Contributions of freshwater mussels (Unionidae) to nutrient cycling in an urban river: filtration,recycling, storage,and removal

Physical separation of soil into different soil organic matter (SOM) fractions is widely used to identify organic carbon pools that are differently stabilized and have distinct chemical composition. However, the mechanisms underlying these differences in stability and chemical composition are only partly understood. To provide new insights into the stabilization of different chemical compound classes in physically-separated SOM fractions, we assessed shifts in the biomolecular composition of bulk soils and individual particle size fractions that were incubated in the laboratory for 345 days. After the incubation, also the incubated bulk soil was fractionated. The chemical composition of organic matter in bulk soils and fractions was characterized by ¹³C-CPMAS nuclear magnetic resonance spectroscopy and sequential chemical extraction followed by GC/MS measurements. Plant-derived lipids and lignin were abundant in particulate organic matter (POM) fractions of sand-, silt-, and clay-size and the mineral-bound, clay-sized organic matter. These results indicate that recent conceptualizations of SOM stabilization probably understate the contribution of plant-derived organic matter to stable SOM pools. Although our data indicate that inherent recalcitrance could be important in soils with limited aggregation, organo-mineral interactions and aggregation were responsible for long-term SOM stabilization. In particular, we observed consistently higher concentrations of plant-derived lipids in POM fractions that were incubated individually, where aggregates were disrupted, as compared to those incubated as bulk soil, where aggregates stayed intact. This finding emphasizes the importance of aggregation for the stabilization of less ‘recalcitrant’ biomolecules in the POM fractions. Because also the abundance of lipids and lignin in clay-sized, mineral-associated SOM was substantially influenced by aggregation, the bioavailability of mineral-associated SOM likely increases after the destruction of intact soil structures.     

Biological and physico-chemical processes influence cutin and suberin biomarker distribution in two Mediterranean forest soil profiles

Recent investigations have shown macromolecules, such as cutins, and suberins as effective markers for above and belowground plant tissues. These biopolyesters contain structural units specific for different litter components and for root biomass. The aim of this work was to understand the fate of plant organic matter (OM) in Mediterranean forest soils by evaluating the incorporation of cutin and suberin by measuring specific biomarkers. Soil and plant tissue (leaves, woods and roots) samples were collected in two mixed Mediterranean forests of Quercus ilex (holm oak) in costal stands in Tuscany (central Italy), which have different ecological and edaphic features. Ester-bound lipids of mineral and organic horizons and the overlying vegetation were analysed using the saponification method in order to depolymerise cutins and suberins and release their specific structural units. Cutin and suberin specific aliphatic monomers were identified and quantified by gas chromatographic techniques. The distribution of cutin and suberin specific monomers in plant tissue suggested that mid-chain hydroxy acids can be used as leaf-specific markers and α,ω-alkanedioic acids and ωC_18:1 as root-specific markers. Differences in the distributions of biomarkers specific for above and belowground plant-derived OM was observed in the two types of soils, suggesting contrasted degradation, stabilisation and transport mechanisms that may be related to soil physico-chemical properties. The acidic and dry soil appeared to inhibit microbial activity, favouring stabilization of leaf-derived compounds, while, in the more fertile soil, protection within aggregates appeared to better preserve root-derived compounds.     

The Complementarity of Extractable and Ester-Bound Lipids in a Soil Profile Under Pine

Extractable and solvent insoluble, ester-bound lipids were analysed in an acid, sandy soil profile under Corsican pine. The n-alkanes and alkanoic acids from the soil profile showed rather poor correlations with those from the pine needles and roots, while the n-alkanol composition in the mineral horizons strongly indicated the presence of lipids derived from a previous grass vegetation. Although the ester-bound lipids (ω-hydroxyalkanoic acids and α,ω-alkanedioic acids (>C₂₄)) suggested that plant sources other than pines were present in the mineral soil horizons their composition was less contaminated and a clear distinction between needle and root input could be discerned. The divergent clustering of soil horizons and plant materials by individual and combined compound classes emphasized the usefulness of both extractable lipids and cutin/suberin in unravelling (past) vegetation and tissue history and contributions to soil organic matter.     

