Publications

Input of available substrates to soil can alter microbial activity resulting in accelerated turnover of native soil organic matter (SOM), i.e. cause priming effects (PE). Following to Fountaine et al. (2007) we hypothesized that the stability of SOM in deep soil horizons is due to the lack of input of fresh organic substrates. We also hypothesized greater PE in mineral versus organic soil horizons. These hypotheses were checked by the comparison of priming effects induced by 14C-glucose in organic and mineral horizons of modern as well as of paleo-soils (podzol sandy soil Yamalo-Nenezky region, Tumen). The following variables were determined in 50-days incubation experiment: 1) dynamics of CO2 evolution; 2) 14CO2 originated from the added glucose; 3) microbial biomass C by substrate-induced respiration; 4) activities of extracellular enzymes (β-glucosidase, chitinase, cellobiogidrolase and xylanase) with fluorogenically labeled substrates. Maximal intensity of SOM mineralization as well as of enzyme activities was observed at 2 -7 days after glucose application. The absolute values of PE were 10 times greater in modern as compared with buried horizons of paleo-soils. However, the relative increase in carbon mineralization (as compared with control soil without glucose amendment) was greater in buried than in modern soils, especially in mineral soil horizons. In organic horizons the PE amounted for 20 and 50 % of untreated control in modern and in paleo-soils, respectively. In mineral horizons the PE amount (in % of control) reached 60 % for modern and 250 % for paleo-soils. We conclude that the input of fresh organic matter in paleo-soils as well as in deep soil horizons can induce greater PE as compared with topsoil layers. This conclusion was further confirmed by the increased activity of hydrolytic enzymes during PE in modern and in buried soils.

Soil organic matter (SOM) formation involves microbial transformation of plant materials of various quality with physico-chemical stabilisation via soil aggregation. Land use and vegetation type can affect the litter chemistry and bioavailability of organic carbon (OC), and consequently influence the processing and stabilisation of OC into SOM. We used 13C nuclear magnetic resonance (13C NMR) and hot-water extraction to assess the changes in chemical composition and labile OC fractions during the transformation processes from leaf to litter to SOM depending on land use and vegetation type. The hot-water-extractable OC (HWEOC) decreased from leaf (43–65 g kg−1) to litter (19–23 g kg−1) to SOM (8–16 g kg−1) similar in four land use types: grassland, sugarcane, forest and banana. These trends demonstrated the uniform converging pathways of OC transformation and increasing stability by SOM formation. The preferential decomposition and decrease of labile OC fractions (∑% di-O-alkyl, O-alkyl and methoxyl) from leaf (54–69%) to SOM (41–43%) confirmed the increasing stability of the remaining compounds. Despite differences in the biochemical composition of the leaf tissues among the vegetation types, the proportions of labile OC fractions in SOM were similar across land uses. The OC content of soil was higher in forest (7.9%) and grassland (5.2%) compared to sugarcane (2.3%) and banana (3.0%). Consequently, the HWEOC per unit of soil weight was higher in forest and grassland (2.0 and 1.2 g kg−1 soil, respectively) compared to sugarcane and banana (0.3 and 0.4 g kg soil−1, respectively). The availability of labile SOM is dependent on the quantity of SOM not the chemical composition of SOM. In conclusion, labile OC fractions in SOM, as identified by 13C NMR, were similar across land use regardless of vegetation type and consequently, SOM formation leads to convergence of chemical composition despite diversity of OC sources.

To investigate the effects of shrubification—the global phenomena of an increase of shrubs in grasslands—on C, N, and P cycles, the changes in

