Production of CO<Subscript>2</Subscript> by Paleosol Samples Taken in the Steppe Zone under Kurgans Constructed in the Periods of Relative Aridization and Optimum of the Climate

The studies of recent soils and paleosols buried under kurgans created in the periods of long-term aridization (3000–2000 BC) and climatic optimum (13th–14th centuries AD) were performed in steppes of the southeastern part of the East European Plain (Privolzhskaya Upland and Caspian Lowland) in order to determine the rate of carbon dioxide production by the soil samples at the natural moisture and after moistening up to 60% of the total moisture capacity. The CO₂ emission from the samples of paleosols corresponding to the period of climatic aridization in the Lower Volga River at their natural moisture status was lower than that from the samples of background surface soils, whereas the CO₂ emission from the samples of paleosols buried under optimum climatic conditions was higher than that from the samples of background surface soils. After moistening of the samples, the increase in the CO₂ emission from the paleosol samples depended on the actual humidity of the climate in the corresponding period.     

Seasonal Dynamics of Soil CO<Subscript>2</Subscript> Concentration and CO<Subscript>2</Subscript> Fluxes from the Soil of the Former Lake Texcoco,Mexico

Seasonal changes of the soil CO₂ concentration and the rate of CO₂ fluxes emission from the soil formed on the sediments of the former Lake Texcoco, which occupied a significant part of the Mexico Valley until the mid-17th century, were studied. The soils (Fluvic Endogleyic Phaeozems) were characterized by a low CO₂ fluxes rate, which is related to their high alkalinity. The mean values of soil respiration were 6.0–14.1 mg C/(m² h) depending on vegetation type, which corresponds to 60–157 g C/(m² yr). The contribution of plants to the CO₂ fluxes insignificantly varied by seasons and depended on the species composition of vegetation. The soil CO₂ concentration and soil respiration in eucalypt (Eucalyptus globulus Labill.) plantation were two times higher than those in the grass–subshrub area, the ground cover of which consisted of Distichlis spicata (L.) Greene and Suaeda nigra (Raf.) J.F. Macbr. species. This can be related to the significant volumes of gas production during the respiration of eucalypt roots and associated rhizosphere community. The contribution of the root systems of grass cover to the soil CO₂ fluxes in eucalypt plantation slightly varied within the year and was equal to 24% on the average. In the grass–subshrub area, its value varied from 41% in the cold season to 60% in the warm season. The spatial variability of soil CO₂ concentration and its flux rate to the atmosphere was due to the differences in plant species composition and hydrothermal conditions, and their temporal trend was closely related to the seasonal accumulation of plant biomass and soil temperature.     

Evaluation of organic carbon stocks and СО<Subscript>2</Subscript> fluxes in grasslands of Western Transbaikalia

The stocks of organic carbon and mean rates of the CO₂ emission during the growing season (May–September) and the entire year were estimated in a sequence of grass ecosystems along the transect encompassing chestnut and meadow-chestnut steppe soils, marsh and meadow alluvial soils, and a haloxerophytic community on a typical solonchak. The total stocks of organic carbon comprised 6.17–9.70 kg С/m² in steppe, 7.41–10.04 kg С/m² in floodplain, and 4.74 kg С/m² in haloxerophytic ecosystems. The portion of humus carbon in the upper 50-cm-thick soil layer comprised 79–92% of the total carbon stock. The mean daily CO₂ emission (С–CO₂/(m² day)) from alluvial soils was moderate (3.3–4.9) or low (1.5–2.5). The dependence of the CO₂ emission on the moistening of steppe soils, temperature of alluvial soils, and temperature and moistening of solonchak was revealed. In comparison with the CO₂ emission from the zonal chestnut soil, its mean values during the growing season and the entire year were 1.2 times higher for the meadowchestnut soil, 3.3 times higher for the marsh alluvial soil, 2.3 times higher for the meadow alluvial soil, and 1.7 times higher for the solonchak. The portion of the CO₂ emission beyond the growing season in the mean annual emission averaged 19.8–24.2% and depended on the type of grass ecosystem and on weather conditions of particular years. The sink of carbon in the grass ecosystems exceeded carbon emission, especially in the steppe ecosystems.     