Root-Derived Short-Chain Suberin Diacids from Rice and Rape Seed in a Paddy Soil under Rice Cultivar Treatments

Suberin-derived substituted fatty acids have been shown to be potential biomarkers for plant-derived carbon (C) in soils across ecosystems. Analyzing root derived suberin compounds bound in soil could help to understand the root input into a soil organic carbon pool. In this study, bound lipids were extracted and identified in root and topsoil samples. Short-chain suberin diacids were quantified under rice (Oryza sativa L.) and rape (Brassica campestris) rotations with different cultivar combinations in a Chinese rice paddy. After removal of free lipids with sequential extraction, the residual bound lipids were obtained with saponification and derivatization before analysis using gas chromatography–mass spectrometry (GC-MS). Diacids C16 and C18 in bound lipids were detected both in rice and rape root samples, while diacids C20 and C22 were detected only in rape root samples. Accordingly, diacids were quantified in both rhizosphere and bulk soil (0–15 cm). The amount of total root-derived diacids in bulk soil varied in a range of 5.6–9.6 mg/kg across growth stages and crop seasons. After one year-round rice-rape rotation, root-derived suberin diacids were maintained at a level of 7–9 mg/kg in bulk soil; this was higher under a super rice cultivar LY than under a hybrid cultivar IIY. While concentrations of the analyzed diacids were generally higher in rhizosphere than in bulk soil, the total diacid (DA) concentration was higher at the time of rape harvest than at rice harvest, suggesting that rape roots made a major contribution to the preservation of diacids in the paddy. Moreover, the net change in the concentration and the ratios of C_16:0 DA to C_18:1 DA, and of C_16:0 DA to C_18:0 DA, over a whole growing season, were greater under LY than under IIY, though there was no difference between cultivars within a single growth stage. Overall, total concentration of root-derived suberin diacids was found to be positively correlated to soil organic carbon concentration both for bulk soil and rhizosphere. However, the turnover and preservation of the root suberin biomolecules with soil property and field conditions deserve further field studies.     

Decadal cycling within long-lived carbon pools revealed by dual isotopic analysis of mineral-associated soil organic matter

Long-lived soil organic matter (SOM) pools are critical for the global carbon (C) cycle, but challenges in isolating such pools have inhibited understanding of their dynamics. We physically isolated particulate (>53 μm), silt-, and clay-sized organic matter from soils collected over two decades from a perennial C₃ grassland established on long-term agricultural soil with a predominantly C₄ isotopic signature. Silt- and clay-sized fractions were then subjected to a sequential chemical fractionation (acid hydrolysis followed by peroxide oxidation) to isolate long-lived C pools. We quantified ¹⁴C and the natural ¹³C isotopic label in the resulting fractions to identify and evaluate pools responsible for long-lived SOM. After removal of particulate organic matter (~14% of bulk soil C) sequential chemical treatment removed 80% of mineral-associated C. In all mineral-associated fractions, at least 55% of C₄-derived C was retained 32 years after the switch to C₃ inputs. However, C₃–C increased substantially beginning ~25 years after the switch. Radiocarbon-based turnover times ranged from roughly 1200–3000 years for chemically resistant mineral-associated pools, although some pools turned over faster under C₃ grassland than in a reference agricultural field, indicating that new material had entered some pools as early as 14 years after the vegetation switch. These findings provide further evidence that SOM chemistry does not always reflect SOM longevity and resistance to microbial decomposition. Even measureable SOM fractions that have extremely long mean turnover times (>1500 years) can have a substantial component that is dynamic over much shorter timescales.     

