Biochar application can mitigate NH3 volatilization in acidic forest and upland soils but stimulates gaseous N losses in flooded acidic paddy soil

Biochar (BC) has attracted attention for carbon sequestration, a strategy to mitigate climate change and alleviate soil acidification. Most meta-analyses have insufficiently elaborated the effects of BC on soil N transformation so the prac-tical importance of BC could not be assessed. In this study, a 15N tracing study was conducted to investigate the effects of BC amendment on soil gross N transformations in acidic soils with different land-use types. The results show that the BC amendment accelerated the soil gross mineralization rate of labile organic N to NH4+ (MNlab) (3 %-128 %) which was associated with an increase in total nitrogen. BC mitigated NH3 volatilization (VNH3) (52 %-99 %) in upland and forest soils due to NH4+/NH3 adsorption, while it caused higher gaseous N losses (NH3 and N2O) in flooded paddy soils. An important function was the effect of BC addition on NH4+ oxidation (ONH4). While ONH4 increased (4 %-19 %) in upland soils, it was inhibited (34 %-71 %) in paddy soils and did not show a response in forest soils. Overall, the BC amendment reduced the potential risk of N loss (PRL), especially in forest soils (82 %-98 %). This study also shows that the BC effect on soil N cycling is land-use specific. The suitability of practices including BC hinges on the effects on gaseous N losses.