Response of labile soil organic matter to changes in forest vegetation in subtropical regions

Labile soil organic matter (SOM) can sensitively respond to changes in land use and management practices, and has been suggested as an early and sensitive indicator of SOM. However, knowledge of effects of forest vegetation type on labile SOM is still scarce, particularly in subtropical regions. Soil microbial biomass C and N, water-soluble soil organic C and N, and light SOM fraction in four subtropical forests were studied in subtropical China. Forest vegetation type significantly affected labile SOM. Secondary broadleaved forest (SBF) had the highest soil microbial biomass, basal respiration and water-soluble SOM, and the pure Cunninghamia lanceolata plantation (PC) the lowest. Soil microbial biomass C and N and respiration were on average 100%, 104% and 75%, respectively higher in the SBF than in the PC. The influence of vegetation on water-soluble SOM was generally larger in the 0–10 cm soil layer than in the 10–20 cm. Cold- and hot-water-soluble organic C and N were on average 33–70% higher in the SBF than in the PC. Cold- and hot-soluble soil organic C concentrations in the coniferous-broadleaved mixed plantations were on average 38.1 and 25.0% higher than in the pure coniferous plantation, and cold- and hot-soluble soil total N were 51.4 and 14.1% higher, respectively. Therefore, introducing native broadleaved trees into pure coniferous plantations increased water-soluble SOM. The light SOM fraction (free and occluded) in the 0–10 cm soil layer, which ranged from 11.7 to 29.2 g kg⁻¹ dry weight of soil, was strongly affected by vegetation. The light fraction soil organic C, expressed as percent of total soil organic C, ranged from 18.3% in the mixed plantations of C. lanceolata and Kalopanax septemlobus to 26.3% in the SBF. In addition, there were strong correlations among soil organic C and labile fractions, suggesting that they were in close association and partly represented similar C pools in soils. Our results indicated that hot-water-soluble method could be a suitable measure for labile SOM in subtropical forest soils.     

Characterization of cultivable heterotrophic bacterial communities in Cr-polluted and unpolluted soils using Biolog and ARDRA approaches

The bacterial communities of two soils with different chromium levels were characterized by Biolog carbon substrate utilization patterns and amplified 16S ribosomal DNA restriction analysis (ARDRA). For each bacterial community sample, cell suspensions containing 10,000 or 100 colony-forming units (CFU) were inoculated in each well of Biolog-GN microplates. The number of carbon compounds utilized by the bacterial community consisting of 100 CFU from unpolluted soil was significantly lower than that detected for the bacterial community consisting of 10,000 CFU. The size of inoculum did not substantially influence the percentage of carbon sources utilized by the Cr-polluted soil bacterial community. ARDRA approach was applied to about 100 bacterial isolates for each soil sample. A similar number of clusters for Gram-negative bacteria were found in both soils, but there were differences in percentages of isolates belonging to each group and specific genomic groups were found in each soil. Pseudomonas was the dominant taxon in both soils. Comparing the ARDRA clusters obtained from Gram-positive isolates it was evident that the culturable bacterial communities of Cr-polluted and unpolluted soils were dominated by the genus Arthrobacter and the genus Bacillus, respectively.     

Do boundaries of soil animal and plant communities coincide? A case study of a Mediterranean forest in Russia

We studied the relationship between plant and soil animal communities by geostatistical analysis in a piedmont forest close to Novorossiysk (Southern Russia). Vegetation on the slope of a hill was an oak-ash-hornbeam forest, while the vegetation on the foot of the hill was a maple-ash-hornbeam forest. Two plots were studied each including both slope and foot habitats. On every plot samples collected formed a grid of 10 × 5 units with a 5 m distance between them. Soil macroinvertebrates were hand-sorted from the samples, and several soil parameters (soil, pebble, and litter mass, soil moisture) were measured.The analysis did not reveal coincidence between the boundaries of plant and soil animal communities on the bend of the hill. Soil animal communities of the plots were dominated by woodlice, diplopods, and insect larvae, reaching an abundance of 680–990 individuals m⁻² throughout the plots. Number of taxonomic groups per sample and overall animal abundance in the bend were the highest in both plots, whilst these parameters on the slope were the lowest. Variograms and maps of spatial distribution indicated that the boundary between soil animal communities was situated further up on the slope than the vegetation boundary. The size of the animal community was smaller than the size of plots sampled, what probably explained the lack of coincidence between the boundaries. There was a significant correlation between distribution of litter mass and parameters of soil animal communities, which was modulated by depth of soil layer and soil moisture. Soil parameters were more important for explaining boundaries between soil animal communities than plant communities in the forest considered.     