Soil Carbon Dioxide Emissions from Sorghum–Sunflower Rotation in Rainfed Semi-arid Tropical Alfisols: Effects of Fertilization Rate and Legume Biomass Incorporation

The long-term effects of plant legume [horse gram (Macrotyloma uniflorum)] biomass incorporations were assessed in terms of carbon dioxide (CO₂) emissions, soil quality parameters, and climatically influenced soil parameters in a dryland Alfisol under varying soil fertility conditions. Six selected treatments consisted of off-season legume incorporation (I) and no incorporation?/?fallow (F), each under three varying nitrogen and phosphorus fertilizer levels (viz., N₀P₀, N₂₅P₃₀, and N₅₀P₃₀). Soil moisture, soil temperature, soil surface carbon dioxide emission, soil dehydrogenases, and microbial biomass carbon (MBC) were monitored at three different crop situations [viz., Kharif period (KP), legume/fallow period (LP), and no crop period (NP)] at 14 different periods of the year. Incorporation practices resulted in greater rates of CO₂ emission over fallow conditions during the Kharif and legume periods, whereas the emission rate was greater in fallow soils during the end of the legume and no crop periods. The increased rates of fertilizer doses also significantly increased the soil CO₂ flux during the majority of the measurements. Beneficial effects of incorporation practices were observed in terms of high soil moisture (5–11%), low soil temperature (3–7%), and high content of MBC over without incorporations. Correlation studies indicated that the soil property MBC was found to be the greatest significant variable with CO₂ emission in all the fertilizer treatments under biomass-incorporated soils. These results indicated the undesirable (in terms of CO₂ fluxes) and desirable (soil biological and other parameters) effects of legume biomass incorporation and fertilizer application and their significance in improving soil quality and greenhouse gas (GHG) emissions in dryland Alfisols of semi-arid tropics.     

Effect of Stover Management and Nitrogen Fertilization on N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> Emissions from Irrigated Maize in a High Nitrate Mediterranean Soil

A high soil nitrogen (N) content in irrigated areas quite often results in environmental problems. Improving the management practices of intensive agriculture can mitigate greenhouse gas (GHG) emissions. This study compared the effect of maize stover incorporation or removal together with different mineral N fertilizer rates (0, 200 and 300 kg N ha^?1) on the emission of nitrous oxide (N₂O) and carbon dioxide (CO₂) on a sprinkler-irrigated maize (Zea mays L.). The trail was conducted in the Ebro Valley (NE Spain) in a high nitrate-N soil (i.e. 200 g NO₃–N kg^?1). Nitrous oxide and CO₂ emissions were sampled weekly using a semi-static closed chamber and quantified using the photoacoustic technique in 2011 and 2012. Applying sidedress N fertilizer tended to increase N₂O emissions whereas stover incorporation did not have any clear effect. Nitrification was probably the main process leading to N₂O. Denitrification was limited by the low soil moisture content (WFPS <?54%), due to an adequate irrigation management. Emissions ranged from ??0.11 to 0.36% of the N applied, below the IPCC (2007) values. Nitrogen fertilization tended to reduce CO₂ emission, but only in 2011. Stover incorporation increased CO₂ emission. Nitrogen use efficiency decreased with increasing mineral fertilizer supply. The application of N in high N soils of the Ebro Valley is not necessary until the soil restores a normal mineral N content, regardless of stover management. This will combine productivity with keeping N₂O and CO₂ emissions under control provided irrigation is adequately managed. Testing soil NO₃ ^?–N contents before fertilizing would improve N fertilizer recommendations.     

Increases in soil CO<Subscript>2</Subscript> and N<Subscript>2</Subscript>O emissions with warming depend on plant species in restored alpine meadows of Wugong Mountain,China

Purpose

Ecosystem restorations can impact carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions which are important greenhouse gasses. Alpine meadows are degraded worldwide, but restorations are increasing. Because their soils represent large carbon (C) and nitrogen (N) pools, they may produce significant amounts of CO₂ and N₂O depending on the plant species used in restorations. In addition, warming and N deposition may impact soil CO₂ and N₂O emissions from restored meadows.

