Grasslands

Grasslands, including steppes, savannas, prairies, and tundra, are important terrestrial ecosystems covering about a quarter of the Earth's land surface. Grasslands build soil systems that differ from those of forests and other vegetation types. A key feature of grasslands, especially those that are more fertile, is their high turnover of shoot and root biomass, and the consequent large pool of labile organic matter at the soil surface. In contrast to many terrestrial ecosystems, heavy herbivore loads, both above- and below-ground, are also a characteristic feature of fertile grasslands. This significantly influences plant growth and species composition, and the structure of grassland, since herbivores consume high proportions of annual aboveground (McNaughton 1983) and below-ground (Stanton 1988) net primary production.

Aboveground herbivores also have a profound influence on nutrient pathways in grasslands through short circuiting both litter return and soil recycling processes (Bard-gett & Wardle 2003). A large percentage of nutrients taken up by plants in grazing ecosystems is cycled directly through animal excreta, resulting in accelerated soil incorporation, particularly of nitrogen and phosphorus (Ruess & McNaughton 1987). Therefore, soils of grazed grasslands tend to have relatively small amounts of litter on the soil surface, but large amounts of organic nitrogen and carbon in the soil. These features combine to produce a soil environment that sustains, or is sustained by, an abundant and diverse faunal and microbial community.

In high latitude and high altitude grasslands, as well as in grasslands of semi-arid regions, soil processes and ecosystem goods and services are likely to be controlled more by abiotic factors than by biotic ones. This may be different in lowland temperate or tropical regions where soil processes are not so limited by climate. Carbon sequestration, for example, across the Great Plains of the United States, depends on interactions between annual precipitation and soil type (Burke et al. 1989). Nutrients can be lost due to a number of factors. For example, in tall-grass prairie, the major local pathways for nutrient loss are either abiotic (hydrologic fluxes and fire-induced losses) or biotic (gaseous fluxes via denitrification and ammonia volatilization) (Blair et al. 1998). In wet grassland systems, hydrology is indisputably the dominant controlling factor. However, in waterlogged soils, there can be considerable release of gaseous products from denitrification, while at the same time waterlogging limits nitrification and mineralization (Nijburg & Laanbroek 1997).

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