Differences in the composition and diversity of bacterial communities from agricultural and forest soils

Differences in the bacterial communities of soils caused by disturbances and land management were identified in rRNA gene libraries prepared from conventional tilled (CT) and no tilled (NT) cropland, a successional forest after 30 y of regrowth (NF) and an old forest of >65 y (OF) at Horseshoe Bend, in the southern Piedmont of Georgia (USA). Libraries were also prepared from forests after 80 y of regrowth at the Coweeta Long Term Ecological Research site (CWT) in the Southern Appalachians of western North Carolina (USA). The composition of the bacterial communities in cropland soils differed from those of the Horseshoe Bend OF and CWT forest soils, and many of the most abundant OTUs were different. Likewise, the diversity of bacterial communities from forest was less than that from cropland. The lower diversity in forest soils was attributed to the presence of a few, very abundant taxa in forest soils that were of reduced abundance or absent in cropland soils. After 30 y of regrowth, the composition of the bacterial soil community of the NF was similar to that of the OF, but the diversity was greater. These results suggested that the bacterial community of soil changes slowly within the time scale of these studies. In contrast, the composition and diversity of the bacterial communities in the Horseshoe Bend OF and Coweeta soils were very similar. Thus, this forest soil bacterial community was widely distributed in spite of the differences in soil properties, vegetation, and climate as well as resilient to disturbances of the above ground vegetation.     

Edge effects and trampling in boreal urban forest fragments – impacts on the soil microbial community

Fragmentation of forest ecosystems increases the proportion of edge habitat and is accompanied by a change in plant species composition. The recreational use of urban forests leads to decreased vegetation cover and the formation of paths, and thus, to fragmentation at small scales. We studied the impacts of forest and path edge effects on the soil microbial community structure (by using the phospholipid fatty acid (PLFA) method) and microbial activity (measured as basal respiration) in 34 mesic boreal urban forest fragments in Finland. We sampled the humus layer 1) from the forest edge into the interior (0–80 m), and 2) at different distances from paths. Microbial community structure was only slightly affected by the forest edge but differences were found between distances of 0–10 m and over 50 m from the edge. These changes correlated with changes in soil pH. Although changes in the microbial community structure were not pronounced, microbial biomass and activity were 30–45% lower at the first 20 m into the forest fragments, due to a low moisture content of the humus near the edge. The decreased microbial activity detected at forest edges implies decreased litter decomposition rates, and thus, a change in ecosystem nutrient cycling. The microbial community structure differed between paths and surrounding areas and correlated with changes in soil pH. Paths also supported approximately 25–30% higher microbial biomass with a transition zone of at least 1 m from the path edge. Path associated disturbances (mainly alterations in vegetation and soil pH) were reflected in the soil microbial community structure up to 1.5 m from the paths.     

Temperature and substrate effects on C & N mineralization and microbial community function of soils from a hybrid poplar chronosequence

Substrate input as well as climatic factors affect C and N cycling and microbial properties in forest soils. We used a microcosm approach to investigate the response of CO₂ efflux, net N mineralization, and microbial community-level physiological profile (CLPP) to temperature (5 vs. 15 °C) and substrate (with and without sucrose addition) addition in surface mineral soils collected from 4-, 6-, 13-, and 15-year old (ages in 2007) hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) stands in northern Alberta. In the early stage of incubation (0–2 h), CO₂ efflux was higher at 5 °C than at 15 °C with little effect from substrate addition, while 24 h after the addition of substrate, CO₂ efflux became higher under the 15 °C incubation. After 72 h incubation, temperature and substrate addition effects on CO₂ efflux subsided and CO₂ efflux rates tended to converge among the treatments. Net N mineralization was significantly affected by substrate addition and stand age, while rates of net ammonification were higher at 5 °C than at 15 °C. Net N mineralization occurred without sucrose addition while net immobilization occurred with sucrose addition. The soil from the youngest stand had the lowest N mineralization rate among the stands for each corresponding substrate-incubation temperature treatment. We used Ecoplates from Biolog™ to study sole-carbon-source-utilization profiles of microbial communities at the end of the incubation. Principal component analysis of C utilization data separated microbial communities with respect to substrate addition, incubation temperature and stand age. Our data showed that organic matter mineralization and microbial substrate utilization were affected by incubation temperature, substrate availability and stand age, indicating that the responses of microbial communities in the studied hybrid poplar plantations to temperature changes were strongly mediated by labile C availability and stand development.     

