Legume-oilseed intercropping in mechanised broadacre agriculture in a Mediterranean climate

Intercropping is a system of farming whereby multiple crop species are grown together for a significant period of time. There are many types of intercropping, differing in layout and species combination, as well as the ecosystem services they provide. In large-scale mechanised systems, intercropping has traditionally utilised cereal-legume pairings with the aim of overyielding (increased combined intercrop yield per unit area compared with the respective monocrops). However, as the price of synthetic inputs such as fertilisers and fungicides rises globally, interest in legume-oilseed intercropping as a low input system is increasing. Legume-oilseed intercropping entails growing a legume and an oilseed species together, often with the goal of harvesting the grain of both crops. Legume-oilseed intercropping provides the same benefits as cereal-legume intercrops whilst mitigating many of the logistical challenges that have previously stymied widespread adoption of the system in broadacre farming. Although oilseed-legume intercropping is becoming increasingly popular in broadacre agriculture, its adoption has not been uniform. The majority of research is conducted in northern Europe, Canada and China, with data from Mediterranean-type climates lacking. As such, this thesis investigated the potential of legume-oilseed intercropping as a low input system in a Mediterranean climate, with a focus on yield and nutrient uptake efficiency and economic productivity, relative to their respective sole crops. The role of arbuscular mycorrhizal fungi (AMF) in facilitating improved intercrop phosphorus nutrition was also investigated. It was hypothesised that the legume-oilseed intercrops would be more productive, in both a yield and economic sense, under low input scenarios than their respective sole crops and have greater nutrient uptake and nutrient uptake efficiency than their respective sole crops. It was also hypothesised that mycorrhizal colonisation of roots would improve phosphorus acquisition and that intercropping would affect mycorrhizal colonisation, with the direction of this affect dependent on intercrop species combinations. Over the 2019, 2020, and 2021 winter growing seasons a total of seven field experiments were conducted in the Mediterranean climate zone of southern Australia. Three field experiments were conducted in 2019, three in 2020, and one in 2021. Chickpea (Cicer arietinum L.) was intercropped with linseed (Linum usitarissimum L.) under varying fertiliser regimes across three sites during 2019 and 2020. There were six fertiliser application treatments combining three levels of nitrogen application (0 kg N ha-1, 25 kg N ha-1, and 50 kg N ha-1) and two levels of phosphorus application (0 kg P ha-1 and 20 kg P ha-1) (0N0P, 0N20P, 25N0P, 25N20P, 50N0P, 50N20P). Chickpea and linseed were also intercropped under varying fungicide regimes (nil, foliar fungicide, foliar fungicide + desiccant) in 2019 and 2020 at Hart field site, as were chickpea and canola (Brassica napus L.) in 2020. Finally, chickpea-linseed, chickpea-canola, and lentil (Lens. culinaris L.)-canola intercropping field trials were sampled in 2020 and 2021 at Hart field site. A glasshouse study utilising the same intercrop species combinations and the respective sole crops was also conducted. Fertiliser treatment did not affect yield in the chickpea-linseed intercrops, and there was no significant difference between total intercrop yield and sole crop yield, with the intercrop yielding 78% and 103% of sole chickpea and linseed, respectively. Intercrop nitrogen uptake land use efficiency was improved, with the sole crops needing 27% more land to achieve the same nitrogen uptake. Intercropping also increased nitrogen fixation in the chickpea, with 36% and 44% Ndfa in the sole and intercrop chickpea, respectively. Conversely, intercrop phosphorus uptake land use efficiency was reduced, with the sole crops needing 8% less land to achieve the same phosphorus uptake. The land equivalency ratio (LER) of the chickpea-linseed intercrops under nil fertilisation averaged 1.04 compared with 0.94 under high fertilisation. The LER of the intercrops under nil fungicide averaged 1.39 compared with 0.93 under high fungicide. Intercropping provided gross margin stability across environments, while the gross margins of the sole crops varied depending on environment, a pattern that held even when grain prices and input costs varied. In the chickpea-oilseed and lentil-canola intercrops, mycorrhizal colonisation and the subsequent effect on phosphorus nutrition were host plant dependent. Lentil was the most mycorrhizal plant, followed by linseed, chickpea, and then canola. Only in lentil in the glasshouse was there a correlation between mycorrhizal colonisation and shoot phosphorus (R = 0.79, p<0.001). Intercropping did not affect AMF colonisation in the field, but in the glasshouse intercropping with canola reduced mycorrhizal colonisation of lentil. The interaction between intercropping and AMF had a limited effect on crop growth and shoot phosphorus. Outside of the AMF-intercropping interaction, intercropping with chickpea increased canola shoot phosphorus. In summary, I found that legume-oilseed intercropping appears well suited as a low-input system in broadacre cropping in Mediterranean environments. Although the actual yield of the intercrops was similar to those of the respective sole crops, intercropping chickpea with the oilseed species linseed and canola resulted in improved land use efficiency (LER) relative to the sole crops, particularly under reduced fertiliser and fungicide inputs. The intercrops also provided greater gross margins under low input scenarios, and improved yield stability across environments. The results show that legume-oilseed intercropping can be utilised as a risk mitigation strategy and a profitable alternative to traditional high input monocultures in a global environment of rising synthetic input costs.

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