The latest issue of Journal of Applied Ecology includes a Special Feature, Functional traits in agroecology. To accompany the feature, we’re introducing a series of blog posts from the authors themselves. The first of these comes from Jennifer Blesh and discusses her article, Functional traits in cover crop mixtures: Biological nitrogen fixation and multifunctionality.

JPE-Agroecology-200x200Global climate, energy, and water crises pose immense challenges for agricultural systems. At the same time, industrial agriculture is a major contributor to these problems. Low species diversity on farms has meant replacing ecological processes and functions with chemical inputs, which cause environmental costs such as surface water pollution, losses of soil carbon, and greenhouse gas emissions. In response, there is growing interest in assessing the resilience and sustainability of a wide range of agroecosystem practices to inform transitions to more sustainable food systems.

Agricultural ecology (agroecology) applies principles from ecological science to manage species interactions to support crop yield and other functions, such as nutrient retention and pest control, allowing for reductions in non-renewable inputs and associated costs. Since farm fields typically have lower diversity compared to natural ecosystems, small increases in agroecosystem biodiversity can lead to large benefits for function, especially if functional diversity is increased. In agroecology, functional diversity refers to the diversity of functional traits that influence agroecosystem function. Some plant traits of potential importance to agriculture are root-to-shoot ratio, the ability to convert nitrogen (N) gas in the atmosphere into plant-available forms (e.g., biological N fixation by legume species), and plant litter chemistry, which affects decomposition and soil nutrient cycling processes.

In this experiment, I manipulated functional diversity in eight vegetable farm fields in Michigan through cover crop mixtures. Cover crops are non-harvested plants, many of which can be grown in the off season between primary crops to provide different functions. This study tested mixtures that included legume species, such as Austrian winter pea, Crimson clover, and Medium red clover, along with grasses (spring wheat, oats, and cereal rye), and a couple of treatments with brassicas (daikon radish and yellow mustard). Mixtures of two or three species were designed to combine contrasting plant traits: biological N fixation, carbon-to-nitrogen ratio of plant shoots, and cold tolerance (species that survive the winter versus those that do not).

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The experimental plots on a collaborating farm (image: Jennifer Blesh)

Nitrogen frequently limits crop productivity. Biological N fixation by legumes is therefore one of the most valuable plant traits in agroecosystems, because legumes can supply a new N source and reduce the most fossil fuel intensive agricultural input, synthetic N fertilizer. Legume N sources are part of an ecological approach to nutrient management with potential to enhance agroecosystem sustainability. And, cover crops that combine legumes with other species may provide multiple functions at once (multifunctionality), such as N supply, nutrient retention, and weed suppression. To date, research exploring how biodiversity affects multifunctionality is limited, and most studies have tested single ecosystem functions.

The eight collaborating farms spanned a gradient in soil fertility due to different management legacies. Across the farms, I tested the hypothesis that increasing functional diversity with cover crop mixtures would increase multifunctionality compared to a cereal rye monoculture and to a no cover crop control. Cereal rye is the most common cover crop in the Midwest since the seed is low cost and it reliably survives the winter. This study also asked whether differences in soil properties on farms would predict variation in N supplied by biological N fixation in the cover crop treatments. I calculated multifunctionality using three different ecosystem functions: N retention in aboveground biomass, N supply from biological N fixation, and weed suppression compared to the no cover crop control plot. I assessed multifunctionality at three different thresholds (i.e., proportions of the highest observed level of each function across farms: 30%, 50% or 75%). For example, the maximum rate of biological N fixation was 154 kg N ha-1, so if a treatment passed the 30% threshold for multifunctionality it supplied at least 46 kg N ha-1.

Several mixture treatments provided greater multifunctionality than the no cover crop control at the 30% and 50% thresholds. These mixes also had significantly greater multifunctionality than the cereal rye monoculture, but only at the lowest threshold (30%), indicating potential trade-offs among different functions. Treatments passing the lowest threshold of multifunctionality still provided management-relevant levels of the three different agroecosystem functions (e.g., an N input of approximately 46 kg N ha-1). Further, the study design was informed by participating farmers’ interests, which meant testing several unusual mixture combinations where some species performed poorly. In other words, there is potential for further enhancing multifunctionality with better performing mixtures. These results point to the promise of cover crop mixtures for providing multiple functions at once, and to the need to optimize species or variety selection, and management practices, for more reliable outcomes.

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Sampling a cover crop treatment on one of the partnering farms (image: Jennifer Blesh)

To achieve this goal, we need to understand how cover crop mixtures perform across farms with differing soil and environmental characteristics. There was large variability in N supply from biological N fixation across treatments and farms. As expected, this variation was mostly explained by differences in the amount of legume biomass produced. So, what drove variation in biomass production across farms?

Winter pea biomass was positively correlated with soil phosphorus availability, and both crimson clover and pea biomass were negatively correlated with two different measures of N availability from soil organic matter. These findings highlight the ecological efficiency of legume N sources: biological N fixation inputs are lower when soil N availability is higher. A similar approach across many farms could provide critical information for understanding the ecology of biological N fixation and for optimizing management of legume N sources on farms.

Currently, only a small proportion of Michigan farmers (approximately 6%) regularly use cover crops, but interest is growing with increasingly widespread interest in food system sustainability. As more farmers plant cover crop mixtures on their farms, they will likely need to modify species selection and management for different environmental conditions and management goals. On-farm studies such as this one are helpful for assessing ecological outcomes in variable and realistic conditions, and for developing better information and predictive tools for farmers. There remains a need to refine understanding of links between plant traits and functions, and to improve the design of cover crop mixtures based on principles of functional ecology.

The full article,  Functional traits in cover crop mixtures: Biological nitrogen fixation and multifunctionality is a part of the Special Feature, Functional traits in agroecology and is available in Journal of Applied Ecology.

Read more blogs from the series: