To evaluate the effect of green waste compost on nitrous oxide emissions from horticulture

Abstract

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 (N 2O). 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 N 2O emissions via nitrification. Additionally, the increased oxygen demand of the microbial biomass, from GWC addition, could increase N 2O 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.