Biomass and Nitrogen Accumulation of Hairy Vetch-Cereal Rye Cover Crop Mixtures: A Meta-Analysis.

Legume–grass cover crop mixtures may deliver the benefits of each component species and out-perform monocultures in biomass and nitrogen (N) accumulation. The objective of this review was to determine how site conditions and cover crop management practices affect the biomass and N accumulation of hairy vetch (Vicia villosa Roth.)–cereal rye (Secale cereale L.) mixtures relative to monocultures. We collected data on hairy vetch and cereal rye monocultures and mixtures as reported in ten peer-reviewed journal articles and from one unpublished study performed by the authors. Linear mixed models were used to evaluate the effects of growing degree days, soil organic carbon concentration, study location, and seeding rate on the biomass, N content, and N concentration of mixtures relative to monocultures. We found that, on average, the mixtures produced 1.5 times the biomass of hairy vetch monocultures and 1.3 times the biomass of cereal rye monocultures. The N content of mixtures was similar to that of hairy vetch monocultures, but 2.9 times that of cereal rye monocultures. The extent of biomass and N content overyielding varied across growing degree days and mixture seeding rates. The N concentrations of mixtures were approximately twice those of cereal rye monocultures, but were lower than those of hairy vetch monocultures. The meta-analysis confirmed that hairy vetch–cereal rye mixtures generally accumulate as much or more biomass and N than monocultures, and suggested that growing degree days and seeding rates affect the performance of mixtures relative to monocultures.