Soil respiration and its determinants on a sub-Antarctic island

The effect of temperature on soil respiration at field moisture holding capacity was assessed for 100 sites representing 21 habitats on sub-Antarctic Marion Island (47°S, 38 °E). Respiration rates were compared across habitats and related to soil chemistry, soil microrganism counts and botanical characteristics. Median Q₁₀ across the 100 sites was 2.0, in the lower part of the range reported for soils elsewhere. Q₁₀ did not differ with temperature between 5 and 20 °C, indicating a mixed community of soil microorganisms having different responses to temperature. Respiration rates are about an order of magnitude higher than those reported at the same temperature for surface soils from Northern Hemisphere tundra. Edaphic richness (high concentrations of available P, inorganic N and total N), associated with large soil microbial populations and substantial relative covers of nitrophilous or coprophilous plants and caused by manuring by seals and seabirds, is the main determinant of soil respiration rate. The island's habitats were originally defined on the basis of canonical correspondence analysis of structural (vegetation and soil chemistry) variables. Since habitat-mean soil respiration rate correlated highly positively with the mean positions of the habitats on a canonical axis interpreted as representing a gradient in the intensity of animal influence, it is concluded that the habitat classification reflects differences in at least one ecosystem functional attribute, soil respiration.     

CO2 emission from soils under different uses and flooding conditions

The emission of CO₂ from Galician (NW Spain) forest, grassland and cropped soils was studied in a laboratory experiment, at different temperatures (10-35 °C) and at moisture contents of 100% and 160% of the field capacity (FC) of each soil (the latter value corresponds to saturated conditions, and represents between 120% and 140% of the water holding capacity, depending on the soil). In the forest soil, respiration in the flooded samples at all temperatures was lower than that at 100% field capacity. In the agricultural (grassland and cropped) soils the emission was higher (particularly at the highest incubation temperatures) in the soils wetted to 160% of the field capacity than in those wetted to 100% of the field capacity. In all cases the emission followed first order kinetics and the mineralization constants increased exponentially with temperature. In the forest soil, the Q₁₀ values were almost the same in the soils incubated at the two moisture contents. The grassland and cropped soils displayed different responses, as the Q₁₀ values were higher in the soils at 160% than in those at 100% of field capacity. In addition, and particularly at the highest temperatures, the rate of respiration increased sharply 9 and 17 days after the start of the incubation in the grassland and in the cropped soil, respectively. The above-mentioned anomalous response of the grassland and cropped soils under flooding conditions may be related to the agricultural use of the soils and possibly to the intense use of organic fertilizers in these soils (more than 150 kg N ha⁻¹ year⁻¹ added as cattle slurry or manure, respectively, in the grassland and cropped soils). The observed increase in respiration may either be related to the development of thermophilic facultative anaerobic microbes or to the formation during the incubation period of a readily metabolizable substrate, possibly originating from the remains of organic fertilizers, made accessible by physicochemical processes that occurred during incubation under conditions of high moisture.     

Short-term temperature dependence of heterotrophic soil respiration after one-month of pre-incubation at different temperatures

Quantification of microbial activities involved in soil organic carbon (SOC) decomposition is critical for the prediction of the long-term impact of climate change on soil respiration (SR) and SOC stock. Although the temperature sensitivity of SR is especially critical in semi-arid regions, such as North West Tunisia, where the SOC stock is low, little research has been carried out in these environments. More needs to be known about factors, such as SOC availability that influence temperature sensitivity. In this study, soil samples were incubated with and without glucose addition for 28 days after a 28-day pre-incubation period. Pre-incubation and incubation was carried out at 20 °C, 30 °C, 40 °C and 50 °C. Respiration measurements were taken with temperature, glucose addition and incubation time as independent variables. The highest pre-incubation temperature reduced the temperature sensitivity of SR during the subsequent incubation period, both with and without glucose addition. Soil samples pre-incubated at 50 °C had the lowest SR at all subsequent incubation temperatures and the lowest temperature sensitivity of SR, even after glucose addition. However, after glucose addition, the effect of a high pre-incubation temperature on soil respiration lasted only two days. Measuring the water-soluble carbon (WSC) in soil samples suggested that the high pre-incubation temperature may have killed part of the microbial biomass, modified microbial communities or solubilized SOC. For quantifying the possible effect of global warming, in particular heat waves, on soil respiration in the soil studied, the results indicate a moderate response of soil respiration to temperature at high temperatures, as shown by Q₁₀ close to 1.7, even in the range 40-50 °C.     

