Greenhouse-aged biochar increases nitrogen removal in riparian soils: Disentangling abiotic and biotic controls

Riparian zones play a crucial role in mitigating agricultural nitrogen (N) pollution. Biochar is considered a promising tool to remove N from agricultural soils; however, few studies have investigated N removal and its underlying mechanisms when riparian soils are amended with biochar. Biochar effectiveness differs depending on whether it is fresh or aged biochar. Previous studies mainly investigated artificial aging methods, such as chemical oxidation and freeze-thaw cycling. Very few examined biochar naturally aged in a greenhouse. In this study, N removal, the water parameters, soil properties, functional groups, and N-cycling microorganisms were determined after differently aged biochar (i.e., fresh (FBC), greenhouse-aged (GBC), and soil-aged biochar (SBC)) had been applied. The total nitrogen (TN) removal efficiency increased in all the biochar treatments, with the highest increment found in the GBC treatment and the lowest in the SBC treatment. This was attributed to increased ammonium (NH 4 + -N) adsorption by the abundant oxygen-containing functional groups in GBC. Additionally, higher nitrite (NO 2 – -N) concentrations and an increased cation exchange capacity (CEC) following GBC application promoted the proliferation of microorganisms involved in ammonia oxidation, assimilatory nitrate reduction, and denitrification, thereby enhancing TN removal efficiency. Conversely, the lower DOC and NO 2 – -N levels, and the reduced CEC in SBC-amended soil constrained the growth of these microorganisms and decreased their contribution towards TN removal efficiency. These results improve understanding about FBC, GBC, and SBC effects on N removal in riparian zones and GBC could potentially be used to reduce the N pollution entering riparian zones.