The counteracting effects of elevated atmospheric CO2 concentrations and drought episodes: Studies of enchytraeid communities in a dry heathland

The potential impacts of interactions of multiple climate change factors in soil ecosystems have received little attention. Most studies have addressed effects of single factors such as increased temperature or atmospheric CO₂ but little is known about how such environmental factors will interact. In the present study we investigate the effects of in situ exposure to elevated atmospheric CO₂ concentration, increased temperatures and prolonged drought episodes on field communities of Enchytraeidae (Oligochaeta) in a dry heathland (Brandbjerg, Denmark). Increased CO₂ had a positive effect on enchytraeid biomass, whereas drought significantly reduced it. Elevated temperature did not result in any detectable effects. No interactions between the three factors were observed. Interestingly, the positive effect of increased CO₂ and the negative effect of drought were cancelled out when applied in combination. Thus, in the combined drought and CO₂ treatment, and when additionally combined with increased temperature, the total biomass of enchytraeids was similar to those in the ambient plots. The positive effect of increased CO₂ seemed to be influenced by season, as the response was most pronounced during summer, and less so in autumn. The impact of the drought was more consistent, with enchytraeids reduced at all sampling occasions. Moreover, the negative effect of drought seemed to depend on the inter-annual variability of precipitation. The year with a dry summer and autumn (2006) showed a stronger impact of drought on the enchytraeids, compared to the year with a wet summer and autumn (2007). Our study emphasises the importance of multi-factorial experimental design as a means to investigate effects of climatic changes.     

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.     

Elevated CO2 increases the effect of an arbuscular mycorrhizal fungus and a plant-growth-promoting rhizobacterium on structural stability of a semiarid agricultural soil under drought conditions

In arid and semiarid Mediterranean regions, an increase in the severity of drought events could be caused by rising atmospheric CO₂ concentrations. We studied the effects of the interaction of CO₂, water supply and inoculation with a plant-growth-promoting rhizobacterium (PGPR), Pseudomonas mendocina Palleroni, or inoculation with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith), on aggregate stabilisation of the rhizosphere soil of Lactuca sativa L. cv. Tafalla. The influence of such structural improvements on the growth of lettuce was evaluated. We hypothesised that elevated atmospheric CO₂ concentration would increase the beneficial effects of inoculation with a PGPR or an AM fungus on the aggregate stability of the rhizosphere soil of lettuce plants. Leaf hydration, shoot dry biomass and mycorrhizal colonisation were decreased significantly under water-stress conditions, but this decrease was more pronounced under ambient vs elevated CO₂. The root biomass decreased under elevated CO₂ but only in non-stressed plants. Under elevated CO₂, the microbial biomass C of the rhizosphere of the G. intraradices-colonised plants increased with water stress. Bacterial and mycorrhizal inoculation and CO₂ had no significant effect on the easily-extractable glomalin concentration. Plants grown under elevated CO₂ had a significantly higher percentage of stable aggregates under drought stress than under well-watered conditions, particularly the plants inoculated with either of the assayed microbial inocula (about 20% higher than the control soil). We conclude that the contribution of mycorrhizal fungi and PGPR to soil aggregate stability under elevated atmospheric CO₂ is largely enhanced by soil drying.     

Population responses of oribatids and enchytraeids to ectomycorrhizal and saprotrophic fungi in plant-soil microcosms

