Effects of cattle grazing on soil nitrogen dynamics in high-elevation grasslands

Tropical mountains are facing a host of environmental stressors, including warming temperatures and land-use change. How hydrologic and biogeochemical cycles respond will impact the provision of key ecosystem services and is a critical research topic with implications from local to global scales. The Cordillera Blanca is a prime example of a tropical mountain range facing complex challenges. Located in central Peru, the Cordillera Blanca contains some of the Andes’ highest (> 6,000 masl), glaciated peaks. These mountains provide drinking and irrigation water, hydropower, grazing lands, and other ecosystem services to hundreds of thousands of people, but are facing numerous threats from climate change, industrialization, and land-use modification.

 One pervasive land-use change that has notably altered the ecology of CB ecosystems is grazing by sheep and cattle. The predominant high-elevation vegetation types, including tall and short grass meadows (puna), shrublands, Polylepis forest, and mountain wetlands (bofedales), are all susceptible to degradation from livestock overgrazing. Haphazard grazing management is compounded by the fact that Andean mountain ecosystems did not evolve in the presence of hard-hooved animals (sheep and cows), but rather low-impact camelid grazers with soft-padded feet (i.e. llamas and alpacas). Soil compaction, trampling, and herbivory by introduced livestock has led to erosion, reduced plant vigor, decreased litter layers, and community shifts toward undesirable plant species. These shifts have implications for the sustainability and yield of grazing activities as well as the ecology of rare, high-elevation ecosystems. While grazing-induced degradation of above-ground properties and processes has received attention, consequences for belowground dynamics and nutrient cycling are unknown.

 The goal of this project is to gain insight into the belowground biogeochemical consequences of livestock grazing in a high-mountain tropical ecosystem. While N cycling in grazed landscapes has been studied in some areas, most research has been conducted at low-elevation, temperate sites. In contrast, the montane tropics are an area where grazing pressure continues to increase, surface waters may be highly sensitive to increased N inputs, and a rapidly changing climate makes it essential to have baseline data on biogeochemical dynamics. In this study, we used both natural grazing gradients as well as a grazing exclusion experiment in the Cordillera Blanca to examine the degree to which grazing alters soil N cycling and is linked to shifts in N concentrations in adjacent surface waters as well as denitrification dynamics in soil and leachate.

 We collected an extensive set of soil and foliar samples from inside and outside of cattle grazing exclosures and in areas of high and low grazing impact across an elevational gradient in the Ulta Valley in Huascaran National Park, Peru. Water samples were also collected from adjacent areas. Data on soil and foliar nitrogen and is currently being processed at the University of Utah. This information, along with data on vegetation community dynamics, will provide useful information on the impacts of grazing in high mountain grasslands and help guide management of these delicate ecosystems.

This research is being led by Dr. Samantha Weintraub (University of Utah) & Dr. Rebecca Cole (University of Hawaii at Manoa).

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