Earthworms, litter and soil carbon in a northern hardwood forest

The important role of soil carbon (C) in the global C cycle has stimulated interest in better understanding the mechanisms regulating soil C storage and its stabilization. Exotic earthworm invasion of northern forest soils in North America can affect soil C pools, and we examined their effects on these mechanisms by adding ¹³C labeled leaf litter to adjacent northern hardwood forests with and without earthworms. Two types of labeled litter were produced, one with the ¹³C more concentrated in structural (S) components and the other in non-structural (NS) components, to evaluate the role of biochemical differences in soil C stabilization. Earthworm invasions have reduced soil C storage in the upper 20 cm of the soil profile by 37 %, mostly by eliminating surface organic horizons. Despite rapid mixing of litter into mineral soil and its incorporation into aggregates, mineral soil C has not increased in the presence of earthworms. Incorporation of litter C into soil and microbial biomass was not affected by biochemical differences between S versus NS labeled litter although NS litter C was assimilated more readily into earthworm biomass and S litter C into fungal hyphae. Apparently, the net effect of earthworm mixing of litter and forest floor C into mineral soil, plus stabilization of that C in aggregates, is counterbalanced by earthworm bioturbation and possible priming effects. Our results support recent arguments that biochemical recalcitrance is not a major contributor to the stabilization of soil C.     

Morphology and discrimination features of pollen from Italian olive cultivars (Olea europaea L.)

Pollen morphology of 14 cultivars of Olea europaea subsp. europaea var. europaea was analysed in order to discriminate main pollen types. The cultivars were selected from the most spread and early flowering crops grown in Italy. Morphometric parameters were observed on acetolysed pollen by means of light microscopy and scanning electron microscopy. Polar axis (P), equatorial diameter (E), P/E ratio, maximum distance between colpi in mesocolpium, distance between the apices of two colpi, exine thickness, maximum length of lumina in mesocolpium and in apocolpium, and exine reticulum thickness in mesocolpium have been measured. According to P and E, the 14 olive cultivars of this study can be divided into the three groups of small (P: 21.75 µm, E: 22.55 µm; ‘Manna’ and ‘Tonda di Cagliari’), large (P: 25.1 µm, E: 26.1 µm; ‘Pescarese’ and ‘Rotondella di Sanza’) and medium size (P: 23.49 µm, E: 24.54 µm, ‘Carolea’, ‘Grossa di Cassano’, ‘Giarraffa’, ‘Nocellara messinese’, ‘Nocellara del Belice’, ‘Santagatese’, ‘Intosso’, ‘Maiatica di Ferrandina’, ‘Nostrale di Fiano Romano’, ‘Santa Caterina’). Maximum length of lumina and exine thickness are useful parameters for further distinction of olive pollen groups, since these parameters are able to provide a specific pollen profile for each cultivar.     

Comparative analysis of the structure,suberin and wax composition and key gene expression in the epidermis of ‘Dangshansuli’ pear and its russet mutant

Mature fruit of ‘Dangshansuli’ pear has yellow-green skin, while its mutant ‘Xiusu’ has russet fruit skin, which is a genetic variation. To explore the mechanism underlying the russet formation, the fruit spot and epidermal structure were observed, the color, texture, and wax and suberin components were evaluated, and the gene expression levels were confirmed. In the present study, the color, texture and fruit spot of the epidermis differed significantly between ‘Dangshansuli’ and ‘Xiusu’ at 25 days after full bloom (DAFB). The cuticular wax components were alkanes, olefins, alkanoic acids, alcohols and terpenes, and the suberin was composed of fatty acid, α,ω-diacids, ω-hydroxy fatty acids, mainly ferulic acid and primary alcohols in the epidermis of ‘Dangshansuli’ and ‘Xiusu’, which exhibited significant differences at most stages of the development of pear fruits. Moreover, the expression levels of genes involved in wax and suberin were consistent with morphological and biochemical analyses. The results indicated that the suberization of epidermal cells occurred when pear fruit was young and that wax and suberin might contribute to the russet formation on the epidermis of ‘Xiusu’, leading to the significant differences in color, texture, fruit spot, and exocarp structure between ‘Dangshansuli’ and ‘Xiusu’ pears.     

