首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Exudates are part of the total rhizodeposition released by plant roots to soil and are considered as a substantial input of soil organic matter. Exact quantitative data concerning the contribution of exudates to soil C pools are still missing. This study was conducted to reveal effects of 13C‐labeled exudate (artificial mixture) which was regularly applied to upper soil material from two agricultural soils. The contribution of exudate C to water‐extractable organic C (WEOC), microbial biomass C (MBC), and CO2‐C evolution was investigated during a 74 d incubation. The WEOC, MBC, and CO2‐C concentrations and the respective δ13C values were determined regularly. In both soils, significant incorporation of artificial‐exudate‐derived C was observed in the WEOC and MBC pool and in CO2‐C. Up to approx. 50% of the exudate‐C amounts added were recovered in the order WEOC << MBC < CO2‐C in both soils at the end of the incubation. Newly built microbial biomass consisted mainly of exudates, which substituted soil‐derived C. Correspondingly, the CO2‐C evolved from exudate‐treated soils relative to the controls was dominated by exudate C, showing a preferential mineralization of this substrate. Our results suggest that the remaining 50% of the exudate C added became stabilized in non‐water‐extractable organic fractions. This assumption was supported by the determination of the total organic C in the soils on the second‐last sampling towards the end of the incubation. In the exudate‐treated soils, significantly more soil‐derived C compared to the controls was found in the WEOC on almost all samplings and in the MBC on the first sampling. This material might have derived from exchange processes between the added exudate and the soil matrix. This study showed that easily available substrates can be stabilized in soil at least in the short term.  相似文献   

2.
ABSTRACT

Hot-water- and water-extractable organic matter were obtained from soil samples collected from a rice paddy 31 years after the start of a long-term rice experiment in Yamagata, Japan. Specifically, hot-water-extractable organic carbon and nitrogen (HWEOC and HWEON) were obtained by extraction at 80°C for 16 h, and water-extractable organic carbon and nitrogen (WEOC and WEON) were obtained by extraction at room temperature. The soil samples were collected from surface (0–15 cm) and subsurface (15–25 cm) layers of five plots that had been treated with inorganic fertilizers alone or with inorganic fertilizers plus organic matter, as follows: PK, NPK, NPK plus rice straw (RS), NPK plus rice straw compost (CM1), and NPK plus a high dose of rice straw compost (CM3). The soil/water ratio was 1:10 for both extraction temperatures. We found that the organic carbon and total nitrogen contents of the bulk soils were highly correlated with the extractable organic carbon and nitrogen contents regardless of extraction temperature, and the extractable organic carbon and nitrogen contents were higher in the plots that were treated with inorganic fertilizers plus organic matter than in the PK and NPK plots. The HWEOC and WEOC δ13C values ranged from ?28.2% to ?26.4% and were similar to the values for the applied rice straw and rice straw compost. There were no correlations between the HWEOC or WEOC δ13C values and the amounts of HWEOC or WEOC. The δ13C values of the bulk soils ranged from ?25.7% to ?23.2% and were lower for the RS and CM plots than for the PK and NPK plots. These results indicate that HWEOC and WEOC originated mainly from rice plants and the applied organic matter rather than from the indigenous soil organic matter. The significant positive correlations between the amounts of HWEOC and HWEON and the amount of available nitrogen (P < 0.001) imply that extractable organic matter can be used as an index for soil fertility in this long-term experiment. We concluded that the applied organic matter decomposed more rapidly than the indigenous soil organic matter and affected WEOC δ13C values and amounts.  相似文献   

3.
Although considerable research has been conducted on the importance of recent litter compared with older soil organic matter as sources of dissolved organic carbon (DOC) in forest soils, a more thorough evaluation of this mechanism is necessary. We studied water‐extractable organic carbon (WEOC) in a soil profile under a cool‐temperate beech forest by analysing the isotopic composition (13C and 14C) of WEOC and its fractions after separation on a DAX‐8 resin. With depth, WEOC became more enriched in 13C, which reflects the increasing proportion of the hydrophilic, isotopically heavier fraction. The 14C content in WEOC and its fractions decreased with depth, paralleling the 14C trend in soil organic matter (SOM). These results indicate a dynamic equilibrium of WEOC and soil organic carbon. The dominant process maintaining the WEOC pool in the mineral soil appears to be the microbial release of water‐soluble compounds from the SOM, which alters in time‐scales of decades to centuries.  相似文献   

