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1.
Glass transitions have been reported for purified humic acids only. In this study, a glass transition was detected in a sample of a sandy forest soil by Differential Scanning Calorimetry (DSC). The glass transition temperature was 79°C for air‐dried samples and 77°C for pre‐moistened samples. In addition to the glass transition, an exothermic process around 30°C was detected in pre‐moistened samples. This could be due to water loss of soil organic matter. However, the nature of this process is not yet understood. This study showed that the macromolecular behaviour of SOM, as indicated by DSC, reacts to the moisture state of soil organic matter.  相似文献   

2.
The Ultuna long-term soil organic matter experiment in Sweden (59′82° N, 17′65° E) was started in 1956 to study the effects of different N fertilisers and organic amendments on soil properties. In this study, samples were taken from 11 of the treatments, including unfertilised bare fallow and cropped fallow, straw with and without N addition, green manure, peat, farmyard manure, sawdust, sewage sludge, calcium nitrate and ammonium sulphate, with n = 4 for each treatment. Samples were taken from topsoil (0–20 cm) and subsoil (27–40 cm depth) and analysed for concentrations of phospholipid fatty acids (PLFAs), organic C, total N and pH. The results showed that the subsoil samples reflected the total PLFA content of the topsoil, but not the microbial community structure. Total PLFA content was well correlated with total organic C and total N in both topsoil and subsoil. Total PLFA content in topsoil samples was highest in the sewage sludge treatment (89 ± 22 nmol PLFA g dw−1). This contradicts earlier findings on microbial biomass in this sewage sludge-treated soil, which indicated inhibition of microorganisms, probably by heavy metals added with sludge. A switch towards microbial growth and faster decomposition of organic matter occurred around 2000, coinciding with lowered heavy metal content in the sludge. According to the PLFA data, the microbial community in the sewage sludge treatment is now dominated by Gram-positive bacteria. A lack of Gram-negative bacteria was also observed for the ammonium sulphate treatment, obviously caused by a drop in pH to 4.2.  相似文献   

3.
The effect of two methods for the preparation of soil samples for sorption experiments—hard (dehydration at 105°C) and mild (drying over P2O5 at 20°C in vacuum) drying—on the values of the vaporphase sorption of p-xylene was studied depending on the content of organic matter in the soil. It was shown with dark gray forest and chernozemic soils as examples that the hard drying of soil samples taken from the upper layer of the humus profile with a high content (>4%) of organic carbon decreased their sorption capacity in the range of 0–5% by 7–81%. Therefore, the method is unsuitable for these soils. It was also found that the mild method of soil preparation had obvious analytical advantages.  相似文献   

4.
The time evolution of the content and composition of carbohydrates was studied in the surface layer of forest soils non-affected and affected by wildfires. The low- and high-severity fires caused an immediate reduction of the C present as carbohydrates of 34% and 47–55%, respectively, which was due to the decrease of both hexoses and pentoses in two hydrolysis fractions (hydrolysate-A, non-cellulosic polysaccharides; hydrolysate-B, cellulosic polysaccharides). Carbohydrates tended to recover with time; however, values had still not reached the amounts found in the corresponding unburnt samples after 12–15 months. No difference between the unburnt and burnt samples was observed in the distribution of the neutral sugars in the hydrolysates over time. On a percentage basis, 72–92% of the total neutral sugars was extracted in hydrolysate-A (59 ± 7% hexoses; 24 ± 7% pentoses) and the rest, 8–28%, in hydrolysate-B (15 ± 5% hexoses; 2 ± 1% pentoses). The contribution of hexoses and pentoses to the neutral sugar pool was different between the two hydrolysis fractions being the hexoses/pentoses ratio higher for hydrolysate-B than for hydrolysate-A. The results also showed that the proportion of soil C present as carbohydrates-C rather than the total carbohydrates content should be used for monitoring short- and medium-term changes induced by fire in soil organic matter quality.  相似文献   

