Influence of warming and enchytraeid activities on soil CO2 and CH4 fluxes

To determine the sum of ‘direct’ and ‘indirect’ effects of climatic change on enchytraeid activity and C fluxes from an organic soil we assessed the influence of temperature (4, 10 and 15 °C incubations) on enchytraeid populations and soil CO₂ and CH₄ fluxes over 116 days. Moisture was maintained at 60% of soil dry weight during the experimental period and measurements of enchytraeid biomass and numbers, and CO₂ and CH₄ fluxes were made after 3, 16, 33, 44, 65, 86 and 116 days. Enchytraeid population numbers and biomass increased in all temperature treatments with the greatest increase produced at 15 °C (to over threefold initial values by day 86). Results also showed that enchytraeid activity increased CO₂ fluxes by 10.7±4.5, 3.4±4.0 and 26.8±2.6% in 4, 10 and 15 °C treatments, respectively, with the greatest CO₂ production observed at 15 °C for the entire 116 day incubation period (P<0.05). The soil respiratory quotient analyses at lower temperatures (i.e. 4-10 °C) gave a Q₁₀ of 1.7 and 1.9 with and without enchytraeids, respectively. At temperatures above 10 °C (i.e. 10-15 °C) Q₁₀ significantly increased (P<0.01) and was 25% greater in the presence of enchytraeids (Q₁₀=3.4) than without (Q₁₀=2.6). In contrast to CO₂ production, no significant relationships were observed between net CH₄ fluxes and temperature and only time showed a significant effect on CH₄ production (P<0.01).Total soil CO₂ production was positively linked with enchytraeid biomass and mean soil CO₂-C production was 77.01±6.05 CO₂-C μg mg enchytraeid tissue⁻¹ day⁻¹ irrespective of temperature treatment. This positive relationship was used to build a two step regression model to estimate the effects of temperature on enchytraeid biomass and soil CO₂ respiration in the field. Predictions of potential CO₂ production were made using enchytraeid biomass data obtained in the field from two upland grassland sites (Sourhope and Great Dun Fell at the Moor House Nature Reserve, both in the UK). The findings of this work suggest that a 5 °C increase in atmospheric temperature above mean ambient temperature could have the potential to produce a significant increase in enchytraeid biomass resulting in a near twofold increase in soil CO₂ release from both soil types. The interaction between temperature and soil biology will clearly be an important determinant of soil respiration responses to global warming.     

Strong temperature dependence and no moss photosynthesis in winter CO2 flux for a Kobresia meadow on the Qinghai-Tibetan plateau

We examined the CO₂ exchange of a Kobresia meadow ecosystem on the Qinghai-Tibetan plateau using a chamber system. CO₂ efflux from the ecosystem was strongly dependence on soil surface temperature. The CO₂ efflux-temperature relationship was identical under both light and dark conditions, indicating that no photosynthesis could be detected under light conditions during the measurement period. The temperature sensitivity (Q₁₀) of the CO₂ efflux showed a marked transition around −1.0 °C; Q₁₀ was 2.14 at soil surface temperatures above and equal to −1.0 °C but was 15.3 at temperatures below −1.0 °C. Our findings suggest that soil surface temperature was the major factor controlling winter CO₂ flux for the alpine meadow ecosystem and that freeze-thaw cycles at the soil surface layer play an important role in the temperature dependence of winter CO₂ flux.     

Uncoupling of microbial CO2 production and release in frozen soil and its implications for field studies of arctic C cycling

