Self-functionalization of soil-aged biochar surfaces increases nitrate retention

Nutrient retention in biochar amended soil has yielded variable results, with poorly understood mechanisms. Identification of changes on biochar surfaces during in situ soil aging can provide mechanistic information on the role of biochar on nutrient retention. In the current greenhouse study, we analyzed changes of biochar surface characteristics from aging in two soils with different iron levels and amended with two types of manure under corn. On pristine biochar surfaces, we detected no iron species. In contrast, after soil aging (70 days), a self-functionalization of biochar surfaces with iron oxides was observed, which can be explained by soil redox cycles allowing reduced iron(II) to migrate on biochar surfaces followed by its re-oxidation. This self-functionalization is proposed as an underlying mechanism explaining the significantly (p < 0.01) increased nitrate retention by 29-180 % in biochar amended soil. Significant (p < 0.05) reductions in leachate phosphate (18-41 %) and dissolved organic carbon (8.8-55 %) were also observed after biochar surface functionalization. Our results indicate that redox-driven iron oxide formation on surfaces of biochar in the soil can be a critical process explaining the dynamic nature of nutrient retention observed in biochar amended soils. Identifying soil environmental conditions most beneficial for such surface functionalization, which has the potential to increase nutrient retention, is critical for implementing efficient biochar amendment strategies and for increased resource efficiency in agroecosystems.

Automated summary

The study investigates the mechanisms underlying nutrient retention in biochar amended soil. It reveals that aging processes in the soil lead to the self-functionalization of biochar surfaces with iron oxides, which explains the increased nitrate retention and decreased leachate phosphate and dissolved organic carbon after biochar surface functionalization. The findings emphasize the importance of identifying optimal soil conditions for this surface functionalization to enhance nutrient retention and resource efficiency in agroecosystems.