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1.
Endogeic earthworm activities can strongly influence soil structure. Although soil microorganisms are thought to be central to earthworm-facilitated aggregate formation, how and where within the soil matrix earthworm-facilitated influences on soil microbial communities are manifested is poorly defined. In this study we used 16S rRNA gene-based terminal restriction fragment polymorphism (T-RFLP) analyses to examine bacterial communities associated with different aggregate size fractions (macroaggregates, microaggregates-within-macroaggregates and inner-microaggregates-within-macroaggregates) of soils incubated for 28 d with and without earthworms. We hypothesized that bacterial communities in different soil aggregate size fractions are differentially influenced by earthworm activities. Our results indicate significantly enhanced aggregate formation (both macroaggregates and microaggregates within macroaggregates) in earthworm-worked soils relative to soils receiving only plant litter. Although significant differences were found between bacterial communities of earthworm and litter-only treatments for all soil fractions, communities associated with earthworm-worked macroaggregate fractions exhibited the least similarity to all other soil fractions regardless of treatment. In addition to differences in terminal restriction fragment (T-RF) size distributions, T-RFLP profiles of earthworm-worked soil macroaggregates had significantly fewer T-RF sizes, further suggesting less species evenness and more extensive alteration of bacterial communities within this fraction. These findings suggest that, due to rapid occlusion of organic materials, microbial communities associated with microaggregates-within-macroaggregates formed during or shortly after passage through the earthworm gut are relatively inactive, and therefore change relatively little over time compared to macroaggregate populations as a whole.  相似文献   

2.
We determined if the structure and function of microbial communities associated with different aggregate size classes was influenced when the aggregate formation occurred under either nitrogen (N) limitation (straw only incubation treatment) or carbon (C) limitation (straw+N incubation treatment). Using a combination of community-level physiological (BD Oxygen Biosensor assay) and molecular (terminal restriction fragment length polymorphism; T-RFLP) profiling methods, we found differences in both microbial community composition and the physiological response of these communities between different aggregate size classes. The response of fungal and bacterial communities to ‘straw only’ and ‘straw+N’ treatments differed in that bacterial community composition was affected by the treatments, whereas fungal community composition was not. The magnitude of change in the bacterial community response increased with decreasing aggregate size. However, there were no significant differences in the mean bacterial community richness (number of different terminal restriction fragments; TRFs) between different aggregate size classes for the two treatments. In general, microbial communities associated with larger aggregate size fractions (large and small macroaggregates) were found to have a significantly faster respiratory response than the communities associated with microaggregates. Application of the fungal inhibitor cycloheximide resulted in a significant reduction in the utilization of cellulose, chitin, mannose, xylan, and xylose by the microbial communities associated with all aggregate size classes, indicating that fungi are significant contributors to the utilization of these compounds. Our results demonstrate that the BD Oxygen Biosensor assay offers a valuable new tool for community level physiological profiling. When used in combination with census-based methods such as T-RFLP, a greater level of resolution can be achieved.  相似文献   

3.
Cultivation is known to influence the organic matter status and structural stability of soil. We investigated the effects of 69 yr of cultivation on the nature, distribution and activity of microbial biomass (MB) in different aggregate size classes of an Orthic Brown Chernozemic soil. Cultivation decreased MB content, its activity and enzyme activity in soil. Microaggregate (<0.25mm) size classes in both native and cultivated soils contained lower organic-C, MB-C, fungal biomass, arylsulfatase, acid phosphatase and respiratory activities as compared to macroaggregates. However, the negative effects of cultivation were more pronounced on macroaggregate size classes. Nutrient ratios of both whole aggregates and microbial biomass were narrower in aggregates from cultivated soil as compared to native soil. In both native and cultivated soils, mineralization of C. N and S was greater in macroaggregates as compared to that in microaggregates. The greatest effect of cultivation on nutrient and microbial characteristics was observed in the 0.25 to 1.00 mm dia size classes. These results suggest that microbial biomass, especially fungal biomass, plays an important role in the formation of macroaggregates and is the labile organic matter that serves as the primary source of C and nutrients released following cultivation.  相似文献   

