Sources of N

Biogeochemical effects

Physiological responses

Community responses

Case studies

Future study

References

Links

Nitrogen Cycle

Need for Further Study

While much progress had been made in our understanding of the effects of nitrogen deposition, there are many fundamental questions that need to be addressed.  First, much of the research on nitrogen saturation has been conducted in a limited geographic area; specifically, the New England area of the USA, and a regional study in Europe known as NITREX (Gunderson 1991).  The responses of other ecosystems such as the southeast USA and the Rocky Mountains (Baron et al 1994; Williams et al 1996)have also been investigated to a limited extent.  Other temperate ecosystems, such as grasslands need to be studied to see how they will respond to N deposition.

The response of tropical forests may differ greatly from temperate forests (Matson et al 1999), and this region is expected to see rapid increases in nitrogen deposition as a result of increased agricultural activities in the near future.  Tropical forests are typically limited by nutrients other than nitrogen.  If the nitrogen saturation hypothesis is applicable, these systems may naturally be in later stages of nitrogen saturation;  any additional inputs of nitrogen are expected to result in increasing nitrate leaching and gas fluxes.  As nitrogen deposition increases, tropical forests may become an important source of nitrous oxide and nitric oxide (Hall and Matson 1999).  The ecosystem consequences of nitrogen saturation in the tropics are largely unknown.

Many recent studies of nitrogen saturation have used stable isotopes to trace the fate of nitrogen inputs (Nadelhoffer et al 1995; Nadelhoffer et al 1999).  These studies have been highly useful in understanding ecosystem processes, however, a large portion of the tracers has not been recovered.  Researchers have also been surprised at the high capacity of forests to retain nitrogen, even following five years of high nitrogen fertilization rates (Aber et al 1998). 

It is especially important to improve our understanding of sinks for nitrogen deposition because of interactions between the carbon and nitrogen cycles.  Many researchers have proposed that increasing nitrogen deposition would increase NPP, thus sequestering large amounts of anthropogenic CO2.  However, recent evidence suggests that this is not an important carbon sink, and that most of the excess nitrogen is stored in soils, which have a low C:N ratio compared to vegetation. 

Numerous hypotheses have been offered for the missing nitrogen sink including microbial immobilization, abiotic incorporation into soil organic matter, and conversion to organic nitrogen by mycorrhizae (see Aber et al 1998 for discussion).  I hypothesize that a significant amount of unaccounted nitrogen is lost through N2 gas flux during denitrification.