The release of organic compounds from plant roots (rhizodeposits) and their subsequent utilization by soil microbial communities is a key process linking atmospheric and terrestrial carbon (C) and nutrient cycling. However, the effects of land-use types on rhizodeposit-mediated alteration of microbial community structure and its consequences for soil C cycle are still, largely, unknown. Therefore, we used 13C-PLFA-SIP to evaluate the rhizosphere C fluxes to microorganisms in a monoculture crop (oilseed rape, Brassica napus L.), a mixed grassland (dominated by Lolium perenne L. mixed with clover Trifolium repens L.), and a tree (willow, Salix schwerinii E.L. Wolf and Salix viminalis L.) within an agroforestry system. Though harboring similar total microbial biomass, less 13C was recovered in microorganisms in soil under willow (0.017 % of 13C input) after 14 days as compared to rape (0.03 %) and grassland (0.09 %). Across three land use types, gram negative bacteria incorporated most of the 13C (28–42 % of total 13C labeled PLFAs), followed by fungi and gram positive bacteria (12–18 %) and less abundance groups (e.g. actinomycetes, <5 %). The arbuscular mycorrhizal fungal biomarker 16:1ω5c was enriched with 13C in soil under grassland (8 % of total 13C labeled PLFAs). The 13C incorporation into gram positive bacteria was higher in soil under willow than under grassland and rape. By contrast, 42 % of total 13C labeled PLFAs were attributed to gram negative bacteria in soil under rape. Overall, our results demonstrated that pronounced differences in microbial community structure and subsequent microbial C utilization between land-use types could affect soil C cycles and storage.

Extensive, progressive rock emergence causes localized variations in soil biogeochemical and microbial properties that may influence the capacity for the regeneration of degraded karst ecosystems. It is likely that karst ecosystem recovery relies on the persistence of soil functions at the microbial scale, and we aimed to explored the role of interactions between soil bacterial taxa and identify keystone species that deliver key biogeochemical functions, i.e. carbon (C) and nutrient (nitrogen, N and phosphorus, P) cycling. We applied high-throughput sequencing and phylogenetic molecular ecological network approaches to topsoils sampled at rock-soil interfaces and adjacent bulk soil along an established gradient of land-use intensity in the Chinese Karst Critical Zone Observatory. Bacterial α-diversity was greater under increased perturbation and at the rock-soil interface compared to bulk soils under intensive cultivation. However, bacterial ecological networks were less intricate and connected fewer keystone taxa as human disturbance increased and at the rock-soil interface. Co-occurrence within the bacterial community in natural primary forest soils was 13% larger than cultivated soils. The relative abundances of keystone taxa Acidobacteria, Bacteroidetes and Chloroflexi increased with land-use intensity, while Proteobacteria, Actinobacteria and Verrucomicrobia decreased by up to 6%. In general, Bacteroidetes, Verrucomicrobia and Chlorobi were related to C-cycling, Proteobacteria, Actinobacteria and Chloroflexi were related to N-cycling, and Actinobacteria and Nitrospirae were related to both N- and P-cycling. Proteobacteria and Chlorobi affected C-cycling and multiple functionality indexes in the abandoned land. We conclude that increasing land-use intensity changed the soil bacterial community structure and decreased bacterial interactions. However, increases in α-diversity at the rock-soil interface in cultivated soils indicated that major soil functions related to biogeochemical cycling were maintained within keystone taxa in this microenvironment. Our study provides foundations to test the success of different regeneration practices in restoring soil microbial diversity and the multifunctionality of karst ecosystems.

Sustainable agricultural production is essential to ensure an adequate food supply, and optimal farm management is critical to improve soil quality and the sustainability of agroecosystems. Integrated soil–crop management based on crop models and nutrient management designs has proven useful in increasing yields. However, studies on its effects on the chemical composition of soil organic carbon (SOC) and microbial community composition, as well as their linkage with crop yield, are lacking. Here, we investigated the changes in SOC content, its chemical functional groups, and bacterial communities, as well as their association with crop yield under different farmland management based on four farmland management field trials over 12 years (i.e., FP: farmer practice; IP: improved farmer practice; HY: high-yield system; and ISSM: integrated soil–crop system management). The crop yield increased by 4.1–9.4% and SOC content increased by 15–87% in ISSM compared to other farmland management systems. The increased proportion of Methoxy C and O-alkyl C functional groups with a low ratio of Alkyl C/O-alkyl C, but high Aliphatic C/Aromatic C in ISSM hints toward slow SOC decomposition and high soil C quality. The relative abundances of r-strategists (e.g., Firmicutes, Myxobacteria, and Bacteroidetes) was highest under the ISSM. Co-occurrence network analysis revealed highly complex bacterial communities under ISSM, with greater positive links with labile SOC functional groups. The soil fertility index was the main factor fueling crop yields, as it increased with the relative abundance of r-strategists and SOC content. Our results indicated that crop yield advantages in ISSM were linked to the high C quality and shifts in bacterial composition toward r-strategists by mediating nutrient cycling and soil fertility, thereby contributing to sustainability in cropping systems.