Soil respiration in eucalypt forests of southeastern Australia

Summary Soil respiration was measured in five eucalypt forests of southeastern Australia. Regardless of the type of forest, the rate of respiration in soils responded to the addition of an available C source (glucose) and did not to the addition of N or P. Addition of glucose, at up to 100% of the glucose equivalent in soil, increased the rate of respiration sigmoidally. The concentration of glucose needed to achieve the maximum rate of respiration in the topsoil (0–2 cm) of an Eucalyptus regnans forest was at least an order of magnitude greater than its equivalent in the soil. The results indicate that microbial respiration in soils from eucalypt forests is limited by an available source of C.     

Response of soil respiration to precipitation during the dry season in two typical forest stands in the forest-grassland transition zone of the Loess Plateau

Forest ecosystems on the Loess Plateau are receiving increasing attention for their special importance in carbon fixation and conservation of soil and water in the region. Soil respiration was investigated in two typical forest stands of the forest-grassland transition zone in the region, an exotic black locust (Robinia pseudoacacia) plantation and an indigenous oak (Quercus liaotungensis) forest, in response to rain events (27.7 mm in May 2009 and 19 mm in May 2010) during the early summer dry season. In both ecosystems, precipitation significantly increased soil moisture, decreased soil temperature, and accelerated soil respiration. The peak values of soil respiration were 4.8 and 4.4 μmol CO₂ m⁻² s⁻¹ in the oak plot and the black locust plot, respectively. In the dry period after rainfall, the soil moisture and respiration rate gradually decreased and the soil temperature increased. Soil respiration rate in black locust stand was consistently less than that in oak stand, being consistent with the differences in C, N contents and fine root mass on the forest floor and in soil between the two stands. However, root respiration (R_r) per unit fine root mass and microbial respiration (R_m) per unit the amount of soil organic matter were higher in black locust stand than in oak stand. Respiration by root rhizosphere in black locust stand was the dominant component resulting in total respiration changes, whereas respiration by roots and soil microbes contributed equally in oak stand. Soil respiration in the black locust plantation showed higher sensitivity to precipitation than that in the oak forest.     

A method for determining microbially available N and P in an organic soil

Summary A bioassay of microbially available soil N and P is described. It is based on the addition of glucose together with N or P to soil, followed by monitoring of the respiration rate. The addition of glucose + N resulted in an immediate increase in the soil respiration rate followed by a short period of exponential increase, reflecting the growth of microorganisms on the added substrate. The exponential phase levelled off, when lack of P prevented further growth of the soil microorganisms. The soil respiration rate then remained constant for several hours before decreasing, when glucose became limiting. The addition of glucose + P resulted in a lower plateau of the soil respiration rate, indicating that microbial growth was more limited by N than P in this forest soil (0.28 and 0.79 mg CO₂ g^-1 organic matter h^-1, respectively). Additions of the limiting nutrient resulted in a proportional increase in the constant level of the soil respiration rate. This was used to calculated the increase in the soil respiration rate per mg N (0.71 mg CO₂ h^-1) or mg P (4.6 mg CO₂ h^-1) added to this particular soil. Microbially available N was then calculated in two ways from the regression equation (0.15 or 0.40 mg g^-1 organic matter) and P (0.13 or 0.17 mg g^-1 organic matter). A comparison with 2 M KCl extraction showed that in nutrient-poor forest soils the microbially available N was 6.3 or 18.5 times higher than the KCl extractable N.     

