Carbon turnover in Alaskan tundra soils: effects of organic matter quality, temperature, moisture and fertilizer

Abstract

SummaryNorthern ecosystems may lose large amounts of soil C as the global climate warms over the next few decades. This study describes how soil C loss is related to temperature, moisture and chemical composition of organic matter in Alaskan tundra soils, including soils that were fertilized annually for 8 years prior to the study.Fertilized and unfertilized soils from four vegetation types (tussock, intertussock, sedge and heath) were incubated at 7 or 15 °C, under saturated or well‐drained conditions, through four 100‐day ‘seasons’ separated by 25‐ to 45‐day frozen periods.Losses of CO2were monitored and total C loss was determined by difference between initial and final C stocks. Initial and final organic matter composition was determined by separation into non‐polar extractable (NPE, mainly fats, oils, and waxes), water‐soluble (WS, mainly soluble carbohydrates and phenolics), acid‐soluble (AS, mainly cellulose and related compounds), and acid‐insoluble (AIS, ‘lignin’) fractions. An isotopic label (99%13C‐enriched glucose) was added to track transformations among the C fractions.Total C loss during the experiment was 3–32% of initial C mass depending on soil type and treatment, with most of the loss as CO2. Wet sedge tundra soils, with the largest AS and AIS fractions, lost the least CO2and total C. The added13C ended up in all C fractions, indicating production, as well as loss of all fractions.The greatest CO2and total C losses occurred under warm, well‐drained conditions, in all soils. The effects of fertilizer treatment were occasionally significant but never large relative to the other treatments.Despite the long incubation under standard conditions, there was no evidence for convergence in C chemistry among soils as indicated by changes in relative abundances of the four C fractions.Large and constant rates of C loss even after 4 ‘seasons’ of incubation suggest that a large portion of the C pool is potentially mineralizable in all soil types.Warming of the Arctic climate and associated thawing of permafrost and the increase in soil drainage have the potential to cause a large release of C. This C, currently stored in soil organic matter, will be released to the atmosphere, creating a positive feedback on future climate changes.