Decrease in heathland soil labile organic carbon under future atmospheric and climatic conditions

Characterization of the impacts of climate change on terrestrial carbon (C) cycling is important due to possible feedback mechanisms to atmospheric CO₂ concentrations. We investigated soil organic matter (SOM) dynamics in the A1 and A2 horizons (~0–5.1 and ~5.1–12.3 cm depth, respectively) of a shrubland grass (Deschampsia flexuosa) after 8 years of exposure to: elevated CO₂ (CO₂), summer drought (D), warming (T) and all combinations hereof, with TDCO₂ simulating environmental conditions for Denmark in 2075. The mean C residence time was highest in the heavy fraction (HF), followed by the occluded light fraction and the free light fraction (fLF), and it increased with soil depth, suggesting that C was stabilized on minerals at depth. A2 horizon SOM was susceptible to climate change whereas A1 horizon SOM was largely unaffected. The A2 horizon fLF and HF organic C stocks decreased by 43 and 23% in response to warming, respectively. Organic nitrogen (N) stocks of the A2 horizon fLF and HF decreased by 50 and 17%, respectively. Drought decreased the A2 horizon fLF N stock by 38%. Elevated CO₂ decreased the A2 horizon fLF C stock by 39% and the fLF N stock by 50%. Under TDCO₂, A2 horizon fLF C and N stocks decreased by 22 and 40%, respectively. Overall, our results indicate that shrubland SOM will be susceptible to increased turnover and associated net C and N losses in the future.     

The effects of vegetation types and microhabitats on carabid beetle community composition in cool temperate Japan

The effects of vegetation types and environmental factors on carabid beetle (Coleoptera: Carabidae) communities were studied. Carabid beetles were collected using pitfall traps (total 2844 trapping days) and seven microenvironmental factors were measured in four vegetation types: grassland, natural evergreen coniferous forest (Pinus densiflora), deciduous broad-leaved natural forest (Quercus crispula, Betula platyphylla, Alnus japonica, or Fagus crenata), and deciduous coniferous plantation (Larix kaempferi) in cool temperate Japan. These four vegetation types provided a novel comparison between natural forests and plantations because the vast majority of related studies have investigated only deciduous broad-leaved natural forests and evergreen coniferous plantations. PERMANOVA indicated that vegetation types affected carabid community composition. Ordination plots showed that community composition differed greatly between grassland and forest vegetation types, but that community composition in the plantation forest overlapped with that of natural forest types. Characteristics differentiating the grassland included a high proportion of winged species and a low mean carabid body weight. Among the examined environmental factors, litter depth, soil water content, and depth of the soil A-horizon had large effects on carabid communities. These results suggest that the effect of afforestation on carabid communities in cool temperate Japan might be insignificant compared with the effects of cover types (deciduous vs. evergreen) and microenvironmental factors.     

Effects of afforestation on phosphorus dynamics and biological properties in a New Zealand grassland soil

Selected chemical, biochemical and biological properties of mineral soil (0–30 cm) were measured under a 19 year old forest stand (mixture of Pinus ponderosa and Pinus nigra) and adjacent unimproved grassland at a site in South Island, New Zealand. The effects of afforestation on soil properties were confined to the 0–10 cm layer, which reflected the distribution of fine roots (< 2 mm) in the soil profile. Concentrations of organic C, total N and P and all organic forms of P were lower under the forest stand, while concentrations of inorganic P were higher under forest compared with grassland, supporting the previously described suggestion that afforestation may promote mineralisation of soil organic matter and organic P. On the other hand, microbial biomass C and P, soil respiration and phosphatase enzyme activity were currently all lower and the metabolic quotient was higher in soil under forest compared with grassland, which is inconsistent with increased mineralisation in the forest soil. Reduced biological fertility by afforestation may be mainly attributed to changes in the quantity, quality and distribution of organic matter, and reduction in pH of the forest soil compared with the grassland soil. We hypothesize that the lower levels of C, N and organic P found in soil under forest are due to enhanced microbial and phosphatase activity during the earlier stages of forest development. Forest floor material (L and F layer) contained large amounts of C, N and P, together with high levels of microbial and phosphatase enzyme activity. Thus, the forest floor may be an important source of nutrients for plant growth and balance the apparent reduction in C, N and P in mineral soil through mineralisation and plant uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Linking Above- and Belowground Responses to 16 Years of Fertilization,Mowing, and Removal of the Dominant Species in a Temperate Grassland