4.
The relative contributions of sources of carbon in soils, such as throughfall, litter, roots, microbial decay products and stable organic fractions, to dissolved organic C are controversial. To identify the origin of dissolved organic C, we made use of a 4‐year experiment where spruce and beech, growing on an acidic loam and on a calcareous sand, were exposed to increased CO2 that was depleted in 13C. We traced the new C inputs from trees into dissolved organic C, into water‐extractable organic C, and into several particle‐size fractions. In addition, we incubated the labelled soils for 1 year and measured the production of dissolved organic C and CO2 from new and old soil C. In the soil solutions of the topsoil, the dissolved organic C contained only 5–10% new C from the trees. The δ13C values of dissolved organic C resembled those of C pools smaller than 50 µm, which strongly suggests that the major source of dissolved organic C was humified old C. Apparently, throughfall, fresh litter and roots made only minor contributions to dissolved organic C. Water‐extractable organic C contained significantly larger fractions of new C than did the natural dissolved organic C (25–30%). The δ13C values of the water‐extractable organic C were closely correlated with those of sand fractions, which consisted of little decomposed organic carbon. The different origin of dissolved and water‐extractable organic C was also reflected in a significantly larger molar UV absorptivity and a smaller natural 13C abundance of dissolved organic C. This implies that the sampling method strongly influences the characteristics and sources of dissolved organic C. Incubation of soils showed that new soil C was preferentially respired as CO2 and only a small fraction of new C was leached as dissolved organic C. Our results suggest that dissolved organic C is produced during incomplete decomposition of recalcitrant native C in the soils, whereas easily degradable new components are rapidly consumed by microbes and thus make only a minor contribution to the dissolved C fraction.  相似文献   

5.
Two processes contribute to changes of the δ13C signature in soil pools: 13C fractionation per se and preferential microbial utilization of various substrates with different δ13C signature. These two processes were disentangled by simultaneously tracking δ13C in three pools - soil organic matter (SOM), microbial biomass, dissolved organic carbon (DOC) - and in CO2 efflux during incubation of 1) soil after C3-C4 vegetation change, and 2) the reference C3 soil.The study was done on the Ap horizon of a loamy Gleyic Cambisol developed under C3 vegetation. Miscanthus giganteus - a perennial C4 plant - was grown for 12 years, and the δ13C signature was used to distinguish between ‘old’ SOM (>12 years) and ‘recent’ Miscanthus-derived C (<12 years). The differences in δ13C signature of the three C pools and of CO2 in the reference C3 soil were less than 1‰, and only δ13C of microbial biomass was significantly different compared to other pools. Nontheless, the neglecting of isotopic fractionation can cause up to 10% of errors in calculations. In contrast to the reference soil, the δ13C of all pools in the soil after C3-C4 vegetation change was significantly different. Old C contributed only 20% to the microbial biomass but 60% to CO2. This indicates that most of the old C was decomposed by microorganisms catabolically, without being utilized for growth. Based on δ13C changes in DOC, CO2 and microbial biomass during 54 days of incubation in Miscanthus and reference soils, we concluded that the main process contributing to changes of the δ13C signature in soil pools was preferential utilization of recent versus old C (causing an up to 9.1‰ shift in δ13C values) and not 13C fractionation per se.Based on the δ13C changes in SOM, we showed that the estimated turnover time of old SOM increased by two years per year in 9 years after the vegetation change. The relative increase in the turnover rate of recent microbial C was 3 times faster than that of old C indicating preferential utilization of available recent C versus the old C.Combining long-term field observations with soil incubation reveals that the turnover time of C in microbial biomass was 200 times faster than in total SOM. Our study clearly showed that estimating the residence time of easily degradable microbial compounds and biomarkers should be done at time scales reflecting microbial turnover times (days) and not those of bulk SOM turnover (years and decades). This is necessary because the absence of C reutilization is a prerequisite for correct estimation of SOM turnover. We conclude that comparing the δ13C signature of linked pools helps calculate the relative turnover of old and recent pools.  相似文献   

