields. Co-composted biochar slightly reduced spelt yields in the first trial year (e.g., with manure compost, 4.20, 4.00, 3.98, 3.88 t ha −1 at biochar C levels of 0, 500, 1,000, and 2,000 kg ha −1 ). No biochar effects were observed for grass-clover ley yield, but co-composted biochar marginally decreased winter wheat yields. In the fourth year of the trial, slight increase in maize yield was observed as a result of co-composting. Both compost and biochar applications led to increases in SOC stocks, with the applied biochar amounts matching the observed increases in SOC levels. The combination of biochar and compost resulted in SOC increases of up to + 10 t ha −1 after three trial years. These findings indicate potential long-term yield benefits resulting from enhanced soil properties.