Species-rich oligotrophic meadows are affected by a wide range of management interventions that influence their functioning and capacity to deliver ecosystem services, but long-term studies on the above- and belowground adaptations to different management tools are still scarce. We focused on the interactive effects of NPK fertilization, mowing, and removal of the initially dominant species (Molinia caerulea) on plant, soil, and microbial responses in wet oligotrophic grassland in a 16-year full-factorial manipulative experiment. Changes in vegetation composition, soil pH, and nutrient availability were accompanied by altered microbial phospholipid fatty acid (PLFA) composition, whereas treatment effects on soil microbial biomass and carbon (C) mineralization were mainly related to changes in soil organic matter (SOM) content and nutrient availability. Fertilization decreased plant species richness aboveground and lowered SOM storage and microbial activity belowground. Mowing preserved high plant diversity and led to more efficient recycling of N within the grassland, whereas Molinia removal significantly affected only plant community composition. Mowing combined with fertilization maintained high species richness only in the short term. Belowground, mowing reduced N leaching from the fertilized system but did not prevent SOM depletion, soil acidification, and concomitant adverse effects on soil microbes. We conclude that annual mowing is the appropriate type of extensive management for oligotrophic species-rich meadows, but the concomitant nutrient depletion should not be compensated for by regular NPK fertilization due to its adverse effects on soil quality.     

The ectomycorrhizal status of a tropical black bolete, <Emphasis Type="Italic">Phlebopus portentosus</Emphasis>, assessed using mycorrhizal synthesis and isotopic analysis

Phlebopus portentosus is one of the most popular wild edible mushrooms in Thailand and can produce sporocarps in the culture without a host plant. However, it is still unclear whether Phlebopus portentosus is a saprotrophic, parasitic, or ectomycorrhizal (ECM) fungus. In this study, Phlebopus portentosus sporocarps were collected from northern Thailand and identified based on morphological and molecular characteristics. We combined mycorrhizal synthesis and stable isotopic analysis to investigate the trophic status of this fungus. In a greenhouse experiment, ECM-like structures were observed in Pinus kesiya at 1 year after inoculation with fungal mycelium, and the association of Phlebopus portentosus and other plant species showed superficial growth over the root surface. Fungus-colonized root tips were described morphologically and colonization confirmed by molecular methods. In stable isotope measurements, the δ¹³C and δ¹⁵N of natural samples of Phlebopus portentosus differed from saprotrophic fungi. Based on the isotopic patterns of Phlebopus portentosus and its ability to form ECM-like structures in greenhouse experiments, we conclude that Phlebopus portentosus could be an ECM fungus.     

Driving Factors Behind Litter Decomposition and Nutrient Release at Temperate Forest Edges

Forest edges have become important features in landscapes worldwide. Edges are exposed to a different microclimate and higher atmospheric nitrogen (N) deposition compared to forest interiors. It is, however, unclear how microclimate and elevated N deposition affect nutrient cycling at forest edges. We studied litter decomposition and release of N, phosphorus (P), total cations (TC) and C/N ratios during 18 months via the litterbag technique along edge-to-interior transects in two oak (Quercus robur L.) and two pine (Pinus nigra ssp. laricio Maire and ssp. nigra Arnold) stands in Belgium. Furthermore, the roles of edge conditions (microclimate, atmospheric deposition, soil fauna and soil physicochemical conditions), litter quality and edge decomposer community were investigated as underlying driving factors for litter decomposition. Litter of edge and interior was interchanged (focusing on the influence of edge conditions and litter quality) and placed in open-top chamber (OTC), which create an edge (warmer) microclimate. As the decomposer macrofauna was more abundant at the edge than in the interior, the OTCs were used to isolate the effects of warming versus soil fauna. Oak litter at the edge lost 87 and 37% more mass than litter in the interior. We demonstrated an edge effect on litter decomposition and nutrient release, caused by an interplay of edge conditions (atmospheric deposition of N and TC, soil pH and C/N ratio), litter quality and soil fauna. Consequently, edge effects must be accounted for when quantifying ecosystem processes, such as litter decomposition and nutrient cycling in fragmented landscapes.     