Knowledge of seasonal trends and controls of soil CO₂ emissions to the atmosphere is important for simulating atmospheric CO₂ concentrations and for understanding and predicting the global carbon cycle. This is particularly the case for high arctic soils subject to extreme fluctuating environmental conditions. Based on field measurements of soil CO₂ efflux, temperature, water content, pore gas composition in soil and frozen cores as well as detailed temperature experiments performed in the laboratory, we evaluated seasonal controls of CO₂ effluxes from a well-drained tundra heath site in NE-Greenland. During the growing season, near-surface temperatures correlated well with observed CO₂ effluxes (r²>0.9). However, during intensive thawing of near-surface layers we observed up to 1.5-fold higher effluxes than expected due to temperature alone. These high rates were consistent with high CO₂ concentrations in frozen soil (>10% CO₂) and suggested a spring burst event during soil thawing and a corresponding trapping of produced CO₂ during winter. Laboratory experiments revealed that microbial soil respiration continued down to a least −18 °C and that up to 80% of the produced CO₂ was trapped in soil at temperatures between 0 and −9 °C. The trapping of CO₂ in frozen soil was positively correlated with soil moisture (r²=0.85) and led to an abrupt change of the temperature sensitivity (Q₁₀) observed for soil CO₂ release at 0 °C with Q₁₀ values below 0 °C being up to 100-fold higher than above 0 °C. The results of sub-zero CO₂ production allowed us to predict the microbial soil respiration throughout the year and to evaluate to what extent burst events during thawing can be explained by the release of CO₂ being produced and trapped during winter. Taking only the upper 20 cm of the soil into account, winter soil respiration accounted for about 40% of the annual soil respiration. At least 14% of the winter CO₂ production was trapped during the winter 2000-2001 and observed to be released upon thawing. Thus, the site-specific winter soil respiration is an important part of the annual C cycle and CO₂ trapping should be accounted for in future field and modelling studies of soil respiration dynamics in arctic ecosystems. In conclusion, we have discovered a soil moisture dependent uncoupling of CO₂ production and release in frozen soils with important implications for future field studies of Arctic C cycling.     

Elevated CO2 concentration and nitrogen fertilisation effects on N2O and CH4 fluxes and biomass production of Phleum pratense on farmed peat soil

The effects of elevated CO₂ supply on N₂O and CH₄ fluxes and biomass production of Phleum pratense were studied in a greenhouse experiment. Three sets of 12 farmed peat soil mesocosms (10 cm dia, 47 cm long) sown with P. pratense and equally distributed in four thermo-controlled greenhouses were fertilised with a commercial fertiliser in order to add 2, 6 or 10 g N m⁻². In two of the greenhouses, CO₂ concentration was kept at atmospheric concentration (360 μmol mol⁻¹) and in the other two at doubled concentration (720 μmol mol⁻¹). Soil temperature was kept at 15 °C and air temperature at 20 °C. Natural lighting was supported by artificial light and deionized water was used to regulate soil moisture. Forage was harvested and the plants fertilised three times during the basic experiment, followed by an extra fertilisations and harvests. At the end of the experiment CH₄ production and CH₄ oxidation potentials were determined; roots were collected and the biomass was determined. From the three first harvests the amount of total N in the aboveground biomass was determined. N₂O and CH₄ exchange was monitored using a closed chamber technique and a gas chromatograph. The highest N₂O fluxes (on average, 255 μg N₂O m⁻² h⁻¹ during period IV) occurred just after fertilisation at high water contents, and especially at the beginning of the growing season (on average, 490 μg N₂O m⁻² h⁻¹ during period I) when the competition of vegetation for N was low. CH₄ fluxes were negligible throughout the experiment, and for all treatments the production and oxidation potentials of CH₄ were inconsequential. Especially at the highest rates of fertilisation, the elevated supply of CO₂ increased above- and below-ground biomass production, but both at the highest and lowest rates of fertilisation, decreased the total amount of N in the aboveground dry biomass. N₂O fluxes tended to be higher under doubled CO₂ concentrations, indicating that increasing atmospheric CO₂ concentration may affect N and C dynamics in farmed peat soil.     

Carbon, nitrogen and temperature controls on microbial activity in soils from an Antarctic dry valley