5.
Freeze–thaw cycles can promote soil N losses as a result of microbial and root cell lysis; however, minimal freeze–thaw effects have typically been observed in studies that have imposed moderate temperature cycles. We conducted laboratory incubations on surface soil (top 3 cm) collected in a temperate old field from late fall through mid-winter to examine how variation in freeze–thaw amplitude, number, timing of collection, and freezing rate altered soil extractable N. We varied freeze–thaw amplitude by imposing minimum cycle temperatures of 0, −1, −2, −5, and −10°C for a series of either one or two cycles and held control samples constant at 3°C. We also examined the effects of freezing rates of 1, 3, and 30°C h−1. We hypothesized that extractable N would be highest for both the maximum freezing amplitudes and rates. While multiple freeze–thaw cycles at −10°C and freeze–thaw cycles associated with artificially high freezing rates increased extractable N, freeze–thaw cycles representative of field conditions at our site had no effect on extractable N in late fall and early winter. By mid-winter there was a significant freeze–thaw cycle effect but, contrary to our prediction, less N was extracted from freeze–thaw treated samples than from the control samples, which remained thawed over the treatment period. Increased extractable N in control samples was driven by increased organic N rather than increased inorganic N. Our results suggest that freeze–thaw damage to soil organisms does not contribute substantially to N release in our system. Instead, soil extractable N may increase during mid-winter thaws as a result of increased soil proteolytic activity above freezing temperatures.  相似文献   

6.
Climate change is predicted to reduce or delay annual wintertime snow pack formation in the forests of the northeastern US. Any delay in snowpack formation could increase soil freezing in winter and, thereby, alter soil characteristics and processes. We examined the hypothesis that delayed snowpack would disrupt soil structure and change organic matter bioavailability in an experimental snow removal study at the Hubbard Brook Experimental Forest (HBEF), NH, USA. Pairs of reference and snow removal treatment plots were studied in four different sites at HBEF. Snow was removed from November–January of two winters, inducing soil freezing throughout both winters. Size class distribution and organic matter concentration and content of aggregates, and carbon and nitrogen mineralization potential of size fractions were quantified for surface mineral soils in the spring of both years immediately after snowmelt. In the first year of sampling, the only significant effect of snow removal was an increase in the smallest (<53 μm) size fraction of mineral soil. In the second year, snow removal increased organic matter concentrations of macroaggregate (250–2,000 μm) and microaggregate (53–250 μm) size fractions. This change corresponded to an increase in net N mineralization potential and the ratio of N to C mineralized in the macroaggregate fraction, but there were no effects of snow removal on C mineralization. We propose that soil freezing increases the movement of organic matter from organic to mineral soil horizons and increases the N content of mineralizable substrates in mineral soil following years with delayed snowpack formation.  相似文献   

7.
 Long-term experiments on different crop management systems provide essential information about turnover of soil organic matter and changes in microbial properties over a period of time. A long-term field site trial, which was established in 1967 near Vienna, Austria, to document the fate of 14C-labelled manure (straw and farmyard) under different crop management systems (crop rotation, spring wheat and bare fallow), was investigated. Soil samples were taken in 1997 and separated into size fractions (>250 μm, 250–63 μm, 63–2 μm, 2–0.1 μm and <0.1 μm) after aggregate dispersion using low-energy sonication. Organic C, total N and 14C content were measured in the bulk soil and the size fractions and microbial properties were analysed in the bulk soil. Additionally, C mineralization in bulk soil samples was monitored at 20 °C over a period of 28 days, and subsequently 14C-CO2 content was analysed. The distribution of organic C and N within the size fractions was similar between crop rotation and spring wheat; the highest amounts of organic C and N were found in the clay-sized fraction. The amounts of C and N were significantly smaller in the bare fallow, which was depleted of organic matter in the coarse-sized fractions. 14C distribution differed significantly from unlabelled C distribution, labelled C was accumulated in the silt-sized fraction, indicating weak humification of the applied manure C. The highest rate of C mineralization was measured in the crop rotation and spring wheat, whereas the emission rate of the bare fallow was about 40% lower. The higher 14C:C ratio of the bulk soil in comparison to the emitted CO2 indicated that labelled C compounds still remained mineralizable after a period of 30 years. Microbial properties showed a great difference between crop management systems and bare fallow, particularly regarding urease and xylanase activity. Received: 31 May 1999  相似文献   