Different Response Patterns of Soil Respiration to a Nitrogen Addition Gradient in Four Types of Land-Use on an Alluvial Island in China

It has been well documented that nitrogen (N) additions significantly affect soil respiration (R _s) and its components [that is, autotrophic (R _a) and heterotrophic respiration (R _h)] in terrestrial ecosystems. These N-induced effects largely result from changes in plant growth, soil properties (for example, pH), and/ or microbial community. However, how R _s and its components respond to N addition gradients from low to high fertilizer application rates and what the differences are in diverse land-use types remain unclear. In our study, a field experiment was conducted to examine response patterns of R _s to a N addition gradient at four levels (0, 15, 30, and 45 g N m^?2 y^?1) in four types of land-use (paddy rice–wheat and maize–wheat croplands, an abandoned field grassland, and a Metasequoia plantation) from December 2012 to September 2014 in eastern China. Our results showed that N addition significantly stimulated R _s in all four land-use types and R _h in croplands (paddy rice–wheat and maize–wheat). R _s increased linearly with N addition rates in croplands and the plantation, whereas in grassland, it exhibited a parabolic response to N addition rates with the highest values at the moderate N level in spite of the homogeneous matrix for all four land-use types. This suggested higher response thresholds of R _s to the N addition gradient in croplands and the plantation. During the wheat-growing season in the two croplands, R _h also displayed linear increases with rising N addition rates. Interestingly, N addition significantly decreased the apparent temperature sensitivity of R _s and increased basal R _s. The different response patterns of R _s to the N addition gradient in diverse land-use types with a similar soil matrix indicate that vegetation type is very important in regulating terrestrial C cycle feedback to climate change under N deposition.     

Manure and mineral fertilization change enzyme activity and bacterial community in millet rhizosphere soils

Fertilization is a key agricultural practice for increasing millet yields and influencing soil properties, enzyme activities and rhizosphere bacterial communities. High throughput Illumina sequencing of the 16S rDNA was applied to compare the bacterial community structures and diversities among six different soil samples. The experiments involved the following: no fertilizer (CK), phosphate (P) and potassium (K) plus organic manure (M) (PKM), nitrogen (N) and K plus M (NKM), NPM, NPK and NPKM fertilization. The results showed that the NPKM fertilization of the millet field had a maximal yield of 3617 kg ha^?1 among the six different treatments. The abundances of the Actinobacteria and Bacteroidetes phyla, especially the Devosia, Mycobacterium, Opitutus and Chitinophaga genera, were higher in NPKM than those in the other samples. Redundancy analysis showed that the soil organic matter (SOM), available phosphorus (AP), and urease (UR) activity were significantly correlated with bacterial communities, while SOM and AP were strongly correlated with soil enzyme activities. Pearson’s correlation showed that the available nitrogen was strongly correlated with Devosia and Mycobacterium, and SOM was strongly correlated with Opitutus and Chitinophaga. Besides, catalase was significantly related to Iamia, the UR was significantly related to Devosia, phosphatase was significantly related to Luteimonas and Chitinophaga. Based on the soil quality and millet yield, NPKM treatment was a better choice for the millet field fertilization practices. These findings provide a better understanding of the importance of fertilization in influencing millet yield, soil fertility and microbial diversity, and they lead to a choice of scientific fertilization practices for sustainable development of the agroecosystem.     

