DIVERSE - DIVerse crop residues Engender Resilience of Soil functions and Ecosystem services

This project will test the idea that cover crop green manure mixtures are a tractable means to increase the diversity of carbon compounds entering soils and that this increased biochemical diversity leads to a greater diversity of enzymes produced by the soil microbial community, and, in turn, the potential to make more nutrients available to crops during and after disturbances such as floods and droughts.

There is a recognised need to sustainably intensify our food production systems this century to meet global demand. The increases in crop yield potential achieved by genetic improvements have not translated into on-farm yield increases of the same magnitude due to sub-optimal growing conditions. Sub-optimal quality and health of the soils on which crops are grown is a major constraint to the realisation of the genetic yield potential. Crops have failed to achieve their yield as a result of unsustainable soil management that has reduced the ability of the soil microbial community to deliver underpinning functions and services (e.g. nutrient mineralisation). Sustainable soils are those that can consistently achieve crop yield potential by delivering ecological functions despite disturbances (e.g. floods and drought) which are expected to become more frequent and severe as our climate changes. The energy required for the soil microbial community to deliver these functions in agricultural soils is provided by organic compounds entering the soil in the form of root exudates, crop residues or organic manures.

When a single carbon substrate is added to a soil (e.g. by incorporating the residues of the previous crop in an arable rotation), a microbial community that is most adapted to exploiting that substrate is activated. When applying several biochemically contrasting substrates, the number of ecological niches increases and the groups of organisms capable of exploiting each of these niches are stimulated. The functional diversity of the soil microbial community is thus increased and the capability to deliver ecosystem functions (e.g. nitrogen mineralisation) is more secure since the suppression of one group of organisms due to a disturbance (e.g. a flood or a drought) can be compensated by other groups, thereby enhancing resistance and resilience of the soil microbial community to disturbance.

Cover crop green manures offer a tractable means to introduce biochemically contrasting crop resides into arable crop rotations because their inclusion does not significantly change the agronomic management of the cash crop. Crop residue diversity can be increased further by incorporating cover crop mixtures comprised of species from a diverse range of plant families since each plant family produces unique plant secondary metabolites. Thus, the incorporation of diverse cover crop mixtures provides biochemically diverse organic compounds that may alter the structure of the soil microbial community and increase the range of organisms capable of metabolising nitrogen-containing organic compounds. This greater diversity and functional redundancy of the soil microbial community may enable the mineralisation of nitrogen associated with the soil organic matter to be maintained after a flood or drought event, when plant demand is restored.

This project will track the fate of carbon applied to soils as unary, binary, tertiary and quaternary mixtures of contrasting cover crop residues using by labelling them and determining which members of the soil microbial community incorporate them into their biomass. Pot and field experiments will investigate whether increasing the diversity of cover crop residues incorporated into soils alters the structure of the soil microbial community, increases the potential for the soils to mineralise nitrogen, before, during and after flooding and drought, and increase the delivery of beneficial ecosystem services (crop yield and nutrient use efficiency).

The nature of the relationship between biodiversity and ecosystem functioning in soils has remained an open question in soil ecology for decades. This project will directly test whether resource partitioning of biochemically contrasting crop residues contributes to the diversity-function relationship in agricultural soils and will develop strategies for exploiting the relationship to provide more sustainable soil management. The project is based on the premise that when providing a single source of carbon to soil, r-strategists within the microbial community capable of producing extracellular enzymes to exploit that resource will be activated, but when applying a mixture of biochemically contrasting sources of carbon, the number of ecological niches increases, and r-strategists capable of competitively exploiting multiple niches are stimulated and extracellular enzymes produced. These groups of organisms are, by definition, functionally different to one another because they differ in their capability to metabolise biochemically contrasting sources of carbon. This increased metabolic diversity would allow soils to metabolise a greater range of carbon substrates and mineralise soil organic nitrogen, especially after a disturbance when r-strategists are supressed and K-strategists continue to catabolise soil organic matter depolymerised by persistent extracellular enzymes. Experiments will be undertaken to determine whether the composition of the soil microbial community can be altered by providing it with a greater biochemical diversity of cover crop residues, to use as a carbon and energy source. The altered soil microbial community will be monitored to determine whether the potential net nitrogen mineralisation rate has a greater resistance and resilience to disturbances (flooding and drought) and increases the yield of commercial crops.

This research proposal is timely because, although cover crop use in the UK is growing rapidly, the design of cover crop mixtures is under-researched. This research will identify novel strategies for creating cover crop green manure mixtures that enhance the potential for soil organic nitrogen mineralisation in arable soils, and ultimately improve the yield of crops. The finding that a greater biochemical diversity of crop residues increases nutrient use efficiency will lead to a change in the philosophy applied when designing cover crop mixtures. Whereas mixtures are currently designed to ensure a range of different functions (e.g. nitrogen fixation, pollination, soil structural improvement), this research will encourage growers and seed suppliers to also consider the biochemical diversity of the residues produced by each cover crop and to formulate new mixtures comprising of plant species from contrasting plant families.

The primary beneficiaries to this research are the seed suppliers themselves, that will, as a result of these findings, offer novel cover crop seed mixtures to their customers and provide scientific evidence for the rationale behind the composition of their mixtures. However, there will also be benefits to growers themselves as consumers of cover crop seed mixtures and to organisations advising growers (e.g. agronomists, farming press, and levy bodies) by equipping them with evidence and empowering them to request custom designed seed mixtures from suppliers that increase the biochemical diversity of their green manures. This project will ultimately provide benefits to society as a whole, due to an increased security of food supply in an uncertain market.

Seed suppliers
Seed suppliers have seen a considerable rise in the sales of cover crops recently as awareness in the importance of 'soil health' for sustainable land management has increased. The construction and marketing of cover crop mixtures has become rapidly more sophisticated and is primarily based on the combination of individual plants that are known to provide benefits when grown in monoculture. Very little research has tested whether these mixtures provide benefits that are greater than the sum of the parts. By providing this evidence, the choice of plant species used in cover crop mixtures can be informed by fundamental science.

Growers
Individual growers can request custom designed cover crop mixtures from seed suppliers to ensure that a mix of species is selected that suits their objectives. Currently these choices are made primarily based on the functions provided by the individual plants (and the price of the seed), with the assumption that the mixture will equal the sum of the parts. This research will identify principles that growers can apply when selecting cover crop green manure mixtures that ensures that the mixture provides benefits that are greater than the sum of the parts.

Agronomists and advisors
Growers make decisions based upon the advice provided by a myriad of organisations including individual agronomists, fertiliser and agrochemical companies, seed suppliers, supermarkets, levy bodies, farming press, online forums, and CPD workshops. This research will provide fundamental science that these organisations can apply to advise clients or members on how to select cover crop green manure mixtures that enhance the ecosystem functions provided by the soil microbial community.

Society as a whole
There is a recognised need to sustainably intensify our food production systems this century to meet global demand in uncertain markets, driven by population growth and shifting diets. Because this research is expected to deliver strategies that promote sustainable soil management and increase crop yields, it will contribute to greater agricultural sustainability and improve the security of food supply in the future. These outcomes benefit society as a whole.