4.
The aggregate formation and stability are controlled by the dynamics of soil organic matters (SOM), but how it is related to SOM chemical composition within different‐sized aggregates is largely unknown during manure fertilization. In this study, the variations of intra‐aggregate organic carbon (OC), including intra‐particulate organic matter (iPOM) and mineral‐associated organic matter, were quantitatively and qualitatively analysed, and then, their effects on aggregate formation and stability were assessed under four treatments: control (CK), mineral fertilizer (NPK), reduced manure (30%M) and manure fertilizers (M). Manure application (M) significantly increased macroaggregate proportion, mean weight diameter (MWD), and OC contents within different‐sized aggregates compared to CK, NPK, and 30%M. The OC accumulation of macroaggregate in M was attributed to OC content increase in silt plus clay subfraction rather than iPOM with more labile organic groups; oppositely, in microaggregate it was located in the relatively stable fine iPOM. The macroaggregate formation and stability were controlled by the fine iPOM within macroaggregates, whose abundant polysaccharide‐C and aliphatic‐C after manure fertilization advanced the microbial growth except for Gram‐positive bacteria, which further promoted macroaggregate formation and stability. The free silt plus clay fraction also affected macroaggregate formation and stability, and its polysaccharide‐C derived from microorganisms or decomposing SOM was positively associated with MWD and macroaggregate proportion. Because polysaccharide‐C can be easily associated with mineral particles, further improving micro‐ or macroaggregation. We conclude that continuous manure fertilization could increase labile SOM accumulation within aggregates and then facilitate microbial growth, which collectively are responsible for aggregate formation and stabilization.  相似文献   

5.
It is increasingly believed that substantial soil organic carbon (SOC) can be sequestered in conservation tillage system by manipulating the functional groups of soil biota. Soil aggregates of different size provide diverse microhabitats for soil biota and consequently influence C sequestration. Our objective was to evaluate the contributions of soil biota induced by tillage systems to C sequestration among different aggregate size fractions. Soil microbial and nematode communities were examined within four aggregate fractions: large macroaggregates (>2 mm), macroaggregates (2–1 mm), small macroaggregates (1–0.25 mm) and microaggregates (<0.25 mm) isolated from three tillage systems: no tillage (NT), ridge tillage (RT) and conventional tillage (CT) in Northeast China. Soil microbial and nematode communities varied across both tillage systems and aggregate fractions. The activity and abundance of microbes and nematodes were generally higher under NT and RT than under CT. Among the four aggregate fractions, soil microbial biomass and diversity were higher in microaggregates, while soil nematode abundance and diversity were higher in large macroaggregates. Structural equation modelling (SEM) revealed that the linkage between microbial and nematode communities and their contributions to soil C accumulation in >1 mm aggregate fractions were different from those in <1 mm aggregate fractions. Higher abundance of arbuscular mycorrhizal fungi (AMF) could enhance C retention within >1 mm aggregates, while more gram-positive bacteria and plant-parasitic nematodes might increase C accumulation within <1 mm aggregates. Our findings suggested that the increase in microbial biomass and nematode abundance and the alteration in their community composition at the micro-niche within aggregates could contribute to the higher C sequestration in conservation tillage systems (NT and RT).  相似文献   

6.
The hypothesis that soil light fraction and heavy fraction harbor distinct eubacterial communities and have differing numbers and sizes of bacterial cells was tested in three agronomic cropping systems. This hypothesis would imply that these soil fractions are distinct microbial habitats. Shoot residue and rhizosphere soil were also included in the analysis. Terminal restriction fragment length polymorphism (T-RFLP) of 16S ribosomal DNA was used to assay eubacterial community structure. T-RFLP profiles were affected by both soil fraction and cropping system, accounting for 35-50% of the variance in the profiles. T-RFLP profiles separated samples into two distinct eubacterial habitats: soil heavy fraction, which includes the mineral particles and associated humified organic matter, and soil light fraction/shoot residue and rhizosphere, which includes particulate soil organic matter. Differences were not based on organic C content of fractions alone; T-RFLP profiles were also differentiated by cropping system and by rhizosphere versus light fraction/shoot residue. Heavy fraction communities had the least amount of random variability in T-RFLP profiles, resulting in the clearest cropping system effects, while rhizosphere and shoot residue communities were the most variable. Profiles from organically managed corn soil were more variable than for either conventionally managed corn or alfalfa. The log number of bacterial cells per gram fraction was affected by soil fraction but not cropping system, being highest in the light fraction. The percentage of cells >0.18 μm3 was also greater in the light fraction than in other fractions. While bacterial cell density was generally correlated with C content of the soil fraction, heavy fraction did have a significantly greater number of cells per μg C than other soil fractions. The results show that habitat diversity in soil, related both to the amounts and types of organic matter, as well as other potential factors, are important in maintaining the high soil bacterial species diversity and evenness that is found in soil.  相似文献   