Root-exudate flux variations among four co-existing canopy species in a temperate forest,Japan

Plants allocate carbon to root exudates to mine nitrogen (N) from soil organic matter (SOM). Little is known about how the root-exudation rate varies among co-existing woody species. We conducted an in situ experiment in a warm temperate forest on two dominant species, Quercus serrata and Ilex pedunculosa, and two of their congeneric species, Quercus glauca and Ilex macropoda, respectively. We hypothesized that the root-exudation rate varies among these species because of their distinct functional traits and N demands. Root-exudation rates were measured using a non-soil culture method during the growing season from June 2013 to May 2014. We also measured foliar N concentrations and the activities of N-degrading enzymes in the rhizosphere soils. The annual demand for N was calculated from the growth rate and allometric equations for biomass. The root-exudation rates of Q. serrata and I. macropoda were consistently greater than those of their congeneric evergreen species on root-length, root-weight, and individual-tree bases. The variations of the annual N demand of these species mirrored this pattern. Within a species, root-exudation rates correlated positively to leaf N contents, suggesting a physiological linkage between photosynthetic capacities and belowground carbon allocation. Root-exudation rates also correlated positively to the activities of polyphenol oxidase, an enzyme that decomposes N from recalcitrant SOM. Our results suggest that the variations of the root-exudation among co-existing species relate to their functional traits and demand for N.     

Context-dependent tree species effects on soil nitrogen transformations and related microbial functional genes

Although it is generally accepted that tree species can influence nutrient cycling processes in soils, effects are not consistently found, nor are the mechanisms behind tree species effects well understood. Our objectives were to gain insights into the mechanism(s) underlying the effects of tree species on soil nitrogen cycling processes, and to determine the consistency of tree species effects across sites. We compared N cycling in soils beneath six tree species (ash, sycamore maple, lime, beech, pedunculate oak, Norway spruce) in common garden experiments planted 42 years earlier at three sites in Denmark with distinct land-use histories (forest and agriculture). We measured: (1) net and gross rates of N transformations using the ¹⁵N isotope pool-dilution method, (2) soil microbial community composition through qPCR of fungal ITS, bacterial and archaeal 16S, and (3) abundance of functional genes associated with N cycling processes—for nitrification the archaeal and bacterial ammonia-monooxygenase genes (amoA AOA and amoA AOB, respectively) and for denitrification, the nitrate reductase genes nirK and nirS. Carbon concentrations were higher in soils under spruce than under broadleaves, so N transformation rates were standardized per g soil C. Soil NH₄⁺ parameters (gross ammonification, gross NH₄⁺ consumption, net ammonification (net immobilization in this case), and NH₄⁺ concentrations, per g C) were all lowest in soils under spruce. Soils under spruce also had the lowest gene abundance of bacteria, bacterial:fungal ratio, denitrifying microorganisms, ammonia-oxidizing archaea and ammonia-oxidizing bacteria. Differences in N-cycling processes and organisms among the five broadleaf species were smaller. The ‘spruce effect’ on soil microbes and N transformations appeared to be driven by its acidifying effect on soil and tighter N cycling, which occurred at the previously forested sites but not at the previously agricultural site. We conclude that existing characteristics of soils, including those resulting from previous land use, mediate the effects of tree species on the soil microbial communities and activities that determine rates of N-cycling processes.     

Shrub Expansion of <Emphasis Type="Italic">Alnus viridis</Emphasis> Drives Former Montane Grassland into Nitrogen Saturation

The N₂-fixing shrub Alnus viridis is currently encroaching on montane grasslands in the Alps as a result of reduced land management and complete abandonment. Alnus introduces large amounts of nitrogen (N) into these formerly N-poor grasslands and restricts the succession to montane forests. We studied pools and fluxes of N and the associated C pools in pastures (controls) and adjacent Alnus shrublands at two elevations (1650 versus 1950 m a.s.l.) in three valleys in the Swiss central Alps. The total N and C pools stored in 50-year-old Alnus shrubland did not exceed those in adjacent pastures with a total of approximately 610 g N m^?2 in phytomass plus soil (down to 30 cm) at both elevations. In Alnus stands, reduced soil N pools balanced the gain in phytomass N pools, a likely result of a faster turnover of soil N. The soil solution under Alnus was continuously enriched with nitrate, with a total N leaching of 0.79 g N m^?2 season^?1 (June–October) under 50-year-old stands at both elevations and the highest flux of 1.76 g N m^?2 season^?1 in 25-year-old shrubland at low elevation, clearly indicating an excess of available N in Alnus shrubland. In contrast, N leaching across all pastures was close to zero (0.08 g N m^?2) throughout the season. At the catchment scale, streamlet water showed increased nitrate concentrations with typical flushing peaks in spring and autumn, provided more than one fifth of the catchment area was covered by Alnus shrubs. We conclude that the expansion of Alnus rapidly converts centuries-old, N-poor grassland into N saturated shrubland, irrespective of elevation, and it reduces the C storage potential of the landscape because the Alnus dominance constrains re-establishment of a natural montane forest.     