6.
An incubation experiment was carried out to examine the N‐immobilizing effect of sugarcane filter cake (C : N = 12.4) and to prove whether mixing it with compost (C : N = 10.5) has any synergistic effects on C and N mineralization after incorporation into the soil. Approximately 19% of the compost‐C added and 37% of the filter cake–C were evolved as CO2, assuming that the amendments had no effects on the decomposition of soil organic C. However, only 28% of the added filter cake was lost according to the total‐C and δ13C values. Filter cake and compost contained initially significant concentrations of inorganic N, which was nearly completely immobilized between day 7 and 14 of the incubation in most cases. After day 14, N remineralization occurred at an average rate of 0.73 µg N (g soil)–1 d–1 in most amendment treatments, paralleling the N mineralization rate of the nonamended control without significant difference. No significant net N mineralization from the amendment N occurred in any of the amendment treatments in comparison to the control. The addition of compost and filter cake resulted in a linear increase in microbial biomass C with increasing amounts of C added. This increase was not affected by differences in substrate quality, especially the three times larger content of K2SO4‐extractable organic C in the sugarcane filter cake. In most amendment treatments, microbial biomass C and biomass N increased until the end of the incubation. No synergistic effects could be observed in the mixture treatments of compost and sugarcane filter cake.  相似文献   

7.
A deeper understanding of the contribution of carbon (C) released by plant roots (rhizodeposition) to soil organic matter (SOM) can help to increase our knowledge of global C-cycling. These insights can eventually lead to sustainable management of SOM especially in agricultural systems. This study was conducted to determine the fate of 13C labelled rhizodeposit-C of maize and wheat plants. They were grown in a greenhouse in permeable nylon bags filled with upper soil material from two agricultural soils of the same location, but with different crop yields. The bags were placed into pots, which were also filled with soil surrounding the bags. Soil inside the bags was considered as rhizosphere soil, wheras the one outside the bags represented bulk soil. The contributions of rhizodeposits to water extractable organic carbon (WEOC), microbial biomass-C (MB-C), CO2-C evolution, and total organic carbon (Corg) were investigated during a 7-week growing period. The WEOC, MB-C, CO2-C, Corg contents and the respective δ13C values were determined regularly, and a newly developed method for determining δ13C values in soil extracts was applied.In both soils, regardless of crop yield potential, significant incorporation of rhizodeposition-derived C was observed in the MB-C, CO2-C, and Corg pool, but not in the WEOC. The pattern of C incorporation into the different pools was the same for both soils with both plants, and rhizodeposit-derived C was recovered in the order MB-C<Corg<CO2-C. This showed that rhizodeposits were mainly respired, but since Corg was the second largest pool of the overall balances, they were also stabilized in the soils at least in the short term. It is suggested that the increased SOM mineralization observed in this study (positive priming effects) was probably induced by C exchange processes between the soil matrix and soluble rhizodeposits. Moreover, soluble rhizodeposit-C was detected in MB-C and CO2-C evolved outside the direct root zone, showing the availability of these C-components in the bulk soil.  相似文献   

8.
Elevated atmospheric carbon dioxide (CO2) levels generally stimulate carbon (C) uptake by plants, but the fate of this additional C largely remains unknown. This uncertainty is due in part to the difficulty in detecting small changes in soil carbon pools. We conducted a series of long-term (170-330 days) laboratory incubation experiments to examine changes in soil organic matter pool sizes and turnover rates in soil collected from an open-top chamber (OTC) elevated CO2 study in Colorado shortgrass steppe. We measured concentration and isotopic composition of respired CO2 and applied a two-pool exponential decay model to estimate pool sizes and turnover rates of active and slow C pools. The active and slow C pools of surface soils (5-10 cm depth) were increased by elevated CO2, but turnover rates of these pools were not consistently altered. These findings indicate a potential for C accumulation in near-surface soil C pools under elevated CO2. Stable isotopes provided evidence that elevated CO2 did not alter the decomposition rate of new C inputs. Temporal variations in measured δ13C of respired CO2 during incubation probably resulted mainly from the decomposition of changing mixtures of fresh residue and older organic matter. Lignin decomposition may have contributed to declining δ13C values late in the experiments. Isotopic dynamics during decomposition should be taken into account when interpreting δ13C measurements of soil respiration. Our study provides new understanding of soil C dynamics under elevated CO2 through the use of stable C isotope measurements during microbial organic matter mineralization.  相似文献   