The oribatid mites Oppiella nova, Tectocepheus velatus and Nothrus silvestris and the enchytraeid worm Cognettia sphagnetorum are four common animal species in boreal forest soils. According to the literature, they respond differently to clear-cutting of forest stands. O. nova responds with population decreases, T. velatus and N. silvestris with small changes and C. sphagnetorum with population increases. We hypothesised that the presence/absence of ectomycorrhizal (EM) fungi is a major factor in explaining these reactions. The population responses of these soil animals to inoculation of five species of EM fungi growing in symbiosis with their host tree (Pinus sylvestris L.) and one saprotrophic fungus, Hypholoma capnoides, growing on wood were tested in pot microcosms with artificial soil (peat and vermiculite) for 70-84 days. Additionally, plants without inoculation of EM fungi, plants growing in forest soil (FS) and plant-free peat and vermiculite (PV) were included. O. nova increased significantly in abundance in the treatments with the EM fungi Suillus variegatus and Paxillus involutus, but not in the other treatments. T. velatus increased significantly in abundance in FS, but declined in most of the other treatments, and N. silvestris showed a similar, albeit not significant, response. C. sphagnetorum did not increase in abundance in any of the treatments with EM fungi but increased its abundance four times in both FS and PV. The results show that the fungivore O. nova preferentially feeds on certain EM fungi, especially S. variegatus, whereas the EM fungus Piloderma fallax and the saprotrophic H. capnoides are not preferred. In contrast, C. sphagnetorum is restricted in its population growth by the EM fungi studied, and this dominant microbi-detritivore is clearly favoured by the absence of EM fungi.     

Nitrous oxide emissions from agricultural soils at low temperatures: a laboratory microcosm study

We studied in laboratory microcosms (intact soil cores) N₂O and CO₂ emissions from four different agricultural soil types (organic soil, clay, silt and loam) at low temperatures with or without freezing-thawing events. When the temperature of the frozen soil cores was increased stepwise from −8 °C the N₂O emissions began to increase at −0.5 °C, and peaked at −0.1 °C in the organic, clay and silt soils, and at +1.6 °C in the loam soils. However, a stepwise decrease in soil temperature from +15 °C also induced an increase in the N₂O emissions close to the 0 °C. These emissions peaked between −0.4 and +2.5 °C depending on the soil type and water content. However, the emission maxima were from 2 to 14.3% of those encountered in the experiments where frozen soils were thawed. Our results show that in addition to the well-documented thawing peak, soils also can have a maximum in their N₂O emission near 0 °C when soil temperature decrease. These emissions, however, are less than those emitted from thawing soils. The correlations between the N₂O and CO₂ emissions were weak. Our results suggest that N₂O is produced in soils down to a temperature of −6 °C.     

Autumn growth and cold hardening of winter wheat under simulated climate change

Abstract

Plant responses to elevated CO₂ are governed by temperature, and at low temperatures the beneficial effects of CO₂ may be lost. To document the responses of winter cereals grown under cold conditions at northern latitudes, autumn growth of winter wheat exposed to ambient and elevated levels of temperature (+2.5°C), CO₂ (+150 µmol mol^?1), and shade (?30%) was studied in open-top chambers under low light and at low temperatures. Throughout the experiment, temperature dominated plant responses, while the effects of CO₂ were marginal, except for a positive effect on root biomass. Increased temperature resulted in increased leaf area, total biomass, total root biomass, total stem biomass, and number of tillers, but also a lower content of total sugars and a weaker tolerance to frost. The loss of frost tolerance was related to the larger size of plants grown at elevated temperature. The 30% light reduction under shading did not affect the growth, sugar content, or frost tolerance of winter wheat. At the low temperatures found at high latitudes during autumn, the atmospheric CO₂ increase is unlikely to enhance autumn growth of winter wheat to any significant extent, while a temperature increase may have important and major effects on its development and growth.     

The quantitative influence of enchytraeids (Oligochaeta) and microarthropods on decomposition of coniferous raw humus in microcosms

