Soil Carbon and Nitrogen Turnover at and below the Elevational Treeline in Northern Fennoscandia

In Swedish Lapland, summer in situ decomposition weight losses of mountain birch (Betula pubescens Ehrh. ssp. tortuosa [Ledeb.] Nyman) litter originating from valley and forest limit sites were significantly greater than losses from treeline litter (12-13% versus 8-9%), regardless of whether treeline litter was incubated at the treeline or in the valley. Carbon dioxide flux rates of valley and forest limit soils (expressed on a dry organic matter basis) increased exponentially (~800%) over a temperature range of 5 to 30°C under nonsaturated conditions (40 to 75% of saturation water content). Under the same nonsaturated conditions, soil carbon dioxide flux rates of treeline soils were significantly lower (by 65-75%). Seasonal fluctuations of KCl-extractable ammonium concentrations over a 1-yr period (July 1988 to June 1989) were not statistically significant nor were net changes in site ammonium concentrations detected. However, mean annual concentrations of ammonium were significantly higher in valley and forest limit soils than in treeline soil (148 and 191 μg g-1 dry soil, respectively, versus 11 μg g-1, organic horizon). Nitrate was not detectable in most samples, but did reach 4 μg g-1 in some treeline samples. Nitrate, thus, was not important for intersite fertility comparisons, but did constitute a major source of nitrogen at the treeline. Daily soil temperature fluctuations and mean soil temperatures were similar in 1988 for the treeline and forest limit sites, while in 1989 the treeline site was considerably warmer than the forest limit. In both summers, the valley mean soil temperatures were coolest and temperatures were less extreme. Despite similarities in soil temperature, organic matter content at the treeline was significantly higher than at the forest limit, indicating that carbon was not being cycled as rapidly at the treeline. Decomposition at the treeline was, thus, not temperature-limited during the activity season. Possible limitations due to low soil nitrogen availability and litter quality are discussed.