Vegetation change and the biogeochemistry of grassland and savanna ecosystems
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Woody plant encroachment into grass-dominated ecosystems has been a globally significant land-cover change during the last century, and has been documented in grasslands and savannas of North America, South America, Africa, Europe, and Asia. This dramatic land cover change appears to be driven largely by livestock grazing and the suppresion of natural fire regimes. Both woody encroachment and livestock grazing have strong potential to influence the storage of key soil nutrients, including carbon, nitrogen, and phosphorus. In addition, grazing and woody encroachment may also modify the biodiversity and functional attributes of the bacterial and fungal communities present in the soil environment. Thus, woody encroachment and livestock grazing may interact to modify nutrient cycles, the availability of limiting nutrients, and the structure and function of soil microbial communities at scales ranging from the ecosystem to the globe, and potentially influence the sustainability of livestock production systems. To better understand how woody encroachment and livestock grazing may interact to modify soil nutrient storage and cycling, we will quantify the magnitude of changes in soil carbon, nitrogen, and phosphorus storage in soils following woody encroachment in both grazed and ungrazed ecosystems. In these same study areas, we will also examine the impact of woody encroachment, grazing, and their interaction on the structure and function of soil bacterial and fungal communities. Results of this study will afford a unique opportunity to explore potential interactions between grazing and woody plant encroachment on belowground portions of the ecosystem that are critical for soil nutrient storage and availability. At present, these interactions remain largely unexplored and limit our understanding of the structure and function of grasslands, savannas, and other dryland ecosystem types. Results should also contribute to the development of more science-based methodologies for managing livestock grazing in these ecosystems that will help ensure the long-term sustainability of ecosystem services provided by these ecosystems.