The response of soil organic matter (OM) decomposition to increasing temperature is a critical aspect of ecosystem responses to global change The impacts of climate warming on decomposition dynamics have not been resolved due to apparently contradictory results from field and lab experiments, most of which has focused on labile carbon with short turnover times But the majority of total soil carbon stocks are comprised of organic carbon with turnover times of decades to centuries Understanding the response of these carbon pools to climate change is essential for forecasting longer-term changes in soil carbon storage Herein, we briefly synthesize information from recent studies that have been conducted using a wide variety of approaches In our effort to understand research to-date, we derive a new conceptual model that explicitly identifies the processes controlling soil OM availability for decomposition and allows a more explicit description of the factors regulating OM decomposition under different circumstances It explicitly defines resistance of soil OM to decomposition as being due either to its chemical conformation (quality )o r its physico-chemical protection from decomposition The former is embodied in the depolymerization process, the latter by adsorption/desorption and aggregate turnover We hypothesize a strong role for variation in temperature sensitivity as a function of reaction rates for both We conclude that important advances in understanding the temperature response of the processes that control substrate availability, depolymerization, microbial efficiency, and enzyme production will be needed to predict the fate of soil carbon stocks in a warmer world

https://www.fs.fed.us/rm/pubs_other/rmrs_2011_conant_r001.pdf

Temperature and soil organic matter decomposition rates – synthesis of current knowledge and a way forward

Time is divided by geologists according to marked shifts in Earth's state. Recent global environmental changes suggest that Earth may have entered a new human-dominated geological epoch, the Anthropocene. Here we review the historical genesis of the idea and assess anthropogenic signatures in the geological record against the formal requirements for the recognition of a new epoch. The evidence suggests that of the various proposed dates two do appear to conform to the criteria to mark the beginning of the Anthropocene: 1610 and 1964. The formal establishment of an Anthropocene Epoch would mark a fundamental change in the relationship between humans and the Earth system.

Defining the Anthropocene

Soil organic matter (SOM) represents one of the largest reservoirs of carbon on the global scale. Thus, the temperature sensitivity of bulk SOM and of different SOM fractions is a key factor determining the response of the terrestrial carbon balance to climatic warming. We condense the available knowledge about the potential temperature sensitivity and the actual temperature sensitivity of decomposition in situ, which ultimately depends on substrate availability. We review and evaluate contradictory results of estimates of the temperature sensitivity of bulk SOM and of different SOM fractions. The contradictory results demonstrate a need to focus research on biological and physicochemical controls of SOM stabilisation and destabilisation processes as a basis for understanding strictly causal relationships and kinetic properties of key processes that determine pool sizes and turnover rates of functional SOM pools. The current understanding is that temperature sensitivity of SOM mineralisation is governed by the following factors: (1) the stability of SOM, (2) the substrate availability, which is determined by the balance between input of organic matter, stabilisation and mineralisation of SOM, (3) the physiology of the soil microflora, its efficiency in substrate utilisation and its temperature optima and (4) physicochemical controls of destabilisation and stabilisation processes, like pH and limitation of water, oxygen and nutrient supply. As soil microflora is functionally omnipotent and most SOM is of high age and stability, the temperature dependence of stable SOM pools is the central question that determines C stocks and stock changes under global warming.

Temperature sensitivity of soil organic matter decomposition—what do we know?

For decades, ecosystem scientists have used global warming potentials (GWPs) to compare the radiative forcing of various greenhouse gases to determine if ecosystems have a net warming or cooling effect on climate. On a conceptual basis, the continued use of GWPs by the ecological community may be untenable because the use of GWPs requires the implicit assumption that greenhouse gas emissions occur as a single pulse; this assumption is rarely justified in ecosystem studies. We present two alternate metrics—the sustained-flux global warming potential (SGWP, for gas emissions) and the sustained-flux global cooling potential (SGCP, for gas uptake)—for use when gas fluxes persist over time. The SGWP is generally larger than the GWP (by up to ~40%) for both methane and nitrous oxide emissions, creating situations where the GWP and SGWP metrics could provide opposing interpretations about the climatic role of an ecosystem. Further, there is an asymmetry in methane and nitrous oxide dynamics between persistent emission and uptake situations, producing very different values for the SGWP vs. SGCP and leading to the conclusion that ecosystems that take up these gases are very effective at reducing radiative forcing. Although the new metrics are more realistic than the GWP for ecosystem fluxes, we further argue that even these metrics may be insufficient in the context of trying to understand the lifetime climatic role of an ecosystem. A dynamic modeling approach that has the flexibility to account for temporally variable rates of greenhouse gas exchange, and is not limited by a fixed time frame, may be more informative than the SGWP, SGCP, or GWP. Ultimately, we hope this article will stimulate discussion within the ecosystem science community about the most appropriate way(s) of assessing the role of ecosystems as regulators of global climate.

