Contribution of white grubs (Scarabaeidae: Coleoptera) to N2O emissions from tropical soils

Soil biological processes that produce greenhouse gases, such as N2O, are more intense in tropical soils because of the warm and humid climate; however, the role played by the wide diversity of fauna in these soils in soil N2O production is still poorly understood. This study attempts to assess the role of scarabaeid grubs (Coleoptera), a major faunal group in tropical soils, in emissions of atmospheric N2O. It was hypothesized that (i) the guts of these grubs are important sites of N2O-genesis, since they present environmental conditions (anoxia, high labile C and N mineral contents) that are suitable for N2O production; and (ii) rates of N2O emissions will vary according to the density of gut microbial communities that are involved in N2O emission (i.e. ammonia-oxidizers and denitrifiers). Through laboratory microcosm experiments, in vitro emissions of N2O were determined directly from live grubs of different scarabaeid species (collected from tropical soils of Madagascar and Mexico) and from their surrounding parent soils. Quantitative PCR was used to determine the abundance of the total bacterial community (using the 16S rRNA gene) as well as the ammonia-oxidizing (bacterial AOB and archaeal AOA) and denitrifying (nirK, nirS and nosZ) microbial communities in the grub guts and surrounding soils. The mineral N contents of grub guts and parent soils were also determined using a continuous flow analysis technique. All of the studied grub species emitted significantly higher N2O than the parent soils and presented a high gut ammonium to nitrate ratio (16:1). Their guts harbor a higher density of total bacterial (4.5-fold) and nitrite reductase (nirK) genes (1.5-fold) than the parent soils. However, with the exception of nirK, the relative and absolute abundances of all ammonia-oxidizer and denitrifier genes were higher in soils than in the grub gut environment. The average gene abundance of AOA was 10-fold higher than that of its bacterial counterpart (AOB). Emission of N2O from grubs correlated significantly with the gene abundance of their gut ammonia-oxidizers (AOA and AOB) and denitrifiers (nirS), but not with mineral N contents. Based on average biomass values, these scarabaeid grubs are estimated to contribute between 0.2 and 1.8% of total soil N2O emissions in tropical areas.