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

Abstract

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.