/pdf/moving-beyond-global-warming-potentials-to-quantify-the-nlce0f48rr.pdf

Moving Beyond Global Warming Potentials to Quantify the Climatic Role of Ecosystems

This synthesis discusses the emissions of carbon dioxide from fossil-fuel combustion and cement production. While much is known about these emissions, there is still much that is unknown about the details surrounding these emissions. This synthesis explores our knowledge of these emissions in terms of why there is concern about them; how they are calculated; the major global efforts on inventory- ing them; their global, regional, and national totals at differ- ent spatial and temporal scales; how they are distributed on global grids (i.e., maps); how they are transported in mod- els; and the uncertainties associated with these different as- pects of the emissions. The magnitude of emissions from the combustion of fossil fuels has been almost continuously in- creasing with time since fossil fuels were first used by hu- mans. Despite events in some nations specifically designed to reduce emissions, or which have had emissions reduction as a byproduct of other events, global total emissions con- tinue their general increase with time. Global total fossil- fuel carbon dioxide emissions are known to within 10 % un- certainty (95 % confidence interval). Uncertainty on individ- ual national total fossil-fuel carbon dioxide emissions range from a few percent to more than 50 %. This manuscript con- cludes that carbon dioxide emissions from fossil-fuel com- bustion continue to increase with time and that while much is known about the overall characteristics of these emissions, much is still to be learned about the detailed characteristics of these emissions.

/pdf/a-synthesis-of-carbon-dioxide-emissions-from-fossil-fuel-5gfkhoudax.pdf

A synthesis of carbon dioxide emissions from fossil-fuel combustion

A 7-year-long data set of integrated high-precision 14 CO 2 observations combined with occasional hourly 14 CO 2 flask data from the Heidelberg sampling site is presented. Heidelberg is located in the highly populated and industrialized upper Rhine valley in southwestern Germany. The 14 CO 2 data are used in combination with hourly carbon monoxide (CO) observations to estimate regional hourly fossil fuel CO 2 (ΔFFCO 2 ) mixing ratios. We investigate three different 14 C calibration schemes to calculate ΔFFCO 2 : (1) the long-term median ΔCO/ΔFFCO 2 ratio of 14.6 ppb ppm -1 (mean: 15.5 ± 5.6 ppb ppm -1 ), (2) individual (2-)week-long integrated ΔCO/ΔFFCO 2 ratios, which take into account the large week-to-week variability of ±5.6 ppb ppm -1 (1σ; interquartile range: 5.5 ppb ppm -1 ), and (3) a calibration which also includes diurnal changes of the ΔCO/ΔFFCO 2 ratio. We show that in winter a diurnally changing ΔCO/ΔFFCO 2 ratio provides a much better agreement with the direct 14 C-based hourly ΔFFCO 2 estimates whereas summer values are not significantly improved with a diurnal calibration. Using integrated 14 CO 2 samples to determine weekly mean ΔCO/ΔFFCO 2 ratios introduces a bias in the CO-based ΔFFCO 2 estimates which can be corrected for with diurnal grab sample data. Altogether our 14 C-calibrated CO-based method allows determining ΔFFCO 2 at a semi-polluted site with a precision of approximately ±25%. DOI: 10.1111/j.1600-0889.2010.00477.x

https://www.tandfonline.com/doi/pdf/10.1111/j.1600-0889.2010.00477.x

Implication of weekly and diurnal 14C calibration on hourly estimates of CO‐based fossil fuel CO2 at a moderately polluted site in southwestern Germany

Soils represent the largest carbon pool in the terrestrial biosphere, and climate change might affect the main carbon fluxes associated with this pool. These fluxes are the production of aboveground litter and root litter, and decomposition of the soil organic matter (SOM) pool by soil microorganisms. Knowledge about the temperature sensitivity of the decomposition of different SOM fractions is crucial in order to understand how climate change might affect carbon storage in soils. In this study, the temperature sensitivity of the turnover times of three different SOM fractions (labile, intermediate, and stabilized) was investigated for 11 forest sites along a temperature gradient. Carbon-14 isotope analyses of the SOM fractions combined with a model provided estimates of their turnover times. The turnover times of the labile SOM fraction were not correlated with mean annual soil temperature. Therefore it was not possible to estimate temperature sensitivity for the labile SOM fraction. Given considerable e...

Temperature sensitivity of the turnover times of soil organic matter in forests

The new accelerator mass spectrometry (AMS) Radiocarbon Analysis Laboratory in Jena is described. The laboratory developed a combustion system for solid samples and a CO2 extraction system for air samples. Thus far, sample preparation, including graphitization, was performed in the laboratory, and the samples were measured subsequently by other AMS facilities. Currently, the laboratory owns a 3MV AMS system from HVEE (Netherlands) that has passed the acceptance tests and will be used for routine 14C determinations in the near future. The AMS system is equipped with 2 ion sources, one suitable for graphite targets and the second for both graphite and CO2 targets.

/pdf/the-new-14c-analysis-laboratory-in-jena-germany-1pvcjcqkll.pdf

The new 14C analysis laboratory in Jena, Germany

The different categories of samples and their treatments at the radiocarbon (14C) laboratory in Jena, Germany, are described. The Jena 14C laboratory is dedicated to studying the global carbon cycle through the analysis of soil, plant, gas, charcoal, and wood samples. The respective preparation procedures for different samples types are presented, including wet-chemistry pretreatments for soil decalcification, charcoal purification using an acid–base–oxidation procedure (ABOx), and α-cellulose extraction from wood and leaves. In particular, investigations of the background corrections for each sample type are emphasized. Furthermore, an investigation of the contamination resulting from the storage of graphite is presented. Four storage conditions are investigated for storage times up to 30 months.

Axel Steinhof

Papers

Implication of weekly and diurnal 14C calibration on hourly estimates of CO‐based fossil fuel CO2 at a moderately polluted site in southwestern Germany

Temperature sensitivity of the turnover times of soil organic matter in forests

The new 14C analysis laboratory in Jena, Germany

Sample Preparation at the Jena 14C Laboratory

The local marine reservoir effect at Kalba (UAE) between the Neolithic and Bronze Age: An indicator of sea level and climate changes