7.
《Soil biology & biochemistry》2001,33(12-13):1599-1611
Aggregate dynamics and their relationship to the microbial community have been suggested as key factors controlling SOM dynamics. Dry–wet (DW) cycles are thought to enhance aggregate turnover and decomposition of soil organic matter (SOM), particularly in tilled soils. The objective of this study was to evaluate the effects of DW cycles on aggregate stability, SOM dynamics, and fungal and bacterial populations in a Weld silt loam soil (Aridic Paleustoll). Samples, taken from 250 μm sieved air-dried soil (i.e. free of macroaggregates > 250 μm), were incubated with 13C-labeled wheat residue. In one set of soil samples, fungal growth was suppressed using a fungicide (Captan) in order to discern the effect of dry–wet cycles on fungal and bacterial populations. Aggregate formation was followed during the first 14 d of incubation. After this period, one set of soil samples was subjected to four DW cycles, whereas another set, as a control, was kept at field capacity (FC). Over 74 d, total and wheat-derived respiration, size distribution of water stable aggregates and fungal and bacterial biomass were measured. We determined native and labeled C dynamics of three particulate organic matter (POM) fractions related to soil structure: the free light fraction (LF), and the coarse (250–2000 μm) and fine (53–250 μm) intra-aggregate POM fraction (iPOM). In the fungicide treated soil samples, fungal growth was significantly reduced and no large macroaggregates (> 2 mm) were formed, whereas without addition of fungicide, fungi represented the largest part of the microbial biomass (66%) and 30% of the soil dry weight was composed of large macroaggregates. During macroaggregate formation, labeled free LF-C significantly decreased whereas labeled coarse iPOM-C increased, indicating that macroggregates are formed around fresh wheat residue (free LF), which is consequently incorporated and becomes coarse iPOM. The first drying and wetting event reduced the amount of large macroaggregates from 30 to 21% of the total soil weight. However, macroaggregates became slake-resistant after two dry-wet cycles. Fine iPOM-C was significantly lower in soil after two dry–wet cycles compared to soil kept at FC. We conclude that more coarse iPOM is decomposed into fine iPOM in macroaggregates not exposed to DW cycles due to a slower macroaggregate turnover. In addition, when macroaggregates, subjected to dry–wet cycles, became slake-resistant (d 44) and consequently macroaggregate turnover decreased, fine iPOM accumulated. In conclusion, differences in fine iPOM accumulation in DW vs. control macroaggregates are attributed to differences in macroaggregate turnover.  相似文献   