Evidence for Covalently Attached p-Coumaric Acid and Ferulic Acid in Cutins and Suberins

p-Coumaric acid (4-hydroxycinnamic acid) and ferulic acid (4-hydroxy-3-methoxycinnamic acid) have been identified as constituents of cutin. Their reduction products were isolated from a phenolic fraction released from the cutin of the fruits of apple, peach, pear, and two varieties of tomato and apple leaf by treatment with LiAlH(4) or LiAlD(4). They were identified by combined gas chromatography and mass spectrometry. p-Coumaric acid was present in all samples of cutin (0.07-0.53% by weight), whereas only peach and pear cutin contained measurable amounts of ferulic acid (0.007% and 0.035%, respectively). Both p-coumaric acid and ferulic acid were identified to be constituents of the insoluble material recovered after partial hydrolysis (12-42% loss) of cutin in 1 m NaOH at 80 C. A significant part (48%) of the p-coumaric acid contained in tomato cutin was contained in the insoluble material recovered after partial degradation (7.4%) of this cutin with 0.01 m NaOH. These data indicate that these phenolic components are tightly (possibly covalently) bound to cutin. Similar analysis of the phenolic fractions from the suberins of potato, sweet potato, turnip, rutabaga, carrot, and red beet revealed that they contained only ferulic acid (0.05-0.22%). Ferulic acid was identified as a constituent of the insoluble material recovered after partial hydrolysis of potato and beet suberins (34% and 32% loss, respectively) in 1 m NaOH at 80 C. A major part (65%) of the ferulic acid contained in potato suberin was contained in the insoluble material recovered after partial (26.8% loss) degradation of this suberin with 0.01 m NaOH. Ferulic acid appears to be tightly (probably covalently) bound to suberin.     

Molecular-level changes in soil organic matter composition after 10 years of litter,root and nitrogen manipulation in a temperate forest

With climate change, forests are expected to receive increased inputs of carbon (C) and nitrogen (N) but it is unclear how this will modify forest C cycling and storage at the molecular-level. To investigate the response of forest soil organic matter (SOM) to changes in soil inputs, a study area was established in a Michigan hardwood forest as part of the Detrital Input and Removal Treatments (DIRT) network. Experimental treatments were comprised of both exclusions of detrital inputs (No Litter, No Roots, No Inputs) and additions of C and N (Double Litter, N-Addition, Double Litter?+?N, Wood). After 10 years of treatment, the soils were characterized using elemental analysis, molecular biomarker techniques, nuclear magnetic resonance spectroscopy, and microbial biomass C measurements. Although manipulation of detrital inputs did not significantly change the soil C and N content after 10 years, alterations in the cycling and distribution of SOM components were observed. Root exclusion enhanced SOM degradation, while doubling litter favoured the degradation of more labile forms of soil C such as unsaturated n-alkanoic acids and simple sugars. N-Addition and Double Litter?+?N increased the concentrations of extractable biomarkers, including aliphatic and cyclic lipids and compounds derived from cutin, suberin, and lignin. Microbial biomass C also varied with experimental litter input manipulations and N addition, and these data were consistent with the observed changes in SOM composition. Overall, the observed shifts in SOM chemistry after 10 years of manipulating ecosystem inputs highlight the sensitivity of natural systems to changes in amounts of C and N inputs from roots and litter, and N inputs from external sources.     

