Spatial Distribution of Mucilage in the Rhizosphere Measured With Infrared Spectroscopy

Mucilage is receiving increasing attention because of its putative effects on plant growth, but so far no method is available to measure its spatial distribution in the rhizosphere. We tested whether the C-H signal related to mucilage fatty acids is detectable by infrared spectroscopy and if this method can be used to determine the spatial distribution of mucilage in the rhizosphere. Maize plants were grown in rhizoboxes filled with soil free of organic matter. Infrared measurements were carried out along transects perpendicular as well as axially to the root channels. The perpendicular gradients of the C-H proportions showed a decrease of C-H with increasing distance: 0.8 mm apart from the root center the C-H signals achieved a level near zero. The measured concentrations of mucilage were comparable with results obtained in previous studies, which encourages the use of infrared spectroscopy to quantitatively image mucilage in the rhizosphere

The complexity of epigenetic diseases

Over the past 30 years, a plethora of pathogenic mutations affecting enhancer regions and epigenetic regulators have been identified. Coupled with more recent genome‐wide association studies (GWAS) and epigenome‐wide association studies (EWAS) implicating major roles for regulatory mutations in disease, it is clear that epigenetic mechanisms represent important biomarkers for disease development and perhaps even therapeutic targets. Here, we discuss the diversity of disease‐causing mutations in enhancers and epigenetic regulators, with a particular focus on cancer. © 2015 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland

Rhizosphere carbon priming: a plant mechanism to enhance soil nitrogen accessibility?

AIMS: Soil priming affects soil N transformation and plant N availability, but few studies have investigated these interactions to date. METHODS: To address this, we reviewed the literature for studies quantifying soil priming, soil N transformation and plant N uptake. RESULTS: Gross N mineralization was strongly controlled by soil priming in studies with plants, while abiotic factors had a minor influence on gross N mineralization. In contrast, soil priming was negatively related to gross N mineralization and had a low explanatory power in incubation studies where substrates are added as surrogates for root exudates. These results indicate that plants support increased N mineralization and that this is not adequately reflected in incubation studies. Additionally, we observed a positive relationship between soil priming and the % of Norg-derived N uptake as well as total N uptake, which demonstrates that priming enhances the availability of N that was previously organically bound and that at least part of the N mineralized during priming was available for plant uptake. CONCLUSION: Our results show that the effect of roots and rhizodeposition leads to a number of processes supporting N mineralization and availability through priming that are not well reflected in incubation studies. To fully capture the interactions between plant roots and their associated microbiota, we recommend focusing research on systems with plants. Additionally, the strong correlation between C and N transformation should be considered in biogeochemical modelling

Application of planar optodes to measure CO2 gradients in the rhizosphere of unsaturated soils

International audienceSoil respiration is tightly linked to rhizosphere processes, which are characterized by high spatial variability. We tested whether planar optodes can be applied to capture this spatial variability of CO2 in the rhizosphere. Maize (Zea mays) was grown in rhizoboxes and CO2 concentration around roots was measured at three volumetric soil water contents (VWC) using planar optodes. Gradients of CO2 were clearly visible around root tips but less pronounced around mature root parts probably due to high root respiration and microbial activity around tips. Saturating the soil from 21% VWC increased the measured CO2 concentration from 0.23 to 1.47 μmol CO2 L−1. Statistical comparisons between VWC levels indicated that small changes in soil WC might strongly affect CO2 measurements. However, provided that the soil moisture is kept constant, optode measurements of CO2 can be used in moist soil samples to quantify relative differences in CO2 concentration between treatments or soil region

Effects of Mucilage on Rhizosphere Hydraulic Functions Depend on Soil Particle Size

Mucilage secreted by roots alters hydraulic properties of soil close to the roots. Although existing models are able to mimic the effect of mucilage on soil hydraulic properties for specific soils, it has not yet been explored how the effects of mucilage on macroscopic soil hydraulic properties depend on soil particle size. We propose a conceptual model of how mechanistic pore-scale interactions of mucilage, water, and soil depend on pore size and mucilage concentration and how these pore-scale characteristics result in changes of macroscopic soil hydraulic properties. Water retention and saturated hydraulic conductivity of soils with different ranges of particle sizes mixed with various mucilage concentrations were measured and used to validate the conceptual model. We found that (i) at low mucilage concentrations, the saturated conductivity of a coarse sand was a few orders of magnitude higher than that of a silt, (ii) at an intermediate concentration, the hydraulic conductivity of a fine sand was lower than of a coarse sand or a silt, and (iii) at a high concentration, all soils had a hydraulic conductivity of the same magnitude. At low matric potentials, mucilage increased the water content in all soilsin all soils. In coarser soils, higher mucilage concentrations were needed to induce an increase in water content of >0.05 g g at low matric potentials. This study shows how pore-scale interactions between mucilage, water, and soil particles affect bulk soil hydraulic properties in a way that depends on soil particle size. Including such effects in quantitative models of root water uptake remains challenging