8.
The soil formation was studied in the 21- to 35-year-old pine plantations created on the overburden dumps of the Nazarovskaya Depression without applying the material of the humus horizon. The surface technogenic formations under the pine plantations belong to the group of naturfabricats (surface formations devoid of the humus horizon and composed of natural substrate). The morphological characteristics, bulk chemical and particle-size compositions, and labile properties of the soils indicate that the accumulation and transformation of organic matter are dominant processes in the artificially planted forests. The accumulation of organic residues and destruction and humification are tightly related to the environmental conditions transformed by the technogenesis and conceal other processes forming the soil profiles. Quasizems created for agricultural production purposes were the objects of the study. They were formed by covering the technically planned overburden rocks with the material of humus layers. The thickness of the humus horizons of the quasizems varies greatly (25–64%); the variation of the humus reserves in them is 34–122%. Middle-profile horizons have not been formed by the present time.  相似文献   

9.
Experimental data on the effect of surface fires on the organic matter transformation in the gray-humus soils of pine forests were obtained in the southwestern part of the Baikal region. The application of methods of thermal analysis (such as differential scanning calorimetry and thermogravimetry) made it possible to obtain qualitative and quantitative information about the decomposition (oxidation) of the components of the soil organic matter upon their heating. It was found that the organic matter content in the soils subjected to fires of high intensity decreased by 1.9 times in comparison with the control. In the litter horizons of the undisturbed soils, thermolabile components (mostly, oligo- and polysaccharides) comprised 61% of the organic matter, and the portion of thermostable components (aromatic compounds) was 39%. A significant decrease in the content of thermolabile components and an increase in the content of thermostable components (up to 62%) were observed in the organic matter of the postpyrogenic forest litter as a result of the charcoal formation during the fire.  相似文献   

10.
The abundance and micro-stratification of bacteria and fungi inhabiting the organic layers of a Scots pine forest (Pinus sylvestris L.) were investigated. An experiment using stratified litterbags, containing organic material of four degradation stages (fresh litter, litter, fragmented litter and humus) was performed over a period of 2.5 years. Dynamics and stratification of fluorescent stained bacteria and fungi, ratios between bacterial and fungal biomass, and relationships with moisture and temperature are described. Average bacterial counts in litter and fragmented litter were similar, i.e., approximately 5×109 bacteriag–1 (dry weight) organic matter, and significantly exceeded those in humus. The mean bacterial biomass ranged from 0.338 to 0.252mg carbon (C) g–1 (dry weight) organic matter. Lengths of mycelia were significantly below the usually recorded amounts for comparable temperate coniferous forests. The highest average hyphal length, 53mg–1 (dry weight) organic matter, was recorded in litter and decreased significantly with depth. The corresponding mean fungal biomass ranged from 0.050 to 0.009mg Cg–1 (dry weight). The abundance of bacteria and fungi was influenced by water content, that of fungi also by temperature. A litterbag series with freshly fallen litter of standard quality, renewed bimonthly, revealed a clear seasonal pattern with microbial biomass peaks in winter. The mean hyphal length was 104mg–1 (dry weight) and mean number of bacteria, 2.40×109 bacteria g–1 (dry weight). Comparable bacterial and fungal biomass C were found in the freshly fallen litter [0.154 and 0.132mgCg–1 (dry weight) organic material, respectively]. The ratio of bacterial-to-fungal biomass C increased from 1.2 in fresh litter to 28.0 in humus. The results indicate the existence of an environmental stress factor affecting the abundance of fungi in the second phase of decomposition. High atmospheric nitrogen deposition is discussed as a prime factor to explain low fungal biomass and the relatively short lengths of fungal hyphae in some of the forest soil layers under study. Received: 26 June 1997  相似文献   