Effect of <Emphasis Type="Italic">Eucalyptus</Emphasis> forests on understory vegetation and soil quality

Purpose

Eucalyptus forest plantations are normally devoid of understory vegetation that is often assumed to be associated with Eucalyptus allelopathic effects. The objective of this study was to determine the influence of high soil compaction and low soil moisture content on inhibition of the germination of understory seeds in Eucalyptus forests and thus would result in the scarcity of understory vegetation.

Materials and methods

The soil water content above the depth of 1 m of six major understory vegetation types was analyzed to determine if there was a correlation between soil water content and understory vegetation. The effects of soil treatment (soil-loosening vs. no soil-loosening) and water supply amount (2500, 2000, 1500, 1000, 500, 250, or 0 ml of water per day) on the seed germination rate of Stylosanthes sp. were explored using an artificial climate chamber experiment. Influence of soil source (five Eucalyptus forest soils vs. two non-Eucalyptus forest soils) and water supply (0, 50, 150, 200, or 400 ml of water every day) on the germination rate of five types of seed were assessed using a three-factor analysis of variance (ANOVA).

Results and discussion

Soil-loosening and water supply significantly (P?<?0.05) increased seed germination rate with the contribution rates of 26.14 and 42.93 %, respectively. Analysis of variance for three-factor experiments revealed a significant (P?<?0.05) effect of water supply and vegetation seed type on the germination rate of plant seeds. No significant effect of soil type was observed on germination rate, indicating that germination rate was not affected by soils in Eucalyptus forest.

Conclusions

The conservation of soil characteristics, such as water content and compaction, during the development of a Eucalyptus forest plantation may be an effective strategy for encouraging the growth of understory vegetation. This study highlights the importance that in dry areas or areas prone to long-term drought, it would be preferable to retain native vegetation.

     

Effects of nitrogen addition and litter properties on litter decomposition and enzyme activities of individual fungi

Litter decomposition is an important process of C and N cycling in the soil. Variation in the response of litter decomposition to nitrogen (N) addition (positive, negative or neutral) has been observed in many field studies. However, mechanism about variability in individual fungal species response to N addition has not yet been well demonstrated in the literature. Therefore, the objective of this study was to investigate the effects of N addition and litter chemistry properties on litter decomposition and enzyme activities of individual fungi. Three fungal species (Penicillium, Aspergillus, and Trichoderma) were isolated from a subtropical mixed forest soil. An incubation experiment was conducted using the individual fungi with two types of litter (leaf of Pinus massoniana and needle of Cryptocarya chinensis) and different N addition levels (0, 50 and 100 for N-deficient treatments, and 500 and 1000 μg N for N-excessive treatments). Cumulative CO₂-C, enzyme activities, and lignin and cellulose loss were measured during the incubation period of 60 days. Litter decomposition and enzyme activities significantly varied with the fungal species, while the N addition and litter types greatly affected fungal enzyme activities. The N treatments significantly increased lignin-rich needle decomposition by lignocellulose decomposers (Penicillium and Aspergillus) but did not affect their leaf decomposition. On the contrary, The N treatments stimulated leaf decomposition by cellulolytic species (Trichoderma) but did not affect its needle decomposition. Correlation analysis showed that lignin in the litter was the key component to affect litter decomposition. Activities of N-acetyl-β-glucosaminidase and phenol oxidase were both positively correlated to litter decomposition. The fungi (Penicillium and Aspergillus) with higher production of N-acetyl-β-glucosaminidase showed higher litter decomposition ability. The low N addition levels stimulated Penicillium and Aspergillus litter decomposition, but they still required more N source (e.g., litter N source) to support decomposition. Depressed fungal litter N uptake (lower N-acetyl-β-glucosaminidase activities) only occurred at the highest N addition level. Litter decomposition of Trichoderma depended more on external N and its litter decomposition capability was the lowest among the three species.     