Materials and methods

We collected soils from degraded meadows and plots restored using three different plant species at Wugong Mountain (Jiangxi, China). We measured CO₂ and N₂O emissions when soils were incubated at different temperatures (15, 25 or 35 °C) and levels of N addition (control vs. 4 g m^?2) to understand their responses to warming and N deposition.

Results and discussion

Dissolved organic C was higher in restored plots (especially with Fimbristylis dichotoma) compared to non-restored bare soils, and their soil inorganic N was lower. CO₂ emission rates were increased by vegetation restorations, decreased by N deposition, and increased by warming. CO₂ emission rates were similar for the three grass species at 15 and 25 °C, but they were lower with Miscanthus floridulus at 35 °C. Soils from F. dichotoma and Carex chinensis plots had higher N₂O emissions than degraded or M. floridulus plots, especially at 25 °C.

Conclusions

These results show that the effects of restorations on soil greenhouse gas emissions depended on plant species. In addition, these differences varied with temperature suggesting that future climate should be considered when choosing plant species in restorations to predict soil CO₂ and N₂O emissions and global warming potential.

     

Formation of microelemental composition and properties of soils under model phytocenoses in soil lysimeters

The soil formation on noncalcareous loam under different phytocenoses in soil lysimeters (Soil Experimental Station of Moscow State University) for 49 years has led to a decrease in acidity and an increase in the content of organic matter, microelements, and heavy metals in the surface soil layer. The rate of microbial CO₂ emission and the microbial biomass content reached the maximum values under the mixed forest stand followed by the broad-leaved forest, then spruce forests, perennial grasses, and fallow. The minimum values of these parameters were characteristic of the black fallow. The percentage of C_mic in the organic carbon content of the soils under the broad-leaved forest was 2.7; in the mixed forest, spruce forest, fallow, and black fallow, it was 1.9, 1.2, 0.9, and 3.3, respectively. The maximum accumulation of heavy metals was recorded in the litter and at the depth of 2–15 cm. The Zn content in the soils under the woody vegetation was 18–20 times higher than in the parent mantle loam; in the soils under perennial grasses and in the plots without plants, it was 14–16 and 5 times higher, respectively. The biogenic accumulation and aerial dust transfer of heavy metals are responsible for the differences in their accumulation between the soils of the model phytocenoses and soils without vegetation. The content of elements in the dust exceeded that in the parent loam by 200–300 times for Zn, 20–40 for lead, 6–60 for nickel, and 20–30 times for strontium and barium. The composition and amount of dust determined the trends in these elements of accumulation in the soils.     

Seasonal and Daily Dynamics of the CO<Subscript>2</Subscript> Emission from Soils of <Emphasis Type="Italic">Pinus koraiensis</Emphasis> Forests in the South of the Sikhote-Alin Range

The presented study shows the results of measuring soil respiration in typical burozems (Dystric Cambisols) under mixed Korean pine–broadleaved forests in the southern part of the Primorskii (Far East) region of Russia growing under conditions of monsoon climate. The measurements were performed in 2014–2016 by the chamber method with the use of a portable infrared gas analyzer. Relative and total values of the CO₂ efflux from the soil surface on four model plots were determined. The intensity of summer emission varied from 2.25 to 10.97 μmol/(m² s), and the total CO₂ efflux from the soils of four plots varied from 18.84 to 25.56 mol/m². It is shown that a larger part of seasonal variability in the soil respiration is controlled by the soil temperature (R² = 0.5–0.7); the soil water content also has a significant influence on the CO₂ emission determining about 10% of its temporal variability. The daily dynamics of soil respiration under the old-age (200 yrs) forest have a significant relationship with the soil temperature (R² = 0.51). The pyrogenic transformation of Pinus koraiensis forests into low-value oak forests is accompanied by an increase in the СО₂ efflux from the soil.     