Carbon assimilation,nitrogen, and photochemical efficiency of different Himalayan tree species along an altitudinal gradient

In the area of Jumla region in Western Nepal, measurements of saturated leaf net photosynthetic rate (P_sat), nitrogen content, leaf fluorescence, carbon isotopic composition, and water status were performed on woody coniferous (Pinus wallichiana, Picea smithiana, Abies spectabilis, Juniperus wallichiana, Taxus baccata), evergreen (Quercus semecarpifolia, Rhododendron campanulatum), and deciduous broadleaved species (Betula utilis, Populus ciliata, Sorbus cuspidata) spreading from 2 400 m up to the treeline at 4 200 m a.s.l. With the exception of J. wallichiana, P_sat values were lower in coniferous than broadleaved species. Q. semecarpifolia, that in this area grows above the coniferous belt between 3 000 and 4 000 m, showed the highest P_sat at saturating irradiance and the highest leaf N content. This N content was higher and P_sat lower than those of evergreen oak species of tempe forests at middle and low altitudes. For all species, P_sat and N content were linearly correlated, but instantaneous nitrogen use efficiency was lower than values measured in lowland and temperate plant communities. The values of carbon isotopic composition, estimated by δ₁₃C, showed the same range reported for temperate tree species. The ranking of δ₁₃C values for the different tree types was conifers < evergreen broadleaved<deciduous, suggesting tighter stomatal closure and higher water use efficiency for the evergreen types, confirming trends found elsewhere. No relevant differences of δ₁₃C were found along the altitudinal gradient. Quantum yield of photochemistry at saturating irradiance, measured by leaf fluorescence (δF/F_m’), was highest in J. wallichiana and lowest in T. baccata. Overall, photochemical efficiency was more strongly related to species than to altitude. Interestingly, changes of .δF/F_m’ along the altitudinal gradient correlated well with the reported altitudinal distribution of the species.     

Soil microarthropods and macrofauna in monsoon tropical forests of Cat Tien and Bi Dup-Nui Ba National Parks,southern Vietnam

The abundance, biomass, vertical distribution, and taxonomic composition of soil invertebrates (springtails, macrofauna, and termites) were studied in forest formations differing in edaphic and climatic conditions: lowland forests dominated by Lagerstroemia spp. or Dipterocarpus spp. in the Cat Tien National Park and in a mountain pine (Pinus kesiya) forest on the Da Lat Plateau, southern Vietnam. In the lowland forests, springtails had a relatively low density (10000–12000 ind./m²), but their diversity was high (41–43 species in each forest). The density of large soil invertebrates (without ants and termites) reached 500–700 ind./m² at a biomass of approximately 30 g/m² (with earthworms accounting for up to 230 ind./m² and 19–28 g/m²). Among termites, species of the genera Macrotermes and Odontotermes were dominant. Their total biomass in some areas exceeded 15–20 g/m². In the mountain pine forest, the total biomass of soil macrofauna was approximately 11 g/m², the abundance and diversity of springtails were low (7500 ind./m², 28 species), and wood-destroying species of the genera Schedorhinotermes sp. and Coptotermes sp. dominated among termites.     

Towards understanding the traits contributing to performance of pearl millet open-pollinated varieties in phosphorus-limited environments of West Africa

Background and aims

Arbuscular mycorrhizal (AM) hyphae represent an important route for input of plant-derived C to soil, but impacts of these inputs on microbial communities and processes are poorly understood. In this study we characterised pathways of C-flow through microbial communities associated with AM hyphae and quantified impacts on mineralisation of native SOM.

