Amendment with biodiesel co-product modifies genes for N cycling ( nirK , nirS , nosZ ) and greenhouse gas emissions (N 2 O, CH 4 , CO 2 ) from an acid soil

When applied to soils, ‘biodiesel co-product’ (BCP) functions as a labile source of organic C for microorganisms and promotes immobilisation of inorganic N (otherwise susceptible to leaching). However, the effects on biological N transformations and greenhouse gas (GHG) emissions, have not yet been investigated. Therefore, we investigated these effects with BCP amendment of soils at water contents ranging from 40 to 100% water holding capacity (WHC). The BCP applications significantly increased soil microbial biomass C, N, and ATP contents at all soil moistures, but maximally at 60% WHC. Amendment with BCP increased the abundance of nosZ genes coding for N 2 O reductase, and temporarily increased emissions of N 2 O up to day 3, but from day 7 onwards, emissions of N 2 O at all water contents almost stopped. Consequently, BCP suppressed total N 2 O emissions from waterlogged soil (100% WHC) by about 40% at 56 days, while the greatest emissions of N 2 O were still observed at 80% WHC. BCP significantly increased the abundance of ammonia oxidising archaea (AOA) and ammonia oxidising bacteria (AOB) at lower moisture contents (40% and 60% WHC) at day 7. In summary, our results suggest that BCP disproportionately increased the genetic capacity for the final reactions of denitrification by nosZ compared with those genes more implicated in early reactions of denitrification ( nirS and nirK ).