11.
Abstract

Dehydrogenase enzymes significantly mediate biological oxidation of organic compounds in soil. Dehydrogenase activities were determined in surface and subsurface horizons of 14 major hill land soils of the Appalachian Region. Soil samples were stored air‐dry (AD) and field‐moist at 4°C prior to determination of dehydrogenase activities. Each soil type had its own level of dehydrogenase activity. The average dehydrogenase activities of surface and subsurface horizons stored at 4°C were 1.4 and 3.3 times higher than soil samples stored AD. Mean dehydrogenase activities of surface horizons were 5.3 and 2.2 times higher than subsurface horizons stored at AD and 4°C, respectively. Depending on storage method (AD or 4°C) and type of horizon (surface or subsurface). dehydrogenase activities were either positively or negatively related to soil moisture status, organic C, total N, soil texture, and forms of ? and S. Most of these relationships were nonsignificant. Dehydrogenase activities were positively related to exchangeable bases. CEC, and ratio of Mg/Mg+Ca; however. the relationships were mostly significant in surface horizons.  相似文献   

12.
On the basis of a large volume of literature and original data, the high content (1–7%) of organic matter in the mineral layer of loamy permafrost-affected soils of coastal lowlands in East Siberia (from the lower reaches of the Lena River to the lower reaches of the Kolyma River) has been statistically proved. In most cases, the reserves of Corg in the mineral layer of these soils exceed those in the surface organic horizons and constitute 60–90% of the total soil pool of Corg. The enrichment of the mineral layer with Corg is due to the cryogenic retention (retenization) of humus (the illuviation and accumulation of colorless humic substances above permafrost) and the cryogenic mass exchange (mechanical admixture of organic matter from the upper organic horizons into the mineral layers). The analysis of 60 soil profiles showed that the accumulation of organic matter above the permafrost table is observed in 43% of cases; in general, the organic matter distribution in the soil profiles is highly variable. A specific type of colorless humus is accumulated above the permafrost table. The mechanisms of its precipitation and transformation in the profile require further studies.  相似文献   

13.
In industrial areas, heavy metals may accumulate in forest soil organic horizons, affecting soil microorganisms and causing changes in the chemical composition of the accumulated organic matter. The objectives of this study were to test the ability of near-infrared spectroscopy (NIRS) to detect heavy metal effects on the chemical composition of forest soil O horizons and to test whether NIRS may be used to quantitatively determine total and exchangeable concentrations of Zn and Pb (Znt, Pbt, Znex, Pbex) and other chemical and microbial properties in forest soil O horizons polluted with heavy metals. The samples of O horizons (n = 79) were analyzed for organic C (Corg), total N and S (Nt, St), Znt, Pbt, Znex, Pbex, basal respiration (BR), microbial biomass (Cmic) and Cmic-to-Corg ratio. Spectra of the samples were recorded in the Vis-NIR range (400–2,500 nm). To detect heavy-metal-induced changes in the chemical composition of O horizons principal components (PC1–PC7) based on the spectral data were regressed against Znt + Pbt values. A modified partial least squares method was used to develop calibration models for prediction of various chemical and microbial properties of the samples from their spectra. Regression analysis revealed a significant relationship between PC3 and PC5 (r = −0.27 and −0.34, respectively) and Znt + Pbt values, indicating an effect of heavy metal pollution on the spectral properties of the O horizons and thus on their chemical composition. For quantitative estimations, the best calibration model was obtained for Corg-to-Nt ratio (r = 0.98). The models for Corg, Nt, and microbial properties were satisfactory but less accurate. NIRS failed to accurately predict St, Corg-to-St, Znt, Pbt, Znex, and Pbex.  相似文献   

14.
Thermal diffusivity of the upper horizons of leached meadow-chernozemic soils varies in dependence on the soil water content within the following limits: 1.20–4.11 × 10−7 m2/s for the Ap horizon, 1.21–3.85 ×10−7 m2/s for the A1 horizon, and 1.35–3.73 × 10−7 m2/s for the A1B horizon. The relationships between the thermal diffusivity and the soil water content are described by S-shape curves with a long gently inclined segment within the range of water contents of <0.20 cm3/cm3, a distinct rise in thermal diffusivity within the water contents from 0.20 to 0.30–0.35 cm3/cm3, and a flattened or somewhat declining segment in the area with the high (>0.30–0.35 cm3/cm3) water contents. The thermal diffusivity of air-dried soil samples correlates with the physical clay (<0.01 mm) content. The Pearson correlation coefficient for these two variables equals −0.67 and is statistically significant at the significance level of 0.05. Regression equations allowing one to calculate the thermal diffusivity of the investigated soil horizons on the basis of data on the soil water content have been obtained.  相似文献   