Spatial Variability of Carbon Dioxide Emission by Soils in the Main Types of Forest Ecosystems at the Zvenigorod Biological Station of Moscow State University

Carbon dioxide (CO₂) emission from the soil surface in forest biogeocenoses of the Zvenigorod Biological Station of Moscow State University in summer varies on average from 120 to 350 mg C–CO₂/(m² h) and depends on the hydrothermal conditions (soil moisture and temperature) and the type of phytocenosis. The intensity of CO₂ emission in the biogeocenosis does not depend on its parcel structure and varies with respect to plant microgroups: it is maximum in oxalis pine–spruce and maple–lime forests and bracken spruce–birch forests and minimum in areas of forest fall without vegetation. The upper (from 0 to 20 cm thick) soil layer provides up to 50% of the total soil CO₂ emission. The role of microbial respiration in the total CO₂ emission from soils is determined by weather conditions and varies from 9–33% in a dry summer to 55–75% in a summer with favorable temperature and moisture.     

Daily and seasonal dynamics of CO<Subscript>2</Subscript> fluxes from soils under different stands of monsoon tropical forest

The analysis of daily, seasonal, and annual dynamics of CO₂ emission from soils under different stands of monsoon tropical tall-tree forest was performed on the basis of field observations conducted at the Russian-Vietnamese Tropical Research and Technology Center of the Russian Academy of Sciences. Under a tropical climate, the main factors responsible for the rate of carbon dioxide emission from the soils are shown to be the soil type and the topographic position of the area studied along with the type of vegetation. Depending on these factors, the rate of CO₂ emission from the soils was 65–178 mg C/(m² h) during the dry season and 123–259 mg C/(m² h) during the wet season. The daily dynamics of CO₂ emissions from the soils of the tropical zone was weakly pronounced in both the wet and the dry season owing to the insignificant diurnal fluctuations of soil temperature. The investigations carried out allowed making an expert evaluation of the annual CO₂ fluxes from the soils under different stands of monsoon tropical tall-tree forest in southern Vietnam. They amounted to 900–2000 g C/(m² yr) depending on the forest type.     

Effect of contrasting changes in hydrothermic conditions on the N<Subscript>2</Subscript>O emission from forest and tundra soils

The effects of intense moistening and alternating freezing-thawing cycles on the N₂O emission from soils of an oak forest (brown forest soil in Lower Saxony, Germany) and southern tundra (cryozem in the area of Tal’nik Station near the city of Vorkuta) were studied in a model experiment. A sharp rise in the N₂O emission reaching 350–670 μg N/m² per h was recorded during the thawing of the brown forest soil, and the loss of nitrogen initiated by the freezing-thawing cycles comprised 74% of the total N₂O emission during the whole experiment. No significant fluxes of N₂O from the tundra soil were recorded during the experiment.     

In situ respiration determination as tool for classifying soils according to soil organic matter content

Abstract

Soil respiration is indicative of biological status of the soil and high respiration is correlated to high contents of available carbon (C) in soil and/or organic matter content. Because of soil respiration's relationship to soil organic matter status and content, soil respiration is considered one measurement that could aid in determining the quality of soil. In the global scale, the cycling of C in soil is important because the rise in CO₂ in the atmosphere is linked to global climate change. In situ measurement of CO₂ using instruments that are portable and perform analyses quickly are important to obtain sufficient number of measurements in the field to overcome spatial variability. Soil respiration tests were conducted in plots amended with fertilizer or organic amendments of agricultural or municipal residues since 1994. Besides CO₂, moisture and temperature were measured over a period where the moisture varied from near saturation to below wilting point. It was found that flux was curvilinearly related to moisture from 5 to 40% (v/v). Maximum flux occurred for all plots between 30 and 40% saturation. The ratio of flux normalized by temperature to the volumetric soil moisture divided soils into two categories, those with soil organic matter (SOM) content above or below 4.5%. The determinations of CO₂ flux, moisture and temperature uses equipment that is portable so that several sites in a field can be analyzed to reduce spatial variation. The only limitation is that the determinations must be performed on soils with less than 40% saturation or 25% moisture (v/v) because the normalized function is no longer linear above this moisture content. More than two SOM categories might be found if studies are expanded to soils with a wider range of SOM content.     

