Moisture modulates rhizosphere effects on C decomposition in two different soil types

While it is well known that soil moisture directly affects microbial activity and soil organic matter (SOM) decomposition, it is unclear if the presence of plants alters these effects through rhizosphere processes. We studied soil moisture effects on SOM decomposition with and without sunflower and soybean. Plants were grown in two different soil types with soil moisture contents of 45% and 85% of field capacity in a greenhouse experiment. We continuously labeled plants with depleted ¹³C, which allowed us to separate plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration measurements. We observed an overall increase in soil-derived CO₂-C efflux in the presence of plants (priming effect) in both soils. On average a greater priming effect was found in the high soil moisture treatment (up to 76% increase in soil-derived CO₂-C compared to control) than in the low soil moisture treatment (up to 52% increase). Greater plant-derived CO₂-C and plant biomass in the high soil moisture treatment contributed to greater priming effects, but priming effects remained significantly higher in the high moisture treatment than in the low moisture treatment after correcting for the effects of plant-derived CO₂-C and plant biomass. The response to soil moisture particularly occurred in the sandy loam soil by the end of the experiment. Possibly, production of root exudates increased with increased soil moisture content. Root exudation of labile C may also have become more effective in stimulating microbial decomposition in the higher soil moisture treatment and sandy loam soil. Our results indicate that moisture conditions significantly modulate rhizosphere effects on SOM decomposition.     

Plant biomass influences rhizosphere priming effects on soil organic matter decomposition in two differently managed soils

We used a continuous labeling method of naturally ¹³C-depleted CO₂ in a growth chamber to test for rhizosphere effects on soil organic matter (SOM) decomposition. Two C3 plant species, soybean (Glycine max) and sunflower (Helianthus annus), were grown in two previously differently managed soils, an organically farmed soil and a soil from an annual grassland. We maintained a constant atmospheric CO₂ concentration at 400±5 ppm and δ¹³C signature at −24.4‰ by regulating the flow of naturally ¹³C-depleted CO₂ and CO₂-free air into the growth chamber, which allowed us to separate new plant-derived CO₂-C from original soil-derived CO₂-C in soil respiration. Rhizosphere priming effects on SOM decomposition, i.e., differences in soil-derived CO₂-C between planted and non-planted treatments, were significantly different between the two soils, but not between the two plant species. Soil-derived CO₂-C efflux in the organically farmed soil increased up to 61% compared to the no-plant control, while the annual grassland soil showed a negligible increase (up to 5% increase), despite an overall larger efflux of soil-derived CO₂-C and total soil C content. Differences in rhizosphere priming effects on SOM decomposition between the two soils could be largely explained by differences in plant biomass, and in particular leaf biomass, explaining 49% and 74% of the variation in primed soil C among soils and plant species, respectively. Nitrogen uptake rates by soybean and sunflower was relatively high compared to soil C respiration and associated N mineralization, while inorganic N pools were significantly depleted in the organic farm soil by the end of the experiment. Despite relatively large increases in SOM decomposition caused by rhizosphere effects in the organic farm soil, the fast-growing soybean and sunflower plants gained little extra N from the increase in SOM decomposition caused by rhizosphere effects. We conclude that rhizosphere priming effects of annual plants on SOM decomposition are largely driven by plant biomass, especially in soils of high fertility that can sustain high plant productivity.     

Soil microbial biomass and activity: the effect of site characteristics in humid temperate forest ecosystems

Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g^–1–8.3 mg C g^–1 and 14–249 g C m^–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO₂‐C m^–2 h^–1; metabolic quotient (qCO₂): 1.4–14.7 mg C (g C_mic)^–1 h^–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing N_tot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of N_tot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of C_mic to C_org was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.     

Short-term biochar-induced increase in soil CO2 release is both biotically and abiotically mediated

The application of biochar to soil has been shown to cause an apparent increase in soil respiration. In this study we investigated the mechanistic basis of this response. We hypothesized that increased CO₂ efflux could occur by: (1) Biochar-induced changes in soil physical properties (bulk density, porosity, moisture content); (2) The biological breakdown of organic carbon (C) released from the biochar; (3) The abiotic release of inorganic C contained in the biochar; (4) A biochar-induced stimulation of decomposition of native soil organic matter (SOM) which could occur both biotically or abiotically; (5) The intrinsic biological activity of the biochar results in the liberation of CO₂. Our results show that most of the extra CO₂ produced after biochar addition to soil came from the equal breakdown of organic C and the release of inorganic C contained in the biochar. Using long-term ¹⁴C-labelled SOM, we show that biochar repressed native SOM breakdown, counteracting the release of CO₂ from the biochar. A range of mechanisms to describe this negative priming response is presented. Although biochar-induced significant changes in the physical characteristics of the soil, overall this made no contribution to changes in soil respiration. Similarly, the evidence from our study suggests that changes in soluble polyphenols do not help explain the respiration response. In summary, biochar induced a net release of CO₂ from the soil; however, this C loss was very small relative to the amount of C stored within the biochar itself (ca. 0.1%). This short-term C release should therefore not compromise its ability to contribute to long-term C sequestration in soil environments.     

