Soluble reactive phosphorus (SRP) and ammonium in waters from tussock tundra and wet sedge plots near Toolik Lake, AK during the summer of 2006.
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Plot location was chosen based on dominate vegetation type (T or W). The tussock tundra plots are located near the Moist Acidic Tundra plots characterized by Shaver and Chapin (1991). The wet sedge plots are located near the outlet of Toolik Lake; all plots have been used by the Arctic LTER project at Toolik Lake.
In wet sedge tundra, three 1 m2-sized plots were positioned randomly in each of two treatments of block 2 of the LTER wet sedge scheme (outlet site), control and N+P-fertilized , respectively, in 2000. Paired control and fertilized plots were labelled with 14C and 15N at one of three different dates of the summer season 2001.
In moist acidic tussock tundra six 1 m2 sized plots were positioned in three of the "X" (=extra") blocks of the LTER scheme, in 2001, inside an area extending 5 m from the boardwalks and, positioning plots so that similar area shares between tussocks and intertussock areas was achieved. In each of the three X blocks two 1m2-sized plots were established. The whole 5 m area of the X blocks extending from the board walks was divided into two halfs, each containing one of the 1m2 sized plots. One half was set aside as the control and the other half was designated to be fertilized. Fertilization with a Hoagland solution (see protocol) started in late summer 2001, and was repeated annually after that , early in the summer season. Paired control and fertilized plots were labelled with 14C and 15N of the summer season 2002; all three pairs were labelled as closely together in time as logistically possible.
Using soil needles and syringes, we pulled soil water for dissolved nutrient analyses. The Water was filtered using a Whatman GF/F filter (0.7μm pore size, nominal) into polypropelyne bottles (SRP and NH4) or glass scint vials (DOC). Samples were stored at 4 degreeC in the dark until analysis. SRP and NH4 were analyzed usually within 24hrs of sampling. DOC samples were acidified to pH ~3 after filtration and sent to the University of Michigan for analysis.
Soluble reactive phosphorus was analyzed using a molybdate colorometric assay (Wetzel and Likens, 1979). Each sample, standard, and blank was duplicated during analysis. We pipetted 10 mLs of each into two P tubes. After the 1 mL SRP reagent was added the samples were incubated in the tlight for 30-90 minutes. The samples were analyzed on the spectrophotometer in the dry lab. All waste was dumped into radactive waste container.
Ammonium ion concentrations were analyzed using a fluorometric assay modified to correct the calculation errors in the original publication. Details of methods, reagents, etc are given in Holmes et al. 1999. The method relies on the the reagent Orthophthadialdehyde (OPA) to bind witht he ammonium causing fluorescence. Each sample under went four treatments: 1. Background Fluorescence (BF), sample + color blank, 2. Sample + Working Reagent, 3. Sample + Working Reagent + Low Spike Addition, and 4. Sample + Working Reagent + Low Spike Addition. The samples were incubated in the dark for at least 8 hours. All treatments were measured on the flourometer in the wet lab. Background fluorescence was subtracted from treatments 2,3,and 4. The concentration of ammonium was calculated using a standard addition curve to account for Matrix Effects (APHA, 1995). All waste was dumped into radactive waste container.
Samples were run on a Shimadzu TOC 5000 Carbon Analyzer using a platinum catalyzed high-temperature combustion to CO2 and infrared detection (Shimadzu TOC-5000; Shimadzu, Columbia, MD).
American Public Health Association. 1995. Standard Methods for the Examination of Water and Wastewater, 19th Edition. American Public Health Association. USA.
Holmes R.M., Aminot A., Kerouel R., Hooker, B.A., Peterson, B.J. 1999. A simple and precise method for measuring ammonium in marine and freshwater ecosystems. Canadian Journal of Fisheries and Aquatic Sciences 56 (10): 1801-1808.
Shaver, G.R., and F.S. Chapin. 1991. Production - biomass relationships and elemental cycling in contrasting arctic vegetation types. Ecological Monographs 61(1): 1-31.
Wetzel, R.G. and G.E. Likens. 1979. Limnological Analyses. 2nd ed. Springer-Verlag, New York. pp 89-92.
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