Abstract
In spite of the numerous studies reporting a decrease in soil nitrous oxide (N2O) 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 N2O 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 N2O 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 aim of improving our understanding of the mechanisms involved in the formation of N2O in these two soils. Evidence suggests that denitrification is the main pathway leading to N2O emissions in soil HP, and ammonia oxidation and nitrifier-denitrification being the major processes generating N2O in soil HC. Biochar systematically stimulated nitrification in soil HC, which was probably the cause of the increased N2O emissions. Here we demonstrate that the effectiveness of using biochar for reducing N2O emissions from a particular soil is linked to its dominant N2O formation pathway.
Highlights
Biochar, a carbonaceous material produced during the pyrolysis of biomass, has been found to decrease N2O emissions from soils (Spokas and Reikosky, 2009; Cayuela et al, 2010; Van Zwieten et al, 2010)
Soil Haplic Phaeozem (HP) was used as a reference soil, since it had been previously used in numerous studies that proved that denitrification was the major process responsible for N2O emissions (Cuhel et al, 2010)
PRE-DOMINANT N2O FORMATION PATHWAYS IN SOIL HP AND Haplic Calcisol (HC) Nitrous oxide emissions patterns and their response to the addition of different N fertilizers were different in soils HP and HC, which clearly reflected the different N2O production pathways involved
Summary
A carbonaceous material produced during the pyrolysis of biomass, has been found to decrease N2O emissions from soils (Spokas and Reikosky, 2009; Cayuela et al, 2010; Van Zwieten et al, 2010). Current knowledge suggests five N2O-genic soil microbial sources (Baggs, 2011; Spott et al, 2011). These are the nitrate or nitrite reducing processes of denitrification and dissimilatory nitrate reduction to ammonium (DNRA), and ammonia oxidation (the first step in nitrification, facilitated by ammonia oxidizing bacteria). No evidence has been reported that would serve to unambiguously define the cause for the observed variations (increase or decline) in soil N2O fluxes This is due to the extremely complex set of reactions leading to N2O formation and consumption in soils and to the fact that the number of studies which analyze how biochar influences specific N2O formation pathways is still very limited
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