Biochar-induced N2O emission reductions after field incorporation in a loam soil

Biochar addition to soils is heralded to reduce N2O emissions, but still, the explanatory mechanisms have not been resolved. Moreover, it is uncertain whether N2O emission reductions would persist after prolonged biochar incorporation in the field. In this study, we incorporated four biochar types in a loam textured cropland field and intact soil cores were sampled to investigate the physical control of biochar on denitrification after 7 months. During a first incubation experiment, we measured N2O emissions from undisturbed and disturbed (i.e. sieved (2 mm) and grounded) soil cores. Both in the disturbed and undisturbed soil cores biochar at water filled pore space (WFPS) of 80% reduced the N2O emissions by 50-90%, refuting the hypothesis that biochar exerts an indirect physical control over soil denitrification several months after incorporation. Secondly, we hypothesized that biochar creates denitrification 'hotspots' in soil, where complete reduction of N2O to N2 is promoted compared to non-amended soil. In these hotspots biochar particles could act as microlocations with local anaerobic conditions and local higher pH, stimulating in this way complete denitrification. Via the acetylene inhibition method we did not observe a reduction in the N2O/(N2O + N2) ratio, which could suggest that biochar did not promote the reduction of N2O to N2. Manipulations likely to promote labile C bioavailability, here either by glucose addition or by soil particulate OM disclosure after disruption of soil aggregates, resulted in the most prominent biochar-induced N2O emission reductions.