Reducing nitrous oxide emissions from sugarcane soil with legume intercropping

Australian sugarcane cropping has low nitrogen (N) use efficiencies, largely due to a mismatch of earlyseason N fertiliser application and later season peak crop N demand, in combination with poor soils and wet climate. To address the problem of N losses via run-off, leaching and N2O emissions, the sugarcane industry is evaluating several avenues. One approach is to improve N use efficiency (NUE) by reducing the use of vulnerable-to-loss N fertiliser, supplementing crop needs with biologically fixed N via sugarcane-legume intercropping. In an optimised system, decomposing legumes would deliver N to sugarcane, synchronised with sugarcane’s long N accumulation phase. We hypothesised that legume intercropping in combination with lower N fertiliser rates will reduce N losses (N2O emissions were quantified here) but not sugar yields. Here we report on one of several field trials with sugarcane grown as monoculture or intercropped with legumes at full N fertiliser or lowered rates (67 or 41% of full N). In the second year of implementation and compared to full N fertiliser, N2O emissions were reduced by 50 to 70% in the 67% N treatments irrespective of legume presence. Highest sugarcane biomass was achieved with full-N rate, 67% N, and 67% N + soybean intercropping. Sugarcane production was reduced in 67% N + mung bean intercropping, 41% N and zero N treatments. Sugar yield was variable but statistically similar across all treatments. These early results indicate that evaluation across different growing regions, fertiliser rates and planting times are needed to optimise sugarcane-legume intercropping systems.