9.
A study was carried out in order to establish the relationship between the water extractable organic carbon (WEOC) content of soils and soil microbial activity, and to determine how variations in the extraction procedure might influence the quantity of WEOC recovered. Concentrations of WEOC were determined in soils taken from 12 different sites in the south east of Scotland, using a procedure in which samples were shaken with distilled water, centrifuged at 5000g and then filtered through 0.45 μm Millipore filters. Filtration resulted in between 30 and 400 μg C g−1 being extracted using this procedure and the concentration of WEOC in the resultant extracts correlated with soil microbial production of CO2 and dehydrogenase activity (P<0.001). Without filtration, although more WEOC was extracted (between 31 and 716 μg C g−1), there was no significant correlation with biological activity. There was also no correlation between WEOC and nitrous oxide release during the incubations. Centrifugation at 20,000g for at least 10 min prior to filtration was required to remove particulate organic materials. Storage of samples at 4 °C or for up to 1 week or freezing for up to 3 months was not found to have a large influence on the concentration of WEOC in extracts, although amounts increased with soil:extractant ratio and increasing extraction time (from 15 to 60 min).  相似文献   

10.
Abstract

Using an Ochrept soil of a forest at climax stage or of an arable site at Kita‐Ibaraki, a city in central Japan, the rates of carbon dioxide (CO2)‐carbon (C) evolution, the amounts of microbial biomass carbon (MBC) and the amounts of dissolved organic carbon (DOC) were measured in a laboratory with special reference to the incubation temperature and the soil water content. The rates of CO2‐C evolution increased exponentially with increase in the incubation temperature in the range of 4–40°C. The temperature coefficients (Q10) were 2.0 for the forest and 1.9 for the arable soil. The amounts of MBC were almost constant of 980 μg g‐1 soil in the incubation temperature up to 25°C for the forest, and 340 μg g‐1 soil in the incubation temperature up to 31 °C for the arable soil. The amounts of DOC in soil solutions were almost constant at 3.1 μg g‐1 soil in the incubation temperature up to 25°C for the forest, and 3.8 μg g‐1 soil in the incubation temperature up to 31°C for the arable soil. The rates of CO2‐C evolution and the amounts of DOC increased with increase in soil water content (% of soil dry weight) up to 91% for the forest or up to 26% for the arable soil. However, the rates of CO2‐C evolution and the amounts of DOC were almost constant within soil water content in the range of 91–160% or 26–53%, respectively. The amounts of MBC of the forest or arable soil were almost constant over a wide range of soil water content in the range of 41–220% or 8–73%, respectively. The rates of CO2‐C evolution of both the forest and the arable soils were highly correlated with the amounts of DOC, but not with the amounts of MBC, under laboratory conditions in the case that the amounts of DOC were changed by various treatments. The regression equation,  相似文献   

11.
Determination of the labile soil carbon (C) and nitrogen (N) fractions and measurement of their isotopic signatures (δ13C and δ15N) has been used widely for characterizing soil C and N transformations. However, methodological questions and comparison of results of different authors have not been fully solved. We studied concentrations and δ13C and δ15N of salt‐extractable organic carbon (SEOC), inorganic (N–NH4+ and N–NO3?) and organic nitrogen (SEON) and salt‐extractable microbial C (SEMC) and N (SEMN) in 0.05 and 0.5 m K2SO4 extracts from a range of soils in Russia. Despite differences in acidity, organic matter and N content and C and N availability in the studied soils, we found consistent patterns of effects of K2SO4 concentration on C and N extractability. Organic C and N were extracted 1.6–5.5 times more effectively with 0.5 m K2SO4 than with 0.05 m K2SO4. Extra SEOC extractability with greater K2SO4 concentrations did not depend on soil properties within a wide range of pH and organic matter concentrations, but the effect was more pronounced in the most acidic and organic‐rich mountain Umbrisols. Extractable microbial C was not affected by K2SO4 concentrations, while SEMN was greater when extracted with 0.5 m K2SO4. We demonstrate that the δ13C and δ15N values of extractable non‐microbial and microbial C and N are not affected by K2SO4 concentrations, but use of a small concentration of extract (0.05 m K2SO4) gives more consistent isotopic results than a larger concentration (0.5 m ).  相似文献   