皆伐对杉木人工林土壤呼吸的影响

应用密闭室碱吸收法对杉木人工林皆伐后的土壤呼吸及各分室呼吸进行为期1年定位研究,结果表明,杉木林皆伐后前4个月土壤呼吸显著高于对照(未伐地)的,皆伐6个月后则显著低于对照的,但伐后1年内的平均土壤呼吸则与对照的无显著差异。皆伐地枯枝落叶层呼吸和矿质土壤呼吸分别在伐后的5个月和6个月内显著高于对照的,但此后则与对照的无显著差异。皆伐地根系呼吸除在伐后当月显著高于对照的外,第3个月迅速降低至消失。皆伐地土壤呼吸、枯枝落叶层呼吸和矿质土壤呼吸最大值出现时间均较对照的有所提前。伐后1年内皆伐地枯枝落叶层呼吸、矿质土壤呼吸和根系呼吸占土壤呼吸的比例分别为34·5%、63·9%和1·6%,而对照的则分别为23·4%、50·1%和26·5%。双因素关系模型拟合结果表明,土壤温度和土壤湿度共同解释皆伐和对照土壤呼吸速率变化的54%和90%。皆伐地土壤呼吸及各分室呼吸对土壤温度的敏感性低于对照的,但对土壤湿度的敏感性则高于对照的。皆伐地土壤呼吸、矿质土壤呼吸和枯枝落叶层呼吸的Q10分别为1·42、1·53和1·34,而对照的土壤呼吸、矿质土壤呼吸、枯枝落叶层呼吸和根系呼吸的Q10则分别为2·42、1·81、2·40和4·41。     

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

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

Soil respiration characteristics of tropical soils from agricultural and forestry land-uses at Wondo Genet (Ethiopia) in response to C, N and P amendments

In most parts of tropical Africa, conversion of forests into agricultural lands is often accompanied by drastic changes in soil properties. However, little study has been done to examine changes in biological properties of soils from different land-uses in response to addition of C and nutrients. We conducted this study with the aim of investigating nutrient limitations for microbial activity in soils from agricultural (farm) and forest land-uses at Wondo Genet (Ethiopia) after amendment with C and limiting nutrients. We measured CO₂ respiration rates from the soils incubated in the laboratory before and after addition of glucose-C together with N and/or P in excess and/or limiting amounts. Based on the respiration kinetics, we determined the basal respiration (BR), substrate-induced respiration (SIR), specific-microbial growth rate (μ), respiration maxima (R_max), % of glucose-C respired, and microbially available N and P in the soils. We found that N was more limiting than P for the micro-biota in the soils considered, suggesting the presence of ample amounts of indigenous P that could be extracted by the micro-biota, if provided with C. Addition of P resulted in a respiration pattern with two peaks, presumably reflecting different N pools being available over time. The SIR, Respiration maxima, μ and microbially available P were higher in soils from the farm, while %C respired was higher in the forest, suggesting increased C costs for micro-biota to be able to utilize nutrients that are strongly bound to organic-matter or clay minerals. Depending on land-use, about 49-69% of added glucose-C was respired during two and a half weeks time, but differences between N or P additions were not significant. The correlation between soil physical and chemical properties and respiration parameters, however, depended on whether N or P was limiting. We concluded that examining the soil respiration kinetics could provide vital information on nutritional status of micro-organisms under different land-uses and on potential availability of nutrients to plants.     

Metabolic and anabolic responses of arable and forest soils to nutrient addition§

The increase in microbial C content, cumulative respiration and changes in ”︁available” C were determined after adding glucose (2 mg glucose-C (g soil)^—1, ”︁C”), glucose + nitrogen (”︁C+N”) or glucose + nitrogen + phosphorus (”︁C+N+P”) to four soils. In two sandy soils, one agricultural and the other from a beech forest in Germany, available C was still present approximately 7 days after C addition. The supplement N and N+P decreased the content of available C and stimulated respiration rate and microbial growth. In two loamy forest soils from Italy, which had a high native content of microbial C, available C was present in the beech soil but not in a silver fir soil treated with C+N. In the Italian beech and fir soil, microbial growth was highest with C+N+P and C+N addition respectively. Available C remaining in the soil was related to some extent to the native microbial C content. However, microbial growth and respiration response varied between soil and treatment. The respiratory coefficient, that is the ratio of assimilated to respired C, varied between 0.0 and 1.45 μg C_mic (μg CO₂-C)^—1 and was generally higher when a large amount of native biomass was present. The eco-physiological strategy of the soil microbiota in using C seemed to shift according to the biomass content, the added concentration and composition of available substrates, and emergent system properties.     