8.
To improve soil structure and take advantage of several accompanying ecological benefits, it is necessary to understand the underlying processes of aggregate dynamics in soils. Our objective was to quantify macroaggregate (> 250 μm) rebuilding in soils from loess (Haplic Luvisol) with different initial soil organic C (SOC) contents and different amendments of organic matter (OM) in a short term incubation experiment. Two soils differing in C content and sampled at 0–5 and 5–25 cm soil depths were incubated after macroaggregate destruction. The following treatments were applied: (1) control (without any addition), (2) OM1 (addition of OM: preincubated wheat straw [< 10 mm, C : N 40.6] at a rate of 4.1 g C [kg soil]–1), and (3) OM2 (same as (2) at a rate of 8.2 g C [kg soil]–1). Evolution of CO2 released from the treatments was measured continuously, and contents of different water‐stable aggregate‐size classes (> 250 μm, 250–53 μm, < 53 μm), microbial biomass, and ergosterol were determined after 7 and 28 d of incubation. Highest microbial activity was observed in the first 3 d after the OM application. With one exception, > 50% of the rebuilt macroaggregates were formed within the first 7 d after rewetting and addition of OM. However, the amount of organic C within the new macroaggregates was ≈ 2‐ to 3‐fold higher than in the original soil. The process of aggregate formation was still proceeding after 7 d of incubation, however at a lower rate. Contents of organic C within macroaggregates were decreased markedly after 28 d of incubation in the OM1 and OM2 treatments, suggesting that the microbial biomass (bacteria and fungi) used organic C within the newly built macroaggregates. Overall, the results confirmed for all treatments that macroaggregate formation is a rapid process and highly connected with the amount of OM added and microbial activity. However, the time of maximum aggregation after C addition depends on the soil and substrate investigated. Moreover, the results suggest that the primary macroaggregates, formed within the first 7 d, are still unstable and oversaturated with OM and therefore act as C source for microbial decomposition processes.  相似文献   

9.
Plant residues, living roots and microbial activity play an important role in aggregate formation and the stabilization of soil organic carbon (SOC), but their impact might differ among soils with different clay mineralogy. We investigated the effect of these organic agents on aggregation and SOC during a 76‐day incubation of 2‐mm sieved soil from an illitic Kastanozem and a kaolinitic Ferralsol, subjected to the following treatments: (i) control (no residue input or plant growth), (ii) residue input, (iii) living plants, and (iv) residue input and living plants. After 46 and 76 days, aggregate size distribution, aggregate‐associated SOC and microbial‐C were measured. In both soils, microbial‐C was less in the control than in the residue and/or plant treatments. After 46 days, new large macroaggregates (> 2000 µm) were formed in the control treatment of the kaolinitic soil, but not of the illitic soil. Control macroaggregates in the kaolinitic soil were formed out of silt and clay particles without accumulating C. Residue input and plant growth had a greater positive effect on macroaggregate formation in the illitic than in the kaolinitic soil. A stronger relation was found between microbial‐C and amount of large macroaggregates in the illitic than in the kaolinitic soil. We conclude that kaolinitic soils can rapidly form macroaggregates independent of biological processes due to physical or electrostatic interactions between the 1:1 clay minerals and oxides. However, biological processes led to stronger organic bonds between the illite compared with the kaolinite clay, resulting in more macroaggregates with long‐term stability in the illitic than in the kaolinitic soil.  相似文献   

10.
Changes in plant cover after afforestation induce variations in litter inputs and soil microbial community structure and activity, which may promote the accrual and physical-chemical protection of soil organic carbon (SOC) within soil aggregates. In a long-term experiment (20 years) we have studied the effects, on soil aggregation and SOC stabilization, of two afforestation techniques: a) amended terraces with organic refuse (AT), and b) terraces without organic amendment (T). We used the adjacent shrubland (S) as control. Twenty years after stand establishment, aggregate distribution (including microaggregates within larger aggregates), sensitive and slow organic carbon (OC) fractions, basal respiration in macroaggregates, and microbial community structure were measured. The main changes occurred in the top layer (0–5 cm), where: i) both the sensitive and slow OC fractions were increased in AT compared to S and T, ii) the percentage and OC content of microaggregates within macroaggregates (Mm) were higher in AT than in S and T, iii) basal respiration in macroaggregates was also higher in AT, and iv) significant changes in the fungal (rather than bacterial) community structure were observed in the afforested soils (AT and T) – compared to the shrubland soil. These results suggest that the increase in OC pools linked to the changes in microbial activity and fungal community structure, after afforestation, promoted the formation of macroaggregates – which acted as the nucleus for the formation and stabilization of OC-enriched microaggregates.  相似文献   

