Because grasses and shrubs may induce different spatial distributions of nutrients in desert soils, this study was initiated to examine the redistribution of nitrogen in grassland and shrubland soils over a long time period. The stable isotope N15 was applied to plots in grassland and shrubland, and the plots were measured annually from 1989-1993 and again in 1999, 2001, and 2002.
Plot establishment - We established four 10 X 10-m plots in grassland and shrubland habitats near the Five Points area of the Sevilleta Long Term Ecological Research Site (LTER) in the northern Chihuahuan Desert, New Mexico, USA.
In the grassland, paired plots (two plots separated by 50-m) were located at sites where Bouteloua eriopoda (Torr.) Torr. (Black grama) and B. gracilis (H.B.K.) Lag. ex Steud. (Blue grama) dominate. In the shrubland, paired plots were located in areas dominated by Larrea tridentata D.C. (Cov.)(creosotebush).
Tracer application - 15NH4Cl tracer was applied to ten 15.25 cm diameter points arranged in a stratified random design in each of the four 10 X 10 m plots in the grassland and shrubland. 0.33 g of 15NH4Cl were dissolved in 500 ml of deionized water and applied to ten sites per plot in 50 mL aliquots in July, 1989.
Sampling - The site of each spike application was sampled annually during the summer in 1989, 1990, 1991, 1992, and 1993. Two samples with a volume of 28.5 cubic centimeters were removed from the site of spike application, which had a total volume of 1815 cubic centimeters to 10-cm depth. Each sample contained a small percents of the total soil volume, about 1.5 percent. All samples were air-dried, sieved through a 2 mm sieve, and shipped to Duke University. Ground soil samples were analyzed for 15N.
Field collections were made every even-numbered month as close to the 15th as possible.
The samples taken in 1989-1993 and 1999 were analyzed by Larry Giles on a mass spectrometer at the Duke University