The Impact of Diesel Oil Pollution on the Hydrophobicity and CO<Subscript>2</Subscript> Efflux of Forest Soils

The contamination of soil with petroleum products is a major environmental problem. Petroleum products are common soil contaminants as a result of human activities, and they are causing substantial changes in the biological (particularly microbiological) processes, chemical composition, structure and physical properties of soil. The main objective of this study was to assess the impact of soil moisture on CO₂ efflux from diesel-contaminated albic podzol soils. Two contamination treatments (3000 and 9000 mg of diesel oil per kg of soil) were prepared for four horizons from two forest study sites with different initial levels of soil water repellency. CO₂ emissions were measured using a portable infrared gas analyser (LCpro+, ADC BioScientific, UK) while the soil samples were drying under laboratory conditions (from saturation to air-dry). The assessment of soil water repellency was performed using the water drop penetration time test. An analysis of variance (ANVOA) was conducted for the CO₂ efflux data. The obtained results show that CO₂ efflux from diesel-contaminated soils is higher than efflux from uncontaminated soils. The initially water-repellent soils were found to have a bigger CO₂ efflux. The non-linear relationship between soil moisture content and CO₂ efflux only existed for the upper soil horizons, while for deeper soil horizons, the efflux is practically independent of soil moisture content. The contamination of soil by diesel leads to increased soil water repellency.     

Evaluation of the soil carbon budget under different upland cropping systems in central Hokkaido,Japan

Abstract

To evaluate the carbon budget in soils under different cropping systems, the carbon dioxide (CO₂) flux from soils was measured in a total of 11 upland crop fields within a small watershed in central Hokkaido over the no snow cover months for 3 years. The CO₂ flux was measured using a closed chamber method at bare plots established in each field to estimate soil organic matter decomposition. Temporal variation in instantaneous soil CO₂ fluxes within the sites was mainly controlled by soil temperature and moisture. Annual mean CO₂ fluxes and cumulative CO₂ emissions had no significant relationship with soil temperature and moisture (P > 0.2). However, there was a significant quadratic relationship between annual mean CO₂ flux or cumulative CO₂ emission and soil clay plus silt content (%) (R² = 0.72～0.74, P < 0.0003). According to this relationship, the optimum condition for soil CO₂ emission is at a clay plus silt content of 63%. The cumulative CO₂ emission during the no snow cover season within each year varied from 1,159 to 7,349 kg C ha^?1 at the different sites. The amount of crop residue carbon retained in the soils following a cropping season was not enough to offset the CO₂ emission from soil organic matter decomposition at all sites. As a consequence, the calculation of the soil carbon budget (i.e. the difference between the carbon added as crop residues and compost and the carbon lost as CO₂ from organic matter decomposition) ranged from –7,349 to –785 kg C ha^?1, except for a wheat site where a positive value of 4,901 kg C ha^?1 was observed because of a large input of organic carbon with compost. The negative values of the soil carbon budget indicate that these cropping systems were net sources of atmospheric CO₂.     

Production of CO<Subscript>2</Subscript> by surface and buried soils of the steppe zone under native and moistened conditions

Modern light chestnut and chestnut soils and their analogues buried under steppe kurgans in the southeastern part of the Russian Plain were studied in order to determine the rates of the CO₂ production by these soils under the native (with the natural moisture content) and moistened (60% of the total water capacity) conditions. It was found that the rates of the CO₂ production by the soil samples in the native state are relatively close to one another and vary from 0.3 to 1.4 μg of C/100 g of soil/h. The rates of the CO₂ production in the moistened state increased by two orders of magnitude for the modern surface soils and by an order of magnitude for the buried soils.     

