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

Global Boundary Lines of N2O and CH4 Emission in Peatlands

International audiencePredicting N2O (nitrous oxide) and CH4 (methane) emissions from peatlands is challenging because of the complex coaction of biogeochemical factors. This study uses data from a global soil and gas sampling campaign. The objective is to analyse N2O and CH4 emissions in terms of peat physical and chemical conditions. Our study areas were evenly distributed across the A, C and D climates of the Köppen classification. Gas measurements using static chambers, groundwater analysis and gas and peat sampling for further laboratory analysis have been conducted in 13 regions evenly distributed across the globe. In each study area at least two study sites were established. Each site featured at least three sampling plots, three replicate chambers and corresponding soil pits and one observation well per plot. Gas emissions were measured during 2–3 days in at least three sessions. A log-log linear function limits N2O emissions in relation to soil TIN (total inorganic nitrogen). The boundary line of N2O in terms of soil temperature is semilog linear. The closest representation of the relationship between N2O and soil moisture is a local regression curve with its optimum at 60–70 %. Semilog linear upper boundaries describe the effects of soil moisture and soil temperature to CH4 best.The global N2O boundary lines revealed a striking similarity with the Southern German N2O boundary lines, as well as with analogous scattergrams for Europe (Couwenberg et al. 2011) and Southern Queensland (Wang and Dalal 2010). This suggests that local rather than global conditions determine land-use-based greenhouse gas emissions.Further work will analyse relationships between the environmental factors and the spatial distribution of the main functional genes nirS, nirK and nosZ regulating the denitrification process in the soil samples currently stored in fridge at −18°. An additional analysis will study the relationships between the intensity of CH4 emissions and methanogenesis-regulating functional genes mcrA, pmoA and dsrAB

Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe

Fluxes of nitrous oxide (NO), methane (CH) and carbon dioxide (CO) were followed at winter-grazed (WG) and ungrazed steppe (UG99) in Inner Mongolia during the winter-spring transition of 2006. Mean fluxes during the period March 12-May 11 were 8.2∈±∈0.5 (UG99) and 1.5∈±∈0.2 μg NO-N m h (WG) for NO, 7.2∈±∈0.2 (UG99) and 3.0∈±∈0.1 mg CO-C m h (WG) for CO and -42.5∈±∈0.9 (UG99) and -14.1∈±∈0.3 μg CH-C m h (WG) for CH. Our data show that NO emissions from semi-arid steppe are strongly affected by freeze-thawing. N O emissions reached values of up to 75 μg NO-N m h at the UG99 site, but were considerably lower at the WG site. The observed differences in NO, CH and CO fluxes between the ungrazed and grazed sites were ascribed to the reduced plant biomass at the grazed site, and-most important-to a reduction in soil moisture, due to reduced snow capturing during winter. Thus, winter-grazing significantly reduced NO emission but on the other hand also reduced the uptake of atmospheric CH. To finally evaluate which of the both effects is most important for the non-CO greenhouse gas balance measurements covering an entire year are needed

Grazing-induced reduction of natural nitrous oxide release from continental steppe

Atmospheric concentrations of the greenhouse gas nitrous oxide (N2O) have increased significantly since pre-industrial times owing to anthropogenic perturbation of the global nitrogen cycle1, 2, with animal production being one of the main contributors3. Grasslands cover about 20 per cent of the temperate land surface of the Earth and are widely used as pasture. It has been suggested that high animal stocking rates and the resulting elevated nitrogen input increase N2O emissions4, 5, 6, 7. Internationally agreed methods to upscale the effect of increased livestock numbers on N2O emissions are based directly on per capita nitrogen inputs8. However, measurements of grassland N2O fluxes are often performed over short time periods9, with low time resolution and mostly during the growing season. In consequence, our understanding of the daily and seasonal dynamics of grassland N2O fluxes remains limited. Here we report year-round N2O flux measurements with high and low temporal resolution at ten steppe grassland sites in Inner Mongolia, China. We show that short-lived pulses of N2O emission during spring thaw dominate the annual N2O budget at our study sites. The N2O emission pulses are highest in ungrazed steppe and decrease with increasing stocking rate, suggesting that grazing decreases rather than increases N2O emissions. Our results show that the stimulatory effect of higher stocking rates on nitrogen cycling4, 7 and, hence, on N2O emission is more than offset by the effects of a parallel reduction in microbial biomass, inorganic nitrogen production and wintertime water retention. By neglecting these freeze–thaw interactions, existing approaches may have systematically overestimated N2O emissions over the last century for semi-arid, cool temperate grasslands by up to 72 per cent