How do tanniferous forages influence soil processes in forage cropping systems?

Authors: Clemensen, Andrea; VILLALBA, JUAN - Utah State University; Lee, Stephen; PROVENZA, FREDERICK - Utah State University; Duke, Sara; REEVE, JENNIFER - Utah State University

Submitted to: Crop, Forage & Turfgrass Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2022
Publication Date: 6/21/2022
Citation: Clemensen, A.K., Villalba, J.J., Lee, S.T., Provenza, F.D., Duke, S.E., Reeve, J. 2022. How do tanniferous forages influence soil processes in forage cropping systems? Crop, Forage & Turfgrass Management. 8. Article e20166. https://doi.org/10.1002/cft2.20166.
DOI: https://doi.org/10.1002/cft2.20166

Interpretive Summary: Nitrogen-fixing legumes such as alfalfa and sainfoin may benefit agricultural systems. In addition to quality forage, both legumes contain plant secondary metabolites that play important roles in agricultural systems. Alfalfa contains triterpenes (saponins), and sainfoin contains phenolic compounds (tannins). These plant secondary metabolites can change the way nutrients are cycled in the soil. This area of research has mostly been done in forest systems, or in controlled laboratory settings. So, research is needed in agricultural systems. Therefore, we did a field study in Lewiston, Utah, USA comparing alfalfa and sainfoin with tall fescue (TF). Our study included a fertilized bale and remove system, a green manure system (leaving plant residue in field), and a no-fertilizer bale and remove system comparing alfalfa and sainfoin with TF. Green manure plots had more soil organic C, greater soil dehydrogenase enzyme activity, and greater soil microbial biomass than the other management systems. Dehydrogenase enzyme activity is an indicator of overall soil microbial activity, which may be used to gauge soil health. Plant biomass was greater in sainfoin than in alfalfa, while soil nitrate was greater in alfalfa than in sainfoin plots. Lower soil nitrate in sainfoin plots could be due to the condensed tannins, which can slow down nitrogen mineralization. Planting forages, like sainfoin, that contain tannins may reduce nitrogen loss, and ultimately enhance agricultural sustainability.

Technical Abstract: Nitrogen-fixing legumes such as alfalfa (Medicago sativa L.) and sainfoin (Onobrychis viciifolia Scop.) may benefit agricultural systems. In addition to quality forage, both legumes contain secondary metabolites that play important roles in agroecological systems. Alfalfa contains triterpenes (saponins), and sainfoin contains phenolic compounds (tannins). Terpenes and phenolics can influence soil nutrient dynamics by inhibiting microbial activity, which could slow nitrogen mineralization and minimize nitrogen loss. However, research evaluating their influence in soil has largely come from silviculture or laboratory studies. Therefore, we initiated a field study in Lewiston, Utah, USA comparing alfalfa and sainfoin with tall fescue [Schedonorus arundinaceus (Schreb.) Dumort.; TF; grass control] for plant and soil characteristics. The split-plot experimental design had three replicates and included a fertilized bale and remove system, a green manure system (leaving plant residue in field), and a no-fertilizer bale and remove system comparing alfalfa and sainfoin with TF. We assessed saponin and condensed tannin (CT) concentrations in alfalfa and sainfoin, respectively, plant biomass, and soil characteristics [e.g., ammonium (NH4), nitrate (NO3), dehydrogenase (DHEA), organic C, soil respiration, etc.]. Green manure plots showed greater (p = 0.04) organic C, DHEA, and microbial biomass than other systems. Plant biomass was greater in sainfoin than in alfalfa (p < 0.001), while soil NO3 was greater in alfalfa than in sainfoin plots (p < 0.01). Soil NO3 differences could be explained by inhibition of mineralization by CT in sainfoin relative to saponins in alfalfa. Planting forages containing tannins may reduce nitrogen loss, thereby enhancing agricultural sustainability.