During the outgoing phase of this project, we (a) analyzed the effects of permafrost thaw with global warming on the amount and quality of soil organic matter pools characterized by different protection mechanisms; and (b) assessed the relative contribution of differently protected soil organic fractions to net C exchange from tundra under permafrost degradation to the atmosphere and detect links with changes in primary production. To achieve this, we used a unique ecosystem warming experiment established in 2008 in a moist acidic tundra ecosystem, located in Healy, Alaska (CiPEHR). The experimental design of CiPEHR includes an ambient (control) treatment and a soil warming treatment, which is applied using fences that increase the snow accumulation and thus trap more heat on one side of the fence.
During the return period, we examined (a) the effects of expected rising temperatures and decreasing rainfall in drylands on the quantity and quality of soil organic matter pools characterized by different protection mechanisms; and (b) how global-warming-induced changes in soil organic matter pools impact dryland ecosystem functioning and net C exchange to the atmosphere. For this purpose, we used an experiment established in November 2008 at a semiarid Mediterranean site in Aranjuez, central Spain. The experimental design was a fully factorial arrangement with three factors, each with two levels. The factors were warming (control vs. temperature increase), rainfall exclusion (control vs. rainfall reduction), and biocrust cover (poorly vs. well-developed biocrust communities). Warming was achieved with open top chambers of hexagonal design. Predicted rainfall reduction treatment was achieved with passive rainfall shelters.
We used physical fractionation techniques to separate soil organic matter from different depths into free (physically and chemically unprotected), intra-aggregate (protected by physical mechanisms), and mineral-associated (protected by chemical mechanisms) pools. We analyzed soil samples and soil organic matter fractions for moisture content, bulk density, C, N, and ash concentration, and 13C and 15N abundance, and by IR and NMR spectroscopy and X-ray powder diffraction. We combined that information with core field measurements, including soil bulk density, temperature and moisture, thaw depth, water table depth, plant productivity, soil nutrients, soil microbial biomass, β-glucosidase activity, abundance of soil bacteria and fungi, and CO2 fluxes.
As a whole, the results obtained show surprisingly high losses of permafrost C upon thaw, which are not compensated by plant biomass uptake nor by mineral-related protection mechanisms of soil organic matter. We also found that warming and rain exclusion may promote the incorporation of biocrust-derived organic compounds into native soil organic matter in dryland ecosystems; however, this may be a transient effect, because of the limited stabilization of the increased soil organic matter fraction and the forecasted decrease of global biocrust cover with climate change. Among other means, these results have been and will be disseminated through international conferences, seminars, and scientific journals. All the data and metadata associated to this project have been made available online in public repositories, with no restrictions and free of charge to ensure maximum dissemination and re-use.
