PLANT CARBON–NUTRIENT INTERACTIONS CONTROL CO2EXCHANGE IN ALASKAN WET SEDGE TUNDRA ECOSYSTEMS

Abstract

We explored the long-term (8-yr) effects of separate field manipulations of temperature and nutrient availability on carbon balance in wet sedge tundra near the Arctic Long Term Ecological Research (LTER) site at Toolik Lake, Alaska. Our goals were (1) to assess the relative importance of chronic warming (with field greenhouses) and increased N and P availability (by fertilization) in controlling gross ecosystem photosynthesis, ecosystem respiration (plant plus heterotrophic respiration), and ultimately ecosystem C balance; and (2) to attempt to partition ecosystem responses to these treatments between plant and soil contributions. We present results of the effects of these manipulations on whole-system CO2 exchange over seasonal and diel cycles, and on nonrhizosphere soil microbial respiration using in situ soil incubations. Wet sedge control plots were, at best, a weak sink for carbon even during the peak growing season. Chronic nutrient additions of N + P shifted wet sedge carbon balance to a strong sink throughout the growing season; nutrient availability regulated seasonal and diel CO2 exchanges in these two wet sedge ecosystems. The N + P plots had significantly higher photosynthesis and ecosystem respiration in spite of the unanticipated effect of ∼30% reduction in thaw depth in these plots, apparently due to a twofold increase in litter accumulation insulating the soil surface and/or possible shading from greater plant cover in these plots. These results highlighted the prevailing importance of nutrient–carbon interactions in controlling ecosystem processes and ecosystem C balance in arctic tundra. In contrast, warming had only subtle effects on CO2 exchanges. Increased temperatures in the warmed plots had little effect on instantaneous rates of photosynthesis or respiration. After eight years of chronic warming with an average 5.6°C higher air temperature over the growing season and a 40–200% increase in net N mineralization rate, it was surprising that warming did not have more profound effects on CO2 exchange and plant cover. If there were an effect of warming, increased temperatures might cause early canopy development and lengthen the growing season, rather than directly affect instantaneous rates of photosynthesis. Based on photosynthesis–light response curves developed from the early- and late-season diel measurements, we demonstrated that the main effect of warming was to accelerate the development of the canopy early in the season. By midseason, however, there were no significant differences in C exchange between warmed and control plots. Perhaps the most important and novel result emerging from this study is the prevailing importance of plant C exchange, not soil processes, in driving ecosystem C fluxes. First, nonrhizosphere soil microbial respiration as estimated CO2 flux from in situ soil incubations was a small fraction of whole-system respiration and did not vary among treatments. This suggests that anaerobic conditions or some other factor may limit soil microbial respiration more than do temperature or nutrients. Second, plant respiration contributed most (90%) of the ecosystem respiration in fertilized plots. This unanticipated and large contribution from plant respiration highlights the critical importance of understanding the response of plant respiration to global environmental change in these wet sedge ecosystems.