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Lecture 9

BIOC63Fall2013 Lecture 9 Notes.docx

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University of Toronto Scarborough
Biological Sciences
Karen Williams

BIOC63Fall2013 Lecture 9 Notes: Global Change Why focus on nitrogen? o N, P, K typically the most limiting nutrients o o N = largest human impact o Global reservoir of N = atmosphere (oxygen is 21% of the atmosphere and 0.004% is carbon dioxide) o Unreactive N = 70% N2 gas o reactive nitrogen o organic N  from plant and animal tissue o NH x NH , 3H ,4Haber-Bosch, microbial fixation o NO x NO, NO , 2ossil fuel combustion, lightning A. Eutrophication (N deposition) Biogenic nitrogen cycle  Lightning  creates huge pressure and high temperatures that can turn unreactive nitrogen to reactive nitreogen in the soil  Soil bacteria can incorporate N in their own cells for  These die and then decompose  Rhizobia  transfer to plants  Plants use it  Animals eat the plant  Excrete it back into the soil in form of nitrate Human influence on the Nitrogen cycle  Reactive N  NH x  N + 3H  2HH 2(g) 2(g) 3(g)  Needed to run HB reaction o German chemist Fritz Haber and Carl Bosch 1909 o High pressure  200-400 atm o High temp  400-600°C 1 o Use fossil fuels to drive the expensive reaction o These conditions are usually only otherwise produced by lightning  Humans have more than doubled global N input  Natural fixation  100 Tg of N/year o More than doubled now thanks to humans o No longer a closed cycle o  Excess atmospheric nitrogen is deposited  Excess nitrogen in terrestrial systems  plants grow along an [N] gradient o There is a different [N] optima for different plant species o Unproductive when N is limiting, productive when N is abundant o When N is limiting there is not enough N to have an abundance of organisms, which means there is a lot of space available, and low competition between species because of this = rich plant communitites because more species of plants can co-exist o when N is abundant = plants are productive in increasing their biomass, but only the dominant species that are most effective at nitrogen fixation will dominate = poor plant communities  Nitrogen overload  decrease in plant diversity o Once the N threshold was reached, D rotundifolia’s population decreased significantly 2 o o D.flexuosa  increased with higher N (because it
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