Markham John H
Department of Biological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada.
Oecologia. 2009 Aug;161(2):353-9. doi: 10.1007/s00442-009-1391-0. Epub 2009 Jun 19.
Traditionally it has been thought that most boreal forest communities lack a significant input of biologically fixed nitrogen. Recent discoveries of nitrogen fixation by cyanobacteria associated with mosses have resulted in a re-evaluation of this view. While it is recognized that rates of nitrogen fixation in mosses can be highly variable, there is little understanding as to why this occurs. I monitored nitrogen fixation, using acetylene reduction, in wet lowland and dry upland boreal forest communities, in central Canada, over a growing season. At the peak of nitrogen fixation in mid summer, Sphagnum capillifolium had an 11 times higher rate of fixation than Pleurozium schreberi. Variation in canopy openness and precipitation had no effect on rates of fixation over the growing season. In P. schreberi fixation rates did not vary between sites. Temperature had a positive effect on fixation rates in both S. capillifolium and P. schreberi, but the effect was 4 times more pronounced in S. capillifolium. Seasonal rates of nitrogen fixation were estimated at 193 mg N m(-2) for S. capillifolium and 23 mg N m(-2) for P. schreberi. With moderate increases in climate warming, predicted increases in nitrogen fixation in S. capillifolium are sufficient to raise its decomposition rate. Increased temperatures may therefore act synergistically to change boreal systems from a sink to a source of carbon.
传统观点认为，大多数北方森林群落缺乏生物固氮的大量输入。最近发现与苔藓相关的蓝细菌能够固氮，这导致了对这一观点的重新评估。虽然人们认识到苔藓中的固氮速率可能变化很大，但对于其发生原因却知之甚少。在加拿大中部的一个生长季节里，我利用乙炔还原法监测了湿润低地和干燥高地北方森林群落中的固氮情况。在仲夏固氮高峰期，细叶泥炭藓的固氮速率比尖叶泥炭藓高11倍。生长季节内，林冠开阔度和降水量的变化对固氮速率没有影响。在尖叶泥炭藓中，不同地点的固氮速率没有差异。温度对细叶泥炭藓和尖叶泥炭藓的固氮速率都有正向影响，但在细叶泥炭藓中的影响更为显著，是尖叶泥炭藓的4倍。细叶泥炭藓的季节性固氮速率估计为193毫克氮每平方米，尖叶泥炭藓为23毫克氮每平方米。随着气候变暖适度增加，预计细叶泥炭藓固氮量的增加足以提高其分解速率。因此，温度升高可能会协同作用，使北方生态系统从碳汇转变为碳源。