Leaf litter is the main driver for changes in bacterial community structures in the rhizosphere of ash and beech

The rhizosphere and the surrounding soil harbor an enormous microbial diversity and a specific community structure, generated by the interaction between plant roots and soil bacteria. The aim of this study was to address the influences of tree species, tree species diversity and leaf litter on soil bacterial diversity and community composition. Therefore, mesocosm experiments using beech, ash, lime, maple and hornbeam were established in 2006, and sampled in October 2008 and June 2009. Mesocosms were planted with one, three or five different tree species and treated with or without litter overlay.Cluster analysis of DGGE-derived patterns revealed a clustering of 2008 sampled litter treatments in two separated clusters. The corresponding treatments sampled in 2009 showed separation in one cluster. PCA analysis based on the relative abundance of active proteobacterial classes and other phyla in beech and ash single-tree species mesocosm indicated an effect of sampling time and leaf litter on active bacterial community composition. The abundance of next-generation sequencing-derived sequences assigned to the Betaproteobacteria was higher in the litter treatments, indicating a higher activity, under these conditions. The Deltaproteobacteria, Nitrospira and Gemmatimonadetes showed an opposite trend and were more active in the mesocosms without litter. The abundance of alphaproteobacterial sequences was higher in mesocosms sampled in 2009 (P = 0.014), whereas the Acidobacteria were more active in 2008 (P = 0.014). At the family level, we found significant differences of the litter vs. non-litter treated group. Additionally, an impact of beech and ash as tree species on soil bacterial diversity was confirmed by the Shannon and Simpson indices. Our results suggest that leaf litter decomposition in pH-stable soils affect the soil bacterial composition, while tree species influence the soil bacterial diversity.     

The bacterial community inhabiting temperate deciduous forests is vertically stratified and undergoes seasonal dynamics

Bacterial communities living in forest soils contribute to the decomposition of organic matter and the recycling of nutrients in these ecosystems and form one of the most diverse habitats on Earth. Unfortunately, due to difficulty in culturing soil bacteria, the understanding of their ecology is still limited. In the case of temperate deciduous forests, soil microbial communities face large seasonal variations in environmental conditions, such as temperature or moisture. Moreover, the supply of nutrients also differs due to seasonal processes, such as the allocation of photosynthates into soil by the roots of primary producers or the seasonal input of fresh litter. The aim of this study was to reveal how the bacterial community responds to these seasonal processes in the litter and soil of a Quercus petraea forest. Bacterial communities from litter and from the organic and mineral horizons of soil were analyzed during the four seasons of the year by 16S rRNA gene pyrosequencing. The results revealed that the composition of the bacterial community is horizon specific. The litter horizon had a higher relative abundance of Proteobacteria and Bacteroidetes than soil, while the organic and mineral horizons had a higher abundance of Acidobacteria, Firmicutes and Actinobacteria than litter. Moreover, the bacterial community was significantly affected by seasonality in all horizons. Bacterial communities in the litter showed significant differences between the vegetation season (May and July) and the autumn and winter seasons (October, February). In mineral soil, bacterial community composition was specific in the summer, when it was significantly different from all other seasons, with a larger number of taxa described as rhizosphere and mycorrhizosphere inhabitants. The results indicate that litter decomposition is the main driver of bacterial community composition in litter horizon. In contrast to reports on fungal communities, bacterial community composition in mineral soil responds to the seasonal peaks of rhizodeposition in the summer.     

