Developing sugarcane - legume companion cropping systems to minimise nitrous oxide emissions

Abstract

Global efforts are underway to reduce greenhouse gas emissions (GHG) from anthropogenic activities. Nitrous oxide (N2O) emissions accounted for 13% of Australia’s National GHG inventory over the period 2016-2017 (NGGI, 2017) with most N2O derived from agricultural soils. Sugarcane soils are high emitters of N2O, and this thesis explores whether legumes, grown as a companion crop with biological N2 fixation (BNF) capacity, can partially replace N fertiliser to lower the emissions of N2O from sugarcane soil.Chapter 2 synthesises published literature on sugarcane intercropping. Most research has focussed on the productivity of sugarcane with intercrops, including legumes. Intercropping can benefit sugarcane yield, have neutral or negative effects. This practice is common in subsistence agriculture, and farm income benefits, but environmental benefits intercropping have not been a research focus.Chapters 3 and 4 explore sugarcane-legume intercropping at three commercial farms in Australia. N2O emissions, soil and crop variables were quantified with different N fertiliser applications and in the presence or absence of legumes. The farms, two Rain-fed, one Irrigated, were located in the dry and wet tropics, and in the subtropics, representing different climate and agronomic settings. Industry-recommended (full) N fertiliser rates were compared with up to 50% reduced N fertiliser rates in the presence or absence of legume and benchmarked against a zero N fertiliser control. We hypothesised that reduced application of N fertiliser limits sugarcane growth and that legumes can alleviate N limitation. However, full and reduced N fertiliser treatments mostly generated similar sugarcane yields, confirming that industry-recommended full N fertiliser rates exceed sugarcane needs. In line with this notion, the reduced N+legume treatments did not improve sugarcane growth. N2O emissions in reduced N+legume were either similar to reduced N fertiliser sugarcane monoculture or higher and similar to the emissions observed with full N fertiliser rates. Soybean strongly benefitted sugarcane yield under N limiting conditions (zero N fertiliser) at one farm, increasing soluble soil N levels and nearly doubling sugarcane yield compared to zero-N sugarcane monoculture, and generating 6-times lower N2O emissions than the full N rate. At the Rain-fed sites, soil nitrate levels explained 81 and 64% of N2O emissions; at the Irrigated site, the interaction of soil nitrate and soil moisture explained 63% of N2O emissions. High N2O emissions factors at the subtropical site were associated with wet, low drainage soil (>70% water filled pore space over summer), conditions that promote denitrification. High N fertiliser rates in Irrigated, well-draining soils had lower N2O emissions, possibly shifting N losses from gaseous to leaching. The promising findings observed with soybean under N limitation require further investigation to explore N2O mitigation options with a view of optimising legume facilitation and the ‘tipping point’ for N fertiliser applications.Chapter 5 presents a glasshouse experiment investigating the effect of N fertiliser rate on competition vs facilitation up to peak N accumulation of soybean. No or low N fertiliser rates enhanced soybean BNF but reduced growth of sugarcane. With moderate to high N fertiliser rates, soybean BNF and growth diminished, while sugarcane growth increased which indicatives increased competitive ability. We conclude that N fertiliser rates substantially impact upon the relative performance of sugarcane and legume intercrop, with a trade-off between N fertiliser application and legume BNF. Under the experimental condition, direct N transfer from soybean to sugarcane was negligible. Rather, decomposition and mineralisation of legume biomass are likely to be the main pathway for increased N accumulation in intercropped sugarcane observed in some instances.Chapter 6 synthesises the findings from literature, field and glasshouse experimentation and discusses future directions for sugarcane-legume intercropping research.