11.
《Applied soil ecology》2001,16(3):251-261
Reduced tillage of agricultural soils has been shown to result in greater macroaggregation, microbial biomass and microbial diversity. While it has been shown that macroaggregates contain more microbial biomass per unit soil mass than microaggregates, it is unclear how microbial diversity varies with soil aggregation. We investigated the functional diversity (catabolic potential) of bacteria, evaluated by calculating Shannon’s diversity index (H′), substrate richness (S) and substrate evenness (E) from potential substrate utilization patterns, in whole soil (i.e. not separated into different aggregate sizes) and aggregates of different sizes (2–4, 1–2, 0.5–1, 0.25–0.5, and 0.1–0.25 mm diameter) in loam and silt loam soils grown to barley and managed for 6 years under conventional tillage (CT) or zero tillage (ZT) systems in northern British Columbia. There were no significant tillage effects on bacterial diversity in whole soils. In soil aggregates, H′ and E were significantly higher under CT than under ZT on the loam at barley planting time, with no significant aggregate size effects. However, at barley-heading stage, all diversity indices in both soils were significantly higher under ZT than under CT, and they tended to increase with increasing aggregate size. Cluster analysis and principal component analysis of substrate utilization patterns also revealed differences in bacterial community structures between CT and ZT, but the substrates that were utilized differently between the two tillage systems were not the same between soil types or sampling times. The results during the cropping cycle imply that deterioration of soil structure is probably one factor that explains the adverse effects of soil tillage on soil microbial biomass and diversity.  相似文献   

12.
Understanding the influence of long-term crop management practices on the soil microbial community is critical for linking soil microbial flora with ecosystem processes such as those involved in soil carbon cycling. In this study, pyrosequencing and a functional gene array (GeoChip 4.0) were used to investigate the shifts in microbial composition and functional gene structure in a medium clay soil subjected to various cropping regimes. Pyrosequencing analysis showed that the community structure (β-diversity) for bacteria and fungi was significantly impacted among different cropping treatments. Functional gene array-based analysis revealed that crop rotation practices changed the structure and abundance of genes involved in C degradation. Significant correlations were observed between the activities of four enzymes involved in soil C degradation and the abundance of genes responsible for the production of respective enzymes, suggesting that a shift in the microbial community may influence soil C dynamics. We further integrated physical, chemical, and molecular techniques (qPCR) to assess relationships between soil C, microbial derived enzymes and soil bacterial community structure at the soil micro-environmental scale (e.g. within different aggregate-size fractions). We observed a dominance of different bacterial phyla within soil microenvironments which was correlated with the amount of C in the soil aggregates suggesting that each aggregate represents a different ecological niche for microbial colonization. Significant effects of aggregate size were found for the activity of enzymes involved in C degradation suggesting that aggregate size distribution influenced C availability. The influence of cropping regimes on microbial and soil C responses declined with decreasing size of soil aggregates and especially with silt and clay micro-aggregates. Our results suggest that long term crop management practices influence the structural and functional potential of soil microbial communities and the impact of crop rotations on soil C turnover varies between different sized soil aggregates. These findings provide a strong framework to determine the impact of management practices on soil C and soil health.  相似文献   

13.
《Applied soil ecology》2001,16(3):195-208
Soil structure mediates many biological and physical soil processes and is therefore an important soil property. Physical soil processes, such as aggregation, can be markedly influenced by both residue quality and soil microbial community structure. Three experiments were conducted to examine (i) the temporal dynamics of aggregate formation and the water stability of the obtained aggregates, (ii) the effect of residue quality on aggregation and microbial respiration, and (iii) the effect of fungi and bacteria on aggregation.In the first experiment, 250 μm sieved air-dried soil, mixed with wheat straw, was incubated for 14 days to allow formation of water-stable macroaggregates (>250 μm). Aggregate stability was measured by wet sieving after four different disruptive treatments: (i) soil at field capacity; (ii) soil air-dried and slowly wetted; (iii) soil air-dried and quickly wetted; (iv) 8 mm sieved soil, air-dried and immersed in water (slaking). After 14 days of incubation, maximum aggregation for soil sieved at field capacity was reached; however, these newly formed aggregates were not yet resistant to slaking.During the second experiment, the effect of low-quality residue (C/N: 108) (with or without extra mineral nitrogen) and high-quality residue (C/N: 19.7) (without extra mineral nitrogen) on macroaggregate formation and fungal and bacterial populations was tested. After 14 days, aggregation, microbial respiration, and total microbial biomass were not significantly different between the low-quality (minus mineral nitrogen) and high-quality residue treatment. However, fungal biomass was higher for the low-quality residue treatment compared to the high-quality residue treatment. In contrast, bacterial populations were favored by the high-quality residue treatment. Addition of mineral N in the low-quality residue treatment resulted in reduced macroaggregate formation and fungal biomass, but had no effect on bacterial biomass. These observations are not conclusive for the function of fungal and/or bacterial biomass in relation to macroaggregate formation. In order to directly discern the influence of soil microflora on aggregation, a third experiment was conducted in which a fungicide (captan) or bactericide (oxytetracycline) was applied to selectively suppress fungal or bacterial populations. The direct suppression of fungal growth by addition of fungicide led to reduced macroaggregate formation. However, suppression of bacterial growth by addition of bactericide did not lead to reduced macroaggregate formation. In conclusion, macroaggregate formation was positively influenced by fungal activity but was not significantly influenced by residue quality or bacterial activity.  相似文献   