Material from the organic layer of a podsol soil in a spruce stand in southern Norway was incubated for 21/2 years at 15°C in microcosms with a volume of 0.1 dm³. The soil was sterilised and reinoculated with a mixed microflora before the incubation start, and given three different faunal communities: (1) A mixed assemblage of microarthropods plus the enchytraeid Cognettia sphagnetorum (“full fauna”, FF), (2) only C. sphagnetorum (“enchytraeids”, E), and (3) no animals added (“no fauna”, NF). The respiration rate was measured during the last 14 months of incubation, and was highest in FF throughout this period. When all respiration analyses were pooled, the value for FF was 33% higher than for NF and 25% higher than for E. The substrate dry mass loss, measured twice, was also highest in FF (17% higher in FF than in both the other treatments after 11/2 years of incubation, and 31% higher than in both the other treatments after 21/2 years). Both for respiration and for mass loss, the difference between FF and the other two treatments was statistically significant, while there was no apparent difference between the E and NF treatments. There was no sign of a general rise or fall in the respiration rate during the 14 months from the first to the last analysis. The ammonium (and total N) concentration in the soil water was higher in FF and E than in NF, whereas the nitrate concentration was lowest in FF and highest in E. The higher mineralisation activity in the FF treatment was probably caused by the higher diversity of mesofauna, and perhaps also by higher diversities of microflora and microfauna accidentally introduced together with the arthropods.     

Denitrification potential in a grassland subsoil: effect of carbon substrates

We examined the potential of a subsoil to denitrify nitrate under optimal anaerobic conditions in a laboratory-based incubation when supplied with a range of C substrates of increasing recalcitrance. Both topsoil and its associated subsoil were supplied with nitrate and either glucose, starch or cellulose. Microbial respiration and the evolution of N₂O and N₂ were measured. The subsoil supported low amounts of microbial activity and responded only to the glucose treatment; with less than one-fifth of the N₂O production measured in the top soil. Overall, our findings demonstrated that the denitrification potential of this particular subsoil is relatively low and that only simple carbohydrates could be utilised readily by the resident microorganisms.     

Influence of Cognettia sphagnetorum (Oligochaeta: Enchytraeidae) on nitrogen mineralization in homogenized mor humus

Summary The influence of the enchytraeid species Cognettia sphagnetorum on N mineralization in homogenized mor humus was examined in a laboratory study. The mor humus was incubated in containers (150 ml) for 8 months at various temperatures and with different moisture levels. Two series were used, one with C. sphagnetorum and one without. The presence of enchytraeids in the cultures increased the level of NH₄ ⁺ and NO₃ ^- by about 18% compared with the cultures without enchytraeids. Almost 40% of this difference was explained by the decomposition of dead enchytraeids. Temperature and soil moisture were the most important factors controlling the mineralization rate. The optimum moisture for N mineralization was between pF 1.6 and 1.1.Dedicated to the late Prof. Dr. W. Kühnelt     

Recovery of enchytraeid populations after severe drought events

The potential impact of changes in precipitation patterns associated with climate changes was investigated in Enchytraeidae (Oligochaeta) in a Danish heathland. The amount of precipitation was manipulated during spring and summer in an experimental field site in order to reveal effects of three different drought regimes: weak drought (WD), medium drought (MD) and high drought (HD). Enchytraeids were sampled every six to eight weeks (0–9 cm depth) for more than the eight months and soil water potential (SWP) and soil water content (SWC) was measured on a regular basis for five months.The enchytraeid communities were generally reduced due to a natural drought spell. The HD treatment significantly reduced the moisture level of the soil further with SWP below −15 bar (5 and 10 cm depth) and SWC around 5% (v/v) for more than two months. As a result almost no enchytraeids were found in HD plots after two months with high drought stress. Nevertheless, the HD-treated enchytraeid communities recovered within two months, as there was no significant difference in biomass and density of the different treatments at that time. During periods with extreme low SWP enchytraeids were practically absent in the top soil (0–3 cm), but a few animals were found in 3–6 cm. During this period SWP was around −15 bar even in 20 cm depth, indicating that active stages could not have survived. Thus, we suggest that the species present must be dependent on a drought tolerant stage, as vertical migration could not have supported the observed recovery.SWP and SWC were both significantly correlated with the total density and biomass of enchytraeids. However, density was better correlated with the SWP and SWC compared to biomass, which can be due to hatching of cocoons and increased fragmentation rate.     

