The LTER moist acidic tussock (Toolik MAT) site was set up in 1989. The experimental design is four blocks of 5 x 20 meter plots with randomly assigned treatments within each block. Treatments include control (CT), nitrogen (N), phosphorus (P) and nitrogen plus phosphorus (NP). Fertilizer is added annually following snowmelt in June as 10 g/m2 nitrogen (as NH4NO3) and 5 g/m2 phosphorous (as triple superphosphate). Exclosure plots were set up at the MAT site in July 1996 on extra 5 x 20 meter plots within the four-block design of the 1989 LTER acidic tussock experimental plots. On each plot a 5 x 10 meter section was fenced with large mesh (4-inch square mesh) and within this fence a 5x5-meter plot was fenced with a small mesh (1/2-inch square mesh). In each block two fenced plots were setup: a plot with no fertilizer and a plot with annual fertilization treatments as described above. Thus the treatments created are no fence, no fertilizer (NFCT); small mesh fence, no fertilizer (SFCT); large mesh fence, no fertilizer (LFCT); no fence, N plus P (NFNP); small mesh fence, N plus P (SFNP); and large mesh fence, N plus P (LFNP). In 2006, a new set of experimental treatments were established in MAT near the earlier plots described above. These include a gradient of fertilization treatments that receive nitrate or ammonium alone, or receive N and P additions with 0.5, 1, 2, 5, or 10 g N added with half of that amount added as P. Only Blocks 1-3 of the 2006 plots were sampled.

For this dataset, only the CT and NP plots were sampled in each of the three sets of experiments (set up in 1989, 1996, and 2006). The 1996 NP plots are the NFNP plots described above, and the 2006 NP plots are the 10gN plots (F10) described above.
Calculations: none Sampling Description.On each sampling date, a metal thaw depth probe with centimeter increments was inserted in 3-5 random locations within each plot. Once the probe hit permafrost, the value for depth of thaw was recorded. Immediately adjacent to this sampling, a hand-held soil thermometer was inserted to a depth of 5 cm and the temperature was recorded. If thaw exceeded 10 cm, a second soil temperature measurement was made at 10 cm depth.