Do plant secondary metabolite-containing forages influence soil processes in pasture systems?

Authors: Clemensen, Andrea; VILLALBA, JUAN - Utah State University; ROTTINGHAUS, GEORGE - University Of Missouri; Lee, Stephen; PROVENZA, FREDERICK - Utah State University; REEVE, JENNIFER - Utah State University

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2020
Publication Date: 8/18/2020
Citation: Clemensen, A.K., Villalba, J.J., Rottinghaus, G.E., Lee, S.T., Provenza, F.D., Reeve, J.R. 2020. Do plant secondary metabolite-containing forages influence soil processes in pasture systems?. Agronomy Journal. 112(5):3744-3757. https://doi.org/10.1002/agj2.20361.
DOI: https://doi.org/10.1002/agj2.20361

Interpretive Summary: Plants naturally produce primary and secondary metabolites. Primary metabolites are directly involved with plant growth and metabolic function. Plant secondary metabolites (PSMs) were once thought of as metabolic waste products, and more recently viewed as toxins to herbivores. However, ongoing research shows that PSMs, such as tannins and terpenes (e.g. saponins), have various benefits to the plants themselves, and are beneficial to herbivores at low doses. Plant secondary metabolites also influence 'grazers' in the soil by slowing nitrogen mineralization, thus reducing nitrogen loss which may increase the sustainability of pasture agroecosystems . We investigated whether cattle-grazed pastures of non-traditional grass-legume forages including tannin-containing sainfoin (Onobrychis viciifolia) and tall fescue (Festuca arundinacea) influenced soil nutrient cycling compared with traditional grass-legume forages of non-tanniferous yet saponin-containing alfalfa (Medicago sativa) and tall fescue. Data from our field and incubation experiments suggest tannins from sainfoin forages, and perhaps saponins from alfalfa forages, may influence soil nutrient cycling by decreasing nitrogen mineralization.

Technical Abstract: Grazed pastures are susceptible to N loss from manure additions, which increases eutrophication and ultimately affects the global N cycle. Plant secondary metabolites (PSMs), such as condensed tannins (CTs) and terpenes, influence soil dynamics in forest systems by generally decreasing N mineralization. We investigated whether cattle-grazed pastures of non-traditional grass-legume forages including CT-containing sainfoin (Onobrychis viciifolia Scop.) and tall fescue (Festuca arundinacea Schreb.; TF) influenced soil dynamics compared with traditional grass-legume forages of non-tanniferous alfalfa (Medicago sativa L.) and TF. Throughout the study, CTs in sainfoin averaged 58.9 g kg-1 whereas saponins in alfalfa averaged 5.7 g kg-1. We observed greater soil microbial respiration (p = 0.01) in TF, indicating greater microbial activity in TF than legumes, and between legumes we found greater soil NO3 (p = 0.01) in alfalfa than in sainfoin, although aboveground biomass and N differences were negligible. We also conducted a laboratory soil-feces incubation study to determine if feces from cattle foraging diets of legumes with CTs, and without, influenced soil dynamics. Both feces treatments showed lower NO3 (p < 0.001) than the control, and between treatments dehydrogenase activity (DHEA) was lower (p = 0.03) in sainfoin than alfalfa, suggesting these PSMs may affect soil processes. To our knowledge this study is the first considering whether pasture forages produce enough PSMs to influence soil dynamics by assessing general differences in soil parameters between CT-containing and non-tanniferous grass-legume systems. More research is needed to determine whether PSMs mitigate N loss in pasture systems by slowing N mineralization.