Characteristics of greenhouse gas emissions from rice paddy fields in South Korea under climate change scenario RCP-8.5 using the DNDC model

Understanding the greenhouse gas(GHG)emission from rice paddy fields is essential to come up with appropriate countermeasure in response to global warming.However,GHG emissions from paddy fields in South Korea are not well characterized.The objectives of this study were to estimate the carbon dioxide(CO₂)and methane(CH₄)emissions from rice paddy fields in South Korea,under the Representative Concentration Pathway 8.5(RCP-8.5)climate change scenario using the DNDC(i.e.,DeNitrification-DeComposition)model at 1-km²resolution.The performance of the model was verified with field data collected using a closed chamber,which supports the application of the model to South Korea.Both the model predictions and field measurements showed that most(>95%)GHG emissions occur in the cropping period,between April and October.As a baseline(assuming no climate change),the national sums of the CO₂and CH₄emissions for the 2020 s and 2090 s were estimated to be 5.8×10⁶and 6.0×10⁶t CO₂-equivalents(CO₂-eq)year^-1for CO₂and 6.4×10⁶and 6.6×10⁶t CO₂-eq year^-1for CH₄,respectively,indicating no significant changes over 80 years.Under RCP-8.5,in the 2090 s,CH₄emissions were predicted to increase by 10.7×10⁶and 14.9×10⁶t CO₂-eq year^-1,for a 10-or 30-cm tillage depth,respectively.However,the CO₂emissions gradually decreased with rising temperatures,due to reduced root respiration.Deep tillage increased the emissions of both GHGs,with a more pronounced effect for CH₄than CO₂.Intermittent drainage in the middle of the cropping season can attenuate the CH₄emissions from paddy fields.The findings of this work will aid in developing nationwide policies on agricultural land management in the face of climate change.     

Palatability trials on hardwood leaf litter grown under elevated CO2: a stable carbon isotope study

The palatability to isopods and microbes of a broad range of hardwood leaf litter, derived from three field CO₂-enrichment experiments in the USA, was investigated, using δ¹³C, to trace the C flow from litter to isopods and to CO₂ respired by microbial decomposition. Leaf litter grown under elevated CO₂ had δ¹³C values ranging from −39 to −45‰, which were significantly different from ambient litter δ¹³C values of around −30‰. Litter palatability to isopods of the Porcellio sp. was tested by incubating ambient- and elevated-CO₂ litter, and a mixture of the two, in the presence of isopods for 14 days, under environmentally controlled conditions; δ¹³C was measured on litter and isopods' body before and after incubation. In an additional experiment, litter was incubated in the absence of fauna for 30 days, and on five occasions the δ¹³C of the CO₂ respired from litter was measured. The ¹³C label was clearly carried from the litter source to the isopods' bodies, and their faeces. For microbial-respired CO₂, δ¹³C was significantly higher than that of the litter source, suggesting preferential degradation of substrates enriched in ¹³C as compared to those in the overall litter. With the exception of Quercus myrtifolia leaf litter, elevated CO₂ did not affect the palatability to isopods nor the microbial degradation of any of the litters, possibly as a result of unaltered litter N concentration. However, significant differences in litter palatability and decay rates were observed among the different species. With this study, the use of isotopically labelled litter material was confirmed as a key methodology that can significantly contribute to the advancement of the understanding of litter decomposition and of the quantification of C fluxes in the process.     

Interpreting the dependence of soil respiration on soil temperature and water content in a boreal aspen stand

