Nitrogen cycling at treeline VII. Laboratory Decomposition

To assess spatial and seasonal patterns of pools and fluxes of dissolved inorganic- N (DIN), amino acid- N (AAN) and microbial biomass N (MBN), we conducted in situ soil incubations at four time periods during May 2001 - May 2002: spring thaw, peak growing season, fall senescence and over-winter. The goal was to be consistent in sampling each of these time periods within each mountain range, which was possible due to the 3-4 week lag in phenology (e.g. budbreak or initiation of senescence) between the southernmost and northernmost sites. Sites were sampled in order from south to north. During the spring 2002 sampling period, soils in the Brooks Range thawed prior to those in the White Mts. and the sampling sequence was adjusted to accommodate this. Within each treeline or forested sub-site, a 50 m transect was established parallel with the slope contour of the mountain. Six points were randomly selected along each transect, and soils were sampled near these points for the entire year. Rates of net DIN mineralization and net AAN production were measured using an in situ buried bag technique (Robertson et al., 1999). We used a 6.7 cm diameter steel corer fitted with a perforated, plastic sleeve to collect paired adjacent soil cores, and sampled below the live moss and detritus layers to a depth of 20 cm. The function of the perforated sleeve was to maintain structural integrity of the soil core during sampling. The perforated sleeve containing the intact core was then placed in a 1 mil breathable polyethylene bag followed by a fine mesh bag, gently returned to the original location, covered with litter and left to incubate. Incubation length was 4 weeks for the spring, growing season, and senescence sampling periods, and from September 2001 to early June 2002 for the over-winter sampling period. The second core in each pair was stored on ice and transported to the laboratory in Fairbanks. Soils were rocky at some sites and sampling to 20 cm was not possible; for these samples, the minimum depth of coring was 10 cm. After harvesting each core, the subsequent pit was backfilled with soil to minimize disturbance to adjacent samples. We examined the decomposability of soil organic matter by measuring mass loss over 1 year at all sites using 15.2 x 1.8 cm birch wood tongue depressors (TDs) as a common litter. TDs were oven dried for 24 hours and weighed before placement in the field. During September 2001, three 20 m transects were randomly established at high, mid and low elevations (spaced 10 m apart) at each forest and treeline sub-site. Five TDs were inserted vertically into the soil profile until flush with the surface of the organic layer at 5 m intervals along each transect. In September 2002, TDs were carefully collected, transported to the laboratory in Fairbanks, rinsed, oven-dried and reweighed. Percent mass loss was calculated. The site at Site Summit on Fort Richardson Military Base was closed during fall 2001 and was not sampled