The Antarctic dry valleys are characterized by extremely low temperatures, dry conditions and lack of conspicuous terrestrial autotrophs, but the soils contain organic C, emit CO₂ and support communities of heterotrophic soil organisms. We have examined the role of modern lacustrine detritus as a driver of soil respiration in the Garwood Valley, Antarctica, by characterizing the composition and mineralization of both lacustrine detritus and soil organic matter, and relating these properties to soil respiration and the abiotic controls on soil respiration. Laboratory mineralization of organic C in soils from different, geomorphically defined, landscape elements at 10 °C was comparable with decomposition of lacustrine detritus (mean residence times between 115 and 345 d for the detritus and 410 and 1670 d for soil organic matter). The chemical composition of the detritus (C-to-N ratio=9:1-12:1 and low alkyl-C-to-O-alkyl-C ratio in solid-state ¹³C nuclear magnetic resonance spectroscopy) indicated that it was a labile, high quality resource for micro-organisms. Initial (0-6 d at 10 °C) respiratory responses to glucose, glycine and NH₄Cl addition were positive in all the soils tested, indicating both C and N limitations on soil respiration. However, over the longer term (up to 48 d at 10 °C) differential responses occurred. Glucose addition led to net C mineralization in most of the soils. In the lake shore soils, which contained accumulated lacustrine organic matter, glucose led to substantial priming of the decomposition of the indigenous organic matter, indicating a C or energetic limitation to mineralization in that soil. By contrast, over 48 d, glycine addition led to no net C mineralization in all soils except stream edge and lake shore soils, indicating either substantial assimilation of the added C (and N), or no detectable utilization of the glycine. The Q₁₀ values for basal respiration over the −0.5-20 °C temperature range were between 1.4 and 3.3 for the different soils, increasing to between 3.4 and 6.9 for glucose-induced respiration, and showed a temperature dependence with Q₁₀ increasing with declining temperature. Taken together, our results strongly support contemporaneous lacustrine detritus, blown from the lake shore, as an important driver of soil respiration in the Antarctic dry valley soils.     

Temperature sensitivity of soil respiration in different ecosystems in China

Understanding the sensitivity of soil respiration to temperature change and its impacting factors is an important base for accurately evaluating the response of terrestrial carbon balance to future climatic change, and thus has received much recent attention. In this study, we synthesized 161 field measurement data from 52 published papers to quantify temperature sensitivity of soil respiration in different Chinese ecosystems and its relationship with climate factors, such as temperature and precipitation. The results show that the observed Q₁₀ value (the factor by which respiration rates increase for a 10 °C increase in temperature) is strongly dependent on the soil temperature measurement depth. Generally, Q₁₀ significantly increased with the depth (0 cm, 5 cm, and 10 cm) of soil temperature measuring point. Different ecosystem types also exhibit different Q₁₀ values. In response to soil temperature at the depth of 5 cm, alpine meadow and tundra has the largest Q₁₀ value with magnitude of 3.05 ± 1.06, while the Q₁₀ value of evergreen broadleaf forests is approximately half that amount (Q₁₀ = 1.81 ± 0.43). Spatial correlation analysis also shows that the Q₁₀ value of forest ecosystems is significantly and negatively correlated with mean annual temperature (R = −0.51, P < 0.001) and mean annual precipitation (R = −0.5, P < 0.001). This result not only implies that the temperature sensitivity of soil respiration will decline under continued global warming, but also suggests that such acclimation of soil respiration to warming should be taken into account in forecasting future terrestrial carbon cycle and its feedback to climate system.     

Microbial activity in soils frozen to below −39 °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 ¹⁴CO₂ release from intact and ¹⁴C-glucose amended tundra soils (Barrow, Alaska) incubated for up to a year at 0 to −39°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 °C. The variation of CO₂ release rate (v) was adequately explained by the double exponential dependence on temperature (T) and unfrozen water content (W) (r²>0.98): v=A exp(λT+kW) and where A, λ and k are constants. The rate of ¹⁴CO₂ 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 °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 °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.     

Long-term effects of seasonal and diurnal temperature fluctuations on carbon dioxide efflux from a forest soil

Temperature fluctuations are a fundamental entity of the soil environment in the temperate zone and show fast (diurnal) and slow (seasonal) dynamics. Responses of soil respiration to temperature fluctuations were investigated in a root-free soil of a mid-European beech-oak forest. First, in laboratory we analysed the efflux of CO₂ from soil microcosms exposed to seasonal (±5 °C of the annual mean) and diurnal fluctuations (±5 °C of the seasonal levels) in a two-factorial design. Second, in field microcosms we investigated effects of smoothing diurnal temperature fluctuations in soil (simulating a possible global trend) on CO₂ efflux. Third, the natural temperature regime was simulated in laboratory microcosms and their CO₂ efflux was compared to the one in the field. The experiments lasted for 1 year to differentiate seasonal and annual responses.Dynamics of CO₂ efflux, microbial basal respiration, biomass and qO₂ varied with seasonal temperature regime. However, in the laboratory the annual cumulative CO₂-C production did not differ between treatments and varied between 10.9% and 11.7% of the total microcosm C, disregarding seasonal and/or diurnal fluctuations. The similarity of cumulative C production suggests that the availability of microbially mobilisable carbon pools rather than the temperature regime limited soil respiration. Diurnal fluctuations generally did not affect CO₂ efflux and microbial activity, though winter Q₁₀ values were increased in their absence. Simulation of the natural temperature regime in the laboratory resulted in CO₂ efflux similar to field microcosms. In the field, rates of CO₂ efflux and microbial activity, seasonal and annual cumulative CO₂-C production were significantly higher at smoothed than at natural temperature conditions (annually 13.1% and 11.0% of total C was respired, respectively). Facing global climate changes the mechanisms regulating responses of soil respiration to temperature fluctuations need further investigation.     