12.
Amino sugars are useful indicators for the accumulation of microbial residues. A 14-day incubation experiment with C4 and C3 sucrose additions was carried out to investigate the relationships between amino sugar-specific shifts in δ13C values and those of CO2 production, microbial biomass C, K2SO4 extractable C and soil organic C (SOC). High performance anion exchange chromatography (HPAEC-IRMS) was able to measure amino-sugar specific δ13C values for muramic acid (MurN), galactosamine (GalN), and glucosamine (GlcN) in the range of natural abundance. At day 7, the initial application of C4 sucrose significantly increased the δ13C value of MurN by 1.0‰ in comparison with the non-amended control treatment, whereas that of GalN and GlcN remained unchanged. This significant increase had disappeared by day 14. This means that the HPAEC-IRMS method is not useful for short-term incubation experiments in the natural abundance range, as the pool size, especially of GalN and GlcN, was too large for a significant response in δ13C values. The δ13C values significantly decreased in the order MurN (−23.2‰) > GalN (−25.7‰) > GlcN (−26.5‰) in the control treatment. Similar δ13C values were measured in GlcN, microbial biomass C, and SOC. MurN exhibited δ13C values similar to the K2SO4 extractable fraction. These results may be caused by differences in the access of bacteria and fungi to different SOC fractions or differences in metabolic fractionation in bacteria and fungi. C3 sucrose application without further nutrient supply seven days after C4 sucrose application together with N and P led to strong mineralization of freshly formed microbial residues.  相似文献   

13.
The availability of C and N to the soil microbial biomass is an important determinant of the rates of soil N transformations. Here, we present evidence that changes in C and N availability affect the 15N natural abundance of the microbial biomass relative to other soil N pools. We analysed the 15N natural abundance signature of the chloroform‐labile, extractable, NO3–, NH4+ and soil total N pools across a cattle manure gradient associated with a water reservoir in semiarid, high‐desert grassland. High levels of C and N in soil total, extractable, NO3–, NH4+ and chloroform‐labile fractions were found close to the reservoir. The δ15N value of chloroform‐labile N was similar to that of extractable (organic + inorganic) N and NO3– at greater C availability close to the reservoir, but was 15N‐enriched relative to these N‐pools at lesser C availability farther away. Possible mechanisms for this variable 15N‐enrichment include isotope fractionation during N assimilation and dissimilation, and changes in substrate use from a less to a more 15N‐enriched substrate with decreasing C availability.  相似文献   

14.
A detailed understanding of the processes that contribute to the δ13C value of respired CO2 is necessary to make links between the isotopic signature of CO2 efflux from the soil surface and various sources within the soil profile. We used density fractionation to divide soils from two forested sites that are a part of an ongoing detrital manipulation experiment (the Detrital Input and Removal Treatments, or DIRT project) into two soil organic matter pools, each of which contributes differently to total soil CO2 efflux. In both sites, distinct biological pools resulted from density fractionation; however, our results do not always support the concept that the light fraction is readily decomposable whereas the heavy fraction is recalcitrant. In a laboratory incubation following density fractionation we found that cumulative respiration over the course of the incubation period was greater from the light fraction than from the heavy fraction for the deciduous site, while the opposite was true for the coniferous site.Use of stable isotopes yielded insight as to the nature of the density fractions, with the heavy fraction solids from both forests isotopically enriched relative to those of the light fraction. The isotopic signature of respired CO2, however, was more complicated. During incubation of the fractions there was an initial isotopic depletion of the respired CO2 compared to the substrate for both soil fractions from both forests. Over time for both fractions of both soils the respired δ13C reflected more closely the initial substrate value; however, the transition from depleted to enriched respiration relative to substrate occurs at a different stage of decomposition depending on site and substrate recalcitrance. We found a relationship between cumulative respiration during the incubation period and the duration of the transition from isotopically depleted to enriched respiration in the coniferous site but not the deciduous site. Our results suggest that a shift in microbial community or to dead microbial biomass as a substrate could be responsible for the transition in the isotopic signature of respired CO2 during decomposition. It is likely that a combination of organic matter quality and isotopic discrimination by microbes, in addition to differences in microbial community composition, contribute to the isotopic signature of different organic matter fractions. It is apparent that respired δ13CO2 cannot be assumed to be a direct representation of the substrate δ13C. Detailed knowledge of the soil characteristics at a particular site is necessary to interpret relationships between the isotopic values of a substrate and respired CO2.  相似文献   