Continuous half-hourly measurements of soil CO₂ efflux made between January and December 2001 in a mature trembling aspen stand located at the southern edge of the boreal forest in Canada were used to investigate the seasonal and diurnal dependence of soil respiration (R_s) on soil temperature (T_s) and water content (θ). Daily mean R_s varied from a minimum of 0.1 μmol m⁻² s⁻¹ in February to a maximum of 9.2 μmol m⁻² s⁻¹ in mid-July. Daily mean T_s at the 2-cm depth was the primary variable accounting for the temporal variation of R_s and no differences between Arrhenius and Q₁₀ response functions were found to describe the seasonal relationship. R_s at 10 °C (R_s10) and the temperature sensitivity of R_s (Q_10Rs) calculated at the seasonal time scale were 3.8 μmol m⁻² s⁻¹ and 3.8, respectively. Temperature normalization of daily mean R_s (R_sN) revealed that θ in the 0–15 cm soil layer was the secondary variable accounting for the temporal variation of R_s during the growing season. Daily R_sN showed two distinctive phases with respect to soil water field capacity in the 0–15 cm layer (θ_fc, 0.30 m³ m⁻³): (1) R_sN was strongly reduced when θ decreased below θ_fc, which reflected a reduction in microbial decomposition, and (2) R_sN slightly decreased when θ increased above θ_fc, which reflected a restriction of CO₂ or O₂ transport in the soil profile.Diurnal variations of half-hourly R_s were usually out of phase with T_s at the 2-cm depth, which resulted in strong diurnal hysteresis between the two variables. Daily nighttime R_s10 and Q_10Rs parameters calculated from half-hourly nighttime measurements of R_s and T_s at the 2-cm depth (when there was steady cooling of the soil) varied greatly during the growing season and ranged from 6.8 to 1.6 μmol m⁻² s⁻¹ and 5.5 to 1.3, respectively. On average, daily nighttime R_s10 (4.5 μmol m⁻² s⁻¹) and Q_10Rs (2.8) were higher and lower, respectively, than the values obtained from the seasonal relationship. Seasonal variations of these daily parameters were highly correlated with variations of θ in the 0–15 cm soil layer, with a tendency of low R_s10 and Q_10Rs values at low θ. Overall, the use of seasonal R_s10 and Q_10Rs parameters led to an overestimation of daily ranges of half-hourly R_s (ΔR_s) during drought conditions, which supported findings that the short-term temperature sensitivity of R_s was lower during periods of low θ. The use of daily nighttime R_s10 and Q_10Rs parameters greatly helped at simulating ΔR_s during these periods but did not improve the estimation of half-hourly R_s throughout the year as it could not account for the diurnal hysteresis effect.     

A new technique to measure soil CO2 efflux at constant CO2 concentration

A new principle for measuring soil CO₂ efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO₂ within the system to achieve a constant CO₂ concentration inside the soil chamber at ambient level, thus balancing the amount of CO₂ entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO₂ efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO₂ within the soil, while allowing for continuous capture of the CO₂ released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO₂ efflux and its δ¹³C, since both variables show sensitivity to a distortion of the soil CO₂ profile commonly found in conventional chamber techniques.     

Conserving marginal populations of the food plant (Impatiens noli-tangere) of an endangered moth (Eustroma reticulatum) in a changing climate

Impatiens noli-tangere is scarce in the UK and probably only native to the Lake District and Wales. It is the sole food plant for the endangered moth Eustroma reticulatum. Significant annual fluctuations in the size of I. noli-tangere populations endanger the continued presence of E. reticulatum in the UK. In this study, variation in population size was monitored across native populations of I. noli-tangere in the English Lake District and Wales. In 1998, there was a crash in the population size of all metapopulations in the Lake District but not of those found in Wales. A molecular survey of the genetic affinities of samples in 1999 from both regions and a reference population from Switzerland was performed using AFLP and ISSR analyses. The consensus UPGMA dendrogram and a PCO scatter plot revealed clear differentiation between the populations of I. noli-tangere in Wales and those in the Lake District. Most of the genetic variation in the UK (H_T=0.064) was partitioned between (G_ST=0.455) rather than within (H_S=0.034) regions, inferring little gene flow occurs between regions. There was similar bias towards differentiation between metapopulations in Wales, again consistent with low levels of interpopulation gene flow. This contrasts with far lower levels of differentiation in the Lake District which suggests modest rates of gene flow may occur between populations. It is concluded that in the event of local extinction of sites or populations, reintroductions should be restricted to samples collected from the same region. We then surveyed climatic variables to identify those most likely to cause local extinctions. Climatic correlates of population size were sought from two Lake District metapopulations situated close to a meteorological station. A combination of three climatic variables common to both sites explained 81-84% of the variation in plant number between 1990 and 2001. Projected trends for these climatic variables were used in a Monte Carlo simulation which suggested an increased risk of I. noli-tangere population crashes by 2050 at Coniston Water, but not at Derwentwater. Implications of these findings for practical conservation strategies are explored.     

