Nutrient retention and release from raw exhausted grape marc biochars and an amended agricultural soil: Static and dynamic investigation

Biochar is the solid by-product of biomass thermochemical conversion via pyrolysis technique. Biochar addition to croplands as an organic amendment can improve soil properties and increase agricultural productivity. However, these positive effects depend largely on biomass feedstock and pyrolysis conditions. In this study, nutrient release from biochars derived from the slow pyrolysis of exhausted grape marc (EGM) at 300, 400 and 500 °C (EGM300, EGM400 and EGM500) was investigated through five successive leaching assays in batch mode for a total duration of 10 days. Then, nutrient leaching/retention kinetics of an agricultural soil amended with EGM500 (1% and 5% w/w) was assessed under dynamic conditions in columns. The batch experiments showed that with the exception of P, the nutrient release efficiency from the three biochars significantly increased with the increase of the number of leaching trials. The highest released amounts were observed at the fifth leaching cycle for K, Ca, P and Mg, which were about 45.5%, 41.5%, 229.5% and 48.9% higher than those registered during the first leaching assay. Regarding the column release experiments, a biochar content of 5% in the agricultural soil resulted in an increase of water leached NO₃ ⁻ and K⁺ by about 181.4% and 521.3%, respectively, and a significant reduction in Na⁺ and Ca^2＋ transport as compared to unamended soil. In a second phase, outcomes of column feeding with a nutrient solution showed that PO₄ ³⁻ and NO₃ ⁻ retention by biochar-amended soils is low. Thus, the use of EGM biochar as a slow release biofertilizer could be considered as a promising agricultural practice and a sustainable solution for biowaste management.