Spatially-distributed microbial enzyme activities at intact, coated macropore surfaces in Luvisol Bt-horizons

Soil macropores serve as preferential pathways for water and solute transport as well as for root growth. They are often coated with organic material and known as "hotspots" of nutrient and C turnover. Differences in the SOM composition between macropores and soil matrix as well as between macropore types (biopores, cracks, pinhole fillings) imply potential differences in the microbial community composition and enzymatic activities. The objective of this work was to detect and assess the spatial distribution of enzyme activities related to C turnover, xylanase (XYL) and phenol oxidase (POX), and the composition of microbial communities in structural components of Luvisol Bt-horizons, developed from loess and glacial till. We applied conventional enzyme assays and phospholipid fatty acids (PLFA) analysis to study materials separated from different types of macropores and soil components as well as bulk soil samples. The spatial distribution of enzyme activities on surfaces of large soil core slices (20 cm in diameter) was quantified by soil zymography. Higher XYL activities were detected in separated burrow wall materials, clay-organic coatings, and pinhole fillings from both sites, as compared to the respective soil matrix or bulk soil samples. The XYL activities correlated with bacteria-specific PLFAs. POX activities were solely found increased for earthworm burrow walls from the loess-derived Bt-horizon, but not for burrows from till samples. Zymograms revealed particularly increased XYL activities at rooted earthworm burrows, emphasising evidence for hotspots of enzyme activity and C turnover. The zymography of POX was hampered by methodological restrictions.10819315

Similar strong impact of N fertilizer form and soil erosion state on N2O emissions from croplands

Soil erosion affects 20% of croplands worldwide. However, understanding the effect of soil erosion on N2O emissions, which is one of the most potent greenhouse gases, is still limited. This limitation is likely because the small-scale differences in soil properties and fertility induced by erosion (i.e. ranges of erosion states) have barely been considered in studies quantifying N2O emissions from croplands. There are, however, indications that the erosion state itself strongly impacts N2O emission, similar to the N fertilizer form. Therefore, our investigations aimed to further explore these indications. We measured N2O fluxes for three years and at five sites within an erosion affected field experiment. N2O emissions were quantified using a manual chamber system. Three sites were established on a summit position (Albic Luvisol; non-eroded) but differed in N fertilizer forms (organic biogas fermented residues, calcium ammonium nitrate and a mixture of both fertilizers). Two additional sites were established on an extremely eroded soil (Calcaric Regosol) and wet depositional soil in a depression (Endogleyic Colluvic Regosol) to measure the effect of soil erosion states on N2O emissions. Both additional sites were fertilized with calcium ammonium nitrate only. In case of the non-eroded soil (summit), organic fertilization resulted in the highest cumulative N2O emission (6.2 ± 0.21 kg N2O-N/ha y−1) compared to mixed (5.5 ± 0.18 kg N2O-N/ha y−1) and mineral (1.9 ± 0.17 kg N2O-N/ha y−1) fertilization. These high emissions were probably caused by soluble C and N substrates from organic fertilizer, resulting in microbial activities favoring high N2O emissions. Regarding the erosion status, we observed the highest N2O emissions in the depositional soil (2.8 ± 0.21 kg N2O-N/ha y−1), followed by the non-eroded (1.9 ± 0.17 kg N2O-N/ha y−1) and the extremely eroded soil (0.6 ± 0.03 kg N2O-N/ha y−1). These differences in N2O emissions were mainly due to the site-specific, erosion induced differences in soil properties such as soil texture, soil organic C and total N contents and stocks, water-filled pore space and soil pH. These results indicate that soil erosion state may indeed be of similar importance, as N fertilizer form, for the magnitude of N2O emissions from croplands