Responses of co-occurring populations of Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) to soil pH, moisture and resource addition

Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) are the two most dominating soil invertebrates in terms of biomass in boreal coniferous forest soils. A microcosm experiment was set up in order to study the influence of pH, moisture and resource addition on D. octaedra and C. sphagnetorum when both species are simultaneously present. Two kinds of coniferous forest humus were used as substrate, pine stand humus (pH 4.2), and spruce stand humus (pH 4.6); in the third treatment the pine stand humus was adjusted with slaked lime (CaOH₂) to the same initial pH as the spruce stand humus. Each substrate was adjusted to water contents of 25%, 42.5% and 60% of WHC (referred to as ‘dry’, ‘moist’ and ‘wet’). In the second part of the experiment, spruce needle litter and birch leaf litter were separately added into the pine stand humus (‘moist’, unlimed) and compared with a control without litter. The microcosms were plastic jars with 75 g (d.m.) of humus, into which 4 specimens of D. octaedra and 70 specimens of C. sphagnetorum were added. D. octaedra showed the highest biomass and C. sphagnetorum the lowest biomass in the spruce stand humus with higher pH. Moisture did not affect earthworms, while C. sphagnetorum thrived best at the highest moisture. Addition of both kinds of litter increased the numbers and biomass of D. octaedra, while on C. sphagnetorum resource addition had little effect. The results help to explain the abundance of these two species in coniferous forests differing in soil acidity, moisture and fertility.     

气候变化下基于DayCent的旱地玉米农田温室气体排放通量模拟

气候变化通过大气CO₂浓度、温度和降雨的改变,直接或间接影响农田温室气体排放,研究未来气候情景下农田温室气体排放对实现农业碳减排具有重要意义。为探究气候变化背景下农田温室气体排放特征,该研究在长期田间定位试验基础上,利用当前大气CO₂浓度与CO₂浓度升高条件下旱作玉米农田温室气体排放通量的田间观测数据,采用“试错法”对DayCent模型进行校验,并利用校验后的模型,根据第六次国际耦合模式比较计划（Coupled Model Intercomparison Project phase 6,CMIP6）气候情景数据,预测未来SSP126与SSP245气候情景下旱地玉米农田温室气体排放通量。结果表明,DayCent模型对不同大气CO₂浓度下N₂O、CH₄和CO₂排放通量的模拟值与观测值高度一致,模拟效率（modeling efficiency,EF）分别为0.58～0.87、0.45～0.65和0.25～0.62,均方根误差（root mean square error,RMSE）分别为0.83～1.33、0.67～0.82和0.58～0.80 g/(hm²·d),决定系数（coefficient of determination,R²）分别为0.80～0.91、0.53～0.80和0.53～0.85。SSP126和SSP245气候情景下,在玉米单作种植模式下旱地农田N₂O和CO₂年排放量均呈现上升趋势,以2001—2020年农田温室气体排放通量为基准,到2060年N₂O年排放量分别增加22.8%和24.9%,CO₂年排放量分别增加6.7%和8.0%;旱地农田CH₄年吸收量呈下降趋势,两个气候情景下分别减少13.6%和13.4%。未来气候情景下旱地农田仍是温室气体排放源,优化氮肥管理和农田耕作措施对实现温室气体减排具有重要意义,模拟结果可以为制定农业适应气候变化对策提供基础数据支持。     

