Determination of N2O reduction to N2 from manure-amended soil based on isotopocule mapping and acetylene inhibition

The quantification of N2O reduction during denitrification is crucial to take measures reducing N2O emissions from arable soils. The analysis of site specific N2O isotopic composition may help to quantify the pathway of N2O reduction to N2 during denitrification process. In manure-amended soil (chicken manure, cow manure and sheep manure), N2O reduction was evaluated by examining the N2O emissions in the presence of acetylene (C2H2) and by analyzing the N2O isotopocule deltas. The results indicated that the three manures had no significant differences for the N2O emission rates and N2O isotopocule deltas. The extent of N2O reduction to N2 was 81–84% using the mapping of Δδ18O(N2O-H2O/N2O-NO3-) (different value of δ18O between the N2O and H2O or NO3−) vs. SP (15N site preference in N2O) and approximately 76% using C2H2 (10% v/v) inhibition method under 60% WFPS (water–filled pore space). Moreover, complete N2O reduction held η18OR and SPR (net isotopocule effect of N2O reduction to N2) values of −13.6‰ and −6.7‰, respectively. It was also found that oxygen exchange between N2O and soil water was approximately 92%. In summary, the mapping of Δδ18O(N2O-H2O/N2O-NO3-) vs. SP was very useful to distinguish the production and consumption pathways of N2O during denitrification. The extent of N2O reduction was underestimated by the C2H2 method. These results provide guidance in correcting the acetylene effect, which is important in evaluating the impact of manure on the N2O reduction.