14.
以贺兰山不同海拔植被下0—20,20—40 cm土层土壤为研究对象,分析不同粒径团聚体含量及团聚体稳定性随海拔升高的变化特征,探讨其与土壤理化性质之间的相关关系。结果表明:0—20 cm土层,0.25~0.053 mm团聚体为主要团聚体类型,随海拔增加,水稳性大团聚体(>0.25 mm)含量增加,<0.053 mm团聚体含量减少,表明土壤团聚体随海拔增加呈现由小粒径向大团聚体转变的趋势。20—40 cm土层,水稳性大团聚体含量在中海拔(2139 m)达到最高,占比为65.73%。平均重量直径(MWD)和几何平均直径(GMD)在0—20,20—40 cm土层均呈现随海拔升高先增加后减小的趋势,并在2139 m处达到峰值。不同海拔梯度土壤团聚体MWD和GMD与土壤有机碳(SOC)、全氮(TN)、全磷(TP)粉粒以及砂粒含量呈正相关,与黏粒含量、pH呈负相关。贺兰山不同海拔植被下土壤团聚体稳定性总体表现为中海拔>高海拔>低海拔,较高含量的大团聚体和土壤养分是团聚体稳定的关键要素。  相似文献   

15.
We hypothesized that nematode and microbial communities vary between soil aggregate fractions due to variations in physical and/or resource constraints associated with each fraction and that this, in turn, contributes to management impacts on whole soil food webs. Nematode and microbial communities were examined within three soil fractions: large macroaggregates (LM; >1000 μm), small macroaggregates (SM; 250-1000 μm) and inter-aggregate soil and space (IS; <250 μm) isolated from soils of four agricultural management systems: conventional tomato (CON), organic tomato (ORG), a minimum till grain-legume intercrop with continuous cover (CC) and an unmanaged riparian corridor (RC). Aggregate fractions appeared to influence nematode assemblages more than did management system. In general the IS and LM fractions contained higher densities of all nematode trophic groups than did SM. Management × fraction interactions for bacterivores and fungivores, however; suggested a non uniform trend across management systems. The IS fraction exhibited stronger trophic links, per the nematode structure index (SI), while the LM and SM fractions had more active fungal decomposition channels as indicated by the channel index (CI). Higher adult to juvenile ratios in the LM and IS than the SM fraction, and a positive correlation between nematode density in the IS fraction and the proportion of macroaggregates in the soil, indicated an association between soil structure and nematode distribution. Microbial communities varied across both aggregate fractions and management systems. Phospholipid fatty acid (PLFA) analysis suggested that the LM fraction contained greater microbial biomass, gram positive bacteria, and eukaryotes than the IS fraction, while SM contained intermediate PLFA associated with these groups. Total PLFA was greater under RC and ORG than under CC or CON. Total PLFA was positively correlated with % C in soil fractions while nematode abundance exhibited no such relationship. Our findings suggest that microbial communities are more limited by resource availability than by habitable pore space or predation, while nematode communities, although clearly resource-dependent, are better associated with habitable pore space for the soil fractions studied here.  相似文献   