Soil CO2 efflux in a temperate deciduous forest: Environmental drivers and component contributions

Soil CO₂ efflux is a large component of total respiration in many ecosystems. It is important to understand the environmental controls on soil CO₂ efflux, in order to evaluate potential responses of ecosystems to climate change. This study investigated the relationship between total soil CO₂ efflux and soil temperature, soil moisture and solar radiation on an interannual basis for a plot of temperate deciduous ancient semi-natural woodland at Wytham Woods in central southern England. We also aimed to quantify the contribution of soil organic matter decomposition (SOM), root-and-rhizosphere respiration, and mycorrhizal respiration components to total soil CO₂ efflux, and determine their environmental correlates. Total soil CO₂ efflux was measured regularly from April 2006 to December 2008 and found to average 4.1 Mg C ha⁻¹ yr⁻¹ in both 2007 and 2008. In addition, we applied a recently developed approach to partition the efflux into SOM, root-and-rhizosphere, and mycorrhizal components in situ using mesh bags. SOM decomposition, root-and-rhizosphere, and mycorrhizal respiration were estimated to contribute 70 ± 6%, 22 ± 6% and 8 ± 3% of total soil CO₂ efflux respectively, equating to 3.0 ± 0.3, 0.9 ± 0.2 and 0.3 ± 0.1 Mg C ha⁻¹ yr⁻¹. In order to avoid the effect of temporal correlation between variables caused by seasonality, we investigated interannual variability by examining the relationship between CO₂ flux anomalies and anomalies in environmental variables. Variation in soil temperature explained 50% of the interannual variance in soil CO₂ efflux, and soil moisture a further 18% of the residual variance. Solar radiation, as a proxy for plant photosynthesis, had no significant effect on total soil CO₂ efflux, but was positively correlated with root-and-rhizosphere respiration, and mycorrhizal respiration. The relationship between anomalies in soil CO₂ efflux and soil temperature was highly significant, with a sensitivity of 0.164 ± 0.023 μmol CO₂ m⁻² s⁻¹ °C⁻¹. For mean peak summer efflux rates (2.03 μmol CO₂ m² s⁻¹), this is equivalent to 8% per °C, or a Q₁₀ temperature sensitivity of 2.2 ± 0.2. We demonstrate the utility of an anomaly analysis approach and conclude that soil temperature is the key driver of total soil CO₂ efflux primarily through its positive relationship with SOM-decomposition rate.     

Temperature sensitivity of respiration differs among forest floor layers in a Pinus resinosa plantation

Decomposer microorganisms contribute to carbon loss from the forest floor as they metabolize organic substances and respire CO₂. In temperate and boreal forest ecosystems, the temperature of the forest floor can fluctuate significantly on a day-to-night or day-to-day basis. In order to estimate total respiratory CO₂ loss over even relatively short durations, therefore, we need to know the temperature sensitivity (Q₁₀) of microbial respiration. Temperature sensitivity has been calculated for microbes in different soil horizons, soil fractions, and at different depths, but we would suggest that for some forests, other ecologically relative soil portions should be considered to accurately predict the contribution of soil to respiration under warming. The floor of many forests is heterogeneous, consisting of an organic horizon comprising a few more-or-less distinct layers varying in decomposition status. We therefore determined at various measurement temperatures the respiration rates of litter, F-layer, and H-layer collected from a Pinus resinosa plantation, and calculated Q₁₀ values for each layer. Q₁₀ depended on measurement temperature, and was significantly greater in H-layer than in litter or F-layer between 5 and 17 °C. Our results indicate, therefore, that as the temperature of the forest floor rises, the increase in respiration by the H-layer will be disproportionate to the increase by other layers. However, change in respiration by the H-layer associated with change in temperature may contribute minimally or significantly to changes of total forest floor respiration in response to changes in temperature depending on the depth and thickness of the layer in different forest ecosystems.     

