Inorganic and organic carbon dynamics in forested soils developed on contrasting geology in Slovenia—a stable isotope approach

Purpose

Soil carbon dynamics were studied at four different forest stands developed on bedrocks with contrasting geology in Slovenia: one plot on magmatic granodiorite bedrock (IG), two plots on carbonate bedrock in the karstic-dinaric area (CC and CD), and one situated on Pleistocene coalluvial terraces (FGS).

Materials and methods

Throughfall (TF) and soil water were collected monthly at each location from June to November during 2005–2007. In soil water, the following parameters were determined: T, pH, total alkalinity, concentrations of Ca²⁺ and Mg²⁺, dissolved organic carbon (DOC), and Cl^? as well as δ¹³C_DIC. On the other hand, in TF, only the Cl^? content was measured. Soil and plant samples were also collected at forest stands, and stable isotope measurements were performed in soil and plant organic carbon and total nitrogen and in carbonate rocks. The obtained data were used to calculate the dissolved inorganic carbon (DIC) and DOC fluxes. Statistic analyses were carried out to compare sites of different lithologies, at different spatial and temporal scales.

Results and discussion

Decomposition of soil organic matter (SOM) controlled by the climate can explain the ¹³C and ¹⁵?N enrichment in SOM at CC, CD, and FGS, while the soil microbial biomass makes an important contribution to the SOM at IG. The loss of DOC at a soil depth of 5 cm was estimated at 1 mol m^?2 year^?1 and shows no significant differences among the study sites. The DOC fluxes were mainly controlled by physical factors, most notably sorption dynamics, and microbial–DOC relationships. The pH and pCO₂ of the soil solution controlled the DIC fluxes according to carbonate equilibrium reactions. An increased exchange between DIC and atmospheric air was observed for samples from non-carbonate subsoils (IG and FGS). In addition, higher δ¹³C_DIC values up to ?19.4?‰ in the shallow soil water were recorded during the summer as a consequence of isotopic fractionation induced by molecular diffusion of soil CO₂. The δ¹³C_DIC values also suggest that half of the DIC derives from soil CO₂ indicating that 2 to 5 mol m^?2 year^?1 of carbon is lost in the form of dissolved inorganic carbon at CC and CD after carbonate dissolution.

Conclusions

Major difference in soil carbon dynamics between the four forest ecosystems is a result of the combined influence of bedrock geology, soil texture, and the sources of SOM. Water flux was a critical parameter in quantifying carbon depletion rates in dissolved organic and inorganic carbon forms.

     

Sources of soil CO2 in calcareous grassland with woody plant encroachment

Purpose

The objective of this study is to estimate the contribution of various sources that influence soil CO₂ concentrations in calcareous grassland.

Materials and methods

The research was performed at the Podgorski Kras plain (45?°33?? N, 13?°55?? E, 400?C430?m.a.s.l.) in the sub-Mediterranean region of Slovenia (SW Slovenia), where many meadows and pastures have been abandoned. In parallel to the measurement of soil respiration R _s, soil gas was sampled for stable isotope analysis. Samples were taken biweekly at two sites, Grassland and Invaded, from July 2008 until November 2010. In addition, daily variations in concentration and stable isotope composition of soil CO₂ were determined in May 2009. The partitioning of soil CO₂ concentrations was performed using stable isotope mass balance calculation.

Results and discussion

The concentration and isotope composition of soil CO₂ exhibited similar seasonal variations at both sites. Lower ??¹³C_CO2 values, ranging from ?28.2 to ?15.2 ??, which occurred during warm periods and higher values, up to ?12.1 ??, were typical of cold winter periods, from December to March. Organic sources were estimated to constitute between 78 and 99?% of total soil CO₂ during warmer periods from May until October. This contribution was lower during the winter, ranging from 46 to 77?%. In winter, the atmospheric component to soil CO₂ dominated, constituting up to 60?%. On average, the inorganic contribution was estimated to comprise 12?% of the soil CO₂ at all sampling locations. The contribution of this source to soil CO₂ concentration, at up to 41?%, was highest in Grassland during the growing season. The inorganic source of soil CO₂ was also an important component during daily variations. The highest contribution was observed during the day, in parallel to the highest respiration rates.

Conclusions

The inorganic pool is shown to be an important part of soil CO₂ in calcareous areas and should be considered as equal to organic CO₂ as a source in soil CO₂ partitioning.     