16.
Our 1988 paper, describing the effects of cultivation on microbial biomass and activity in different aggregate size classes, brought together the ‘aggregate hierarchy theory’ and the ‘microbial biomass concept’. This enabled us to identify the relationships between microbial and microhabitat (aggregate) properties and organic matter distribution and explain some of their responses to disturbance. By combining biochemical and direct microscopy based quantification of microbial abundance with enzyme activities and process measurements, this study provided evidence for the role of microbial biomass (especially fungi) in macroaggregate dynamics and carbon and nutrient flush following cultivation. In the last ten years environmental genomic techniques have provided much new knowledge on bacterial composition in aggregate size fractions yet detailed information about other microbial groups (e.g. fungi, archaea and protozoa) is lacking.We now know that soil aggregates are dynamic entities – constantly changing with regard to their biological, chemical and physical properties and, in particular, their influences on plant nutrition and health. As a consequence, elucidation of the many mechanisms regulating soil C and nutrient dynamics demands a better understanding of the role of specific members of microbial communities and their metabolic capabilities as well as their location within the soil matrix (e.g. aggregates, pore spaces) and their reciprocal relationship with plant roots. In addition, the impacts of environment and soil type needs to be quantified at the microscale using, wherever possible, non-destructive ‘in situ’ techniques to predict and quantify the impacts of anthropogenic activities on soil microbial diversity and ecosystem level functions.  相似文献   

17.
Knowledge of the cycling and compartmentalization of soil C that influence C storage may lead to the development of strategies to increase soil C storage potentials. The objective of this study was to use soil hydrolases and soil aggregate fractionation to explore the relationship between C cycling activity and soil aggregate structure. The prairie chronosequence soils were native prairie (NP) and agricultural (AG) and tallgrass prairies restored from agriculture in 1979 (RP-79) and 1993 (RP-93). Assays for -glucosidase (E.C. 3.2.1.21) and N-acetyl--glucosaminidase (NAGase, EC 3.2.1.30) activities were conducted on four aggregate size fractions (>2 mm, 1–2 mm, 250 m–1 mm, and 2–250 m) from each soil. There were significantly greater amounts of >2-mm aggregates in the RP-79 and RP-93 soils compared to the NP and AG soils due to rapid C accumulation from native plant establishment. Activities for both enzymes (g PNP g–1 soil h–1) were greatest in the microaggregate (2–250 m) compared to the macroaggregate (>2 mm) fraction; however, microaggregates are a small proportion of each soil (<12%) compared to the macroaggregates (75%). The RP soils have a hierarchical aggregate system with most of the enzyme activity in the largest aggregate fractions. The NP and AG soils show no hierarchical structure based on aggregate C accretion and significant C enzyme activity in smaller aggregates. The distribution of enzyme activity may play a role in the storage of C whereby the aggrading restored soils may be more susceptible to C loss during turnover of macroaggregates compared to the AG and NP soils with less macroaggregates.  相似文献   

18.
The effects of tillage on the interaction between soil structure and microbial biomass vary spatially and temporally for different soil types and cropping systems. We assessed the relationship between soil structure induced by tillage and soil microbial activity at the level of soil aggregates. To this aim, organic C (OC), microbial biomass C (MBC) and soil respiration were measured in water-stable aggregates (WSA) of different sizes from a subtropical rice soil under two tillage systems: conventional tillage (CT) and a combination of ridge with no-tillage (RNT). Soil (0–20 cm) was fractionated into six different aggregate sizes (> 4.76, 4.76–2.0, 2.0–1.0, 1.0–0.25, 0.25–0.053, and < 0.053 mm in diameter). Soil OC, MBC, respiration rate, and metabolic quotient were heterogeneously distributed among soil aggregates while the patterns of aggregate-size distribution were similar among properties, regardless of tillage system. The content of OC within WSA followed the sequence: medium-aggregates (1.0–0.25 mm and 1.0–2.0 mm) > macro-aggregates (4.76–2.0 mm) > micro-aggregates (0.25–0.053 mm) > large aggregates (> 4.76 mm) > silt + clay fractions (< 0.053 mm). The highest levels of MBC were associated with the 1.0–2.0 mm aggregate size class. Significant differences in respiration rates were also observed among different sizes of WSA, and the highest respiration rate was associated with 1.0–2.0 mm aggregates. The Cmic/Corg was greatest for the large-macroaggregates regardless of tillage regimes. This ratio decreased with aggregate size to 1.0–0.25 mm. Soil metabolic quotient (qCO2) ranged from 3.6 to 17.7 mg CO2 g− 1 MBC h− 1. The distribution pattern of soil microbial biomass and activity was governed by aggregate size, whereas the tillage effect was not significant at the aggregate scale. Tillage regimes that contribute to greater aggregation, such as RNT, also improved soil microbial activity. Soil OC, MBC and respiration rate were at their highest levels for 1.0–2.0 mm aggregates, suggesting a higher biological activity at this aggregate size for the present ecosystem.  相似文献   

