Biochar increases soil N 2 O emissions produced by nitrification-mediated pathways

Abstract

In spite of the numerous studies reporting a decrease in soil nitrous oxide (N 2 O) emissions after biochar amendment, there is still a lack of understanding of the processes involved. Hence the subject remains controversial, with a number of studies showing no changes or even an increase in N 2 O emissions after biochar soil application. Unraveling the exact causes of these changes, and in which circumstances biochar decreases or increases emissions, is vital to developing and applying successful mitigation strategies. With this objective, we studied two soils [Haplic Phaeozem (HP) and Haplic Calcisol (HC)], which showed opposed responses to biochar amendment. Under the same experimental conditions, the addition of biochar to soil HP decreased N 2 O emissions by 76%; whereas it increased emissions by 54% in soil HC. We combined microcosm experiments adding different nitrogen fertilizers, stable isotope techniques and the use of a nitrification inhibitor (dicyciandiamide) with the aimof improving our understanding of themechanisms involved in the formation of N 2 O in these two soils. Evidence suggests that denitrification is the main pathway leading to N 2 O emissions in soil HP, and ammonia oxidation and nitrifier-denitrification being the major processes generating N 2 O in soil HC. Biochar systematically stimulated nitrification in soil HC, which was probably the cause of the increased N 2 O emissions. Here we demonstrate that the effectiveness of using biochar for reducing N 2 O emissions from a particular soil is linked to its dominant N 2 O formation.