Rapid loop-mediated isothermal amplification assays for grapevine yellows phytoplasmas on crude leaf-vein homogenate has the same performance as qPCR

A fluorescence-based real-time loop-mediated isothermal amplification (LAMP) assay for ‘Candidatus Phytoplasama solani’ (Bois noir phytoplasma; BNp) detection was developed and optimised for rapid laboratory and on-site BNp detection. This assay is highly specific, rapid and as sensitive as qPCR. It was validated according to European and Mediterranean Plant Protection Organisation recommendations. In addition, 286 grapevine leaf samples from the 2015 growing season were tested with this new real-time LAMP assay and an assay previously developed for detection of Flavescence dorée phytoplasma (FDp). These LAMP assays for detection of both BNp and FDp used without any DNA extraction step, which is a required step for qPCR analysis, were comparably effective to qPCR, and positive results were obtained in less than 35 min.     

Methane cycling in a drained wetland soil profile

Purpose

Peatlands have an important role in methane cycling in the natural environment. Methane emissions as a result of methanogenesis and methanotrophy in soil are affected by several environmental factors such as temperature, oxygen and groundwater level. The objective of this study was to analyse methane cycling as a function of soil depth.

Materials and methods

In this study, methane cycling and soil organic matter mineralization were investigated in a drained fen grassland area of Ljubljana marsh, Slovenia that has been subjected to reclamation strategies for several centuries. Potential mineralization, methane production and methane oxidation rates were measured in slurry incubation experiments with soil samples from 10 sampling depths of a 1-m profile. In addition, the extent of iron reduction in the soil was determined.

Results and discussion

The potential for methane production was low in the investigated soil profile, even in constantly flooded layers below the water table fluctuations. During anaerobic incubations, the highest accumulated concentrations and production rates of methane were observed in the upper 10-cm layer and the lowest in deeper soil layers, indicating that plant exudates are the main source of energy for heterotrophic soil microbes and that methanogenesis in deeper layers is limited by the availability of appropriate organic substrates. Methane oxidation was on the other hand active throughout the soil profile, suggesting that the potentially active methane oxidizing community is present despite low methane production. The highest abundance and activity of methanotrophs was detected in the water table fluctuation layers.

Conclusions

Together, these findings have implications for understanding the biogeochemical function of drained peat soils and emphasize the influence of drainage on quality of soil organic matter and consequently on methane production even in flooded soils.

     

Fiber surface characteristics evaluated by principal component analysis

Principal component analysis (PCA) was used to evaluate the results of standard fiber analyses, determinations of charge, electron spectroscopy for chemical analysis (ESCA) measurements, and selective staining of kraft fibers prebleached with oxygen, followed by hydrogen peroxide or ozone. The majority of data variance is explained by the lignin content in fibers and by polarity (hydrophilicity vs hydrophobicity) of functional groups. The lignin determination methods (kappa number, C1 (ESCA), selective staining) gave similar but not equal results, because they measure different parts of lignin. The determination methods of the charged groups (total charge, surface charge, C4 (ESCA), and hexenuronic acids) also gave similar but not equal results. The results of staining by using cationic dyes do not correlate with the quantity of anionic (mainly carboxylic) groups in fibers, regardless of whether the dyes are selective for lignin or hemicellulose. Hydrogen bonding and hydrophobic interactions seem to overrule ionic interactions between dyes and fibers. Therefore, the majority of bonds formed between fibers themselves, as well as between fibers and paper additives, can to a great extent be expected to have the character of hydrogen bonds.     

Nature and lability of northern Adriatic macroaggregates

The key organic constituents of marine macroaggregates (macrogels) of prevalently phytoplankton origin, periodically occurring in the northern Adriatic Sea, are proteins, lipids and especially polysaccharides. In this article, the reactivity of various macroaggregate fractions in relation to their composition in order to decode the potentially »bioavailable« fractions is summarized and discussed. The enzymatic hydrolysis of the macroaggregate matrix, using α-amylase, β-glucosidase, protease, proteinase and lipase, revealed the simultaneous degradation of polysaccharides and proteins, while lipids seem largely preserved. In the fresh surface macroaggregate samples, a pronounced degradation of the α-glycosidic bond compared to β-linkages. Degradation of the colloidal fraction proceeded faster in the higher molecular weight (MW) fractions. N-containing polysaccharides can be important constituents of the higher MW fraction while the lower MW constituents can mostly be composed of poly- and oligosaccharides. Since the polysaccharide component in the higher MW fraction is more degradable compared to N-containing polysaccharides, the higher MW fraction represents a possible path of organic nitrogen preservation. Enzymatic hydrolysis, using α-amylase and β-glucosidase, revealed the presence of α- and β-glycosidic linkages in all fractions with similar decomposition kinetics. Our results indicate that different fractions of macroaggregates are subjected to compositional selective reactivity with important implications for macroaggregate persistence in the seawater column and deposition.