Detrital export

Other outputs include harvested plant biomass and detrital export (removing the nitrogen stored in the biomass, or simply wetland outflows where nutrients are exported in water and POM leaving wetlands such as in tidal marshes or riparian wetlands). 

The main nitrogen losses from marshes are caused by denitrification and detrital export and sedimentation sinks. Denitrification has been reported to be the major cause of nitrogen loss from marshes. Although the potential for denitrification is very high in the reduced marsh soil, the level of in situ denitrification depends on the availability of nitrate. Nitrate must be either formed through nitrification of ammonium or supplied from an extrinsic source such as tidal input, wet deposition, or groundwater flow. 

In the present-day ocean, about half of the PIC exported to oceanic sediments consists of the remains of foraminiferans. These microorganisms are protozoans. They are widespread in the marine environment with some species having a pelagic lifestyle and others benthic. As shown in Figure 15.1a, their calcareous structures have the appearance of a chambered snail shell and are composed of the mineral calcite. Since this hard part is covered by tissue, it is technically a type of skeleton. These detrital remains are referred to as tests or forams. Among the present-day and extinct species of foraminiferans, considerable variation exists in the size, shape, and density of their tests. 

Most modem N-cycle models simulate N export using one or the other of these approaches, but details vary and some models combine aspects of expHcit sinking and reminerahzation with Martin curve parameterizations. For example, Chai et al (1996) modeled a 5-component N-based ecosystem including a detrital N compartment with explicit sinking, but did not permit detrital remineralization within the euphotic zone. In contrast, Christian et al (2002a) used a Martin curve to redistribute detrital N vertically, but rather than remineralizing at the specified depth, it is returned to the... 

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