Nitrogen turnover and greenhouse gas emissions in a tropical alpine ecosystem, Mt. Kilimanjaro, Tanzania

Abstract

Tropical alpine ecosystems are identified as the most vulnerable to global environmental change, yet despite their sensitivity they are among the least studied ecosystems in the world. Despite its important role in constraining potential changes to the carbon balance, soil nitrogen (N) turnover and plant availability in high latitude and high altitude ecosystems is still poorly understood. Here we present a first time study on a tropical alpine Helichrysum ecosystem at Mt. Kilimanjaro, Tanzania, which lies at an altitude of 3880 m. Vegetation composition is characterized and major gross N turnover rates are investigated using the N-15 pool dilution method for three different vegetation cover types. In addition greenhouse gas exchange (CO2, N2O and CH4) was manually measured using static chambers. Gross N turnover rates and soil CO2 and N2O emissions were generally lower than values reported for temperate ecosystems, but similar to tundra ecosystems. Gross N mineralization, NH4 (+) immobilization rates, and CO2 emissions were significantly higher on densely vegetated plots than on sparsely vegetated plots. Relative soil N retention was high and increased with vegetation cover, which suggests high competition for available soil N between microbes and plants. Due to high percolation rates, irrigation/rainfall has no impact on N turnover rates and greenhouse gas (GHG) emissions. While soil N2O fluxes were below the detection limit at all plots, soil respiration rates and CH4 uptake rates were higher at the more densely vegetated plots. Only soil respiration rates followed the pronounced diurnal course of air and soil temperature. Overall, our data show a tight N cycle dominated by closely coupled ammonification-NH4 (+)-immobilization, which is little prone to N losses. Warming could enhance vegetation cover and thus N turnover; however, only narrower C:N ratios due to atmospheric nitrogen deposition may open the N cycle of Helichrysum ecosystems.