We examined changes in the morphological, physicochemical properties and features of the organic matter of forest soils impacted by wildfires on the territory of Russia. Morphological signs of pyrogenesis (pyrogenic horizon formation, partial charring of litter and illuviation of organic compounds) are most evident detected in the first decade after a fire. Ground fires in lichen pine forests, formed on Albic Podzols lead to complete burning of litter. Low intensity ground fires in sphagnum pine forests, developing on Histic Podzols, contribute to partial burning of litter (charring). Fires change the hydrothermal regime of soils, which is most clearly demonstrated for soils formed on permafrost soils / cryosols. Fires lead to hydrophobization of the upper mineral horizons, estimated from the contact angle of wetting. Resistant products of pyrogenesis (charcoals, soot) are retained in soils for several centuries. The most common changes in the physical and chemical properties of soils after fires are a decrease in acidity by 1–2 units of pH, an increase of saturation with base saturation. Fires increase aromaticity of soil organic matter. After fires, the content of polyaromatic hydrocarbons in soils increases, and the concentrations of water-soluble organic compounds decrease. Restoration of soil properties to the prefire state takes a decade to several centuries. The introduction of a universal subtype “pyrogenic” is proposed in describing the morphological characteristics of forest soils.

Mucilage is a mixture of polysaccharides and some lipids which is secreted by the root tip. It facilitates plant nutrient acquisition, stabilizes aggregates, reduces lubrication during plant growth and may increase rhizosphere water content due to its high water holding capacity. So far there is no method to measure the spatial distribution of mucilage in soil around roots. The aim of this study was to test whether infrared spectroscopy can be applied to quantify gradients of mucilage around roots in soil. The C-H to C-O ratio obtained from infrared spectroscopy measurements is an indicator of soil hydrophobicity. As Mucilage turns hydrophobic after drying we hypothesized that mucilage can be detected by the C-H to C-O ratio measured with infrared spectroscopy. We grew maize plants in rhizoboxes filled with quartz silt. Before measurement the planted containers were dried and the roots were removed from soil. Infrared spectroscopy measurements were conducted with a spatial resolution of 50x50 µm a) radially with increasing distance from the root channel center and b) axially with increasing distance from the root channel tip. In parallel, the contact angle, which also indicates soil hydrophobicity, was quantified in the same locations. Both measurements were additionally conducted on glass slides covered with quartz silt mixed with given concentrations of mucilage. The measurements on the glass slides revealed that the C-H to C-O ratio and the contact angle measurements correlated well with the mucilage concentration in soil. Similarly, the infrared spectroscopy measurements in in the rhizoboxes revealed that radial profiles of mucilage around roots can be quantified: while the C-H to C-O ratio was highest inside the root channels, it decreased to the bulk soil values 0.7 mm in radial direction from the border of the root channel. In axial direction the C-H to C-O ratio did not change significantly, indicating that those compounds causing hydrophobicity of mucilage are not easily degraded by soil microorganisms. We showed that infrared spectroscopy can be applied to measure profiles of mucilage around roots in soil. The radial profiles of mucilage were narrower than those reported for other rhizodeposits which may be explained by the viscosity of mucilage.

This study aimed to trace the incorporation of 13C-labeled hydrophilic and hydrophobic compounds extracted from maize straw into bacterial and fungal phospholipid fatty acids (PLFAs) in upland and paddy soils. In both soils, the amounts of bacterial 13C-PLFAs recovered from hydrophilic C were 1.2–2.8 times higher than those from hydrophobic C. By contrast, the amounts of fungal 13C-PLFAs recovered from hydrophobic C were 3.8–12.8 times greater than those from hydrophilic C. These results indicate that bacteria have a preference for the uptake of hydrophilic compounds, whereas fungi prefer hydrophobic compounds in both soils. The fungal preference for hydrophobic compounds is stronger than bacterial preference for hydrophilic compounds. This is partly owing to the efficient incorporation of hydrophobic compounds into actinomycete PLFA of 10Me18:0 in upland soil and bacterial PLFA of 18:1ω7c in paddy soil. Consequently, the specific uptake of hydrophilic and hydrophobic compounds from plant residues by bacteria and fungi will lead to divergent C pathways in soil.

