Influence of freeze-thaw events on carbon dioxide emission from soils at different moisture and land use

BACKGROUND: The repeated freeze-thaw events during cold season, freezing of soils in autumn and thawing in spring are typical for the tundra, boreal, and temperate soils. The thawing of soils during winter-summer transitions induces the release of decomposable organic carbon and acceleration of soil respiration. The winter-spring fluxes of CO(2 )from permanently and seasonally frozen soils are essential part of annual carbon budget varying from 5 to 50%. The mechanisms of the freeze-thaw activation are not absolutely clear and need clarifying. We investigated the effect of repeated freezing-thawing events on CO(2 )emission from intact arable and forest soils (Luvisols, loamy silt; Central Germany) at different moisture (65% and 100% of WHC). RESULTS: Due to the measurement of the CO(2 )flux in two hours intervals, the dynamics of CO(2 )emission during freezing-thawing events was described in a detailed way. At +10°C (initial level) in soils investigated, carbon dioxide emission varied between 7.4 to 43.8 mg C m(-2)h(-1 )depending on land use and moisture. CO(2 )flux from the totally frozen soil never reached zero and amounted to 5 to 20% of the initial level, indicating that microbial community was still active at -5°C. Significant burst of CO(2 )emission (1.2–1.7-fold increase depending on moisture and land use) was observed during thawing. There was close linear correlation between CO(2 )emission and soil temperature (R(2 )= 0.86–0.97, P < 0.001). CONCLUSION: Our investigations showed that soil moisture and land use governed the initial rate of soil respiration, duration of freezing and thawing of soil, pattern of CO(2 )dynamics and extra CO(2 )fluxes. As a rule, the emissions of CO(2 )induced by freezing-thawing were more significant in dry soils and during the first freezing-thawing cycle (FTC). The acceleration of CO(2 )emission was caused by different processes: the liberation of nutrients upon the soil freezing, biological activity occurring in unfrozen water films, and respiration of cold-adapted microflora

Microbial Maintenance: A Critical Review on Its Quantification

Microbial maintenance is an important concept in microbiology. Its quantification, however, is a subject of continuous debate, which seems to be caused by (1) its definition, which includes nongrowth components other than maintenance; (2) the existence of partly overlapping concepts; (3) the evolution of variables as constants; and (4) the neglect of cell death in microbial dynamics. The two historically most important parameters describing maintenance, the specific maintenance rate and the maintenance coefficient, are based on partly different nongrowth components. There is thus no constant relation between these parameters and previous equations on this subject are wrong. In addition, the partial overlap between these parameters does not allow the use of a simple combination of these parameters. This also applies for combinations of a threshold concentration with one of the other estimates of maintenance. Maintenance estimates should ideally explicitly describe each nongrowth component. A conceptual model is introduced that describes their relative importance and reconciles the various concepts and definitions. The sensitivity of maintenance on underlying components was analyzed and indicated that overall maintenance depends nonlinearly on relative death rates, relative growth rates, growth yield, and endogenous metabolism. This quantitative sensitivity analysis explains the felt need to develop growth-dependent adaptations of existing maintenance parameters, and indicates the importance of distinguishing the various nongrowth components. Future experiments should verify the sensitivity of maintenance components under cellular and environmental conditions

Siberian wetlands: Where a sink is a source

[1] A greenhouse gas inventory can for some ecosystems be based solely on the net CO2 exchange with the atmosphere and the export of dissolved organic carbon. In contrast, the global warming effect may be more complex in ecosystems where other greenhouse gases such as CH4 or N2O have significant exchanges with the atmosphere. Through micrometeorological landscape- scale measurements from the largest wetlands on Earth in West Siberia we show that CH4 has a stronger effect than CO2 on the greenhouse gas budget in terms of radiative forcing on the atmosphere. Direct measurements of the CO2 and CH4 exchange during the summer of 1999 show that these wetland ecosystems, on average, acted as net sinks of carbon of 0.5 g C m(-2) day(-1) but large net sources of CH4. Given the high Global Warming Potential of CH4, the Siberian wetlands are an important source of radiative forcing, even in comparison to anthropogenic emissions

Microbial activity in soils frozen to below-39 degrees C

Recent research on life in extreme environments has shown that some microorganisms metabolize at extremely low temperatures in Arctic and Antarctic ice and permafrost. Here, we present kinetic data on CO2 and (CO2)-C-14 release from intact and C-14-glucose amended tundra soils (Barrow, Alaska) incubated for up to a year at 0 to -39 degrees C. The rate of CO2 production declined exponentially with temperature but it remained positive and measurable, e.g. 2-7 ng CO2-C cm(-3) soil d(-1), at -39 degrees C. The variation of CO2 release rate (v) was adequately explained by the double exponential dependence on temperature (T) and unfrozen water content (W) (r(2)> 0.98): v=A exp(lambda T+kW) and where A, lambda and k are constants. The rate of (CO2)-C-14 release from added glucose declined more steeply with cooling as compared with the release of total CO2, indicating that (a) there could be some abiotic component in the measured flux of CO2 or (b) endogenous respiration is more cold-resistant than substrate-induced respiration. The respiration activity was completely eliminated by soil sterilization (1 h, 121 degrees C), stimulated by the addition of oxidizable substrate (glucose, yeast extract), and reduced by the addition of acetate, which inhibits microbial processes in acidic soils (pH 3-5). The tundra soil from Barrow displayed higher below-zero activity than boreal soils from West Siberia and Sweden. The permafrost soils (20-30 cm) were more active than the samples from seasonally frozen topsoil (0-10 cm, Barrow). Finding measurable respiration to -39 degrees C is significant for determining, understanding, and predicting current and future CO2 emission to the atmosphere and for understanding the low temperature limits of microbial activity on the Earth and on other planets. (c) 2005 Elsevier Ltd. All rights reserved

Microbial communities and processes in Arctic permafrost environments

In polar regions, huge layers of frozen ground, termed permafrost, are formed. Permafrost covers more than 25 % of the land surface and significant parts of the coastal sea shelfs. Its habitats are controlled by extreme climate and terrain conditions. Particularly, the seasonal freezing and thawing in the upper active layer of permafrost leads to distinct gradients in temperature and geochemistry. Microorganisms in permafrost environments have to survive extremely cold temperatures, freeze-thaw cycles, desiccation and starvation under long-lasting background radiation over geological time scales. Although the biology of permafrost microorganisms remains relatively unexplored, recent findings show that microbial communities in this extreme environment are composed by members of all three domains of life (Archaea, Bacteria, Eukarya), with a total biomass comparable to temperate soil ecosystems. This chapter describes the environmental conditions of permafrost and reviews recent studies on microbial processes and diversity in permafrost-affected soils as well as the role and significance of microbial communities with respect to global biogeochemical cycles