Monitoring rhizospheric pH,oxygen, and organic acid dynamics in two short‐time flooded plant species

The rhizosphere of two flooding‐resistant plant species (Arundinella anomala Steud., Alternanthera philoxeroides Mart.) from Three Gorges Reservoir area (China) has been examined for reactions to waterlogging and submergence. Rhizosphere parameters were monitored in natural sediment substrate by means of a dual‐access floodable rhizobox, which allows monitoring of oxygen and pH dynamics noninvasively with planar optodes in high temporal and spatial resolution, as well as simultaneous low‐invasive soil‐solution sampling. Analysis of samples for low‐molecular‐weight organic acids (LMWOA) was done by capillary electrophoresis. Roots could be observed easily in situ during growth and exposure to flooding. The floodable rhizobox is therefore considered a valuable tool for root‐reaction monitoring also under flooding conditions. During waterlogging, both species exuded oxygen into their rhizosphere and showed diurnal rhythms of rhizospheric acidification. The pH of the rhizosphere of growing root tips decreased up to 0.8 units corresponding to higher LMWOA concentrations. These rhythms weakened during flooding, but gained maximum amplitude again rapidly after resurfacing. We conclude that the root system was still fully functioning during and after flooding, and that flooding poses no threat to the physiology of the root system of the study species.     

Impact of organic amendments on the dynamics of soil microbial biomass and bacterial communities in cultivated land

In the course of a 1 year study on agricultural plots (mustard-beet) the impact of organic materials and soil management on soil microbial communities was assessed after amendment with three types of organic materials (sewage sludge, turkey manure and compost made of turkey manure and ligneous waste) and a mineral fertilizer. Transient impacts on potential metabolic activity, on genetic structure and on quantities of soil microorganisms, measured with three different methods (C_mic, total DNA content and CFU enumeration), were observed. The extent of these impacts depended on the composition of the amendment. The proportions of mineralisable carbon and lignin in organic materials seemed to influence the behaviour of the bacterial communities during the first 3 months after amendment. After a period of 6 months, no effect specific to the type of organic materials was detected. The dynamics of microbial biomass was assessed by both microbial carbon and by total DNA, but no correlation was found between the results; DNA seemed to be more sensitive to time-related variations in microbial biomass. Finally, although the bacterial functional and genetic structures were strongly modified between 3 and 6 months, this modification was not related to the type of amendment. This result suggests that organic amendments have less effect than seasonal variations or others anthropic factors such as the mechanical management of the soil.     

Phosphorus dynamics in the rhizosphere of two wheat cultivars in a soil with high organic matter content

Soil organic matter (SOM) is a key of most nutrient cycling and its content influences labile phosphorus (P) pool. In order to promote P availability from SOM, some plant strategies could be important to increase organic P mineralization, which may change among cultivars of the same crop. This study evaluated phosphorus dynamics in the rhizosphere of two wheat cultivars in soil with high organic matter content. