Biochar, soil and land-use interactions that reduce nitrate leaching and N₂O emissions: A meta-analysis

Biochar can reduce both nitrous oxide (N₂O) emissions and nitrate (NO₃^-) leaching, but refining biochar's use for estimating these types of losses remains elusive. For example, biochar properties such as ash content and labile organic compounds may induce transient effects that alter N-based losses. Thus, the aim of this meta-analysis was to assess interactions between biochar-induced effects on N₂O emissions and NO₃^- retention, regarding the duration of experiments as well as soil and land use properties. Data were compiled from 88 peer-reviewed publications resulting in 608 observations up to May 2016 and corresponding response ratios were used to perform a random effects meta-analysis, testing biochar's impact on cumulative N₂O emissions, soil NO₃^- concentrations and leaching in temperate, semi-arid, sub-tropical, and tropical climate. The overall N₂O emissions reduction was 38%, but N₂O emission reductions tended to be negligible after one year. Overall, soil NO₃^- concentrations remained unaffected while NO₃^- leaching was reduced by 13% with biochar; greater leaching reductions (>26%) occurred over longer experimental times (i.e. >30 days). Biochar had the strongest N₂O-emission reducing effect in paddy soils (Anthrosols) and sandy soils (Arenosols). The use of biochar reduced both N₂O emissions and NO₃^- leaching in arable farming and horticulture, but it did not affect these losses in grasslands and perennial crops. In conclusion, the time-dependent impact on N₂O emissions and NO₃^- leaching is a crucial factor that needs to be considered in order to develop and test resilient and sustainable biochar-based N loss mitigation strategies. Our results provide a valuable starting point for future biochar-based N loss mitigation studies.