Interactions between physical and biotic factors influence CO2 flux in Antarctic dry valley soils

Soil carbon dioxide (CO₂) flux is an integrative measure of ecosystem functioning representing both biotic and physical controls over carbon (C) balance. In the McMurdo Dry Valleys of Antarctica, soil CO₂ fluxes (approximately −0.1-0.15 μmol m⁻² s⁻¹) are generally low, and negative fluxes (uptake of CO₂) are sometimes observed. A combination of biological respiration and physical mechanisms, driven by temperature and mediated by soil moisture and mineralogy, determine CO₂ flux and, therefore, soil organic C balance. The physical factors important to CO₂ flux are being altered with climate variability in many ecosystems including arid forms such as the Antarctic terrestrial ecosystems, making it critical to understand how climate factors interact with biotic drivers to control soil CO₂ fluxes and C balances. We measured soil CO₂ flux in experimental field manipulations, microcosm incubations and across natural environmental gradients of soil moisture to estimate biotic soil respiration and abiotic sources of CO₂ flux in soils over a range of physical and biotic conditions. We determined that temperature fluctuations were the most important factor influencing diel variation in CO₂ flux. Variation within these diel CO₂ cycles was explained by differences in soil moisture. Increased temperature (as opposed to temperature fluctuations) had little or no effect on CO₂ flux if moisture was not also increased. We conclude that CO₂ flux in dry valley soils is driven primarily by physical factors such as soil temperature and moisture, indicating that future climate change may alter the dry valley soil C cycle. Negative CO₂ fluxes in arid soils have recently been identified as potential net C sinks. We demonstrate the potential for arid polar soils to take up CO₂, driven largely by abiotic factors associated with climate change. The low levels of CO₂ absorption into soils we observed may not constitute a significant sink of atmospheric CO₂, but will influence the interpretation of CO₂ flux for the dry valley soil C cycle and possibly other arid environments where biotic controls over C cycling are secondary to physical drivers.     

Effect of dry-wet cycles on carbon dioxide release from two different volcanic ash soils in a Japanese temperate forest

ABSTRACT

In the present study, two volcanic ash soils (soil A and B) from a temperate broad-leaved forest in eastern Japan were aerobically incubated under repeated dry-wet cycles and continuously constant moisture conditions. The primary aims were to quantify the potential for enhancement of carbon dioxide (CO₂) release owing to increased water fluctuation and to examine differences in the responses of volcanic ash soils with different physicochemical properties. Soil B, rather than soil A, was a typical Andosol. During incubation at 20°C for 120 days with five dry-wet cycles, the CO₂ release rate was measured periodically. Abundance of the stable carbon isotope in CO₂ (δ¹³C-CO₂) was measured to capture changes in the origin of decomposed soil organic matter (SOM) owing to the dry-wet cycles. The CO₂ release rate under the dry-wet cycles was up to 49% higher than the values predicted from a parabolic relationship between CO₂ release and water content during incubation under the continuously constant moisture condition. The magnitude of CO₂ release enhancement was 2.7-fold higher in soil B relative to that in soil A. The δ¹³C-CO₂ value in the dry-wet cycles was enriched by 0.3–2.3‰ compared to that during incubation under the continuously constant moisture conditions, suggesting that the decomposition of well-metabolized and/or old SOM was enhanced by the dry-wet cycles. Thus, the present study suggests that Andosols, which have been believed to have a strong SOM stabilization ability, are vulnerable to dry-wet cycles. Then, increased water fluctuation in a future warmer world would have significant potential to stimulate CO₂ release from soils.     

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.     

