A comparison of ecosystem services produced from intermediate wheatgrass monocultures and legume bicultures

Susanne Wiesner and Paul Stoy, Biological Systems Engineering, University of Wisconsin, Madison, WI, Susanne Wiesner, Atmospheric and Oceanic Sciences, University of Wisconsin, Madison, WI, Alison J. Duff, Kristine Niemann and Heathcliffe Riday, Dairy Forage Research Center, USDA Agricultural Research Service, Madison, WI, Ankur R Desai, Atmospheric and Oceanic Sciences, UW-Madison, Timothy E. Crews, The Land Institute, Salina, KS, Valentin Picasso, Agronomy Department, University of Wisconsin – Madison

Background/Question/Methods
Shifting conventional agriculture to perennial crops could improve the ecological and economic sustainability of agroecosystems because of their ability to provide diverse ecosystem services including soil carbon sequestration. The integration of perennial forages can reduce soil erosion and nutrient runoff, which cost farmers half a billion US dollars annually to replace. Intermediate wheatgrass (IWG; Thinopyrum intermedium) is a stress-tolerant grain and forage species, with an extensive root system. IWG is often grown in legume bicultures for symbiotic nutrient supplies, specifically nitrogen (N), which has been shown to increase forage biomass and nutritive value. An eddy covariance (EC) tower was installed in December 2018 in an IWG field in Wisconsin – of which half of the field was frost-seeded with red clover (Trifolium pratense) – to investigate if functionally diverse perennial grain bicultures can provide greater ecosystem services in the form of carbon accumulation and yields compared to monocultures. We spatially partitioned EC fluxes and collected biomass samples in both treatments to investigate if IWG bicultures exhibit greater forage and grain yields compared to IWG monocultures. We also investigated if IWG bicultures accumulate C more rapidly following harvest due to greater nitrogen availability and compared their annual net biome productivity (NBP).
Results/Conclusions
We found that IWG bicultures had greater forage and grain biomass, which was highly variable due to greater spatial variability in red clover presence. EC data indicated that IWG monocultures were larger carbon sinks (–542-593 g C m-2 yr-1) compared to the bicultures (–456-520 g C m-2 yr-1), due to greater photosynthetic activity during the growing season and following harvests in August 2019 and 2020. Harvest resulted in carbon loss for the subsequent 30 days, which was greater for bicultures. IWG stands showed potential to provide valuable ecosystem services, as annual NBP (248-323 g C m-2, including harvest exports) was greater compared to European grasslands, and similar to US tallgrass prairies. NBP estimates suggested that the IWG monoculture accumulated more carbon belowground, which likely served as a resource pool for biomass regrowth in spring, whereas the biculture was a small carbon source, suggesting ecological tradeoffs of both stands in terms of nutrient availability and carbon sink potential. However, the decision to plant IWG as a monoculture or biculture should be informed by the context of farm operations and field conditions, and the relative importance of carbon sequestration or crop yield targets and nutrient availability.