Periodic Reporting for period 2 - VULCAN (Vulnerability of soil organic carbon to climate change in permafrost and dryland ecosystems)
Reporting period: 2017-07-01 to 2018-06-30
Continued emissions of greenhouse gases, particularly CO2, will cause further increases in global mean temperatures over the 21st century, especially in northern latitudes, which will alter global precipitation patterns and increase the aridity conditions in drylands worldwide. A full understanding of how soil organic matter will respond to these changes in the climate system is crucial. This is because organic matter is a multi-functional component of soil that provides vital ecosystem services, including support of primary production. Furthermore, soil organic matter represents one of the largest C reservoirs on Earth—holding more than three-fold the amount of C that is currently in the atmosphere as CO2—and has implications for the mitigation or exacerbation of climate change. Permafrost soils of northern latitudes and soils of drylands are keystone contributors to this C reservoir. While highly contrasting, permafrost and dryland ecosystems are both extremely vulnerable and under severe pressure due to global warming. With warming and permafrost thaw, soil organic matter previously stabilized by freezing temperatures becomes exposed to microbial decomposition, which may significantly change ecosystem functioning and aggravate climate change by releasing significant amounts of CO2 into the atmosphere. Similarly, rising temperatures and declining rainfall may accelerate soil organic matter decomposition in drylands, which not only may release CO2 to the atmosphere but also may lead to land degradation and desertification. The main research objective of this project is to gain a deeper insight into the vulnerability of soil organic matter to climate change in permafrost and dryland ecosystems, and to explore potential implications related to their functioning and feedback to global warming.
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.
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.
We used novel approaches for permafrost and dryland ecosystems to investigate the response of soil organic matter to warming and rainfall reduction. Unraveling the magnitude of this response is crucial to better understand the pace of global climate change, as well as the implications on the functioning of these ecosystems.