15.
 Nitrification and denitrification are, like all biological processes, influenced by temperature. We investigated temperature effects on N trace gas turnover by nitrification and denitrification in two soils under two experimental conditions. In the first approach ("temperature shift experiment") soil samples were preincubated at 25  °C and then exposed to gradually increasing temperatures (starting at 4  °C and finishing at 40–45  °C). Under these conditions the immediate effect of temperature change was assessed. In the second approach ("discrete temperature experiment") the soil samples were preincubated at different temperatures (4–35  °C) for 5 days and then tested at the same temperatures. The different experimental conditions affected the results of the study. In the temperature shift experiment the NO release increased steadily with increasing temperature in both soils. In the discrete temperature experiment, however, the production rates of NO and N2O showed a minimum at intermediate temperatures (13–25  °C). In one of the soils (soil B9), the percent contribution of nitrification to NO production in the discrete temperature experiment reached a maximum (>95% contribution) at 25  °C. In the temperature shift experiment nitrification was always the dominant process for NO release and showed no systematic temperature dependency. In the second soil (soil B14), the percent contribution of nitrification to NO release decreased from 50 to 10% as the temperature was increased from 4  °C to 45  °C, but no differences were evident in the discrete temperature experiment. The N2O production rates were measured in the discrete temperature experiment only. The contribution of nitrification to N2O production in soil B9 was considerably higher at 25–35  °C (60–80% contribution) than at 4–13  °C (15–20% contribution). In soil B14 the contribution of nitrification to N2O production was lowest at 4  °C. The effects of temperature on N trace gas turnover differed between the two soils and incubation conditions. The experimental set-up allowed us to distinguish between immediate effects of short-term changes in temperature on the process rates, and longer-term effects by which preincubation at a particular temperature presumably resulted in the adaptation of the soil microorganisms to this temperature. Both types of effects were important in regulating the release of NO and N2O from soil. Received: 20 October 1998  相似文献   

16.
Humus forms may be the first tool to assess qualitatively organic matter turnover in soils; as such they should be related to the stocks of organic C a soil can store, to the characteristics of organic matter that affect its stability and, more generally, to the factors of soil formation. In this work, we tested these hypotheses in 27 forest soils of northwestern Italy. Site variables representing the pedogenic factors allowed classifying the plots into three clusters, which were significantly different for soil and humus types. The average stocks of organic C in the humic episolum (organic and top mineral horizons) ranged from 2.7 kg m−2 in Eumulls to 9.5 kg m−2 in Amphimulls. A clear trend in C stocks was visible and related both to the increasing presence of organic layers where the environmental conditions do not favour a rapid turnover of organic matter and to the good mixing of organics and minerals in “bio-macrostructured” A horizons. The characteristics of organic matter were also linked to humus forms: The proportion of humified complex substances was the highest in the most active forms, and conversely, non-humified extracted substances formed a considerable part of organic matter only where the environmental conditions limit organic matter degradation. Humus forms seem therefore to reflect several mechanisms of organic matter stabilisation and are clearly related to the capacity of the soil to store C.  相似文献   

17.
The thermal regime of the different soil types of Georgia has been studied on the basis of soil temperature records obtained at 60 weather stations in 1947–1995. It is shown that the thermal conductivity and temperature gradients in the soil profiles depend on the soil type. In the upper 0–20 cm of the soils in the intermontane depressions, the lowest temperature gradients in the summer are typical of the red ferrallitic soils (0.5–1.1°C/dm), and the highest gradients are observed in the chernozems of eastern Georgia (1.0–1.3°c/dm). The soil temperature gradients are especially high in the late spring (1.4–1.8°c/dm), when the atmospheric convection is formed. In the mountains, the lowest gradients in the summer are observed in the cinnamonic soils (0.4–0.9°c/dm), and the highest gradients are established in the brown forest soils (0.5–1.3°c/dm). The redistribution of the temperatures in the deeper layers depends on the soil texture. The lowest temperature gradient is in the soils of Telavi (3.1°c/m), where a stony layer is found at the depth of 60 cm, which causes the good heating of the deep layers. The highest gradients are seen in the clayey and loamy soils of western (Chakva, 3.8°c/m) and eastern (Tbilisi, 3.9°c/m) Georgia. This is probably related to the high water content in the heavy-textured soils.  相似文献   