Temporal change in spatial variability of soil respiration on a slope of Japanese cedar (Cryptomeria japonica D. Don) forest

Although information regarding the spatial variability of soil respiration is important for understanding carbon cycling and developing a suitable sampling design for estimating average soil respiration, it remains relatively understudied compared to temporal changes. In this study, soil respiration was measured at 35 locations by season on a slope of Japanese cedar forest in order to examine temporal changes in the spatial distribution of soil respiration. Spatial variability of soil respiration varied between seasons, with the highest coefficient variation in winter (42%) and lowest in summer (26%). Semivariogram analysis and kriged maps revealed different patterns of spatial distribution in each season. Factors affecting the spatial variability were relief index (autumn), soil hardness of the A layer (winter), soil hardness at 50 cm depth (spring) and the altitude and relief index (summer). Annual soil respiration (average: 39 mol m⁻² y⁻¹) varied from 26 mol m⁻² y⁻¹ to 55 mol m⁻² y⁻¹ between the 35 locations and was higher in the upper part of the slope and lower in the lower part. The average Q₁₀ value was 2.3, varying from 1.3 to 3.0 among the locations. These findings suggest that insufficient information on the spatial variability of soil respiration and imbalanced sampling could bias estimates of current and future carbon budgets.     

Below ground carbon turnover and greenhouse gas exchanges in a sub-arctic wetland

Here we present results from a field experiment in a sub-arctic wetland near Abisko, northern Sweden, where the permafrost is currently disintegrating with significant vegetation changes as a result. During one growing season we investigated the fluxes of CO₂ and CH₄ and how they were affected by ecosystem properties, i.e., composition of species that are currently expanding in the area (Carex rotundata, Eriophorum vaginatum and Eriophorum angustifolium), dissolved CH₄ in the pore water, substrate availability for methane producing bacteria, water table depth, active layer, temperature, etc. We found that the measured gas fluxes over the season ranged between: CH₄ 0.2 and 36.1 mg CH₄ m⁻² h⁻¹, Net Ecosystem Exchange (NEE) −1000 and 1250 mg CO₂ m⁻² h⁻¹ (negative values meaning a sink of atmospheric CO₂) and dark respiration 110 and 1700 mg CO₂ m⁻² h⁻¹. We found that NEE, photosynthetic rate and CH₄ emission were affected by the species composition. Multiple stepwise regressions indicated that the primary explanatory variables for NEE was photosynthetic rate and for respiration and photosynthesis biomass of green leaves. The primary explanatory variables for CH₄ emissions were depth of the water table, concentration of organic acid carbon and biomass of green leaves. The negative correlations between pore water concentration and emission of CH₄ and the concentrations of organic acid, amino acid and carbohydrate carbon indicated that these compounds or their fermentation by-products were substrates for CH₄ formation. Furthermore, calculation of the radiative forcing of the species expanding in the area as a direct result of permafrost degradation and a change in hydrology indicate that the studied mire may act as an increasing source of radiative forcing in future.     

Sulfur dioxide inhibition of photosynthesis at different CO2 and O2 concentrations in relation to photorespiration

The mechanism of SO₂ inhibition of photosynthesis in intact leaves of tomato and maze was studied to evaluate SO₂ inhibition of photorespiration. Leaf tissues were fumigated with SO₂ under photorespiratory (low CO, and/or high O, concentrations) and non-photo-respiratory conditions. When tomato leaf disks were fumigated with 10 ppm SO₂ at 2, 21 and 100° o O., SO₂ inhibited photosynthesis at 2% O₂ in the same degrees as at 21% O₂. SO₂ inhibition of photosynthesis was depressed at higher CO₂ concentrations when the disks were fumigated with SO₂ at different CO₂ concentrations. High CO₂ concentrations also reduced the photosynthesis inhibition of maize leaf disks. These results suggest that SO₂ inhibits photosynthesis through other mechanisms than photorespiration inhibition and confirm the view that SO₂ competes with CO₂ for the carboxylating enzymes in photosynthesis