Nitrous oxide emissions from soil in mango and banana fields: effects of nitrogen rate, fertiliser type, and ground cover practices

Abstract

Globally, agricultural soils are the dominant source of the greenhouse gas, nitrous oxide (N₂O), and a growing body of evidence indicates that soils in tropical zones may emit disproportionately large amounts to the atmosphere. This is important as N₂O contributes approximately 6% of anthropogenically induced global warming and is also responsible for ozone depletion. These emissions primarily originate from microbial nitrification and denitrification processes in soil, which are driven by soil water content, temperature, available nitrogen (N), organic carbon (OC) and their interactions. This study investigated the effects of N fertiliser application rate and type, and ground cover, on N₂O emissions from soil in mango and banana fields in tropical northern Australia. The fertiliser types were conventional urea and two enhanced efficiency fertilisers (EEFs): urea treated with a nitrification inhibitor (3, 4-dimethylpyrazole phosphate, DMPP) and polymer sulphur-coated (PC) urea mixed with standard urea at a 40/60 ratio (in mangoes only). Ground cover treatments were bare ground versus vegetative ground cover in bananas, and bare ground versus hay mulch in mangoes. A manual chamber technique was used to measure gas emissions from three field experiments with factorial designs (randomised block, four replications of each treatment). The experiments were conducted in 1) a commercial mango orchard on a Yellow Chromosol soil at Mutchilba, 2) a commercial banana farm on a Red Ferrosol soil at East Palmerston, and 3) a banana research farm on a Brown Dermosol soil at South Johnstone. In banana fields (Chapter 3), soil mineral N content, water content, and time since fertiliser application were the primary drivers of N₂O emissions. Low N rate treatments (12 kg N ha⁻¹ mth⁻¹) had consistently lower N₂O emissions than high N rates (18 to 54 kg N ha⁻¹ mth⁻¹), however overall N₂O flux was highest in all treatments when fertiliser was applied during persistently wet conditions (>68% water-filled pore space, WFPS). Urea treated with DMPP had approximately half the N₂O emissions than untreated urea on the Brown Dermosol, but did not significantly reduce emissions on the Red Ferrosol. Vegetative ground cover reduced N₂O emissions compared to bare soil during wet conditions and with higher N rates, presumably due to N uptake by the ground cover decreasing soil mineral N concentrations. In the mango orchard (Chapter 2), N₂O emissions were lower than under bananas at the other sites. The mango site soil had less mineral N, lower water holding capacity and lower OC content. N₂O emissions were not lowered by using EEFs rather than urea at application rates <25 kg N ha⁻¹. However, at a higher fertiliser application rate of 42 kg N ha⁻¹, DMPP approximately halved N₂O emissions. Mulching also lowered N₂O emissions, however sufficient irrigation after fertiliser application to mulch is recommended to reduce potential ammonia volatilisation. Overall, the management factors examined influenced soil mineral N, water content, temperature and possibly OC, all of which played important roles in determining total N₂O emissions in both crops. In banana fields, using lower N rates and DMPP treated urea during wet conditions will reduce N₂O losses. However, vegetative ground covers do not appear to be a reliable or consistent method of N₂O mitigation, as any reduction may be offset by the potential additional N required to compensate for plant competition and to avoid yield decline. In mangoes the most benefit would be gained from mulching, due to the reduction in N₂O and an increase in yield. However, further research is required to substantiate the N₂O reduction of hay mulch over the longer term. There appeared to be little justification for N₂O mitigation measures with EEFs in mangoes, due to generally negotiable N₂O emissions in the Yellow Chromosol, and the additional cost of EEFs. On the whole, more research is required around the mechanisms reducing the efficacy of DMPP-treated urea in Red Ferrosols and during hot conditions (35– 45°C). Finally, the PC urea product in this study needs to be tested in more suitable conditions that favour denitrification (higher N rate and soil water content) in order to more appropriately assess its impact on N₂O production.