Sites that are N-rich either naturally or following disturbance have a high potential to function as sources of most of the reactive N forms identified in Table 13.1 because mineralization and nitrification, the processes that produce most of these reactive forms, occur at high rates.
Nitrogen sinks are defined as habitats that have a high potential to remove reactive N from the environment, preventing its movement into adjacent ecosystems. Ecosystems such as wetlands that are wet and rich in organic materials, for example, have a great potential to function as sinks because of their ability to support denitri-fication. In many cases these sink areas absorb reactive N produced in source areas of the landscape. Riparian buffer zones next to streams, for example, can be managed to absorb nitrate moving out of crop fields in groundwater (Lowrance et al., 1984). This nitrate can be stored in plant tissue or in soil organic matter as organic N or can be denitrified to N gas and thereby released to the atmosphere—preferably as N2, a nonreactive form.
Humans have doubled the circulation of reactive N on earth, creating a nitrogen cascade in which added N flows through the environment, leading to degradation of air and water quality and coastal ecosystems in many areas (Vitousek et al., 1997; Galloway et al., 2003). Solutions to landscape, regional, and global N enrichment problems often rely heavily on managing microbial N transformations. For example, coastal areas of the Gulf of Mexico suffer from eutrophication
TABLE 13.4 Criteria for Determining if a Site Is a Source or a Sink of N in the Landscape (from Groffman, 2000)
Is the site N rich?
Is the site highly disturbed?
Does the site have a high potential for denitrification?
Does the site have a high potential for NH3 volatilization?
Fertilized Fine texture (clay) Legumes Wet tropics
Disturbance of plant uptake (e.g., harvest) Stimulation of mineralization (e.g., tillage) Disturbance of links between plant and microbial processes (e.g., tillage)
High available organic matter
and hypoxia that have been linked to excess N from the Mississippi river basin (Turner and Rabalais, 1994). Proposed solutions to this problem include better management of microbial N- transformations in crop fields as well as the creation of denitrifying wetland sinks for excess N moving out of agricultural areas (Mitsch et al., 2001).
Source-sink dynamics of N ultimately depend on the juxtaposition of different ecosystems in the landscape and the hydrologic and atmospheric transport vectors that link them—a complex topic that requires knowledge of hydrology and atmospheric chemistry in addition to soil ecology and microbiology. Because soil microbes play a crucial role in forming and consuming reactive N in the environment, however, their management can be an important and even crucial means for regulating N fluxes at local, regional, and global scales.
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