Comparing modified substrate-induced respiration with selective inhibition (SIRIN) and N2O isotope approaches to estimate fungal contribution to denitrification in three arable soils under anoxic conditions
The coexistence of many N2O production pathways in soil hampers differentiation of microbial pathways. The question of whether fungi are significant contributors to soil emissions of the greenhouse gas nitrous oxide (N2O) from denitrification has not yet been resolved. Here, three approaches to independently investigate the fungal fraction contributing to N2O from denitrification were used simultaneously for, as far as we know, the first time (modified substrate-induced respiration with selective inhibition (SIRIN) approach and two isotopic approaches, i.e. endmember mixing approach (IEM) using the 15N site preference of N2O produced (SPN2O) and the SP/~d18O mapping approach (SP/~d18O Map)). This enabled a comparison of methods and a quantification of the importance of fungal denitrification in soil. Three soils were incubated in four treatments of the SIRIN approach under anaerobic conditions to promote denitrification. While one treatment without microbial inhibition served as a control, the other three treatments were amended with inhibitors to selectively inhibit bacterial, fungal, or bacterial and fungal growth. These treatments were performed in three variants. In one variant, the 15N tracer technique was used to estimate the effect of N2O reduction on the N2O produced, while two other variants were performed under natural isotopic conditions with and without acetylene. All three approaches revealed a small contribution of fungal denitrification to N2O fluxes (fFD) under anaerobic conditions in the soils tested. Quantifying the fungal fraction with modified SIRIN was not successful due to large amounts of uninhibited N2O production. In only one soil could fFD be estimated using modified SIRIN, and this resulted in 28~c9 %, which was possibly an overestimation, since results obtained by IEM and SP/~d18O Map for this soil resulted in fFD of below 15% and 20 %, respectively. As a consequence of the unsuccessful SIRIN approach, estimation of fungal SPN2O values was impossible. While all successful methods consistently suggested a small or missing fungal contribution, further studies with stimulated fungal N2O fluxes by adding fungal C substrates and an improved modified SIRIN approach, including alternative inhibitors, are needed to better cross-validate the methods.