A high soil potential net nitrification was observed at 4 m depth: What are the driving factors?

Studies about nitrogen (N) mineralization and nitrification in deep soil layers are rare because N processes are considered to occur mainly in topsoil that hosts active and diverse microbial communities. This study aimed to measure the soil potential net N mineralization (PNM) and nitrification (PNN) down to 4 m depth and to discuss factors controlling their variability. Twenty-one soil cores were collected at the Restinclières agroforestry experimental site, where 14-year-old hybrid walnut trees were intercropped with durum wheat. Soil cores were incubated in the dark in the laboratory at both 6 and 25°C. The soil was a deep calcic fluvisol with a fluctuating water table. It featured a black layer that was very rich in organic matter and permanently water saturated at depths between 3.0 and 4.0 m. The mean soil mineral N content was 3 mg N kg⁻¹ soil in the upper 0.0–0.2 m layer, decreasing until a depth of 2 m and increasing to the maximum value of 25.8 mg N kg⁻¹ soil in the black layer. While nitrate (NO₃⁻) was the dominant form of mineral N (89%) in the upper 0.0–0.2 m layer, its proportion progressively decreased with depth until ammonium (NH₄⁺) became almost the only form of mineral N (97%) in the saturated black layer. Laboratory soil incubation revealed that PNM and PNN occurred at all depths, although the latter remained low at 6°C. The soil nitrate content in the black layer was multiplied by 48 times after 51 days of incubation at 25°C, whereas it was almost inexistent at the sampling date. While the soil total N, the pH and the incubation temperature explained 84% of the variation in PNM, only 29% of the percent nitrification variance was explained by the incubation temperature (T_inc) and the soil C-to-N ratio. These results point out the necessity to consider soil potential net N mineralization and nitrification of deep soil layers to improve model predictions.     

Experimental platforms for the investigation of spatiotemporal patterns in the rhizosphere—Laboratory and field scale

The numerous feedback loops between roots, microorganisms, soil chemical and physical properties, and environmental variables result in spatial parameter patterns which are highly dynamic in time. In order to improve our understanding of the related rhizosphere processes and their relevance at the soil–plant system scale, experimental platforms are required. Those platforms should enable (1) to relate small scale observations (nm to dm) to system behaviour, (2) the integration of physical, chemical and biological sampling approaches within the same experiment, and (3) sampling at different time points during the life cycle of the system in question. Here we describe what requirements have to be met and to what extent this has been achieved in practice by the experimental platforms which were set up within the framework of DFG priority programme 2089 “Rhizosphere Spatiotemporal Organisation—a key to rhizosphere functions”. It is discussed to what extent theoretical considerations could be accommodated, in particular for the comparison across scales, i.e., from laboratory to field scale. The latter scale is of utmost importance to overcome the trade‐off between fraction of life cycle covered and the avoidance of unrealistic root length densities.     

Biofilm formation vs. PCB adsorption on granular activated carbon in PCB-contaminated aquatic sediment

Purpose

Granular activated carbon (GAC) that is both an efficient polychlorinated biphenyl (PCB) adsorbent and a good growth support for microorganisms is considered suitable for in situ remediation of PCB-contaminated aquatic sediment. Here, the potential competition between biofilm formation and PCB (Aroclor 1260) adsorption on GAC in a PCB-contaminated aquatic sediment was investigated.

Materials and methods

GACs, both coated and uncoated with biofilm or PCBs, were incubated with sediment. Each was monitored for biofilm development and PCB adsorption by: (a) cryo-scanning electron microscopy, (b) real-time quantitative PCR analysis of the 16S rRNA, (c) terminal restriction fragment length polymorphism and (d) chemical analysis.

Results and discussion

Biofilm formation on the GAC by the sediment's bacteria was rapid and occurred in three stages: (1) initial adherence of discrete bacteria, (2) an increase in biomass associated with a shift in bacterial diversity and (3) exopolymeric matrix production. The density and biodiversity of the multispecies biofilm depended on the biofilm's age. The addition of Aroclor 1260 to the sediment resulted in a decrease of the biofilm biomass, whereas Aroclor 1260 previously adsorbed on the GAC prior to contact with the sediment did not influence the biofilm biomass or its formation dynamics. Similarly, a biofilm previously developed on the GAC did not significantly decrease PCB adsorption, although contact of the GAC with the sediment did hamper adsorption of the higher chlorinated PCB congeners.

