Organic nitrogen increases nitrogen availability, microbial activity but limits carbon priming in soil

Synthetic fertiliser nitrogen (SFN) is immediately available in soil and prone to loss. Combining insect-derived organic N with SFN has potential to regulate mineral nitrogen (N) release in soil by enhancing microbial activity. Nonetheless, addition of organic N and microbial activity may induce positive soil carbon (C) priming and nitrous oxide (N2O) emission. We hypothesised that insect-derived organic N with SFN may increase mineral N release in soil by stimulating microbial activity without positive soil C priming or N2O emission. We also hypothesised that, addition of C with organic N would further enhance mineral N release from the organic N. We incubated a Vertosol for 21 days with ground mealworm (Tenebrio molitor) (MWL) larvae with and without diammonium phosphate (DAP). We also mixed MWL and DAP with sugarcane residues (SCM) to add a C source. Microbial biomass and activities of β-glucosidase and N-acetyl-β-glucosaminidase were significantly higher in MWL, DAP+MWL (1:1 N ratio) and DAP+MWL + SCM (1:10:3.9 N ratio) than control and DAP. Catalytic efficiency of β-glucosidase and net N mineralisation was highest in MWL. Despite high microbial biomass and enzyme activity in DAP+MWL + SCM, it was the MWL and DAP+MWL that produced significantly higher mineral N. While DAP+MWL + SCM significantly increased soil priming, MWL and DAP+MWL did not. The highest N2O emission was in DAP+MWL. These observations demonstrated the potential of organic N to increase mineral N alone or in combination with SFN without destabilising soil organic matter. Such novel hybrid fertiliser formulations may provide benefit to soil health, crop and environment.