Methods

Continuous, steady-state ¹³CO₂ labelling was applied throughout the growth period (60 d) of Lolium perenne. Exclusion meshes were used to control access of roots and AM hyphae to soil, and plant-derived C was quantified within microbial PLFA and NLFA, and soil CO₂ efflux was partitioned into plant- and soil organic matter (SOM) derived components.

Results

Pathways of C-flow through hyphosphere and mycorrhizosphere communities were distinct, as was the fate of plant-derived C from AM hyphae accessing soil through 37 and 1 μm meshes. Mineralisation of native SOM was increased in all treatments, relative to unplanted controls, and this priming effect was largest for AM hyphae accessing soil through the 1 μm mesh size.

Conclusions

We demonstrated that AM hyphae can strongly increase mineralisation of native SOM and identified distinct pathways of C-flow through hyphosphere communities. Our results suggest that, in addition to affecting rates of litter decomposition, AM hyphae may have a significant influence on turnover of native SOM.

     

Maternal inheritance of chloroplast DNA in <Emphasis Type="Italic">Pinus mugo</Emphasis> Turra: a case study of <Emphasis Type="Italic">Pinus mugo</Emphasis> × <Emphasis Type="Italic">Pinus sylvestris</Emphasis> crossing

The opposite modes of chloroplast DNA (cpDNA) inheritance were found to operate in the reciprocal crossings of Scots pine (Pinus sylvestris L.) and mountain dwarf pine (Pinus mugo Turra). The crossings were found to be partially compatible. In P. sylvestris × P. mugo crossing, the paternal transmission of cpDNA to the offspring takes place corroborating the generally acknowledged concept of the paternal cpDNA inheritance in gymnosperms. On the contrary, in P. mugo × P. sylvestris crossing the seed progeny exhibited P. mugo haplotype of the mother tree deviating conspicuously from the above concept. In the open pollination offspring of the putatively hybrid individuals of the Scots and mountain dwarf pines, a biparental inheritance of cpDNA was revealed in mother tree with P. mugo haplotype indicating a loosened control of the maternal inheritance of cpDNA in the putative hybrids. Implications and impacts of this finding for further studies are discussed.     

Effects of both substrate and nitrogen and phosphorus fertilizer on <Emphasis Type="Italic">Sphagnum palustre</Emphasis> growth in subtropical high-mountain regions and implications for peatland recovery

Human activities have recently caused severe destruction of Sphagnum wetlands in subtropical high-mountain regions, calling for urgent efforts to restore Sphagnum wetlands. Through a greenhouse experiment in western Hubei, China, we studied the effects of different substrate types (peat and mountain soil) and different levels of nitrogen (N) (0, 2, 4, 6, 10 g m^?2 year^?1) and phosphorus (P) (0, 0.2, 0.5, 1, 2 g m^?2 year^?1) on the growth of Sphagnum palustre, which was evaluated by four growth indicators: length growth, number of capitula, coverage change and biomass. We aimed to determine the optimal nutrient conditions for S. palustre growth, which would contribute to the rapid colonization and restoration of Sphagnum wetlands. The results showed that the different substrates significantly influenced S. palustre growth. Compared with those of peat, the acidic properties of the local yellow brown soil in the subtropical high-mountain regions were more favorable for S. palustre growth. As N addition increased, the four growth indicators responded inconsistently to the different substrates. While the number of capitula markedly increased, the other three indicators significantly decreased in the mountain soil or exhibited no definitive changes in the peat. The addition of P markedly promoted S. palustre growth in both substrates. However, a threshold for P fertilization existed; the highest productivity occurred at P additions of 0.2 and 0.5 g m^?2 year^?1 in the peat and mountain soil, respectively. The N and P contents in the capitula increased in parallel as the N and P fertilization rates increased, suggesting that these nutrients were absorbed proportionately and were used during the growth of S. palustre.     