15.
Partitioning the root‐derived CO2 efflux from soil (frequently termed rhizosphere respiration) into actual root respiration (RR, respiration by autotrophs) and rhizomicrobial respiration (RMR, respiration by heterotrophs) is crucial in determining the carbon (C) and energy balance of plants and soils. It is also essential in quantifying C sources for rhizosphere microorganisms and in estimation of the C contributing to turnover of soil organic matter (SOM), as well as in linking net ecosystem production (NEP) and net ecosystem exchange (NEE). Artificial‐environment studies such as hydroponics or sterile soils yield unrealistic C‐partitioning values and are unsuitable for predicting C flows under natural conditions. To date, several methods have been suggested to separate RR and RMR in nonsterile soils: 1) component integration, 2) substrate‐induced respiration, 3) respiration by excised roots, 4) comparison of root‐derived 14CO2 with rhizomicrobial 14CO2 after continuous labeling, 5) isotope dilution, 6) model‐rhizodeposition technique, 7) modeling of 14CO2 efflux dynamics, 8) exudate elution, and 9) δ13C of CO2 and microbial biomass. This review describes the basic principles and assumptions of these methods and compares the results obtained in the original papers and in studies designed to compare the methods. The component‐integration method leads to strong disturbance and non‐proportional increase of CO2 efflux from different sources. Four of the methods (5 to 8) are based on the pulse labeling of shoots in a 14CO2 atmosphere and subsequent monitoring of 14CO2 efflux from the soil. The model‐rhizodeposition technique and exudate‐elution procedure strongly overestimate RR and underestimate RMR. Despite alternative assumptions, isotope dilution and modeling of 14CO2‐efflux dynamics yield similar results. In crops and grasses (wheat, ryegrass, barley, buckwheat, maize, meadow fescue, prairie grasses), RR amounts on average to 48±5% and RMR to 52±5% of root‐derived CO2. The method based on the 13C isotopic signature of CO2 and microbial biomass is the most promising approach, especially when the plants are continuously labeled in 13CO2 or 14CO2 atmosphere. The “difference” methods, i.e., trenching, tree girdling, root‐exclusion techniques, etc., are not suitable for separating the respiration by autotrophic and heterotrophic organisms because the difference methods neglect the importance of microbial respiration of rhizodeposits.  相似文献   

16.
Rock fragments in soil can contain significant amounts of organic carbon. We investigated the nature and dynamics of organic matter in rock fragments in the upper horizons of a forest soil derived from sandstone and compared them with the fine earth fraction (<2 mm). The organic C content and its distribution among humic, humin and non‐humic fractions, as well as the isotopic signatures (Δ14C and δ13C) of organic carbon and of CO2 produced during incubation of samples, all show that altered rock fragments contain a dynamic component of the carbon cycle. Rock fragments, especially the highly altered ones, contributed 4.5% to the total organic C content in the soil. The bulk organic matter in both fine earth and highly altered rock fragments in the A1 horizon contained significant amounts of recent C (bomb 14C), indicating that most of this C is cycled quickly in both fractions. In the A horizons, the mean residence times of humic substances from highly altered rock fragments were shorter than those of the humic substances isolated in the fine earth. Values of Δ14C of the CO2 produced during basal respiration confirmed the heterogeneity, complexity and dynamic nature of the organic matter of these rock fragments. The weak 14C signatures of humic substances from the slightly altered rock fragments confirmed the importance of weathering in establishing and improving the interactions between rock fragments and surrounding soil. The progressive enrichment in 13C from components with high‐14C (more recent) to low‐14C (older) indicated that biological activity occurred in both the fine and the coarse fractions. Hence the microflora utilizes energy sources contained in all the soil compartments, and rock fragments are chemically and biologically active in soil, where they form a continuum with the fine earth.  相似文献   

