Nitrogen cycle loss terms

The loss terms in N-cycle models that transform particulate and dissolved organic nitrogen into other forms can include a variety of processes (e.g., phytoplankton exudation, zooplankton grazing, sloppy feeding, phytoplankton and zooplankton mortality, bacterial remineralization, etc.). Different models may differ substantially in terms of which of these are included and their formulation (Christian and Anderson, 2002). Many N-cycle models now include significant phytoplankton exudation loss terms. This is often parameterized by simply specifying that some fixed fraction of the DIN uptake by phytoplankton is shunted directly to the DON pool (e.g., Anderson and Williams, 1998). Sloppy feeding by zooplankton can be similarly accounted for. Many models also include linear loss terms in the phytoplankton equation that represent either natural mortality or phytoplankton respiration (e.g.. Hood et ai, 2001). 

When the impact of process scale is viewed from the planetary boundaries perspective, the inherent multicriteria nature of any sustainability assessment is indispensable. Even when only environmental LCA impacts are accounted for, studies have shown that certain boundaries have been crossed or are very close to the limit (i.e., with respect to climate change, biodiversity loss, and nitrogen and phosphorous cycles), while others are stiU reasonably well safeguarded (i.e., stratospheric ozone depletion, ocean acidification, and freshwater use) [64]. It is therefore possible that different production sectors may have an impact on different planetary boundaries some of which may be within or already outside their safe operating space. For instance, studies have indicated the severe impacts of plastic debris on marine organisms [65]. Thus, from a cradle-to-grave LCA perspective, fossil-based plastics production may have a more direct or at least a different kind of effect in terms of biodiversity compared to fossil-based fuel production, which is certainly in higher production scales. 

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