Conversion of grassland to cropland affects microbial transformation of plant derived organic substances and the accumulation of microbially synthesized products in soil. We investigated long-term effects of agricultural use [more than 50 years' rotation with barley (Hordeum vulgare) and rapeseed (Brassica rapa)] after conversion from grasslands (grazed in the winter season) on the contents and composition of non-cellulose sugars and amino sugars in a broad range of soils on the Tibetan Plateau. Soils from two types of croplands (barley or rapeseed) were compared to a reference grassland soil by detailed analysis of plant vs. microbial and bacterial vs. fungal biomarkers. Long-term cultivation decreased total soil organic matter (SOM), light SOM and total non-cellulose sugar contents in the 0–20 cm soil by 27%, 47–72% and 57%, respectively, reflecting decreased root biomass compared to grassland. The ratios of (galactose + mannose)/(arabinose + xylose) and (rhamnose + fucose)/(arabinose + xylose) were both 26% smaller in cropland than in grassland soils, while the ratios of hemicelluloses in shoots or roots were similar between native and cultivated plants. Consequently, net transformation of plant substances to microbially synthesized polysaccharides decreased in cultivated soils. The total amino sugars (muramic acid, glucosamine, mannosamine, galactosamine) in cropland soils decreased by 42% as did their contribution to the SOM pool by 22%, compared to grassland soils, but the ratio of glucosamine/muramic acid in cropland soils doubled when compared to the grassland soil. This shows a strong decrease in microbial residue under cultivation, with the magnitude of the decrease greater in the bacterial than in the fungal components. All the above results from the intensively sampled site were confirmed in seven other sites featuring lower sampling intensity. We concluded that the conversion of grassland to cropland strongly decreases microbial transformation of plant residues and accumulation of the resulting microbial compounds – necromass – in soil (i.e., reduction of microbial input to stable SOM). The conversion also leads to a shift in the composition of microbial compounds towards a decreasing contribution of bacterial compared to fungal necromass.

Positive priming effect following the incorporation of biochar may partially reduce the biochar-induced soil carbon (C) sequestration. We hypothesize that

The conversion from cereal fields to vegetable production in the last three decades represents a significant shift in land use in China. Here, we studied the effects of conversion form cereal fields to vegetable production in north China on soil organic carbon (SOC) and total nitrogen (TN) in both bulk soil and soil aggregates. We used two approaches: (1) measurements of paired soil samples from wheat (Triticum aestivum L.) – maize (Zea mays L.) fields and adjacent greenhouses vegetable fields in three vegetable production areas representing various management intensities in terms of C and N inputs and frequency of tillage; (2) fractionating soil to distinguish intra-aggregate particulate organic matter (iPOM) and organo-mineral complexes (silt+clay). Our results indicated that converting cereal fields to greenhouse vegetable production with intermediate and high management intensity led to increases in SOC and TN and decreases in C:N ratios in the top soil. The accumulation rates of C and N in the surface soil (0–30cm) were estimated to be 1.37MgCha−1 yr−1 and 0.21MgNha−1 yr−1 over an average period of 8 years after cereal fields to greenhouse vegetable production conversion. At the soil aggregate level, only the coarse (>250μm) and fine (53–250μm) iPOM fraction contributed to the increases in soil C (e.g., 49% and 51% of total C increases, respectively), while the coarse and fine iPOM, and silt+clay fraction accounted for 22%, 30% and 48%, respectively, of total N increases. This illustrates how the addition of readily available C (manure) and N (manure and inorganic N) leads to a temporary stabilization of C in relatively labile SOM fractions, but to a preferential stabilization of N in organo-mineral SOM fractions. In conclusion, the conversion to highly intensive vegetable systems in China leads to marked differences in C and N stabilization dynamics.