Livestock grazing promotes legume abundance under increased nutrient loads: Mechanistic evidence from a temperate grassland

Abstract

Question: Legumes are a key component of rangelands because they play an important role in animal nutrition and the entrance of nitrogen (N) into ecosystems through symbiotic fixation. Legume abundance is commonly low in N-enriched environments because of competition with grasses and non-legume forbs. Both haying and livestock grazing remove plant biomass and reduce light limitation to plant growth, with the difference that livestock may selectively consume legumes. This study examines how legumes respond to grazing, haying and fertilization, and what mechanisms explain legume abundance in rangelands. Location: Flooding Pampa, Argentina. Methods: We performed two manipulation field experiments over 3 years. First, a factorial of rangeland management (intact, haying, or grazing) under two nutrient levels (ambient and increased N, phosphorus [P] and potassium [K]); and second, a factorial of rangeland management (intact or haying) and N × P addition. We evaluated legume, grass and non-legume forb abundance and ground-level light in three to five replicates of our experiments over 3 years. Results: NPK fertilization increased legume abundance consistently under grazing, and temporarily under haying, but had no effect in the intact grassland. Also, P addition increased legume abundance only under haying when N was not added. Temporal changes in legume abundance were positively associated with changes in ground-level light, which increased with haying and grazing, but decreased with fertilization in the intact grassland, and negatively with grass abundance. Conclusions: The negative effects of nutrients on legume abundance were offset by the positive effects of livestock. The reduction in grass competition and increase in ground-level light due to grazing and haying explained the positive responses of legume abundance to nutrients in this temperate grassland. Our results highlight the importance of considering the interactive response of legume abundance to grazing and fertilization, which are becoming common practices in rangelands.