Nitrogen transfer and yield effects of legumes intercropped with the perennial grain crop intermediate wheatgrass

Perennial grain crops are being developed to reduce the negative environmental impacts of tillage and chemical inputs related to annual row-crop agriculture. To further improve the ecological benefits of perennial grains like Kernza® intermediate wheatgrass (IWG) [Thinopyrum intermedium (Host.) Barkw. & D.R. Dewey], intercropping with perennial legumes has the potential to diversify grain production systems and reduce mineral N fertilizer requirements; however, the facilitative vs. competitive effects of various legume species on perennial grain yields are unknown. We compared grain and biomass yields, tissue C:N ratio, and δ15N of IWG in response to either mineral fertilizer treatments or intercropping with one of six legume species at three locations for three years. IWG tissue C:N ratio increased through time at all sites suggesting a consistent increase in N limitation. Although no legume intercrop consistently affected grain yields through time or across sites, very rarely did an intercrop reduce grain yields to levels less than fertilized and unfertilized IWG monocultures. However, legume biomass in year 1 was negatively correlated with IWG grain yields in year 1, suggesting that negative effects of competition may outweigh positive effects of N fixation and transfer the year following establishment. The relationship between legume biomass and IWG grain yield became positive by year 3, indicating a potential lag in the positive effects of legume intercrop on grain yield. At one location, red clover (Trifolium repens L.) biomass was higher than all other legume treatments in year 1 and declined through time, giving way to a subsequent increase in IWG biomass and grain yields through time. At this site, N transfer from legumes to IWG determined by δ15N was positive by year 3 for red clover and two other legume species. This study provides evidence that legume intercrops can benefit IWG production under certain conditions, but outcomes are site-specific and may depend on conditions related to soil N levels, temperature and precipitation patterns, and weed pressure. Research is needed to identify specific traits that promote legume coexistence and facilitation with IWG, and how these traits might rank in importance depending on environmental conditions.