Scientists from Wageningen University & Research demonstrate that the productivity of a next main crop can be manipulated through the choice of species in a preceding winter cover crop mixture. They report their latest findings in the Journal of Applied Ecology of 2nd of June.
With their publication, the scientist agree with recommendations of FAO to included cover crops in rotations, on top of that they open new avenues for the design of good species mixtures. Their advice: chose a mixture of winter cover crops which is productive and high in nitrogen, avoid the succession of closely related crops.
Biomass and nitrogen concentration, those are the keywords for a successful application of winter cover crop mixtures. The positive effects of cover crops on soil quality is recognised for some time now, however how the potential benefits of mixtures over monoculture cover crops relate to the mixture composition received little attention to date.
The soil legacies of plants are two-fold. On the one hand, nutrients released from plant residues through decomposition influence soil fertility. On the other hand, living plants attract and stimulate soil organisms thereby influencing soil health. In crop rotations the two mechanisms always act in concert, and are shaped by the properties of the plant that leaves the legacy. In order to test which of the pathways is most important and how the pathways are shaped by plant properties, PhD candidate at Wageningen University & Research, Janna Barel and colleagues under the lead of Gerlinde De Deyn embarked on a two-year field experiment.
Diversity in time
The experiment focussed on two principles: diversity in time and space. Effects of diversity in time were investigated by rotating two main crops (oat and endive) with 6 winter cover crop treatments. The researchers tested which combination of species during summer-winter-summer rotation resulted in the highest productivity of the final main crop. The motto of avoiding closely related crops proved to be a valuable guideline. Oat productivity was negatively affected by a history of winter grown English ryegrass and previously grown oat. For both oat and endive, growing a cover crop in winter instead of keeping it fallow is recommended. Biomass and nitrogen concentrations of the cover crop plants are both relevant selection criteria, and depending on the following crop either biomass or nitrogen concentration is more important.
Diversity in space
Diversity in space was tested by comparing the legacies of winter cover crop mixtures with those of their monocultures. Previous research in natural grasslands showed that an increase in plant diversity generally leads to increased productivity. The reason for this is that plant diversity helps reduce diseases and damage by pest-organisms. After all, the target plants for specialist pests and diseases are harder to find in mixtures. Moreover plant species mixtures are more efficient in taking-up resources when the species differ in their nutrient uptake strategies. These mechanisms of mixing complementing species could therefore provide agriculture with sustainable management practices.
In the current experiment, mixtures of fodder radish and vetch were more productive than was expected based on the average performance of their monocultures. This contrasted with mixtures of English ryegrass and white clover. The productivity gain of radish-vetch mixtures came without a penalty for nitrogen concentration in the plant material, whereas the nitrogen concentration of the grass-clover mixtures were lower than expected. Surprisingly, despite the high productivity of radish-vetch the abundance of plant-feeding nematodes was kept at levels similar to the monocultures. This makes the radish-vetch mixture interesting as a bio-control for nematodes.
Furthermore, the main crops differed in their responses to winter cover crop legacies. Despite its high biomass, fodder radish limited the growth of succeeding oat, whereas endive growth was stimulated. However, this limiting effect was not observed for radish-vetch mixture indicating that species mixtures can alleviate potential negative impacts of species in monoculture.
This research was supported by the Netherlands Organisation for Scientific Research, NWO-ALW Vidi.