Changes in soil respiration in the course of the postagrogenic succession on sandy soils in the southern taiga zone

The dynamics of the CO₂ emission from sandy soils in the course of the postagrogenic succession in the southern taiga zone were studied. We measured total emission from the soil surface and, separately, respiration from the litter and mineral soil horizons during the warm (snowless) seasons of 2010 and 2011 on differently aged fallow plots: 0 years (cropland) and 7, 23, 55, 100, and 170 years. It was demonstrated that changes in the CO₂ emission in the course of the succession have a nonlinear pattern: the emission sharply increases in the first decade, then somewhat decreases, and then gradually increases again up to the maximum values. This is explained by the dependence of the rate of the emission on the soil carbon pools (humus + litter + underground phytomass) that are also subjected to nonlinear changes. Initially, the emission is mainly due to mineralization of labile organic substances added to the plowed soils in the form of organic fertilizers. Then, in parallel with a gradual increase in the pools of litter and underground phytomass, the total pool of soil organic carbon increases, and its role in the emission becomes more pronounced. The seasonal dynamics of the soil respiration are mainly controlled by the soil temperature; the soil moistening plays an important role only during the initial meadow stage of the succession.     

Fluxes of N2O from farmed peat soils in Finland

Agricultural peat soils are important sources of nitrous oxide (N₂O). Emissions of N₂O were measured from field plots of grass, barley, potatoes and fallow on a peat field in northern Finland during 2000–2002 and in southern Finland in 1999–2002. In the north the mean annual fluxes of N₂O (with their standard errors) during 2 years were 4.0 (±1.2), 13 (±3.0) and 4.4 (±0.8) kg N ha^?1 from the plots of grass, barley and fallow, respectively. In the north there were no significant thaw periods in the middle of winter. As a result, the thawing in the spring did not induce especially large N₂O emissions. Emissions of N₂O were larger in the south than in the north. In the southern peat field the mean annual fluxes during 3 years were 7.3 (±1.2), 15 (±2.6), 10 (±1.9) and 25 (±6.9) kg N₂O‐N ha^?1 for grass, barley, potato and fallow plots, respectively. Here, the largest single episodes of emission occurred during the spring thaw each year, following winter thaw events. An emission factor of 10.4 kg N₂O‐N ha^?1 year^?1 for the N₂O emission from the decomposition of the peat results from these data if the effect of fertilization according to the IPCC default emission factor is omitted. The direct effect of adding N as fertilizer on N₂O emissions was of minor importance. On average, 52% of the annual N₂O flux entered the atmosphere outside the cropping season (October–April) in the north and 55% in the south. The larger N₂O fluxes from the peat soil in the south might be due to the more humified status of the peat, more rapid mineralization and weather with more cycles of freezing and thawing in the winter.     

Carbon Dioxide Flux in a Subtropical Agricultural Soil of China

Red soils, one of the typical agricultural soils in subtropical China, play important roles in the global carbon budget due to their large potential to sequester C and replenish atmospheric C through soil CO₂ flux. Soil CO₂ emission was measured using a closed chamber method to quantify year-round soil flux and to determine the contribution of soil temperature, dissolved organic carbon (DOC) and soil moisture content to soil CO₂ flux. Soil flux was determined every 10 d during the experiment from August 1999 to July 2000, at the Ecological Station of Red Soil (the Chinese Academy of Sciences). In addition, diurnal flux measurements for 24 hr were made on August 5 and November 5, 1999 during this experiment. The average soil fluxes from 2 hr measurements between 9:00 and 11:00 can be regarded as the representative of daily averages. Soil CO₂ fluxes were generally higher in summer and autumn than in winter and spring, averaged 7.16 and 0.86 g CO₂ m^-2 d^-1 for the former and latter two seasons, and had a seasonal pattern more similar to soil temperature and DOC than soil moisture. The annual soil CO₂ flux was estimated as 1.65 kg CO₂ m^-2 yr^-1. Regressed separately, the reasons for soil flux variability were 86.6% from soil temperature, 58.8% from DOC, and 26.3% from soil moisture, respectively. Regressed jointly, a multiple equation was developed by the above three variables that explained 85.2% of the flux variance, but only soil temperature was the dominant factor affectingsoil flux, with significant partial correlation coefficient (r² = 0.804, p ≤ 0.05), through stepwise regression analysis. Based on the exponential equation using soil temperature, the predicted fluxes were calculated and were essentially equal to the measured ones throughout the experiment. No significant difference was detected between the predicted average and the measured one. The exponential relationship describing the response of soil CO₂ flux to the changes in soil temperature should accurately predict soil CO₂ flux from red soils in subtropical China.     