18.
Abstract

The cation exchange capacity (CEC) at pH 7 was measured for samples of 347 A horizons and 696 B horizons of New Zealand soils. The mean CEC was 22.1 cmolc/kg for the A horizons and 15.2 cmolc/kg for the B horizons. Multiple regressions were carried out for CEC against organic carbon (C), clay content, and the content of seven groups of clay minerals. The results, significant at p <0.001, were consistent with most of the CEC arising from soil organic matter. For the samples of A horizon, the calculated CEC was 221 cmolc/kg per unit C and for the B horizons was 330 cmolc/kg C. There was also a contribution from sites on clay minerals. Multiple regression indicated that smectite had a higher CEC (70 cmolc/kg) than other minerals but it was not as high as that of type smectites; kaolin minerals had the lowest CEC. There was a significant effect of interaction between organic matter and some clay minerals on the CEC. Samples from B horizons containing allophane had lower CEC than those not containing allophane which is consistent with allophane reacting with carboxyl groups on organic matter. For the samples from the A horizons, however the CEC was higher when allophane was present.  相似文献   

19.
The results of long-term studies (1957–2007) of the changes in the morphology of soil profiles and in the reserves and fractional composition of the humus in the soils of the Ingulets irrigation system are discussed. After 50 years of irrigation, the boundaries of the genetic horizons shifted downward by 15–30 cm. The redistribution of the humus took place: its content decreased to a low level in the plow layer of the irrigated and rainfed soils and significantly increased in the layer of 60–100 cm so that the reserves of humus in the layer of 0–100 cm somewhat increased and corresponded to a moderate level. The distribution of humus in the soil profiles was characterized by the gradual lowering down the soil profile. The concentration of nitrogen in the humus of the irrigated southern chernozems was very low. The degree of humification of the soil organic matter was high. The humus was of the humate type in the upper horizons and of the fulvate-humate type in the lower horizons.  相似文献   

20.
Andosols are characterised by high organic matter (OM) content throughout the soil profile, which is mainly due to the stabilisation of soil organic matter (SOM) by mineral interactions. The aim of the study was to examine whether there were differences in the chemical composition of mineral-associated SOM and free OM in the top A horizon and in the subsoil (horizons below the A11 horizon). Our experimental approach included the replicated sampling of a fulvic and an umbic Andosol under pine and laurel forest located on the island of Tenerife with a Mediterranean sub-humid climate. We determined the extent of the organo-mineral interactions by comparing the sizes of the light (free) and heavy (dense) soil fractions obtained by physical separation through flotation in a liquid with a density of 1.9 g cm–3. We determined the elemental and isotopic composition of both fractions and analysed their chemical composition by analytical pyrolysis. The elemental and isotopic composition showed similar values with depth despite the different vegetation and climatic conditions prevailing at the two sites. Carbon (C) stabilised by mineral interactions increased with depth and represented 80–90% of the total C in the lowest horizons. The heavy fractions mainly released N-containing compounds upon analytical pyrolysis, whereas lignin-derived and alkyl compounds were the principal pyrolysis products released from the light fractions of the top- and subsoil horizons. Principal component analysis showed that the chemical composition of OM stabilised by mineral interaction differs in the different horizons of the soil profile. In the A horizons, the chemical composition of this OM was similar to those of the light fractions, i.e. litter input. There was a gradual change in the bulk molecular composition from a higher contribution of plant-derived molecules in the light and heavy fractions of the A horizon to more microbial-derived molecules as well as black C-derived molecules at depth. We conclude that transport processes in addition to decomposition and possibly in situ ageing affect the chemical composition of mineral-associated OM in subsoils.  相似文献   

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