Grazing intensity alters soil respiration in an alpine meadow on the Tibetan plateau

Grazing intensity may alter the soil respiration rate in grassland ecosystems. The objectives of our study were to (1) determine the influence of grazing intensity on temporal variations in soil respiration of an alpine meadow on the northeastern Tibetan Plateau; and (2) characterise the temperature response of soil respiration under different grazing intensities. Diurnal and seasonal soil respiration rates were measured for two alpine meadow sites with different grazing intensities. The light grazing (LG) meadow site had a grazing intensity of 2.55 sheep ha⁻¹, while the grazing intensity of the heavy grazing (HG) meadow site, 5.35 sheep ha⁻¹, was approximately twice that of the LG site. Soil respiration measurements showed that CO₂ efflux was almost twice as great at the LG site as at the HG site during the growing season, but the diurnal and seasonal patterns of soil respiration rate were similar for the two sites. Both exhibited the highest annual soil respiration rate in mid-August and the lowest in January. Soil respiration rate was highly dependent on soil temperature. The Q₁₀ value for annual soil respiration was lower for the HG site (2.75) than for the LG site (3.22). Estimates of net ecosystem CO₂ exchange from monthly measurements of biomass and soil respiration revealed that during the period from May 1998 to April 1999, the LG site released 2040 g CO₂ m⁻² y⁻¹ to the atmosphere, which was about one third more than the 1530 g CO₂ m⁻² y⁻¹ released at the HG site. The results suggest that (1) grazing intensity alters not only soil respiration rate, but also the temperature dependence of soil CO₂ efflux; and (2) soil temperature is the major environmental factor controlling the temporal variation of soil respiration rate in the alpine meadow ecosystem.     

Winter soil CO2 efflux and its contribution to annual soil respiration in different ecosystems of a forest-steppe ecotone, north China

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative winter soil CO₂ fluxes are reported to be a significant component of their annual carbon budgets. However, little information on winter soil CO₂ efflux is known from mid-latitude ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of ecosystem carbon budgets and the response of soil CO₂ efflux to climate changes. In this study we measured winter soil CO₂ efflux and its contribution to annual soil respiration for seven ecosystems (three forests: Pinus sylvestris var. mongolica plantation, Larix principis-rupprechtii plantation and Betula platyphylla forest; two shrubs: Rosa bella and Malus baccata; and two meadow grasslands) in a forest-steppe ecotone, north China. Overall mean winter and growing season soil CO₂ effluxes were 0.15-0.26 μmol m⁻² s⁻¹ and 2.65-4.61 μmol m⁻² s⁻¹, respectively, with significant differences in the growing season among the different ecosystems. Annual Q₁₀ (increased soil respiration rate per 10 °C increase in temperature) was generally higher than the growing season Q₁₀. Soil water content accounted for 84% of the variations in growing season Q₁₀ and soil temperature range explained 88% of the variation in annual Q₁₀. Soil organic carbon density to 30 cm depth was a good surrogate for SR₁₀ (basal soil respiration at a reference temperature of 10 °C). Annual soil CO₂ efflux ranged from 394.76 g C m⁻² to 973.18 g C m⁻² using observed ecosystem-specific response equations between soil respiration and soil temperature. Estimates ranged from 424.90 g C m⁻² to 784.73 g C m⁻² by interpolating measured soil respiration between sampling dates for every day of the year and then computing the sum to obtain the annual value. The contributions of winter soil CO₂ efflux to annual soil respiration were 3.48-7.30% and 4.92-7.83% using interpolated and modeled methods, respectively. Our results indicate that in mid-latitude ecosystems, soil CO₂ efflux continues throughout the winter and winter soil respiration is an important component of annual CO₂ efflux.     

