When nutrients become less limiting in tundra ecosystems, a suite of changes result. In moist acidic tundra in northern Alaska, long-term experimental fertilization shifts the plant community from an even mixture of sedges, deciduous and evergreen shrubs, and mosses, to one dominated by one species of deciduous shrub and a forb. The fertilized community has significantly greater primary productivity and plant litter production, resulting in cooler soils because of the shading and insulative changes to the soil surface. These changes in ecosystem properties may further alter ecosystem processes which control the rates of carbon and nitrogen cycling in arctic ecosystems To better understand how decomposition of three abundant vascular plant species is affected by nutrient addition, including the resulting vegetation changes, we conducted two 3-year decomposition experiments in fertilization treatments lasting 6, 16, and 23 years. We tested the following hypotheses: (1) Changes in the decomposition microenvironment in fertilized plots slow decomposition, and these effects are intensified with duration of experiment. (2) Plant community shifts in response to greater nutrients affect decomposition via species-specific differences in leaf decomposition rates. (3) Leaves that have been fertilized and have higher tissue quality decompose faster than those grown under ambient soil nutrients.
Results/Conclusions
Concurrent environmental measurements in the treatment plots confirmed that fertilized plots had shallower depth of thaw and cooler soil temperatures than control plots. As predicted, decomposition was slower for all three species in fertilized plots, but this was not detected until after a decade of nutrient addition. Supporting our second hypothesis, the three study species differed in leaf decomposition rate across all experiments, with the sedge (Eriophorum vaginatum) decomposing more slowly than both the deciduous shrub (Betula nana) and the forb (Rubus chamaemorus). Finally, fertilized leaves consistently decomposed faster than leaves grown under ambient nutrients, but results were affected by nutrient status of the treatment plot for two of the species. Since Eriophorum is excluded from fertilized plots after a decade, species specific differences could be speeding up community level decomposition, counteracting the negative effects on decomposition from the cooler soils. These results emphasize the challenges in determining ecosystem-level effects of increased nutrient availability in arctic tundra that is occurring with climate warming. Not only do the plant communities shift, but plant tissue quality and decomposition microenvironment also change. Understanding the net effect of these processes over long time scales will inform more accurate predictions of regional responses to warming.