Conclusions

A mature multispecies bacterial biofilm developed in 1 month on GAC in contact with aquatic sediment did not hamper PCB adsorption, and PCB adsorption did not influence biofilm formation. These findings are important for the application of remediation strategies.     

Carbon isotopic signature of CO2 emitted by plant compartments and soil in two temperate deciduous forests

? Context

The carbon isotope composition of the CO₂ efflux (δ¹³C_E) from ecosystem components is widely used to investigate carbon cycles and budgets at different ecosystem scales. δ¹³C_E, was considered constant but is now known to vary along seasons. The seasonal variations have rarely been compared among different ecosystem components.

? Aims

We aimed to characterise simultaneously the seasonal dynamics of δ¹³C_E in different compartments of two temperate broadleaved forest ecosystems.

? Methods

Using manual chambers and isotope ratio mass spectrometry, we recorded simultaneously δ¹³C_E and δ¹³C of organic matter in sun leaves, current-year twigs, trunk bases and soil in an oak and a beech forest during 1 year.

? Results

In the two forests, δ¹³C_E displayed a larger variability in the tree components than in the soil. During the leafy period, a pronounced vertical zonation of δ¹³C_E was observed between the top (sun leaves and twigs with higher values) and bottom (trunk and soil with lower values) of the ecosystem. No correlation was found between δ¹³C_E and δ¹³C of organic matter. Causes for these seasonal variations and the vertical zonation in isotope signature are discussed.

? Conclusion

Our study shows clear differences in values as well as seasonal dynamics of δ¹³C_E among different components in the two ecosystems. The temporal and local variation of δ¹³C_E cannot be inferred from organic matter signature or CO₂ emission rates.     

Relationship between tree morphology and growth stress in mature European beech stands

Aims

In European Beech (Fagus sylvatica L.) large growth stresses lead to severe log end splitting that devaluate beech timber. Our study aimed at detecting relationships between growth stress and some morphology parameters in trees.

Methods

Growth stress indicators were recorded for 440 mature trees in nine stands from five European countries, together with morphology parameters.

Results

Most trees displayed an uneven distribution of growth stress around the trunk. Moreover, growth stress intensity varied largely between individual trees. Geometry of the trunk was a poor predictor of growth stress intensity. Crown asymmetry resulted in a larger stress dissymmetry within trees. Trunk inclination was not correlated to max tension stress, contrary to what is usually found in younger trees. In the case of small inclination, growth stress was close to expected from biomechanics of restoring verticality. Trees exhibiting a larger inclination probably evolved a different mechanical solution: a rather large crown, lower tree slenderness and a sufficient asymmetry in growth stress as to prevent a higher inclination due to growth.

Conclusion

A large slenderness is the best accurate predictor of a large growth stress, although variations in the ratio height/diameter at breast height explained only 10 % of the variability of growth stress. A large crown surface was the best predictor of a low level of growth stress. A large spacing between trees seems a good solution to lower the risk of growth stress in mature beech.     

High rate of transplacental infection and transmission of Neospora caninum following experimental challenge of cattle at day 210 of gestation

In order to investigate the pathogenesis of neosporosis following a primary infection in late pregnancy, cattle were subcutaneously challenged with 5 × 10⁸Neospora caninum (NC1 isolate) tachyzoites at day 210 of gestation and serial necropsies were then carried out at 14, 28, 42 and 56 days post-infection (dpi). No abortions occurred and all the foetuses were viable at the time of euthanasia. There was a high rate of vertical transmission, as parasites were detected by immunohistochemical labelling and PCR in all the foetuses from 28 dpi. Focal necrotic lesions were observed in the placentomes of the placenta from 28 dpi and showed resolution during later time points, denoted by infiltration of inflammatory cells at 42 dpi and fibrosis at 56 dpi. Foetuses at 28 and 42 dpi showed scarce and isolated lesions which are unlikely to represent a threat to foetal viability. No lesions were observed in the foetuses at 14 or 56 dpi suggesting control of the infection and resolution of the lesions by maternal and foetal immune responses. Once infection was established, it could not be cleared from the host and vertical transmission of the parasite occurred in all infected hosts. Parasite was detected in the placenta at 28 dpi, while in previous experimental infections of cattle at day 70 and 140 of gestation using the same challenge model, it was already present at day 14 post infection. This suggests that a change in the maternal immune response plays a crucial role in limiting the initial infection during the last term of pregnancy.     