Annual soil CO2 effluxes in the High Arctic: The role of snow thickness and vegetation type

Little work has been done to quantify annual soil CO₂ effluxes in the High Arctic region because of the difficulty in taking winter measurements. Since the effects of climate change are expected to be higher in Arctic than in temperate ecosystems, it is important that summer measurements are extended to cover the entire year. This study evaluates the quantity and quality of soil organic C (SOC) and seasonal controls of soil CO₂ effluxes in three soils under three dominating types of vegetation (Dryas, Cassiope, and Salix) at Svalbard. Measurements included soil CO₂ effluxes in the field and the laboratory, temperature, water content, and snow thickness. About 90% of the variation in soil respiration throughout 1 year was due to near-surface soil temperatures which ranged from −12 to +12 °C. Total annual soil CO₂ effluxes varied from 103 g C m⁻² at soils under Cassiope, 152 g C m⁻² under Dryas sites, and 176 g C m⁻² under Salix, with 20%, 14%, and 30%, respectively, being released during a 6-month winter period. The sensitivity of soil respiration with respect to soil temperature was the same year round and differences in winter CO₂ effluxes at the three vegetation types were mainly related to subsurface soil temperatures controlled by snow depth. The quantity and quality of soil organic matter varied under the different vegetation types. Soils under Salix had the largest and most labile pool of SOC and were characterized by a long period of snow cover. In contrast, soils under Cassiope were more nutrient-poor, more acidic and held the smallest amount of total and labile SOC, whereas soils under Dryas remained snow-free most of the winter and therefore had the coldest winter conditions. Thus, winter soil respiration rates under Dryas and Cassiope were significantly lower than those under Salix; under Dryas this was mainly due to snow depth, under Cassiope this was a combination of snow depth and poor litter quality. It is concluded that winter respiration is highly variable across Arctic landscapes and depends on the spatial distribution of snow, which acts as a direct control on soil temperatures and indirect on vegetation types and thereby, the amount and quality of soil organic matter, which serve as additional important drivers of soil respiration.     

Respiration pattern and microbial use of field-grown transgenic Bt-maize residues

A 49-day incubation experiment was carried out with the addition of field-grown maize stem and leaf residues to soil at three different temperatures (5, 15, and 25 °C). The aim was to study the effects of two transgenic Bt-maize varieties in comparison to their two parental non-Bt varieties on the mineralization of the residues, on their incorporation into the microbial biomass and on changes in the microbial community structure. The stem and leaf residues of Novelis-Bt contained 3.9 μg g⁻¹ dry weight of the Bt toxin Cry1Ab and those of Valmont-Bt only 0.8 μg g⁻¹. The residues of the two parental non-Bt varieties Nobilis and Prelude contained higher concentrations of ergosterol (+220%) and glucosamine (+190%) and had a larger fungal C-to-bacterial C ratio (+240%) than the two Bt varieties. After adding the Bt residues, an initial peak in respiration of an extra 700 μg CO₂-C g⁻¹ soil or 4% of the added amount was observed in comparison to the two non-Bt varieties at all three temperatures. On average of the four varieties, 19-38% of the maize C added was mineralized during the 49-day incubation at the three different temperatures. The overall mean increase in total maize-derived CO₂ evolution corresponded to a Q₁₀ value of 1.4 for both temperature steps, i.e. from 5 to 15 °C and from 15 to 25 °C. The addition of maize residues led to a strong increase in all microbial properties analyzed. The highest contents were always measured at 5 °C and the lowest at 25 °C. The variety-specific contents of microbial biomass C, biomass N, ATP and adenylates increased in the order Novelis-Bt ? Prelude<Valmont-Bt ? Nobilis. The mineralization of Novelis-Bt residues with the highest Bt concentration and lowest N concentration and their incorporation into the microbial biomass was significantly reduced compared to the parental non-Bt variety Nobilis. These negative effects increased considerably from 5 to 25 °C. The transgenic Bt variety Valmont did not show further significant effects except for the initial peak in respiration at any temperature.     

