Response of nitrogen cycling to simulated climate change: differential responses along a subalpine ecotone

In situ nitrogen (N) transformations and N availability were examined over a four-year period in two soil microclimates (xeric and mesic) under a clim ate-warming treatment in a subalpine meadow/sagebrush scrub ecotone. Experi mental plots that spanned the two soil microclimates were exposed to an in situ infrared (IR) climate change manipulation at the Rocky Mountain Biolog ical Laboratory, near Crested Butte, Colorado. Although the two microclimat es did not differ significantly in their rates of N transformations in the absence of heating, they differed significantly in their response to increa sed IR. Under a simulated warming in the sagebrush-dominated xeric microcli mate, gross N mineralization rates doubled and immobilization rates increas ed by up to 60% over the first 2 years of the study but declined to predist urbance rates by the fourth year. This temporal pattern of gross mineraliza tion rates correlated with a decline in SOM. Concurrently, rates of net min eralization rates in the heated plots were 60% higher than the controls aft er the first year. There were no differences in gross or net nitrification rates with heating in the xeric soils. In contrast to the xeric microclimat e, there were no significant effects of heating on any N transformation rat es in the mesic microclimate. The differing responses in N cycling rates of the two microclimate to the increased IR is most certainly the result of d ifferences in initial soil moisture conditions and vegetation type and cove r.