Dynamics of soil organic matter turnover and soil respired CO2 in a temperate grassland labelled with 13C

The fate of carbon (C) in grassland soils is of particular interest since the vast majority in grassland ecosystems is stored below ground and respiratory C‐release from soils is a major component of the global C balance. The use of ¹³C‐depleted CO₂ in a 10‐year free‐air carbon dioxide enrichment (FACE) experiment, gave a unique opportunity to study the turnover of the C sequestered during this experiment. Soil organic matter (SOM), soil air and plant material were analysed for δ¹³C and C contents in the last year of the FACE experiment (2002) and in the two following growing seasons. After 10 years of exposure to CO₂ enrichment at 600 ppmv, no significant differences in SOM C content could be detected between fumigated and non‐fumigated plots. A ¹³C depletion of 3.4‰ was found in SOM (0–12 cm) of the fumigated soils in comparison with the control soils and a rapid decrease of this difference was observed after the end of fumigation. Within 2 years, 49% of the C in this SOM (0–12 cm) was exchanged with fresh C, with the limitation that this exchange cannot be further dissected into respiratory decay of old C and freshly sequestered new C. By analysing the mechanistic effects of a drought on the plant‐soil system it was shown that rhizosphere respiration is the dominant factor in soil respiration. Consideration of ecophysiological factors that drive plant activity is therefore important when soil respiration is to be investigated or modelled.     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

Relationships of soil respiration to microbial biomass, substrate availability and clay content

The roles of microbial biomass (MBC) and substrate supply as well as their interaction with clay content in determining soil respiration rate were studied using a range of soils with contrasting properties. Total organic C (TOC), water-soluble organic carbon, 0.5 M K₂SO₄-extractable organic C and 33.3 mM KMnO₄-oxidisable organic carbon were determined as C availability indices. For air-dried soils, these indices showed close relationship with flush of CO₂ production following rewetting of the soils. In comparison, MBC determined with the chloroform fumigation-extraction technique had relatively weaker correlation with soil respiration rate. After 7 d pre-incubation, soil respiration was still closely correlated with the C availability indices in the pre-incubated soils, but poorly correlated with MBC determined with three different techniques—chloroform fumigation extraction, substrate-induced respiration, and chloroform fumigation-incubation methods. Results of multiple regression analyses, together with the above observations, suggested that soil respiration under favourable temperature and moisture conditions was principally determined by substrate supply rather than by the pool size of MBC. The specific respiratory activity of microorganisms (CO₂-C/MBC) following rewetting of air-dried soils or after 7 d pre-incubation was positively correlated with substrate availability, but negatively correlated with microbial pool size. Clay content had no significant effect on CO₂ production rate, relative C mineralization rate (CO₂-C/TOC) and specific respiratory activity of MBC during the first week incubation of rewetted dry soils. However, significant protective effect of clay on C mineralization was shown for the pre-incubated soils. These results suggested that the protective effect of clay on soil organic matter decomposition became significant as the substrate supply and microbial demand approached to an equilibrium state. Thereafter, soil respiration would be dependent on the replenishment of the labile substrate from the bulk organic C pool.     

Photodegradation effects on CO2 emissions from litter and SOM and photo-facilitation of microbial decomposition in a California grassland

Decomposition of soil organic matter (SOM) and plant litter has been shown to be affected by high solar radiation; this could partly explain why biogeochemical models underestimate decomposition in arid and semi-arid ecosystems. We set out to test the effect of using traditional PVC chambers for measuring soil gas fluxes versus quartz chambers that allowed passage of light during field measurements in a dry-land field in Davis, CA. Results showed that fluxes from quartz-top chambers were on average 29% higher than from opaque chambers. We also studied the effect of solar light exposure on decomposition of native grass litter and SOM in a field experiment where plots were shaded or left exposed for 157 days during summer; litter did not seem to be affected by exposure to light. However, we concluded that SOM decomposition was affected by light exposure since shaded soil had similar respiration to sunlight-exposed soil indicating that microbial respiration occurred under the shade while photo-degradation likely occurred under the sun. Additionally, ¹⁵N-labeled grass was placed in litter bags in the field with either clear filters to allow light or aluminum covers to block light; 3-month exposure caused a change in lignin degradability as indicated by the change in the Ad/Al ratio. Incubation of that litter showed 9.3% more CO₂ produced from litter in clear and aluminum bags than unexposed litter. This showed that photo-facilitation occurred although to a small degree and was a result of light exposure and/or heat degradation. We attributed the similar respiration from clear- and aluminum-exposed litter to heat degradation of the aluminum-exposed litter. In conclusion, our results show that in hot dry ecosystems conventional PVC chambers underestimate measured CO₂ flux rates; sunlight exposure changes litter chemistry and appears to affect the degradation of soil organic matter, but the magnitude of degradation depends on an interaction of factors such as soil temperature and moisture.     