Effects of vegetation on soil microbial C,N, and P dynamics in a tropical forest and savanna of Central Africa

The forest–savanna transition zone is widely distributed on nutrient-poor oxisols in Central Africa. To reveal and compare the nutrient cycle in relation to soil microbes for forest and savanna vegetation in this area, we evaluated seasonal fluctuations in microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) for 13 months as well as soil moisture, temperature, soil pH levels, and nutrients for both vegetation types in eastern Cameroon. Soil pH was significantly lower in forest (4.3) than in savanna (5.6), and soil N availability was greater in forest (87.1 mg N kg⁻¹ soil) than in savanna (32.9 mg N kg⁻¹ soil). We found a significant positive correlation between soil moisture and MBP in forest, indicating the importance of organic P mineralization for MBP, whereas in savanna, we found a significant positive correlation between soil N availability and MBP, indicating N limitation for MBP. These results suggest that for soil microbes, forest is an N-saturated and P-limited ecosystem, whereas savanna is an N-limited ecosystem. Additionally, we observed a significantly lower MBN and larger MB C:N ratio in forest (50.7 mg N kg⁻¹ soil and 8.6, respectively) than in savanna (60.0 mg N kg⁻¹ soil and 6.5, respectively) during the experimental period, despite the rich soil N condition in forest. This may be due to the significantly lower soil pH in forest, which influences the different soil microbial communities (fungi-to-bacteria ratio) in forest versus savanna, and therefore, our results indicate that, in terms of microbial N dynamics, soil pH rather than soil substrate conditions controls the soil microbial communities in this area. Further studies should be focused on soil microbial community, such as PLFA, which was not evaluated in the present study.     

Structural and functional diversity of bacterial community in soil treated with the herbicide napropamide estimated by the DGGE,CLPP and r/K-strategy approaches

Napropamide is one of the most commonly used herbicide in agricultural practice and can exhibit toxic effect to soil microorganisms. Therefore, the main objective of this study was to examine the genetic and functional diversity of microbial communities in soil treated with napropamide at field rate (FR, 2.25 mg kg⁻¹ of soil) and 10 times the FR (10 × FR, 22.5 mg kg⁻¹ of soil) by the denaturing gradient gel electrophoresis (DGGE) and the community level physiological profile (CLPP) methods. In addition, the r/K-strategy approach was used to evaluate the effect of this herbicide on the community structure of the culturable soil bacteria. DGGE patterns revealed that napropamide affected the structure of microbial community; however, the richness (S) and genetic diversity (H) values indicated that the FR dosage of napropamide experienced non-significant changes. In turn, the 10 × FR dosage of herbicide caused significant changes in the S and H values of dominant soil bacteria. DGGE profiles suggest an evolution of bacteria capable of degrading napropamide among indigenous microflora. Analysis of the CLPPs indicated that the catabolic activity of microbial community expressed as AWCD (average well-color development) was temporary positively affected after napropamide application and resulted in an increase of the substrate richness (S_R) as well as functional biodiversity (H) values. Analysis of the bacterial growth strategy revealed that napropamide affected the r- or K-type bacterial classes (ecotypes). In treated-soil samples K-strategists dominated the population, as indicated by the decreased ecophysiological (EP) index. Napropamide significantly affected the physiological state of culturable bacteria and caused a reduction in the rate of colony formation as well as a prolonged time of growth rate. Obtained results indicate that application of napropamide may poses a potential risk for soil functioning.     

Microbial properties and soil respiration in submontane forests of Venezuelian Guyana: characteristics and response to fertilizer treatments

