Document: MIC-3-59-94

Effects of soil organic matter quality and microclimate on soil nitrogen transformations in arctic treeline ecosystems.

MACK, M.*, J.BERINGER, C.COPASS and F.S.CHAPINIII

University of Alaska Fairbanks, Fairbanks, AK 99775-7000 USA ¹

Abstract:
Climate warming is expected to impact soil nutrient cycling in arctic ecosystems directly through effects on soil temperature, and indirectly though changes in vegetation composition. To compare the relative effects of these two factors on net N transformations, we reciprocally transplanted intact soil cores among nearby tundra, shrub, and forest ecosystems that span treeline on the Seward Peninsula of Alaska. We assessed the effects of soil organic matter (SOM) quality (where a core originated) and microclimate (where a core was incubated) on rates of net nitrogen mineralization and nitrification, and the percent of mineralized N that was nitrified over a six-week incubation. In situ rates of mineralization were highest in forest and shrubland, and lowest in tundra. Nitrification rates were an order of magnitude lower in forest and tundra than in shrubland, where 80% of mineralized N was nitrified. In the transplant experiment, mineralization was highly sensitive to both SOM quality and microclimate, and there was no interaction between these factors. Soils originating in forest and shrubland mineralized two times more N than tundra soils wherever they were incubated. In the shrubland microclimate, all cores mineralized two times more N than in the forest or tundra. Nitrification was affected only by SOM quality, and was five times higher in shrubland soil than in forest or tundra soils. The percent of mineralized N that was nitrified, however, was strongly affected by SOM and microclimate, and these effects were interactive. Tundra and shrubland SOM nitrified a higher percent of mineralized N than forest and the forest microclimate induced the highest percent nitrified for all soils but its own, which had the lowest percent nitrified overall. These results suggest that climate warming is likely to have the greatest effect on N mineralization from forest soils because these soils have the highest SOM quality, but a depressive microclimate. Low nitrification potential and large plant stature make it likely that increased N availability will be immobilized in plant biomass. In contrast, high nitrification potential and small plant biomass in tundra and shrub systems may lead to increased N loss in a warming climate, due to increased leaching or denitrifiction.

Keywords: Microclimate, soil organic matter quality, Net nitrogen mineralization, nitrification, Treeline

This abstract is being presented at: 9:30 AM in session:
Oral Session #41: N Dynamics: Additions, Retention and Transformations.