Influence of tillage and plant residue management on respiration of a Florida Everglades Histosol

Subsidence of drained, high organic matter Histosols in the Everglades Agricultural Area (EAA) is a concern for the sustainability of crop production in southern Florida. Histosol subsidence is primarily due to oxidation of organic matter by aerobic microorganisms, but far less is known about the influence of agricultural practices. The use of shallow tillage, as opposed to deep tillage, combined with proper plant residue management, may help to reduce the present rate of subsidence and soil CO₂ emissions. The present study was conducted on a Lauderhill soil (euic, hyperthermic, Lithic Haplosaprist) previously cropped in sugarcane (Saccharum spp.). The objectives were to (1) determine the effects of tillage depth on short-term CO₂ losses in a herbicide-killed weedy residue covered field and another field kept fallow without residue cover, and (2) compare soil respiration measurements made with two different dynamic closed-system portable chamber techniques. Four tillage practices common to the EAA were used to produce soil disturbance ranging in depth from approximately 20 to 300 mm. These practices included switch plowing, disk harrowing, and single and multiple tine cultivation. Twenty-four hours after tillage, cumulative CO₂ loss from the deepest tillage treatment (switch plow; 300 mm deep) was as much as 33 times greater than that from the no-till (control) treatment. Cumulative CO₂ loss following intermediate tillage (disk harrow; 78–145 mm deep) was as much as 2.3-fold greater than the no-till treatment, but shallower tillage (tine cultivation; 20–41 mm deep) was generally not different. Short-term tillage-induced CO₂ loss was primarily related to soil moisture content and soil porosity. Soil respiration measurements made with the two chamber techniques agreed well with each other except for the deepest tillage treatment, where the larger chamber measured CO₂ flux that was approximately 10 times greater than for the smaller chamber. Results indicate that minimum or no-tillage may reduce short-term tillage-induced CO₂ emissions on organic soils, thus minimizing soil subsidence.     

Does adding microbial mechanisms of decomposition improve soil organic matter models? A comparison of four models using data from a pulsed rewetting experiment

Contemporary soil organic matter (SOM) models have been successful at simulating decomposition across a range of spatial and temporal scales using first-order kinetics to represent the decomposition process; however, recent work suggests the simplicity of the first-order representation of decomposition is not adequate to capture the microbially-driven dynamics of SOM decomposition over short timescales. For example, the response of soils to drying-rewetting events may best be explained by microbial and/or exoenzyme controls on decomposition. To test if adding these microbial mechanisms improves the ability of SOM models to simulate the response of soils to short-term environmental changes, we developed four different SOM decomposition models with varying mechanistic complexity and compared their ability to simulate soil respiration from a pulsed drying-rewetting laboratory-based experiment. Specifically, we tested the ability of the models to capture the timing and magnitude of soil CO₂ efflux in response to rewetting or constant moisture conditions. The results of the comparison suggest that the inclusion of exoenzyme and microbial controls on decomposition can improve the ability to simulate pulsed rewetting dynamics; however, less mechanistic first-order models prevail under steady-state moisture conditions. These modeling results may have implications for understanding the long-term response of soil carbon stocks in response to local and regional climate change.     

Increased CO2 emission and organic matter decomposition by leaf-cutting ant nests in a coastal environment

Leaf-cutting ants perform a vital role in the cycling of carbon and nutrients in tropical ecosystems. Nests have high levels of organic matter and refuse dumps host up to two times more soil micro-organisms than non-nest soil. The increased levels of organic matter in the soil of nests, however, can affect CO₂ emissions from soil and alter the balance of atmospheric CO₂. We aimed at assessing the effect of nests of the leaf-cutting ant Acromyrmex balzani on CO₂ emissions in a coastal area of Northeast Brazil. Results show that A. balzani nests emitted up to four times more CO₂ than the surrounding soil and emissions were positively correlated with soil moisture and soil organic matter (SOM) content. In addition, field experiments demonstrated that refuse material has a lower residence time than the leaf material brought to the colonies. Despite the high density of nests and high content of SOM compared to adjacent control soil, CO₂ emissions by A. balzani nests represent only 0.3% of the total CO₂ efflux by the studied ecosystem. Although these effluxes account for a relative small portion of the total soil CO₂ emission, they are still important for the understanding of C balance, especially when one considers the thousands of tons of CO₂ emitted each day, across entire Neotropical regions where leaf-cutting ants occur.     