Nitrogen fertilization inhibits soil microbial respiration regardless of the form of nitrogen applied

We tested how amendments of different forms of nitrogen (N) affect microbial respiration rates by adding six different forms of N (NH₄NO₃, (NH₂)₂CO (urea), KNO₃, NH₄Cl, (NH₄)₂SO₄, Ca(NO₃)₂) to three distinct soils. All inorganic N forms led to a net reduction in microbial respiration, and the magnitude of the observed response (up to 60 % reduction) was consistent across all soils and negatively correlated with N concentration. Urea also reduced respiration rates in nearly all cases, but the effect was attenuated by the associated input of labile organic carbon. We observed decreases in respiration regardless of soil type, the specific N counter ion, N added as NH₄⁺ or NO₃⁻, or the effects of N form on soil pH, suggesting that decreases in respiration rates were mainly a direct result of the increase in soil N availability, rather than indirect effects caused by the form of N added.     

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.     

Using perlite as a substrate carrier for measuring microbial available phosphorus by respiration kinetics in soils

The release of CO₂ by soil microorganisms after the addition of nitrogen and glucose in excess and calibration additions of phosphorus has successfully been used to assess microbial available P, assuming the native soil P pool is then limiting respiration. However, in P-fixing soils and soils with high P content, carbon can be exhausted before the available soil P pool. It is not possible to simply increase the amount of glucose as then the glucose concentration would be lethal for microorganisms. A modified method was tested where soil is mixed with perlite. It was hypothesised that perlite, having a high water holding capacity, would dilute the concentration of glucose, while maintaining the bioavailability of added nutrients, thus avoiding carbon limitation. Factorial combinations of amount of soil and perlite (both adjusted to −25 kPa water potential) were tested to examine if perlite as such had any effect on the respiration. Five tropical soil samples with a sharp gradient in P availability and one N-limited compost material were used. The method successfully reduced the risk of carbon limitation. Microbial indices, such as basal respiration, substrate-induced respiration and maximum P-limited respiration, were directly proportional to the amount of soil in the experiments but unrelated to the amount of perlite, showing that perlite did not affect microbial measurements.     

Spatial and seasonal variations in soil respiration in a temperate deciduous forest with fluctuating water table

Our objectives were to determine both spatial and temporal variations in soil respiration of a mixed deciduous forest, with soils exhibiting contrasting levels of hydromorphy. Soil respiration (R_S) showed a clear seasonal trend that reflected those of soil temperature (T_S) and soil water content (W_S), especially during summer drought. Using a bivariate model (RMSE=1.03), both optimal soil water content for soil respiration (W_SO) and soil respiration at both 10 °C and optimal soil water content (R_S10) varied among plots, ranging, respectively, from 0.25 to 0.40 and from 2.30 to 3.60 μmol m⁻² s⁻¹. Spatial variation in W_SO was related to bulk density and to topsoil N content, while spatial variation in R_S10 was related to basal area and the difference in pH measured in water or KCl suspensions. These results offer promising perspectives for spatializing ecosystem carbon budget at the regional scale.     

The effects of N and P additions on soil microbial properties in paired stands of temperate secondary forests and adjacent larch plantations in Northeast China

The conversion of secondary forests to larch plantations in Northeast China has resulted in a significant decline in soil available nitrogen (N) and phosphorus (P), and thus affects plant productivity and ecosystem functioning. Microbes play a key role in the recycling of soil nutrients; in turn, the availability of soil N and P can constrain microbial activity. However, there is little information on the relationships between available soil N and P and the microbial biomass and activity in larch plantation soil. We studied the responses of soil microbial respiration, microbial biomass and activity to N and P additions in a 120-day laboratory incubation experiment and assessed soil microbial properties in larch plantation soil by comparing them with the soil of an adjacent secondary forest. We found that the N-containing treatments (N and N + P) increased the concentrations of soil microbial biomass N and soluble organic N, whereas the same treatments did not affect microbial respiration and the activities of β-glucosidase, N-acetyl-β-glucosaminidase and acid phosphatase in the larch plantation. In addition, the concentration of microbial biomass P decreased with N addition in larch plantation soil. In contrast, N and N + P additions decreased microbial respiration, and N addition also decreased the activity of N-acetyl-β-glucosaminidase in the secondary forest soil. The P treatment did not affect microbial respiration in either larch plantation or secondary forest soils, while this treatment increased the activities of β-glucosidase and acid phosphatase in the secondary forest soil. These results suggested that microbial respiration was not limited by available P in either secondary forest or larch plantation soils, but microbial activity may have a greater P demand in secondary forest soil than in larch plantation soil. Overall, there was no evidence, at least in the present experiment, supporting the possibility that microbes suffered from N or P deficiency in larch plantation soil.     

