Effects of N Application Rate and Dicyandiamide on the Fate of 15 N Fertilizer and the Abundance of Microbial Genes in a Sandy Soil Amended with Sugarcane Litter

Renewable energy produced from sugarcane ( Saccharum spp.) has clear advantages over fossil fuels in mitigating climate change. However, N 2 O emissions and NO 3 − leaching from fertilizer nitrogen (N) addition can jeopardize the sustainability of sugarcane production worldwide. One of the possible approaches to mitigating these N-loss pathways is the use of nitrification inhibitors such as dicyandiamide (DCD). However, the effects of DCD on the fate of fertilizer-derived N in soil pools and functional N genes (e.g., nitrifiers and denitrifiers) remain unclear. We performed a laboratory incubation study for 54 days to evaluate the influence of DCD and fertilizer N rate (( 15 NH 4 ) 2 SO 4 applied at 40, 80, 120, and 160 µg N g −1 ) on 15 N transformation and abundance of total prokaryotes (16S rRNA), nitrifiers (ammonia-oxidizing archaea (AOA) and bacteria (AOB)), and denitrifiers ( nirK and nosZ ) in a tropical sandy soil amended with sugarcane residues. Especially for high N rates (120 and 160 µg N g −1 ), DCD not only increased the NH 4 + - 15 N recovery by inhibiting nitrification but also stimulated microbial immobilization. However, the nitrification inhibitor decreased N losses (i.e., unrecovered 15 N). The abundance of total archaea and bacteria was unaffected by both the fertilizer N rate and DCD. While the abundance of AOB and nirK was suppressed by DCD, AOA and nosZ abundance increased, with the latter gene being the highest at 160 µg g −1 . We conclude that DCD mitigates N losses from ammonium-based fertilizer by inhibiting NO 3 − production and changing nitrifying and denitrifying microorganisms in sandy acidic soil.