Forest Continuity as a Key Determinant of Soil Carbon and Nutrient Storage in Beech Forests on Sandy Soils in Northern Germany

Forest (or tree) age has been identified as an important determinant of the carbon (C) storage potential of forest soils. A large part of Central Europe’s current forested area was affected by land use change with long periods of cultivation in past centuries suggesting that the organic C stocks in the soil (SOC) under recent forest may partly be legacies of the past and that stand age effects have to be distinguished from forest continuity effects (that is, the time since re-afforestation). We examined the influence of mean tree age and forest continuity on the SOC pool and the stores of total N and available P, Ca, Mg, and K in the soil (mineral soil and organic layer) across a sample of 14 beech (Fagus sylvatica) forests on sandy soil with variable tree age (23–189 years) and forest continuity (50-year-old afforestation to ancient (‘permanent’) forest, that is, >230 years of proven continuity). Ancient beech forests (>230 years of continuity) stored on average 47 and 44% more organic C and total N in the soil than recent beech afforestation (50–128 years of continuity). Contrary to expectation, we found large and significant C and N pool differences between the forest categories in the mineral soil but not in the organic layer indicating that decade- or century-long cultivation has reduced the subsoil C and nutrient stores while the organic layer element pools have approached a new equilibrium after only 50–128 years. PCA and correlation analyses suggest that forest continuity cannot be ignored when trying to understand the variation in soil C stocks between different stands. Forest clearing, subsequent cultivation, and eventual re-afforestation with beech resulted in similar relative stock reductions of C and N and, thus, no change in soil C/N ratio. We conclude that the continuity of forest cover, which may or may not be related to tree age, is a key determinant of the soil C and nutrient stores of beech forests in the old cultural landscape of Central Europe.     

Decomposition and stabilization of root litter in top- and subsoil horizons: what is the difference?

Mechanisms leading to high mean residence times of organic matter in subsoil horizons are poorly understood. In lower parts of the soil profile root material contributes greatly to soil organic matter (SOM). The objective of this study was to elucidate the decomposition dynamics of root-derived C and N in different soil depths during a 3 year field experiment and to examine the importance of different protection mechanisms as well as abiotic factors for the decomposition dynamics. Additionally, we assessed the effect of root litter addition on native SOM. Our conceptual approach included the exposure of litterbags with ¹³C and ¹⁵N labeled wheat root material mixed to loamy agricultural soil at three different soil depths (30, 60 and 90 cm). During the incubation period, we monitored soil temperature, humidity and the incorporation of root derived C and N into the soil microbial biomass and physical SOM fractions. Our results showed that abiotic decay conditions were better in subsurface horizons compared to the topsoil. Root litter addition significantly increased the size of microbial biomass in all three soil horizons. In the topsoil, root-derived C decomposition was significantly higher in the first 6 months of incubation compared to subsoil horizons. In 60 and 90 cm depths, a lag phase with development of soil microbial biomass seemed to be prevailing before decomposition was activated. For root-derived N, similar decomposition kinetics could be observed in top- and subsoil horizons. Despite of higher SOM contents, better soil structure and higher microbial activity in the topsoil horizon compared to subsoil horizons, the amounts of root-derived C and N remaining after 3 years were similar for all three depths. Most of the root-derived C and N was present as organo-mineral complexes or occluded in soil aggregates (oPOM), illustrating similar importance of these two protection mechanisms in all three soil depths. Addition of fresh root litter caused small losses of native soil C whereas native N was retained. We conclude that despite of similar SOM protection mechanisms, there are distinct differences in decomposition dynamics of root litter between top- and subsoil horizons. In the long run, the better abiotic decay conditions prevailing in subsoil horizons may compensate for their poorer physico-chemical characteristics.     

Integrating plant litter quality,soil organic matter stabilization,and the carbon saturation concept

Labile, ‘high‐quality’, plant litters are hypothesized to promote soil organic matter (SOM) stabilization in mineral soil fractions that are physicochemically protected from rapid mineralization. However, the effect of litter quality on SOM stabilization is inconsistent. High‐quality litters, characterized by high N concentrations, low C/N ratios, and low phenol/lignin concentrations, are not consistently stabilized in SOM with greater efficiency than ‘low‐quality’ litters characterized by low N concentrations, high C/N ratios, and high phenol/lignin concentrations. Here, we attempt to resolve these inconsistent results by developing a new conceptual model that links litter quality to the soil C saturation concept. Our model builds on the Microbial Efficiency‐Matrix Stabilization framework (Cotrufo et al., 2013) by suggesting the effect of litter quality on SOM stabilization is modulated by the extent of soil C saturation such that high‐quality litters are not always stabilized in SOM with greater efficiency than low‐quality litters.     