19.
Copper (Cu) is accumulating in agricultural soils worldwide creating concern for adverse impacts on soil microbial communities and associated ecosystem services. In order to evaluate the structural and functional resilience of soil microbial communities to increasing Cu levels, we compared a Cu-adapted and a corresponding non-adapted soil microbial community for their abilities to resist experimental Cu pollution. Laboratory soil microcosms were set-up with either High-Cu soil from Cu-amended field plots (63 g Cu m−2) or with Low-Cu control soil from the same five-year field experiment. Laboratory treatments consisted of Cu amendments in the presence or absence of pig manure. Microbial activities (soil respiration, substrate-induced respiration, [3H]leucine incorporation), bacterial community structure (terminal restriction fragment length polymorphism, T-RFLP), community-level physiological profiles, and pollution-induced bacterial community tolerance (PICT detected using the [3H]leucine incorporation technique) were monitored for 12 weeks. The High-Cu and Low-Cu soil microbial communities initially exhibited almost identical structure and function and could only be distinguished from each other by their differential Cu tolerance. Experimental Cu pollution inhibited microbial activities, affected bacterial community structure, and induced further bacterial community tolerance to Cu. However, Low-Cu and High-Cu soil microbial communities showed essentially identical responses. Manure amendment did not protect against Cu toxicity and slightly increased Cu bioavailability as measured by a Cu-specific whole-cell bacterial biosensor. Our results indicate convergence of bacterial community structure and function in the High-Cu and Low-Cu soils during the five-year field experiment. We conclude that soil bacterial communities can exhibit structural and functional resilience to a five-year Cu exposure by virtue of their ability to develop Cu tolerance without affecting overall community structure. The observed increased Cu tolerance may involve phenotypic adaptation or selection at the micro-diversity level, for example an increased proportion of Cu-resistant strains within each bacterial species, which go undetected by T-RFLP community fingerprinting. Finally, our results indicate that Cu-dissolved organic matter complexes contribute to microbial toxicity in manure-amended soils implying that free Cu may comprise a poor predictor of metal toxicity.  相似文献   

20.
Recent studies have concluded that the dynamics of soil structure are central to the understanding of soil organic matter (SOM) cycling and the ensuing soil‐water–nutrient relationships. Aggregate turnover directly controls the stabilization and physical protection of SOM. Therefore, quantifying aggregate dynamics will improve our ability to predict SOM behaviour as affected by ecosystem management and global change. We present an approach to directly quantify aggregate dynamics using rare‐earth oxides as tracers. A 6‐week laboratory incubation was set up to measure aggregate dynamics at different times. We made samples in which each different aggregate size‐fraction contained a different tracer. By following the redistribution of these tracers into the other aggregate size‐fractions, we could quantify all soil mass transfers between aggregate size‐fractions. A comparison with a control soil showed that the tracer did not affect soil respiration or the aggregation process itself. Tracer mixing homogeneity, recovery and immobility were tested and validated. While initially macroaggregate formation occurred rapidly, microaggregate formation occurred more slowly during the experiment. Subsequent aggregate stabilization was more pronounced for the newly formed microaggregates than for the newly formed macroaggregates. Calculated turnover times were smaller for macroaggregates than for microaggregates (i.e. 30 vs. 88 days). Further research is needed to investigate to what extent these results can be extrapolated to the field. Our results confirmed existing qualitative views and concepts on aggregate dynamics in a quantitative way and will be valuable in directly linking aggregate turnover to the stabilization and protection of SOM.  相似文献   

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