Pete Iannetta and colleagues based at the Agroecology Group of the James Hutton Institute discuss their experiences in research on ‘plant teams’ and why they think this approach holds huge potential for the sustainable intensification of agriculture…

The concept of growing two or more crops together, in the same space and time, is termed “intercropping[1]” by researchers, though the approach may also be characterised by the term “plant teams”. Whichever is preferred, they both refer to what was a standard practice historically. The use of plant teams is still widespread today, but only among farmers who wish to maximise natural chemical cycling, nutrient use efficiency and biological pest control. That is, users of plant teams aim to maximise productivity whilst minimising, and even avoiding completely, their reliance on external inputs such as inorganic fertilisers and pesticides.

Such an aim is admirable, since in today’s age farmers are under greater pressure to be more productive whilst also improving the qualities of their farmed system such as soil structure and biodiversity, and whilst maintaining a profit. This challenge can be worrying and as the author of the book Soul Searching (Keith Caserta) penned, “worry is the interest you pay on a debt you may not owe”. Nevertheless, every year farmers respond positively to the gauntlet which history has thrown before them. This gauntlet has been characterised as “sustainable intensification” by farming policy makers. This term is, in fact, an oxymoron, and approaches to maximise outputs whilst minimising inputs should rather be termed “sustainable deintensification”, which is more scientifically accurate.

Placing such contextual and philosophical considerations aside and re-focussing on farming practice, there are many agronomic measures which may be used to increase efficiency and the use of plant teams is among the most promising of methods. However, it does not feature strongly as common practice in the UK. There are a number of reasons for that, but first let us look at the research findings which aim to understand why plant teams work, and provide examples of results from agronomic studies.

Plant teams are usually deployed by organic farmers and proponents of conservation agriculture. One of the more common types of plant team comprises legume and non-legume species. The interplay here exploits the capacity of the legume to meet its own nitrogen requirements via biological nitrogen fixation, or BNF. BNF is a natural symbiotic process whereby soil bacteria, referred to collectively as rhizobia, infect legume roots and become entrapped within the core of legume root nodules. In exchange for plant sugars, the rhizobia fix atmospheric nitrogen in biologically useful forms. BNF can be highly efficient and, for example, experiments at the Centre for Centre for Sustainable Cropping have shown that faba bean crops can fix up to 300 kg of nitrogen, with up to 100 kg being left in-field in residues after harvest. In a plant team, BNF by legume-rhizobia association is enhanced as a function of the non-legume companion plant’s capacity to compete for, and so deplete, available soil nitrogen.

Such positive plant-plant interactions are allied to other facilitative processes. For example, the bacteria which live around the roots of plants (rhizobacteria), are a complex population of species, or microbiome, that serve important functions. A well balanced microbiome can help optimise crop fitness. In 2016, it was found that the rhizobacteria community of the legume species Lotus japonicus L., a model species for researchers, harbours a high density of symbiotic or plant growth promoting microbes[2]. Although not creating and living in legume root nodules, these microbes may exhibit traits such as BNF and they show other attributes. These include a high efficiency to suppress soil-borne pathogens, ability to increase the bioavailability of soil phosphorous and even extend to the secretion of a plant growth hormone, IAA (indole acetic acid)[3]. Thus, a non-legume co-existing with a legume does not simply benefit from the legumes capacity to entrap rhizobia for BNF within root nodules, but also form a “symbiotic microbiome” that extends into the soil rhizosphere.

Such facilitative traits are not simply a feature of legume species, and cultivating different species of non-legumes and even distinct varieties of the same species have shown excellent results for plant productivity. This has been proven by many years of research at the James Hutton Institute showing that growing different varieties of cereals together is a proven method of biological disease control, which can justify reductions in pesticide use[4].