In vitro photoautotrophic acclimatization,direct transplantation and ex vitro adaptation of rubber tree (<Emphasis Type="Italic">Hevea brasiliensis</Emphasis>)

We investigated the effect of carbon dioxide (CO₂)-ambient (350 µmol CO₂ mol^?1) and CO₂-enriched (1500 µmol CO₂ mol^?1) conditions of in vitro photoautotrophic system on two cultivars, ‘RRIM600’ and ‘RRIT413’ of rubber tree (Hevea brasiliensis) in an acclimatization process of 45 days. Survival percentage of in vitro rubber tree plantlets derived from somatic embryos under ambient CO₂ was better than those under CO₂-enriched conditions, especially in cv. ‘RRIT413’. Subsequently, the survival rate of ex vitro transplanted plantlets was similar to the in vitro plantlets and abnormal morphological characters such as light-green leaves (SPAD), small leaves in cv. ‘RRIT413’ acclimatized under CO₂-enriched conditions were demonstrated 30 days after the plantlets were transferred into the soil. Maximum quantum yield of PSII, photon yield of PSII, stomatal conductance and transpiration rate in cv. ‘RRIT413’ acclimatized under CO₂-enriched conditions were sharply declined by 39.0, 50.6, 47.1 and 45.8%, respectively as compared to those acclimatized under ambient CO₂ conditions. In contrast, the in vitro acclimatized plantlets of cv. ‘RRIM600’ were un-responsive under both ambient- and enriched-CO₂ conditions. In conclusion, genotypic dependent in response to CO₂ enriched conditions in in-vitro acclimatization of rubber tree plantlets was evidently demonstrated as a key result to regulate plant growth and development in ex vitro environments. Interestingly, soluble sugar contents (sucrose, glucose and fructose) were increased after transplanting the plantlets of cv. ‘RRIM600’ acclimatized under CO₂-enriched condition into the soil and thus, can be considered as an adaptive indicator of ex vitro adaptation.     

Efficient,one step and cultivar independent shoot organogenesis of potato

An efficient, one step and genotype independent protocol of shoot organogenesis was developed from leaf and internodal explants taken from microshoots of different cultivars of potato (Solanum tuberosum L.). Initially, microshoots were cultured on basal Murashige and Skoog medium additionally supplemented with 10 µM AgNO₃ (MS1 medium) to achieve healthy shoot growth required to get the quality explants. Shoot organogenesis was induced from both types of explants (leaf and internodal) on MS1 medium variously supplemented with 6-benzyladenine (BA) and gibberellic acid (GA₃). Maximum explants were induced shoot organogenesis on MS1 medium supplemented with 10 µM BA and 15.0 µM GA₃ from both the cultivars namely ‘Kufri Chipsona 1’ and ‘Kufri Pukhraj’. Among the types of explants used, better response was observed from internodal segments as compared to leafs. This optimized medium combination was found to be equally effective for all the eight cultivars tested namely ‘Kufri Pukhraj’, ‘Kufri Chipsona 1’, ‘Kufri Chipsona 2’, ‘Kufri Jyoti’, ‘Kufri Surya’, ‘Kufri Chandramukhi’, ‘Kufri Khyati’ and ‘Desiree’. The clonal uniformity of the regenerated shoots was confirmed using random amplified polymorphic DNA and inter-simple sequence repeats markers.     