水稻土基底呼吸与CO2排放强度的日动态及长期不同施肥下的变化

土壤呼吸排放是陆地生态系统土气交换快速而活跃的途径之一,对大气CO2浓度的变化有显著的影响。本文对太湖地区一个代表性水稻土水稻收割后土壤基底呼吸CO2排放进行了昼夜观测和采样分析。结果表明,不同小区平均土壤呼吸与CO2排放速率在CO2-C.12.2～25.2.mg/(m2h)之间,日排放量在CO2-C.327.2～604.1mg/(m2d)之间,低于文献报道的森林和草地及旱作农田的土壤呼吸;与长期有机-无机配施处理相比,长期单施化肥CO2日排放量提高了55%～85%,并且显著提高了土壤呼吸对土壤(5.cm)温度的响应敏感性。相关分析表明,土壤呼吸CO2排放强度与土壤微生物N(Nmic)、微生物C∶N(Cmic/Nmic)和P的有效性有密切的关系;生物有效N和P的有效性显著地影响着土壤呼吸与CO2的生成和排放。本试验结果进一步支持了水稻土的固碳效应。但是,供试不同小区土壤呼吸排放强度的变异隐含着长期不同施肥处理可能使与高呼吸活性有关的微生物群落发生改变,有待于进一步研究。     

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.     

Mineralization of Soil Organic Matter and Added Carbon Substrates in Two Acidic Soils with High non-exchangeable Aluminum

Mineralization of soil organic matter and of added ¹⁴C labelled substrates were studied on samples from two acidic forest soils, “Cademario”-sample from the Bh-horizon of a cryptopodzolic soil rich in humus and nonexchangeable Al and “Sagno”-sample from the A-horizon of a Haplumbrept with moderate humus- and Al-content. The respiration rates for the two soils were not different when related to the content of organic matter. When treated with Na₂CO₃, the CO₂ production rate in the Sagno soil increased about three fold whereas no significant difference was observed for Cademario samples. This is attributed to the more pronounced dissolution of organic matter due to the pH increase in the Sagno soil. N-mineralization was different in the two soils. During a 28 day incubation period, 0.11% and 0.34% of the total organic N was released in the Cademario and Sagno samples, respectively. Na₂CO₃ treatment stimulated N-mineralization in both soils but the mineral N-form was primarily nitrate in the Sagno sample and ammonium in the aluminum-rich sample from Cademario. Glucose, succinate and salicylate added to the soils were mineralized in this order. However, CO₂ evolution was much slower in the case of salicylate, especially in the untreated soils, a fact which is attributed to the Al-complexing power of this substrate.     

Soil organic matter dynamics under grain farming in Northern Kazakhstan

Because of their ability to store a high amount of soil organic matter (SOM), Chernozem soils are one of the most important resources from both agricultural and environmental viewpoints. This study was carried out to determine the SOM budget under grain farming in the Chernozem soil of northern Kazakhstan through the analysis of in situ soil respiration and soil environmental factors such as soil temperature as well as moisture content. Five experimental plots including one fallow field were established at the experimental farm of Barayev Kazakh Research and Production Center of Grain Farming, Shortandy, northern Kazakhstan (mean annual precipitation and average year temperature are 323 mm and 1.6°C, respectively). Mean daily soil temperature increased to above O°C in early April, remaining at above 20°C from mid-June to mid-August, and then sharply decreased to below 5°C at the end of September. Most of the biological activities were considered to be limited from April to September. On the other hand, the soil moisture content remained high after thawing until mid-June and then continuously decreased in the cropped plots except during the rainfall events. The soil respiration rate recorded the highest values from late June to early July and overall fluctuations were similar to those of the soil temperature, unlike the fluctuations of soil microbial C and N contents, which exhibited similar patterns to those of the soil moisture content. In order to represent the daily soil respiration rates using the soil environmental factors, the following relationship was introduced as a model function: Cem = aM ^pbexp(-E/RT). The coefficients, a, b, and E (activation energy in Arrhenius equation), were determined by stepwise multiple regression after logarithm transformation using the measured data, Cem (daily soil respiration rate), M (volumetric soil moisture content), and T (absolute soil temperature). As a result, a significant relationship was always obtained between the soil respiration rate and the activation energy, E, while the contribution of the soil moisture content to the soil respiration rate was uncertain. Using the regression equations and monitored data of soil temperature and moisture content, cumulative soil respiration throughout the cropping period was calculated to be in the range of 2.5 to 3.2 Mg C ha^p-1 On the other hand, the amounts of crop residues in the cropped plots that were expected to be incorporated into the soils ranged from 1.6 to 4.4 Mg C ha^p-1 Except for the plot planted with oats (higher amounts of residues than for wheat), the SOIL budget was slightly negative in this year, that is, the soils lost their organic matter stock. Although it is difficult to generalize the C budget in different years because of the large variations in crop growth due to fluctuating water resources, the disadvantage of summer fallow (no residues) was obvious in terms of SOM budget. The net soil respiration rate in the fallow plot, 2.9 Mg C ha^p-1 was approximately equivalent to 4% of the total SOM stock in the plow layer (30 cm) (70 to 80 Mg C ha^p-1 To reduce further loss of SOM, at least evenly extensive use of summer fallow should be reconsidered.     