The distribution of vegetation types in Venezuelan Guyana (in the ‘Canaima’ National Park) represents a transitional stage in a long term process of savannization, a process considered to be conditioned by a combined chemical and intermittent drought stress. All types of woody vegetation in this environment accumulate large amounts of litter and soil organic carbon (SOC). We hypothesized that this accumulation is caused by low microbial activity. During 1 year we measured microbial biomass carbon (Cmic), microbial respiration and soil respiration of stony Oxisols (Acrohumox) at a tall, a medium and a low forest and with three chemical modifications of site conditions by the addition of NO₃⁻, Ca²⁺ and PO₄³⁻ as possible limiting elements. Due to high SOC contents, mean Cmic was 1 mg g soil⁻¹ in the mineral topsoil and 3 mg g soil⁻¹ in the forest floor. Mean microbial respiration in the mineral topsoil and the forest floor were 165 and 192 μg CO₂-C g soil⁻¹ d⁻¹, respectively. We calculated high mean metabolic quotients (qCO₂) of 200 mg CO₂-C g Cmic⁻¹ d⁻¹ in the litter layer and 166 mg CO₂-C g Cmic⁻¹ d⁻¹ in the mineral topsoil, while the Cmic-to-SOC ratios were as low as 1.0% in the litter layer and 0.8% in the mineral topsoil. Annual soil respiration was 9, 12 and 10 Mg CO₂-C ha⁻¹ yr⁻¹ in the tall, medium and low forest, respectively. CO₂ production was significantly increased by CaHPO₄ fertilization, but no consistent effects were caused by Ca²⁺ and NO₃⁻, fertilization. Our findings indicate that Cmic and microbial respiration are reduced by low nutrient concentrations and low litter and SOC quality. Reduced microbial decomposition may have contributed to SOC accumulation in these forests.     

Responses of soil dissolved organic matter to long-term plantations of three coniferous tree species

Tree species have significant effects on the availability and dynamics of soil organic matter. In the present study, the pool sizes of soil dissolved organic matter (DOM), potential mineralizable N (PMN) and bio-available carbon (C) (measured as cumulative CO₂ evolution over 63 days) were compared in soils under three coniferous species — 73 year old slash (Pinus elliottii), hoop (Araucaria cunninghamii) and kauri (Agathis robusta) pines. Results have shown that dissolved organic N (DON) in hot water extracts was 1.5–1.7 times lower in soils under slash pine than under hoop and kauri pines, while soil dissolved organic C (DOC) in hot water extracts tended to be higher under slash pine than hoop and kauri pines but this was not statistically significant. This has led to the higher DOC:DON ratio in soils under slash pine (32) than under hoop and kauri pines (17). Soil DOC and DON in 2 M KCl extracts were not significantly different among the three tree species. The DOC:DON ratio (hot water extracts) was positively and significantly correlated with soil C:N (R² = 0.886, P < 0.01) and surface litter C:N ratios (R² = 0.768, P < 0.01), indicating that DOM was mainly derived from litter materials and soil organic matter through dissolution and decomposition. Soil pH was lower under slash pine (4.5) than under hoop (6.0) and kauri (6.2) pines, and negatively correlated with soil total C, C:N ratio, DOC and DOC:DON ratio (hot water extracts), indicating the soil acidity under slash pine favored the accumulation of soil C. Moreover, the amounts of dissolved inorganic N, PMN and bio-available C were also significantly lower in soils under slash pine than under hoop and kauri pines. It is concluded that changes in the quantity and quality of surface litters and soil pH induced by different tree species largely determined the size and quality of soil DOM, and plantations of hoop and kauri pine trees may be better in maintaining long-term soil N fertility than slash pine plantations.     

Succession of soil nematodes in pine forests on coal-mining sands near Cottbus,Germany

The succession of soil nematodes from initial planting with Pinus sylvestris seedling to about 30-year-old pine plantations on coal mining sands in the Lusatian lignite-mining district near Cottbus (Germany) was studied and compared with the nematode fauna of a 40-year-old semi-natural pine forest on naturally formed sandy soil. The initial stage was primarily characterised by a very low abundance (20×10³ individuals/m²), which increased over a period of two years to values common in older pine plantations (500–600×10³ individuals/m²). In the semi-natural forest the mean abundance of nematodes was about 1300×10³ individuals/m². Populations of Tardigrada, Rotifera and Enchytraeidae also increased with stand age. Nematode biomass increased from 49 to 543 mg m⁻² in pine plantations and slightly decreased in the semi-natural forest to 301 mg m⁻² over the period of investigation. The early colonisation of the initial stage was by bacterivorous (Acrobeloides) and fungal feeding (Aphelenchoides) nematodes, but the communities diversified as succession progressed with bacterivorous nematodes of the genera Plectus, Wilsonema and Metateratocephalus, root-fungal feeding Filenchus, omnivorous Aporcelaimellus and Eudorylaimus, and predacious Prionchulus becoming abundant. The abundance of plant-parasitic nematodes was very low. The greatest number of nematode genera was found in the semi-natural forest.     

