Assessing Nitrogen Release from Organic Fertilizers with Biochar Addition in a Laboratory Incubation Trial

Abstract In temperate agricultural systems, biochar is commonly added to organic fertilizers before field application. The impact of biochar on nitrogen (N) release from organic fertilizers (based on equivalent N input) remains largely unexplored. To assess net N release from different organic fertilizers (with biochar addition), 104-day laboratory incubations were conducted. Net N release was measured from cattle slurry, dairy manure compost, and grass-clover compost. Each fertilizer was co-fermented or co-composted with biochar at rates of 0, 500, 1,000, or 2,000 kg ha − 1 , derived from two manufacturing processes. All amendments were applied at an N rate of 1,200 kg ha − 1 . In addition, the effect of pristine biochar (0, 500, 1,000, 2,000, and 20,000 kg ha − 1 ) on N release from the experimental soil was investigated. Net N release was highest from cattle slurry (34.4%), followed by grass-clover compost (17.1%) and manure compost (7.4%). Biochar addition reduced net N release from all fertilizers, e.g., for cattle slurry, to 30.2% at 500 kg ha − 1 , 28.6% at 1,000 kg ha − 1 , and 23.5% at 2,000 kg ha − 1 biochar C. Pristine biochar had negligible effects on soil N release at low rates but slightly reduced soil N release at a high rate. These results highlight the role of on-farm biochar pretreatment on the N fertilizer value of organic amendments. To maintain crop yields, fertilizer requirement calculations should account for potential N limitations associated with biochar addition, and N fertilizer rates may need to be tailored.