Carbon and Nitrogen Amendments Lead to Differential Growth of Bacterial and Fungal Communities in a High-pH Soil

Microbial growth in soil is mostly limited by lack of carbon (C). However, adding fresh, C-rich litter can induce nitrogen (N) limitation. We studied the effect of alleviating C and N limitation in high-pH (> 8) soils, soils expected to favor bacterial over fungal growth. Nitrogen limitation was induced by incubating soils amended with C-rich substrate (starch or straw) for 4 weeks. Limiting nutrients and the effects of alleviating limitation were then studied by adding C (as glucose) or N (as NH₄NO₃) and measuring microbial growth and respiration after 4 d. In non-amended, C-limited soils, adding C but not N increased both microbial respiration and bacterial growth. In N-limited, substrate-amended soils, adding C increased respiration, whereas adding N increased both microbial respiration and growth. Inducing N limitation by amending with straw was most easily detected in increased fungal growth after the addition of N, whereas with starch, only bacterial growth responded to alleviating N limitation. Compared to earlier results using a low-pH soil, the effect of substrate used to induce N limitation was more important than pH for inducing bacterial or fungal growth after alleviating N limitation. Furthermore, we found no evidence that alleviating N limitation resulted in decreased respiration concomitant with increased microbial growth in soil, suggesting no drastic changes in C use efficiency.     

Controls on soil respiration in semiarid soils

Soil respiration in semiarid ecosystems responds positively to temperature, but temperature is just one of many factors controlling soil respiration. Soil moisture can have an overriding influence, particularly during the dry/warm portions of the year. The purpose of this project was to evaluate the influence of soil moisture on the relationship between temperature and soil respiration. Soil samples collected from a range of sites arrayed across a climatic gradient were incubated under varying temperature and moisture conditions. Additionally, we evaluated the impact of substrate quality on short-term soil respiration responses by carrying out substrate-induced respiration assessments for each soil at nine different temperatures. Within all soil moisture regimes, respiration rates always increased with increase in temperature. For a given temperature, soil respiration increased by half (on average) across moisture regimes; Q₁₀ values declined with soil moisture from 3.2 (at −0.03 MPa) to 2.1 (−1.5 MPa). In summary, soil respiration was generally directly related to temperature, but responses were ameliorated with decrease in soil moisture.     

沼泽垦殖前后土壤呼吸与CH_4通量变化

湿地是陆地生态系统的重要组成部分,在碳的储存中起着重要作用。湿地垦殖后,在相同季节根层土壤温度明显高于沼泽湿地土壤,但垦殖后土壤有机碳、氮素含量明显降低,C:N比值减小,土壤呼吸通量增大,且具有季节性变化。垦殖8年的农田土壤,呼吸通量大于垦殖15年的农田土壤,弃耕后土壤有机碳含量及土壤呼吸强度有所增加,土壤呼吸通量与土壤温度呈显著正相关关系。沼泽湿地土壤为大气CH4的重要源,通量季节性变化明显,沼泽垦殖后农田土壤成为CH4的汇,不同垦殖年限土壤间CH4通量差异性不大。     

Effects of water amendment on basal and substrate-induced respiration rates of mineral soils

Summary We studied the effects of amending soils with different volumes of water or glucose solution on respiration rates measured as CO₂ evolution. Basal respiration was not significantly affected by the volume of water amendment, but substrate-induced respiration in static soil solutions was significantly reduced by increasing water contents. Inhibition of substrate-induced respiration was removed by continuously agitating the incubation vessels. Estimates of substrate-induced respiration rates for six soils differed markedly, depending on whether the vessels were stationary or agitated during the incubation. Agitation allowed increased discrimination between substrate-induced respiration rates for the soils, while static incubation only differentiated the soil with the highest substrate-induced respiration rate from the other soils.     

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.