Relationships between carbon dioxide emission and soil properties in salt-affected landscapes

Net carbon dioxide (CO₂) emission from soils is controlled by the input rate of organic material and the rate of decomposition which in turn are affected by temperature, moisture and soil factors. While the relationships between CO₂ emission and soil factors are well-studied in non-salt-affected soils, little is known about soil properties controlling CO₂ emission from salt-affected soils. To close this knowledge gap, non-salt-affected and salt-affected soils (0-0.30 m) were collected from two agricultural regions: in India (irrigation induced salinity) and in Australia (salinity associated with ground water or non-ground water associated salinity). A subset (50 Indian and 70 Australian soils) covering the range of electrical conductivity (EC) and sodium adsorption ratio (SAR) in each region was used in a laboratory incubation experiment. The soils were left unamended or amended with mature wheat residues (2% w/w) and CO₂ release was measured over 120 days at constant temperature and soil water content. Residues were added to overcome carbon limitation for soil respiration. For the unamended soils, separation in multidimensional scaling plots was a function of differences in soil texture (clay, sand), SOC pools (particulate organic carbon (POC) and humus-C) and also EC. Cumulative CO₂-C emission from unamended and amended soils was related to soil properties by stepwise regression models. Cumulative CO₂-C emission was negatively correlated with EC in saline soils (R² = 0.50, p < 0.05) from both regions. In the unamended non-salt-affected soils, cumulative CO₂-C emission was significantly positively related to the content of POC for the Indian soils and negatively related to clay content for the Australian soils. In the wheat residue amended soils, cumulative CO₂-C emission had positive relationship with POC and humus-C but a negative correlation with EC for both Indian and Australian soils. SAR was negatively related (β = −0.66, p < 0.05) with cumulative CO₂-C emission only for the unamended saline-sodic soils of Australia. Cumulative CO₂-C emission was significantly negatively correlated with bulk density in amended soils from both regions. The study showed that in salt-affected soils, EC was the main factor influencing for soil respiration but the content of POC, humus-C and clay were also influential with the magnitude of influence depending on whether the soils were salt affected or not.     

The influence of precipitation pulses on soil respiration - Assessing the “Birch effect” by stable carbon isotopes

Sudden pulse-like events of rapidly increasing CO₂-efflux occur in soils under seasonally dry climates in response to rewetting after drought. These occurrences, termed “Birch effect”, can have a marked influence on the ecosystem carbon balance. Current hypotheses indicate that the “Birch” pulse is caused by rapidly increased respiration and mineralization rates in response to changing moisture conditions but the underlying mechanisms are still unclear. Here, we present data from an experimental field study using straight-forward stable isotope methodology to gather new insights into the processes induced by rewetting of dried soils and evaluate current hypotheses for the “Birch“-CO₂-pulse. Two irrigation experiments were conducted on bare soil, root-free soil and intact vegetation during May and August 2005 in a semi-arid Mediterranean holm oak forest in southern Portugal. We continuously monitored CO₂-fluxes along with their isotopic compositions before, during and after the irrigation. δ¹³C signatures of the first CO₂-efflux burst, occurring immediately after rewetting, fit the hypothesis that the “Birch” pulse is caused by the rapid mineralization of either dead microbial biomass or osmoregulatory substances released by soil microorganisms in response to hypo-osmotic stress in order to avoid cell lyses. The response of soil CO₂-efflux to rewetting was smaller under mild (May) than under severe drought (August) and isotopic compositions indicated a larger contribution of anaplerotic carbon uptake with increasing soil desiccation. Both length and severity of drought periods probably play a key role for the microbial response to the rewetting of soils and thus for ecosystem carbon sequestration.