Thermodynamic parameters of enzymes in grassland soils from Galicia, NW Spain

The thermodynamic parameters of the enzymes catalase, dehydrogenase, casein-protease, α-N-benzoyl-l-argininamide (BAA)-protease, urease, Carboxymethyl (CM)-cellulase, invertase, β-glucosidase and arylsulphatase, were investigated in grassland soils from a European temperate-humid zone (Galicia, NW Spain). The effect of temperature on enzyme activity was determined at 5, 18, 27, 37, 57 and 70 °C. The temperature-dependence of the rate of substrate hydrolysis varied depending on the enzyme and soil. In general, the soil containing the least amount of organic matter (OM) showed the lowest enzyme activity for all temperatures and enzymes, whereas soils with similar OM contents showed similar levels of activity for the entire temperature range. Temperature had a noteworthy effect on the activity of oxidoreductases. Product formation in the reaction catalyzed by dehydrogenase increased with increasing temperature until 70 °C, which was attributed to chemical reduction of iodonitrotetrazolium violet (INT) at high temperatures. Catalase activity was not affected above 37 °C, which may be explained either by non-enzymatic decomposition of hydrogen peroxide or by the fact that catalase has reached kinetic perfection, and is therefore not saturated with substrate.The Arrhenius equation was used to determine the activation energy (E_a) and the temperature coefficient (Q₁₀) for all enzymes. The values of E_a and Q₁₀ for each enzyme differed among soils, although in general the differences were small, especially for those enzymes that act on substrates of low molecular weight. In terms of the values of E_a and Q₁₀ and the differences established among soils, the results obtained for those enzymes that act on substrates of high molecular weight differed most from those corresponding to the other enzymes. Thus the lowest E_a and Q₁₀ values corresponded to BAA-protease, and the highest values to CM-cellulase and casein-protease. Except for catalase in one of the soils, the values of E_a and Q₁₀ for the oxidoreductases were similar to those of most of the hydrolases. In general, the effect of temperature appeared to be more dependent on the type of enzyme than on the characteristics of the soil.     

Temperature, water content and wet-dry cycle effects on DOC production and carbon mineralization in agricultural peat soils

Agricultural peat soils in the Sacramento-San Joaquin Delta, California have been identified as an important source of dissolved organic carbon (DOC) and trihalomethane precursors in waters exported for drinking. The objectives of this study were to examine the primary sources of DOC from soil profiles (surface vs. subsurface), factors (temperature, soil water content and wet-dry cycles) controlling DOC production, and the relationship between C mineralization and DOC concentration in cultivated peat soils. Surface and subsurface peat soils were incubated for 60 d under a range of temperature (10, 20, and 30 °C) and soil water contents (0.3-10.0 g-water g-soil⁻¹). Both CO₂-C and DOC were monitored during the incubation period. Results showed that significant amount of DOC was produced only in the surface soil under constantly flooded conditions or flooding/non-flooding cycles. The DOC production was independent of temperature and soil water content under non-flooded condition, although CO₂ evolution was highly correlated with these parameters. Aromatic carbon and hydrophobic acid contents in surface DOC were increased with wetter incubation treatments. In addition, positive linear correlations (r²=0.87) between CO₂-C mineralization rate and DOC concentration were observed in the surface soil, but negative linear correlations (r²=0.70) were observed in the subsurface soil. Results imply that mineralization of soil organic carbon by microbes prevailed in the subsurface soil. A conceptual model using a kinetic approach is proposed to describe the relationships between CO₂-C mineralization rate and DOC concentration in these soils.     

Soil CO2 flux in six monospecific forest plantations in Southern Rwanda

Forest soils contain the largest carbon stock of all terrestrial biomes and are probably the most important source of carbon dioxide (CO₂) to atmosphere. Soil CO₂ fluxes from 54 to 72-year-old monospecific stands in Rwanda were quantified from March 2006 to December 2007. The influences of soil temperature, soil water content, soil carbon (C) and nitrogen (N) stocks, soil pH, and stand characteristics on soil CO₂ flux were investigated. The mean annual soil CO₂ flux was highest under Eucalyptus saligna (3.92 μmol m⁻² s⁻¹) and lowest under Entandrophragma excelsum (3.13 μmol m⁻² s⁻¹). The seasonal variation in soil CO₂ flux from all stands followed the same trend and was highest in rainy seasons and lowest in dry seasons. Soil CO₂ flux was mainly correlated to soil water content (R² = 0.36-0.77), stand age (R² = 0.45), soil C stock (R² = 0.33), basal area (R² = 0.21), and soil temperature (R² = 0.06-0.17). The results contribute to the understanding of factors that influence soil CO₂ flux in monocultural plantations grown under the same microclimatic and soil conditions. The results can be used to construct models that predict soil CO₂ emissions in the tropics.     

Are ecological gradients in seasonal Q10 of soil respiration explained by climate or by vegetation seasonality?

