Effect of krummholz establishment on soil previously occupied by tundra: root ingrowth and nitrogen properties of short-term and seasonal water inputs.

• Format: Dataset
• Language: unknown
• Published: Niwot Ridge LTER/University of Colorado1560 30th Street, CB [e-mail] 2001
• Subjects: NWT; Niwot Ridge LTER Site; LTER; Colorado; N; NH4; NH4+; NO3; krummholz; soil; soil water N inputs; tree island; tundra; Tundra
• Online Access: http://hdl.handle.net/10255/dryad.9464; http://metacat.lternet.edu/knb/metacat/knb-lter-nwt.134.2/xml

Previous work has shown that passage of Englemann spruce (Picea Englemanii) and subalpine fir (Abies lasiocarpa) tree islands across tundra lowers the soil carbon and nitrogen storage capacity of the top 15cm of soil (A horizon) (Pauker and Seastedt 1996). The presence of tree islands (krummholz) on the tundra acts as a 'snowfence' causing snow to drift within the krummholz and in a semi-circular area leeward of the krummholz. This drifting causes soil within and leeward of the krummholz to have a much deeper, more consistent and longer lasting snow-cover than the adjacent tundra soils. The snowdrifts form a permanent cover over the leeward and krummholz sites from November through June, helping to insulate the soils beneath from the extreme low temperatures and supplying significantly more meltwater in the spring. In contrast, high winds cause the adjacent tundra to be free from snow and exposed to extremely cold temperatures for much of the winter. Studies have shown that atmospheric nitrogen deposition accumulates on snowpacks throughout the winter months and that this is released as a 'pulse' of nitrogen ions at snowmelt (Williams et al. 1996). We hypothesized that accumulation of snowpacks in krummholz and leeward sites may alter nitrogen dynamics in these areas relative to tundra, by either (a) affecting microbial activity via its insulating effect or (b) accumulating large amounts of atmospherically deposited N and releasing it as a pulse at snowmelt. This study sought to examine the nitrogen inputs into tundra, krummholz, and leeward soils over an entire winter season (using buried ion exchange resin bags which collect ions percolating down at snowmelt and early spring rains) and over a short period in spring, immediately following snowmelt (using microlysimeters to collect soil water over a 24 hour period). Belowground NPP (determined from root ingrowth cores) and d13C values of tundra and krummholz vegetation are also presented in this file. NH4+ and NO3- concentrations of soil water collected over 24 hours (from microlysimeters) were not significantly different at krummholz and tundra sites. In contrast, inputs of NO3- and NH4+ over the entire winter season were affected by site. The amount of NH4+ collected from tundra soils was significantly lower than from krummholz soils but was not significantly lower than that from leeward soils. In contrast, the amount of NO3- from tundra soils was lower than both krummholz and leeward (although this was not quite statistically significant, p= 0.09). To summarize, consistently (although not significantly) greater winter inputs of NO3- were found in areas with the greatest snow accumulation (i.e. krummholz and leeward sites) but this was not the case for NH4+ inputs. Williams et al (1996) suggest that NH4+ released from snow may be immobilized in underlying soils and this might explain why high levels of NH4+ were not detected at leeward sites. However, in order to fully understand the mechanisms that determine the relative winter N inputs at these sites, all the N fluxes (including N inputs in snow and meltwater and microbial activity) must be investigated. Belowground NPP was significantly lower in krummholz sites than tundra sites. This is consistent with much of the literature in which turnover of fine roots in grassland ecosystems greatly exceeds that of forests. The d13C value of stems of conifers was significantly less negative than that of all other vegetative material collected, indicating less discrimination of heavy isotopes in the stem than in other tissues and in tundra vegetation.