Organic matter properties in soils afforested with Pinus radiata

Aims

Afforestation causes important alterations in SOM content and composition that affect the soil functions and C balance. The aim of this study was to identify the mechanisms that determine the changes in SOM composition following afforestation of grasslands.

Methods

The study included 4 chronosequences and 5 paired plots comprising pastures and land afforested with Pinus radiata. The SOM was characterized by ¹³C CP-MAS NMR spectroscopy and differential scanning calorimetry.

Results

During the first 10–20 year after afforestation, the changes in SOM content varied from slight gains to large losses (>40 %). The analyses revealed that even SOM compounds considered resistant to decomposition were degraded during this time. The SOM gains, observed 20 year after stand establishment, were favoured by the higher recalcitrance of pine litter and possibly by soil acidification. The concentrations of most SOM compounds, particularly the stable compounds, were higher at the end of the rotation. The low degree of protection, along with the favourable climatic conditions, may also explain the rapid decomposition of SOM, including resistant compounds, in these soils. DSC analysis complemented the information about SOM composition provided by other techniques.

Conclusions

The accumulation of stable SOM compounds at the end of the rotation suggests a longer soil C turnover in these afforested soils, which may alleviate the gradual loss of SOC in intensively managed forest soils.     

Successional patterns of key genes and processes involved in the microbial nitrogen cycle in a salt marsh chronosequence

Here, we investigated the patterns of microbial nitrogen cycling communities along a chronosequence of soil development in a salt marsh. The focus was on the abundance and structure of genes involved in N fixation (nifH), bacterial and archaeal ammonium oxidation (amoA; AOB and AOA), and the abundances of genes involved in denitrification (nirS, nirK, nosZ). Potential nitrification and denitrification activities were also measured, and increases in nitrification were found in soils towards the end of succession, whereas denitrification became maximal in soils at the intermediate stages. The nifH, nirK and nirS gene markers revealed increases in the sizes of the respective functional groups towards the intermediate stage (35 years), remaining either constant (for nifH) or slightly declining towards the latest stage of succession (for nirK and nirS). Moreover, whereas the AOB abundance peaked in soils at the intermediate stage, that of AOA increased linearly along the chronosequence. The abundance of nosZ was roughly constant, with no significant regression. The drivers of changes in abundance and structure were identified using path analysis; whereas the ammonia oxidizers (AOA and AOB) showed patterns that followed mainly N availability, those of the nitrogen fixers followed plant diversity and soil structure. The patterns of denitrifiers were group-dependent, following the patterns of plant diversity (nirK and nirS) and belowground shifts (nosZ). The variation observed for the microbial groups associated with the same function highlights their differential contribution at different stages of soil development, revealing an interplay of changes in terms of niche complementarity and adaptation to the local environment.     

Field-based artificial crossings indicate partial compatibility of reciprocal crosses between <Emphasis Type="Italic">Pinus sylvestris</Emphasis> and <Emphasis Type="Italic">Pinus mugo</Emphasis> and unexpected chloroplast DNA inheritance

Crossability relationships between Scots pine (Pinus sylvestris L.) and mountain dwarf pine (Pinus mugo Turra) was studied, using artificial pollination approach. Partial compatibility of the reciprocal crossings of these species was proved experimentally, validating the idea of a spontaneous formation of their hybrid swarms under natural conditions. The hybrids were validated using organellar DNA markers and nuclear DNA microsatellites. Based on the percentage of filled seeds, the interspecific crossings were less efficient than the intraspecific cross-pollinations of P. sylvestris and P. mugo individuals. Both species were found to intercross readily with individuals of their putative hybrid swarm, P. mugo exhibiting a higher hybridological affinity towards putatively hybrid individuals than P. sylvestris. Validation of the hybrids confirmed the paternal inheritance of chloroplast DNA (cpDNA) in the combination P. sylvestris × P. mugo only. Surprisingly, in the reciprocal crossing P. mugo × P. sylvestris, maternal inheritance of cpDNA was revealed. Obtained results offer a new insight into the direction and intensity of gene flow within the hybrid swarms of Scots pine and mountain dwarf pine.