Soil respiration (SR) is highly sensitive to future climate change, and particularly to global warming. However, considerable uncertainties remain associated with the temperature sensitivity of SR and its controlling processes. Using 384 field measurement data from 114 published papers and one book, this study quantifies the variation in the seasonal Q₁₀ values of soil respiration, the multiplier by which respiration rates increase for a 10 °C increase in temperature, and its drivers across different sites. No significant correlation between Q₁₀ and mean annual temperature or mean annual precipitation is found when statistically controlling seasonal changes in vegetation activity, deduced from satellite vegetation greenness index observations (normalized difference vegetation index, or NDVI). In contrast, the seasonal amplitude of NDVI is significantly and positively correlated with the apparent Q₁₀ of SR. This result indicates that the variations of seasonal vegetation activity exert dominant control over the variations of the apparent Q₁₀ of SR across different sites, highlighting the ecological linkage between plant physiological processes and soil processes. It further implies that the seasonal variation of vegetation activity may thus dominate the apparent seasonal temperature sensitivity. We conclude that the apparent Q₁₀ value of SR estimated from field measurements is generally larger than the intrinsic temperature sensitivity of soil organic matter decomposition, and thus cautions should be taken when applying apparent Q₁₀ values directly in ecosystem models. Our regression analysis further shows that when the amplitude of NDVI variation approximates 0 (and thus when the seasonality in vegetation activity is marginal), the residual Q₁₀ of SR for soil temperature measured at 5 cm depth is about 1.5.     

Unfrozen water content moderates temperature dependence of sub-zero microbial respiration

Abrupt increases in the temperature sensitivity of soil respiration below 0 °C have been interpreted as a change in the dominance of other co-dependent environmental controls, such as the availability of liquid-state water. Yet the relationship between unfrozen water content and soil respiration at sub-zero temperatures has received little attention because of difficulties in measuring unfrozen water contents. Using a recently-developed semi-solid ²H NMR technique the unfrozen water content present in seasonally frozen boreal forest soils was quantified and related to biotic CO₂ efflux in laboratory microcosms maintained at temperatures between −0.5 and −8 °C. In both soils the unfrozen water content had an exponential relationship with temperature and was increased by addition of KCl solutions of defined osmotic potential. Approximately 13% unfrozen water was required to release the dependence of soil respiration on unfrozen water content. Depending on the osmotic potential of soil solution, this threshold unfrozen water content was associated with temperatures down to −6 °C; yet if temperature were the predictor of CO₂ efflux, then the abrupt increase in the temperature sensitivity of CO₂ efflux was associated with −2 °C, except in soils amended with −1500 kPa KCl which did not show any abrupt changes in temperature sensitivity. The KCl-amendments also had the effect of decreasing Q₁₀ values and activation energies (Ea) by factors of 100 and three, respectively, to values comparable with those for soil respiration in unfrozen soil. The disparity between the threshold temperatures and the reductions in Q₁₀ values and activation energies after KCl amendment indicates the significance of unfrozen water availability as an environmental control of equal importance to temperature acting on sub-zero soil respiration. However, this significance was diminished when soils were supplied with abundant labile C (sucrose) and the influences of other environmental controls, allied to the solubility and diffusion of respiratory substrates and gases, are considered to increase.     

Modelling decomposition of standard plant material along an altitudinal gradient: A re-analysis of data of Coûteaux et al. (2002)

We explored an alternative method to analyse data of Coûteaux et al. [2002, Soil Biology and Biochemistry 34, 69-78] on the decomposition of a standard organic material in six soils along an altitudinal gradient in the Venezuelan Andes (65-3968 m a.s.l.). Coûteaux et al., fitted separate two-component decomposition models to data of the individual sites, allowing the initial size of the labile and the resistant component to differ between sites. This procedure led them to conclude that the initial size of the resistant component and its decomposition rate depend on temperature while decomposition rate of the labile component does not, which seems biologically unlikely and at variance with literature. As an alternative we fitted a single two-component model to the whole data set, using identical initial component sizes for all sites. We found no statistical ground for using variable initial component sizes. It appeared that the data does not allow a conclusion on the effect of temperature on the decomposition of the labile component. We also investigated alternatives for the values of Q₁₀ and T_opt that were used by Coûteaux et al., and found that temperature explains a larger part of the differences in decomposition rate among sites when using a Q₁₀ value of 3.75 instead of 2.2 and a T_opt value of 27 °C instead of 25 °C. We discuss the arguments used in model selection and the consequences for predictions of long-term accumulation of soil carbon. Our analysis suggests an even stronger positive feedback between global warming and soil carbon emission than the analysis by Coûteaux et al.