Adventitious shoot regeneration from in vitro leaf explants of <Emphasis Type="Italic">Fraxinus nigra</Emphasis>

In order to establish an attractive method for the production of valuable medicinal alkaloids (galanthamine and lycorine), the plants of Leucojum aestivum and L. aestivum ‘Gravety Giant’ grown in bioreactor RITA® were subjected to various concentrations of methyl jasmonate (MeJA), salicylic acid (SA), 1-aminocyclopropane-1-carboxylic acid (ACC) and 2-chloroethylphosphonic acid (ethephon) at different times of culture. The application of MeJA showed a negative effect on L. aestivum and L. aestivum ‘Gravety Giant’ plant growth. We observed that the incubation of plants during 168 h with 100 µM of MeJA resulted above two times lower F.W. (fresh weight) increments compared with control. While SA showed an inhibitory effect only on the growth of L. aestivum cultures. ACC and ethephon had a positive effect on both types of culture. Treatment with 50 µM of MeJA during 168 h stimulated galanthamine and lycorine biosynthesis in L. aestivum and L. aestivum ‘Gravety Giant’ cultures. In addition, the accumulation of galanthamine was increased when 10 µM of ACC were added to both types of culture. 10 µM of ACC stimulated also lycorine biosynthesis by L. aestivum ‘Gravety Giant’. The addition of 10 µM of ethephon had a positive effect only on lycorine production in plants of L. aestivum. SA promoted galanthamine and lycorine biosynthesis in tested plants. Indeed the highest galanthamine (0.8 mg/g dry weight: D.W.) and lycorine (1.53 mg/g D.W.) concentrations were observed in L. aestivum ‘Gravety Giant’ plants treated with 5 µM of SA during 10 h.     

Response of wheat to subsoil salinity and temporary water stress at different stages of the reproductive phase

To examine the effects of subsoil NaCl salinity in relation to water stress imposed at different growth stages, wheat was grown in a heavy texture clay soil (vertosol) under glasshouse conditions in polythene lined cylindrical PVC pots (100 cm long with 10.5 cm diameter) with very low salinity level (EC_e 1.0 dS/m; ESP 1.0 and Cl 30 mg/kg soil) in top 10 cm soil (10–20 cm pot zone) and low salinity level (EC_e 2.5 dS/m, ESP 5, and Cl 100 mg/kg soil) in top 10–20 cm soil (20–30 cm pot zone). The plants were exposed to three subsoil salinity levels in the 20–90 cm subsoil (30–100 cm pot zone) namely low salinity (EC_e: 2.5 dS/m, ESP: 5, Cl: 100 mg/kg soil), medium salinity (EC_e: 4.0 dS/m, ESP: 10, Cl: 400 mg/kg) and high salinity (EC_e: 11.5 dS/m, ESP: 20, Cl: 1950 mg/kg) in the subsoil (20–90 cm soil layer: 30–100 cm pot zone). Watering of plants was withheld for 20 days commencing at either early booting or anthesis or mid grain filling, and then resumed until maturity, and these treatments were compared with no water stress. Water stress commencing at anthesis stage had the most depressing effect on grain yield and water use efficiency of wheat followed by water stress at grain filling stage and early booting stage. High subsoil salinity reduced grain yield by 39.1, 24.3%, and 13.4% respectively in plants water-stressed around anthesis, early booting, and mid grain filling compared with 36.6% in well-watered plants. There was a significant reduction in root biomass, rooting depth, water uptake and water use efficiency of wheat with increasing subsoil salinity irrespective of water regimes. Plants at high subsoil salinity had 64% of their root biomass in the top 0–30 cm soil and there was a marked reduction in subsoil water uptake. Roots also penetrated below the non-saline surface into salinised subsoil and led to attain high concentration of Na and Cl and reduced Ca/Na and K/Na ratio of flag leaf at anthesis stage. Results suggest that high subsoil salinity affects root growth and water uptake, grain yield and water use efficiency even in well water plants. Water stress at anthesis stage had the most depressing effect on wheat.     