The science literature on agronomy for plant teams shows clear benefits: a study of legume supported crop rotations throughout Europe showed that productivity peaked when legume inclusion was 50% in the rotation, with an equal balance of forage and grain legumes, often deployed in a plant team[5]. Such positive findings are realised despite the fact that the modern crop (non-legume) varieties are bred for high input monocropping, and so are unlikely to exhibit traits for optimal performance for co-existence with a companion species. Modern day breeding programmes have mainly selected for yield and disease resistance attributes. Thus, the development of agronomic and breeding approaches to optimise the function of, and the agronomy for, plant teams is in its infancy and takes the form of EU funded industry-science partnerships via projects such as DIVERSify and TRUE (www.true-project.eu). Such research and development efforts are aimed help to realise economic strategies that are better harmonised with parallel policies targeted to safeguard environmental and human well-being.

Projects funded by governments within the UK include, for example, that of barley-pea intercrops for brewing and distilling. Of the Scottish arable area (1,900k ha in 2014), 55% was sown with barley, of which 84% (874 ha) was spring barley - the main raw material of the brewing and distilling industries. These industries are of critical economic importance to the UK, contributing almost equally to the £10 billion annual UK tax revenues. Spring barley normally requires 110 kg of inorganic nitrogen fertiliser per hectare. This nitrogen requirement could be provided exclusively by plant teams. If this happened for the spring barley area (in Scotland), the carbon footprint (carbon dioxide equivalents (CO2e)) and financial savings would be around 1.36 MT and £27.6 million, respectively!

It was also found the barley component of a barley-pea plant team sown at a half seed rate, and without the use of any added mineral nitrogen, pesticides or herbicides, produced high quality yields that were equivalent to that of the barley monocrop sown at half the full seed rate. The barley within the team produced twice as many tillers and the associated grain exhibited a higher level of grain nitrogen than the monocrop at 1.3%, which is a good level for malting for fermentation based processes. Furthermore, the land equivalent ration, was 1.2, that is 20% more yield than would normally have been achieved with a monocrop, and the pea nitrogen (protein) levels were on average 10% higher, depending on which variety was used.

Similarly, plant team based approaches for the production of whole crop forages have proven equally promising, with winter rye, winter oat and winter pea combinations showing high biomass yield, protein and good digestibility under conditions of reduced (50% nitrogen) inputs. Barley and wheat also contribute well to digestibility, and faba beans have potential as an alternative to peas. The winter peas are normally problematic under Scottish conditions but using cereals as a support they grew very successfully. Such combinations sown with ryegrass give a second biomass crop feeding on the nitrogen released from the legume roots after cutting the whole-crop. There is a lot of potential held with these different plant team combinations and we aim to unpick this even further as part of the DIVERSify project.

An invitation: JHIs Agroecology team invite farmers who are already using plant teams, or interested in trialling plant teams, such as cereal-legume mixtures to please get in touch (). We want to further understand farmers’ experiences of using plant teams, and whether these were successful or not.

Acknowledgements

The DIVERSify and TRUE projects are funded from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement numbers 727284 and 727973, respectively. For further information on these projects, or plant teams, please contact Alison, Adrian or Pete by email at [email protected]. Pete Iannetta is a research deliverable (RD) leader of 2.1.8 ‘novel crops and novel cropping systems’, and coordinator of the EU-TRUE project. The James Hutton Institute is financially supported by the Scottish Government.

[1] Brooker, R. W. et al. (2014) Improving intercropping: a synthesis of research in agronomy, plant physiology and ecology. New Phytologist, 206, 107:117.

[2] Zgadzaj, R. et al. (2016) Root nodule symbiosis in Lotus japonicus drives the establishment of distinctive rhizosphere, root, and nodule bacterial communities. PNAS, 113, E7996–E8005, doi: 10.1073/pnas.1616564113.

[3] Turner, T. R. et al. (2013) The plant microbiome. Genome Biology, 14, 209.

[4] Newton, A. C. et al. (2008) Deployment of diversity for enhanced crop function. Annals of Applied Biology, 154, 309-322.

[5] Iannetta, P. M. et al. (2016) A Comparative Nitrogen Balance and Productivity Analysis of Legume and Non-legume Supported Cropping Systems: The Potential Role of Biological Nitrogen Fixation. Frontiers in Plant Science, https://doi.org/10.3389/fpls.2016.01700.