Effects of NH4+ and NO3− on litter and soil organic carbon decomposition in a Chinese fir plantation forest in South China

Soil organic carbon (SOC) dynamics and nutrient availability determine the soil quality and fertility in a Chinese fir plantation forest in subtropical China. Uniformly ¹³C-labeled Chinese fir (Cunninghamia lanceolata) and alder (Alnus cremastogyne) leaf litter with or without 100 mg NH₄⁺ or NO₃⁻ were added to the soil. The purpose was to investigate the influence of N availability on the decomposition of the litter and native SOC. The production of CO₂, the natural abundance of ¹³C–CO₂, and the inorganic N dynamics were monitored. The results showed that Chinese fir (with a high C:N ratio) and alder (with a low C:N ratio) leaf litter caused significant positive priming effects (PEs) of 24% and 42%, respectively, at the end of the experiment (235 d). The PE dynamics showed that positive PE can last for at least 87 d. However, the possible occurrence of a significant negative PE with a sufficient incubation period is difficult to confirm. The application of both NH₄⁺ and NO₃⁻ was found to have a stimulating effect on the decomposition of Chinese fir and alder leaf litter in the early stage (0–15 d) of incubation, but an adverse effect in the late stage. Compared with NO₃⁻, NH₄⁺ caused a greater decrease in the PE induced by both Chinese fir and alder leaf litter. The effects of NH₄⁺ and NO₃⁻ on the PE dynamics had different patterns for different incubation stages. This result may indicate that the stability or recalcitrance of SOC, especially in such plantation forest soils, strongly depends on available leaf litter and application of N to the soil.     

Microbial function in adjacent subtropical forest and agricultural soil

Soil microbial communities and their activities are altered by land use change; however impacts and extent of these alterations are often unclear. We investigated the functional responses of soil microbes in agricultural soil under sugarcane and corresponding native soil under Eucalyptus forest to additions of contrasting plant litter derived from soybean, sugarcane and Eucalyptus in a microcosm system, using a suite of complimentary techniques including enzyme assays and community level physiological profiles (CLPP). Initially agricultural soil had 50% less microbial biomass and lower enzyme activities than forest soil, but significantly higher nitrification rates. In response to litter addition, microbial biomass increased up to 11-fold in agricultural soil, but only 1.8-fold in forest soil, suggesting a prevalence of rapidly proliferating ‘r’ and slower growing ‘K’ strategists in the respective soils. Litter-driven change in microbial biomass and activities were short lived, largely returning to pre-litter addition levels by day 150. Decomposition rates of sugarcane and soybean litter as estimated via CO₂ production were lower in agricultural than in forest soil, but decomposition of more recalcitrant Eucalyptus litter was similar in both soils, contradicting the notion that microbial communities specialise in decomposing litter of the dominant local plant species. Enzyme activities and community level physiological profiles (CLPP) were closely correlated to microbial biomass and overall CO₂ production in the agricultural soil but not the forest soil, suggesting contrasting relationships between microbial population dynamics and activity in the two soils. Activities of enzymes that break down complex biopolymers, such as protease, cellulase and phenol oxidase were similar or higher in the agricultural soil, which suggests that the production of extracellular biopolymer-degrading enzymes was not a factor limiting litter decomposition. Enzyme and CLPP analyses produced contrasting profiles of microbial activity in the two soils; however the combination of both analyses offers additional insights into the changes in microbial function and community dynamics that occur after conversion of forest to agricultural land.     

Response of microbial communities to different doses of chromate in soil microcosms

