Interactive nutrient management: compost and nitrogen effects on subtropical soil emissions

Overproduction of greenhouse gases (GHGs) by agricultural soils is a critical environmental issue, especially in subtropical farming systems, where mixed organic and inorganic nitrogen (N) management is commonly used to maintain soil fertility. A 3×3 factorial design was arranged using nine treatments in a controlled laboratory incubation experiment, using three compost rates (C₀-0, C₁-30, and C₂- 60 T ha⁻¹) and three N fertilizer rates (N₀-0, N₁-60, and N₂-120 kg N ha⁻¹). This study aimed to determine the short-term impacts of different rates of inorganic N fertilizers (N₀: 0, N₁: recommended, N₂: double recommended) and compost application levels (C₀: no compost, C₁: low compost, C₂: high compost) on N₂O and CO₂ emissions on a Vertisol soil collected at Gatton, Queensland, Australia, under controlled incubation conditions. The cumulative N₂O and CO₂ emissions during the incubation period were also measured and given as kg N ha^-1 and Mg C ha^-1, respectively. Findings indicated that the application of inorganic N fertilizer alone did not have any significant influence on the emissions of N²O (N₀C₀: 17.13, N₁C₀: 54.77, N₂C₀: 88.75 kg N ha^-1; p > 0.05), but the co-application of compost and inorganic N had a considerable effect on the emissions of N₂O, with the greatest reduction being observed in the N₂C₁ treatment (1031.96 ± 124 kg N ha⁻¹). The addition of compost also increased CO₂ emissions across all N levels, with the cumulative CO₂ efflux maximized at the N₁C₂ level (69.5 ± 13.4 Mg C ha^-1) due to increased microbial heterotrophic respiration and priming effects. It is interesting to note that high rates of inorganic N application, combined with moderate levels of compost application (N₂C₁), inhibited CO₂ emissions, indicating that nitrogen suppressed microbial decomposition activity. The results of this study suggest that the best strategy to reduce short-term GHG emissions while maintaining soil health and mineralisation potential is to co-apply 60 t ha⁻¹ compost and 120 kg N ha⁻¹ fertilizer (N₂C₂). These results indicate that the combined use of compost and inorganic N fertilizers significantly increased GHG emissions in Vertisol soils, highlighting the importance of optimizing organic-inorganic N input ratios to reduce environmental trade-offs whilst preserving the agronomic productivity of subtropical crop systems. The long-term impacts and field-scale validation of these results should be investigated in future research.