Shrub-encroachment induced alterations in input chemistry and soil microbial community affect topsoil organic carbon in an Inner Mongolian grassland

Shrub encroachment frequently occurs in arid and semi-arid grasslands worldwide and affects the regional carbon balance. Many previous studies have revealed the effects of shrub encroachment on bulk carbon content of grasslands, but molecular evidence is surprisingly lacking. In this study, we examined the chemical composition of plant tissues and soil organic carbon (SOC), and soil microbial communities to identify the effects of shrub (Caragana microphylla) encroachment on SOC storage in the top layer (0–10 cm) along a gradient of natural shrub cover in the grasslands of Inner Mongolia. We found that SOC in the shrub patches was derived mainly from leaves, whereas SOC in the grassy matrix was composed of a mixture of fresh root- and leaf-derived compounds. Compared with pure grassland, the SOC decreased by 29% in the shrub-encroached grasslands (SEGs), and this decrease was enhanced by increasing shrub cover. We also found that free lipids and lignin-derived phenols increased while the ratios of ω-C₁₈/∑C₁₈ and suberin/cutin decreased with increasing shrub cover. In addition, the ratios of fungal to bacterial phospholipid fatty acids (PLFAs) and gram-negative to gram-positive bacterial PLFAs decreased with increasing shrub cover. These results indicate that the encroachment of nitrogen-rich legume shrubs can lead to carbon loss by altering the chemical composition of plant inputs as well as the soil microbial community in grassland ecosystems.     

Soil N and C trace gas fluxes and microbial soil N turnover in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary

During two intensive field campaigns in summer and autumn 2004 nitrogen (N₂O, NO/NO₂) and carbon (CO₂, CH₄) trace gas exchange between soil and the atmosphere was measured in a sessile oak (Quercus petraea (Matt.) Liebl.) forest in Hungary. The climate can be described as continental temperate. Fluxes were measured with a fully automatic measuring system allowing for high temporal resolution. Mean N₂O emission rates were 1.5 μg N m⁻² h⁻¹ in summer and 3.4 μg N m⁻² h⁻¹ in autumn, respectively. Also mean NO emission rates were higher in autumn (8.4 μg N m⁻² h⁻¹) as compared to summer (6.0 μg N m⁻² h⁻¹). However, as NO₂ deposition rates continuously exceeded NO emission rates (−9.7 μg N m⁻² h⁻¹ in summer and −18.3 μg N m⁻² h⁻¹ in autumn), the forest soil always acted as a net NO _x sink. The mean value of CO₂ fluxes showed only little seasonal differences between summer (81.1 mg C m⁻² h⁻¹) and autumn (74.2 mg C m⁻² h⁻¹) measurements, likewise CH₄uptake (summer: −52.6 μg C m⁻² h⁻¹; autumn: −56.5 μg C m⁻² h⁻¹). In addition, the microbial soil processes net/gross N mineralization, net/gross nitrification and heterotrophic soil respiration as well as inorganic soil nitrogen concentrations and N₂O/CH₄ soil air concentrations in different soil depths were determined. The respiratory quotient (ΔCO_{2 resp} ΔO_{2 resp}⁻¹) for the uppermost mineral soil, which is needed for the calculation of gross nitrification via the Barometric Process Separation (BaPS) technique, was 0.8978 ± 0.008. The mean value of gross nitrification rates showed only little seasonal differences between summer (0.99 μg N kg⁻¹ SDW d⁻¹) and autumn measurements (0.89 μg N kg⁻¹ SDW d⁻¹). Gross rates of N mineralization were highest in the organic layer (20.1–137.9 μg N kg⁻¹ SDW d⁻¹) and significantly lower in the uppermost mineral layer (1.3–2.9 μg N kg⁻¹ SDW d⁻¹). Only for the organic layer seasonality in gross N mineralization rates could be demonstrated, with highest mean values in autumn, most likely caused by fresh litter decomposition. Gross mineralization rates of the organic layer were positively correlated with N₂O emissions and negatively correlated with CH₄ uptake, whereas soil CO₂ emissions were positively correlated with heterotrophic respiration in the uppermost mineral soil layer. The most important abiotic factor influencing C and N trace gas fluxes was soil moisture, while the influence of soil temperature on trace gas exchange rates was high only in autumn.