MRI characteristics of cerebral microbleeds in four dogs

Cerebral microbleeds in people are small foci of hemosiderin-containing macrophages in normal brain parenchyma. They are the remnant of previous hemorrhage and occur with greater frequency in older individuals. Our purpose was to describe the magnetic resonance (MR) appearance of cerebral microbleeds in four dogs. These lesions appeared as round, hypointense foci measuring ≤4 mm on T2*-gradient-recalled echo images. They were less conspicuous or absent on T2-weighting, being iso- or hypointense, and uniformly invisible on T1-weighted images. No contrast enhancement was seen in any of the cerebral microbleeds. Necropsy-derived histopathologic analysis of one brain confirmed these lesions to be chronic cerebrocortical infarcts containing hemosiderin. The MR changes seen in dogs were analogous to what has been described in people and will be helpful in distinguishing cerebral microbleeds from other brain lesions.     

Impacts of naturally acquired protozoa and strongylid nematode infections on growth and faecal attributes in lambs

On two separate sampling occasions, faecal samples were collected from lambs (2-5 months of age) grazing pasture on two separate sheep farms in southern Western Australia. Live weight, body condition score (BCS), faecal consistency score (FCS) and faecal dry matter percentage (DM%) were measured. Faecal samples were screened by PCR for Cryptosporidium (18S rRNA, actin and 60 kDa glycoprotein [gp60] loci), Giardia duodenalis (glutamate dehydrogenase [gdh] and β-giardin) and patent strongylid nematode infections (ITS-2 nuclear ribosomal DNA for Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus spp. Chabertia ovina and Oesophagostomum spp.). Faecal worm egg counts (WECs) were performed using a modified McMaster WEC technique. The WECs were adjusted for FCS and transformed using log(10)(adjusted WEC+25) prior to statistical analyses. Cryptosporidium, Giardia and Trichostrongylus spp. detected by PCR were associated with an increased risk of non-pelleted faeces (FCS ≥ 3.0) for both flocks. Cryptosporidium-positive lambs were 2.8-11.6 times more likely to have non-pelleted faeces and Giardia-positive lambs were 2.4-14.0 times more likely to have non-pelleted faeces compared to lambs negative for each respective parasite. Lambs positive for both Cryptosporidium and Giardia were 2.9-11.8 times more likely to have non-pelleted faeces than lambs positive for only one or neither of these parasites. Mixed internal parasite infections were found to have greater impacts on FCS and BCS than single infections. A higher number of internal parasites detected per lamb was associated with lower BCS and more loose faeces. The relationship between parasite detection and live weight or growth rate were inconsistent for both flocks. Adjusted WEC was correlated with FCS and faecal DM% for one flock only, although little or no correlation was found with live weight and growth rate for both flocks. Cryptosporidium ubiquitum and Cryptosporidium parvum were the most prevalent Cryptosporidium species isolated in the two flocks. Giardia assemblage E was the most commonly isolated genotype assemblage from both flocks, while assemblage A was isolated almost as frequently as assemblage E in the one flock. One flock was a potential source of zoonotic Cryptosporidium and the other flock was a potential source of zoonotic Giardia.     

Regional modeling of grassland biogeochemistry using GIS

We used an ecosystem coupled to a Geographic Information System (GIS) to simulate spatial variability in storage and fluxes of C and N within grassland ecosystems. The GIS contained information on driving variables required to run the model. These were soil texture, monthly precipitation and monthly minimum and maximum temperatures. We overlayed polygon maps of the above variables to produce a driving variable map of our study region. The final map had 768 polygons in 160 unique classes. The ecosystem model was run to a steady state for each class and NPP, soil organic matter (SOM), net N mineralization and trace gas emission were mapped back into the GIS for display. Variation in all of the above propertiees occurred within the region. NPP was primarily controlled by climate and patterns followed spatial variation in precipitation closely. Soil organic matter, in contrast, was controlled largely by soil texture within this climatic range. Error associated with aggregation within the study area showed that spatial averages over the study area could be used to drive simulations of NPP, which is linearly related to rainfall. More spatial detail had to be preserved for accurate simulation of SOM, which is nonlinearly related to texture. Mechanistic regional models form a valuable link between process studies and global models.