Summary of info from "Issues in Ecology #1: Human Alteration of the Global Nitrogen Cycle

What are the main anthropogenic sources of fixed nitrogen, and how do they compare with natural sources?

a) Nitrogen fertilizer 80 Tg/year (in what chemical form?)

b) Nitrogen-fixing crops 40 Tg/year

c) Fossil fuel use 20 Tg/year (how does combustion of fossil fuel fix nitrogen?)

d) Burning of forests and grasslands 40 Tg/year

e) Draining of wetlands 10 Tg/year (why does this contribute fixed nitrogen?)

f) Land clearing for crops 20 Tg/year (ditto)

**Over the past 100 years, humans have approximately doubled the amount of nitrogen entering the land-based nitrogen cycle. (where does this fixed nitrogen wind up?)

How does human impact on the N cycle relate to the C cycle?

a) Increased amount of fixed nitrogen leads to increased plant growth, which leads to increased absorption and fixation of carbon dioxide.

b) This fertilization effect may be the "missing sink" for CO2. Estimates say that about 1000 Tg of carbon has been absorbed by an unknown sink; plant growth could account for 100-1300 Tg according to estimates by different groups.

c) Other impacts: N2O (produced by denitrifying bacteria) is a potent greenhouse gas.

Main effects on aquatic ecosystems:

a) Acidification of lakes (through deposition of combustion-related nitric acid in rainfall)

b) Eutrophication: Fertilization with fixed nitrogen compounds (such as what?) causes an algae bloom, the algae blocks light from getting down into the water, plants at the lake bottom die & decompose, the bottom of the lake becomes very depleted in oxygen (why?). Fish and other animals die.

c) Fertilization can cause decreased diversity of plant/animal species in a previously diverse ecosystem.

d) Fixed nitrogen enters groundwater and drinking water and can form a health risk (which form of fixed nitrogen causes methemoglobinemia?)