TO EVALUATE THE EFFECT OF GREEN WASTE COMPOST ON NITROUS OXIDE EMISSIONS FROM HORTICULTURE

A field study was conducted over 9 months to measure the short-term effect of green waste compost (GWC) application on crop yields, nitrogen (N) availability and emissions of the greenhouse gas, nitrous oxide (N₂O). The GWC was applied at 0, 2 or 4 t/C ha as GWC with either 0 or 150 kg N/ha fertiliser. The hypothesis was that GWC would lower the soils mineral N content, via microbial immobilisation, resulting in a reduced substrate for microbially mediated nitrification and denitrification reactions and, therefore, reduced N₂O. However, high N fertiliser rates, the potential re-mineralisation of N from the microbial biomass, and N released from the decomposition of GWC, could provide N substrate for increased N₂O emissions via nitrification. Additionally, the increased oxygen demand of the microbial biomass, from GWC addition, could increase N₂O emissions via denitrification. The trial commenced one month after a postharvest GWC application, at lettuce (Latuca sativa ‘Pattagonia’) transplant and was terminated after the following sorghum (Sorghum bicolour (L.) Moench) crop. The application of GWC did not cause prolonged N immobilisation nor did it affect crop yields. Application of GWC did not significantly affect N₂O flux. The application of N fertiliser (as 3 applications of 50 kg N/ha) significantly increased N₂O and CO₂ fluxes (P<0.05). Cumulative N₂O emissions from the low N environment (0 kg N/ha) were 150 mg/m² compared to 400 mg/m² in the high N environment (150 kg N/ha). The cumulative N₂O emissions from unamended soil which received 150 kg N/ha was 1.7% of applied N, which is lower than the Australian Government’s default emission factor for horticulture of 2.1% of applied N. This demonstrates that intensive horticulture systems which implement good water and fertiliser use practices may emit lower amounts of N₂O than previously estimated. Also addition of GWC to horticulture is likely to be safe to crops and to not increase N₂O emissions.