Based on results from the completed N15 addition experiments, it appears that collection of two additional downstream water samples for analysis of N15 in NO3 pools will greatly aid in determining nitrification rate and nitrate uptake length. Preliminary calculations indicate that nitrification rate accounts for nearly 25% of the total ammonium uptake rate in Walker Branch, and estimates of nitrate uptake length using the N15 data are considerably shorter than estimates calculated from the NO3 additions prior to the start of the N15 experiment. Therefore, please collect two additional 4L water samples for 15NO3 at distances 1.5 times and 2 times the distance of the most downstream station on each of the 4 water collection dates (day 0, day 20, day 41, and 12-16 hours post N15 drip). In other words, if your most downstream routine collection station for organisms, etc. is at 200 m below the N15 dripper, then collect these extra water samples at 300 m and 400 m.