Geologic nitrogen and climate change in the geochemical budget of Karkevagge, Swedish Lapland
by Dixon, John C.; Campbell, Sean W.; Durham, Bill
While nitrogen is an essential nutrient for plant growth in the terrestrial environment and generally fixed by biological processes in soils, in Arctic and alpine environments N is a severely limited nutrient because of both climatic and soil constraints. It is widely believed that geologic nitrogen is such a small component of the nitrogen cycle that this source is generally not viewed as being a significant component. However, it has been estimated that in fact as much as 20% of the total nitrogen budget may be stored in geological materials and where N is severely limited such as in Arctic and alpine environments geologic N may in fact be a dominant source of N for biological processes. Nitrogen saturation may therefore have substantial impacts on geochemical cycles involving both soil and water chemistry. In order to determine the potential for the rocks and soils of Karkevagge to be significant sources of N samples of the principal rock units of the valley along with soil developed on a variety of rock types were sampled. Ammonia/ammonium contents were determined using ion chromatography. Ammonia/ammonium contents of the sampled rock materials display a wide range of abundances with marble containing a mean NH4 abundance of 4300 mg N kg, quartz schist 1600 mg N kg, biotite schist 4300 mg N kg, and garnet mica schist 4500 mg N kg. Soil ammonia/ammonium abundances were found to be 1600 mg N kg. N-pool abundances range from 2.5 kg m(3) for marble to 8.7 for garnet mica schist. Soil N-pools were found to be 3.0 kg m(3). The inorganic N present as ammonia/ammonium measured in bedrock represents substantial N-input into the geochemical budget. The N in bedrock originates from organically bound nitrogen associated with sediment or geothermal waters representing a mixture of sedimentary, mantle and meteoric sources. Nitrogen in the soil originates from the weathering of N-rich rocks, the presence of unweathered nitrate minerals or fluid inclusions released to the soil and ultimately to plants by assimilation by biota or nitrification. Geologic nitrogen in Karkevagge rocks and soils may represent a large and reactive pool with the potential for considerable impact on the geochemical system. To investigate the potential responses of nitrogen release, as well as other nutrients to increasing CO2 concentration scenarios, flow-through leaching experiments were conducted on crushed rock samples and soils from the study area. Leaching experiments revealed two to three fold increases in both N and K release with a tripling of carbon dioxide concentrations. Ammonium shows more rapid release than potassium which suggests substantial changes in nitrogen cycling with increasing CO2 enrichment of the atmosphere. (C) 2012 Elsevier B.V. All rights reserved.
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