The response of organic matter mineralisation to nutrient and substrate additions in sub-arctic soils

Global warming in the Arctic may alter decomposition rates in Arctic soils and therefore nutrient availability. In addition, changes in the length of the growing season may increase plant productivity and the rate of labile C input below ground. We carried out an experiment in which inorganic nutrients (NH₄NO₃ and NaPO₄) and organic substrates (glucose and glycine) were added to soils sampled from across the mountain birch forest-tundra heath ecotone in northern Sweden (organic and mineral soils from the forest, and organic soil only from the heath). Carbon dioxide production was then monitored continuously over the following 19 days. Neither inorganic N nor P additions substantially affected soil respiration rates when added separately. However, combined N and P additions stimulated microbial activity, with the response being greatest in the birch forest mineral soil (57% increase in CO₂ production compared with 26% in the heath soil and 8% in the birch forest organic soil). Therefore, mineralisation rates in these soils may be stimulated if the overall nutrient availability to microbes increases in response to global change, but N deposition alone is unlikely to enhance decomposition. Adding either, or both, glucose and glycine increased microbial respiration. Isotopic separation indicated that the mineralisation of native soil organic matter (SOM) was stimulated by glucose addition in the heath soil and the forest mineral soil, but not in the forest organic soil. These positive ‘priming’ effects were lost following N addition in forest mineral soil, and following both N and P additions in the heath soil. In order to meet enhanced microbial nutrient demand, increased inputs of labile C from plants could stimulate the mineralisation of SOM, with the soil C stocks in the tundra-heath potentially most vulnerable.     

Effluxed CO2-C from sterilized and unsterilized treatments of a calcareous soil

Soil inorganic carbon (C) represents a substantial C pool in arid ecosystems, yet little data exist on the contribution of this pool to ecosystem C fluxes. A closed jar incubation study was carried out to test the hypothesis that CO₂-¹³C production and response to sterilization would differ in a calcareous (Mojave Desert) soil and a non-calcareous (Oklahoma Prairie) soil due to contributions of carbonate-derived CO₂. In addition to non-sterilized controls, soils were subjected to sterilization treatments (unbuffered HgCl₂ addition for Oklahoma soil and unbuffered HgCl₂ addition, buffered HgCl₂ addition, and autoclaving for Mojave Desert soil) to decrease biotic respiration and more readily measure abiotic CO₂ flux. Temperature and moisture treatments were also included with sterilization treatments in a factorial design.The rate of CO₂ production in both soils was significantly decreased (36-87%) by sterilization, but sterilization treatments differed in effectiveness. Sterilization had no significant effect on effluxed CO₂-¹³C values in the non-calcareous Oklahoma Prairie soil and autoclaved Mojave Desert soil as compared to their respective non-sterilized controls. However, sterilization significantly altered CO₂-¹³C values in Mojave Desert soil HgCl₂ sterilization treatments (both buffered and non-buffered). Plots of 1/CO₂ versus CO₂-δ¹³C (similar to Keeling plots) indicated that the source CO₂-δ¹³C value of the Oklahoma Prairie soil treatments was similar to the δ¹³C value of soil organic matter [(SOM); −17.76‰ VPDB] whereas the source for the (acidic) unbuffered-HgCl₂ sterilized Mojave Desert soil was similar to the δ¹³C value of carbonates (−0.93‰ VPDB). The source CO₂-δ¹³C value of non-sterilized and autoclaved (−18.4‰ VPDB) Mojave Desert soil treatments was intermediate between SOM (−21.43‰ VPDB) and carbonates and indicates up to 13% of total C efflux may be from abiotic sources in calcareous soils.