Associate Professor University of Michigan, United States
Background/Question/Methods
Cover crops are an increasingly popular strategy for improving agricultural sustainability. Cover crop mixtures that combine contrasting functional traits between species can be particularly effective for enhancing agroecosystem functions. However, most research on cover crops has focused on their aboveground traits and functions, while those belowground remain poorly understood. There is also significant uncertainty surrounding the context dependency of cover crop performance across variable conditions, such as distinct soil conditions across farms. To address these knowledge gaps, we conducted a field experiment on six farms in Michigan that span a gradient of soil health resulting from distinct management histories and soil types. Specifically, we asked: How do belowground cover crop traits and functions vary across multi- and single-species cover crop treatments, and are these outcomes moderated by soil health status across farms? Treatments included a cereal rye (Secale cereale), crimson clover (Trifolium incarnatum), oat (Avena sativa), and dwarf-essex rapeseed (Brassica napus) mixture, and cereal rye and crimson clover monocultures. In each treatment, we measured root biomass, carbon (C) inputs, and nitrogen (N) assimilation. We also assessed cover crop C:N ratio, and the distribution of root biomass across three size classes (fine, medium, and coarse) based on root diameter.
Results/Conclusions
The mixture outperformed the monocultures on all farms in terms of total root biomass, carbon (C) inputs, and nitrogen (N) assimilation (all p< 0.001). Root C:N ratios also differed markedly across treatments (45.8, 29.7, and 20.9 for mixture, rye, and clover, respectively) (p< 0.001). The high C:N ratio in mixture was driven by coarse roots of rapeseed, which comprised 48% of mixture root biomass with a C:N of 47.2. Importantly, several models had significant interaction effects between treatment and farm, suggesting that cover crop traits and functions varied in response to the soil health gradient. For example, compared to farms with lower phosphorus availability and clay content, total root biomass was, on average, 41.7% higher for rye, but 30.3% lower for clover on farms with higher phosphorus availability and clay content, while mixture root biomass was intermediate on these farms. Fine root N concentration also differed significantly across farms for rye and clover, ranging from 1.49% to 1.8%, and from 1.74% to 2.21%, respectively, but did not differ significantly in mixture. These findings advance understanding of belowground cover crop traits and functions, and underscore the need for adapting cover crop management for desired outcomes across farms with distinct soil conditions.