17.
During the first few days after rewetting of an air-dried soil (AD-RW), microbial activity increases compared to that in the original moist soil, causing increased mineralisation (a flush) of soil organic carbon (C) and other nutrients. The AD-RW flush is believed to be derived from the enhanced mineralisation of both non-biomass soil organic matter (due to its physical release and enhanced availability) and microbial biomass killed during drying and rewetting. Our aim was to determine the effects of AD-RW on the mineralisation of soil organic matter and microbial biomass during and after repeated AD-RW cycles and to quantify their proportions in the CO2-C flushes that resulted. To do this, a UK grassland soil was amended with 14C-labelled glucose to label the biomass and then given five AD-RW cycles, each followed by 7 d incubation at 25 °C and 50% water holding capacity. Each AD-RW cycle increased the amount of CO2-C evolved (varying from 83 to 240 μg g−1 soil), compared to the control with, overall, less CO2-C being evolved as the number of AD-RW cycles increased. In the first cycle, the amount of biomass C decreased by 44% and microbial ATP by 70% while concentrations of extractable C nearly doubled. However, all rapidly recovered and within 1.3 d after rewetting, biomass C was 87% and ATP was 78% of the initial concentrations measured prior to air-drying. Similarly, by 2 d, extractable organic C had decreased to a similar concentration to the original. After the five AD-RW cycles, the amounts of total and 14C-labelled biomass C remaining in the soil accounted for 60 and 40% of those in the similarly incubated control soil, respectively. Soil biomass ATP concentrations following the first AD-RW cycle remained remarkably constant (ranging from about 10 to 14 μmol ATP g−1 biomass C) and very similar to the concentration in the fresh soil prior to air-drying. We developed a simple mathematical procedure to estimate the proportion of CO2-C derived from biomass C and non-biomass C during AD-RW. From it, we estimate that, over the five AD-RW cycles, about 60% of the CO2-C evolved came from mineralisation of non-biomass organic C and the remainder from the biomass C itself.  相似文献   

18.
The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with 14C-labelled ryegrass for 1 year. The plants reduced the loss of 14CO2, from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little 14C label being associated with the shoot. Chemical fractionation of the root showed that the 14C was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of 14CO2, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.  相似文献   

19.
Both plant species and CO2 concentration can potentially affect rhizodeposition and consequently soil microbial activity and community composition. However, the effect differs based on plant developmental stage. We focused on the effect of three plant species (forbs, grasses, and N2‐fixers) at an early stage of development on root C deposition and fate, soil organic matter (SOM) mineralization and soil microbial community composition at ambient (aCO2) and elevated (eCO2) CO2 levels. Plants were grown from seed, under continuous 13C‐labelling atmospheres (400 and 800 µmol mol?1 CO2), in grassland soil for three weeks. At the end of the growth period, soil respiration, dissolved organic C (DOC) and phospholipid fatty acid (PLFA) profiles were quantified and isotopically partitioned into root‐ and soil‐derived components. Root‐derived DOC (0.53 ± 0.34 and 0.26 ± 0.29 µg mL soil solution?1) and soil‐derived CO2 (6.14 ± 0.55 and 5.04 ± 0.44 µg CO2‐C h?1) were on average two times and 22% higher at eCO2 than at aCO2, respectively. Plant species differed in exudate production at aCO2 (0.11 ± 0.11, 0.10 ± 0.18, and 0.58 ± 0.58 µg mL soil solution?1 for Plantago, Festuca, and Lotus, respectively) but not at eCO2 (0.20 ± 0.28, 0.66 ± 0.32, and 0.75 ± 0.15 µg mL soil solution?1 for Plantago, Festuca, and Lotus, respectively). However, no differences among plant species or CO2 levels were apparent when DOC was expressed per gram of roots. Relative abundance of PLFAs did not differ between the two CO2 levels. A higher abundance of actinobacteria and G‐positive bacteria occurred in unplanted (8.07 ± 0.48 and 24.36 ± 1.18 mol%) and Festuca‐affected (7.63 ± 0.31 and 23.62 ± 0.69 mol%) soil than in Plantago‐ (7.04 ± 0.36 and 23.41 ± 1.13 mol%) and Lotus‐affected (7.24 ± 0.17 and 23.13 ± 0.52 mol%) soil. In conclusion, the differences in root exudate production and soil respiration are mainly caused by differences in root biomass at an early stage of development. However, plant species evidently produce root exudates of varying quality affecting associated microbial community composition.  相似文献   

20.
The characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air‐dried soils to compare three extraction methods for water‐extractable organic matter (WEOM) as follows: (i) pressurised hot‐water‐extractable organic carbon (PH‐WEOC), a percolation at high pressure and temperature; (ii) water‐extractable organic carbon (WEOC), a 1‐hour end‐over shaking; and (iii) leaching‐extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48‐day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH‐WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic‐like fluorophores and a tryptophan‐like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH‐WEOC and LEOC while WEOC extracts were depleted in humic‐like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method‐induced differences in WEOM quality vary with soil characteristics.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号