The toxic effect of chromate on soil microbial communities is not well documented, although microorganisms control biogeochemical cycling, contribute to formation of soil structure, regulate the fate of organic matter applied to soil. In this study the effects of short- and middle-term chromate on the soil microbial community were investigated. The shifts in the size and in the diversity of culturable heterotrophic bacterial community, the resistance to Cr(VI) of heterotrophic bacteria, the presence of cyanobacteria, the activity of 19 enzymes, and the ATP content were monitored over time (120 days) in soil microcosms artificially contaminated with three concentrations of chromate (50, 250 and 1000 mg kg⁻¹ soil). The chromate contamination affected the structure and the diversity of the soil bacterial community. Bacterial strains isolated from the microcosm contaminated with the highest concentration of chromate were identified by 16S rDNA gene sequencing. All isolates belonged to the genus Pseudomonas, were able to reduce Cr(VI), and showed a high resistance to chromate. To our knowledge, this is the first report that shows Pseudomonas strains having the capability to resist up to 40 mM of Cr(VI) on minimal medium. The cyanobacterial group was more sensitive to chromate contamination than culturable heterotrophic bacteria. No cyanobacterial growth was detected in enrichment cultures from the soil polluted with the highest chromate concentration. Some enzymes were inhibited by high concentrations of chromate, whereas others were stimulated. The ATP content in microcosms was strongly affected by chromate. We conclude that the soil microbial community responds to chromate pollution through changes in community structure, in metabolic activity, and in selection for Cr(VI)-resistance.     

Species richness and phylogenetic diversity comparisons of soil microbial communities affected by nickel-mining and revegetation efforts in New Caledonia

In order to understand the impact of human activities on soil microbial diversity, we investigated bacterial communities in samples recovered from four New Caledonia environments that have been disturbed by varying degrees of nickel mining associated activities: an undisturbed area with natural soil (characterized by pristine vegetation), a mine spoil (devoid of vegetation), two revegetated mine spoils by endemic plants. For each sample, total DNA was extracted and 16S rDNA clone library were constructed. 442 clones were sequenced and analyzed. Using these clones, diversity was estimated not only in terms of species richness (non-parametric estimators) and evenness (Reciprocal of Simpson's index), but also in terms of phylogenetic diversity (LIBSHUFF program). Statistically significant differences were detected in phylogenetic composition between mine spoils and natural soil (p = 0.001), between revegetated soils and natural soil (p = 0.001), and between revegetated soils and mine spoils (p = 0.001). On the other hand, no significant differences in species richness were observed between the different environmental samples.These findings provide insights into the response of bacterial community following environmental perturbations caused by nickel-mining activities and revegetation efforts.     

Land use influences soil fungal community composition across central Victoria,south-eastern Australia

Current theory expects that fungi, on the one hand, are spatially ubiquitous but, on the other, are more susceptible than bacteria to disturbance such as land use change due to dispersal limitations. This study examined the relative importance of location and land use effects in determining soil fungal community composition in south-eastern Australia. We use terminal restriction fragment length polymorphism (T-RFLP; primer pair ITS1-F–ITS4) and multivariate statistical methods (NMDS ordinations, ANOSIM tests) to compare relative similarities of soil fungal communities from nine sites encompassing three locations (ca 50–200 km apart) and four land uses (native eucalypt forest, Pinus radiata plantation, Eucalyptus globulus plantation, and unimproved pasture). Location effects were generally weak (e.g. ANOSIM test statistic R ≤ 0.49) and were, in part, attributed to minor differences in soil texture. By contrast, we found clear and consistent evidence of land use effects on soil fungal community composition (R ≤ 0.95). That is, soils from sites of the same land use grouped together in NMDS ordinations of fungal composition despite geographic separations of up to ca 175 km (native eucalypt forests) and 215 km (P. radiata plantations). In addition, different land uses from the same location were clearly separate in NMDS ordinations, despite, in one case, being just 180 m apart and having similar land use histories (i.e. P. radiata versus E. globulus plantation both established on pasture in the previous decade). Given negligible management of all sites beyond the early establishment phase, we attribute much of the land use effects to changes in dominant plant species based on consistent evidence elsewhere of strong specificity in pine and eucalypt mycorrhizal associations. In addition, weak to moderate correlations between soil fungal community composition and soil chemical variables (e.g. Spearman rank correlation coefficients for individual variables of 0.08–0.32), indicated a minor contributing role of vegetation-mediated changes in litter and soil chemistry. Our data provide evidence of considerable plasticity in soil fungal community composition over time spans as short as 6–11 years. This suggests that – at least within geographic zones characterised by more-or-less contiguous forest cover – soil fungal community composition depends most on availability of suitable habitat because dispersal propagules are readily available for colonisation after land use change.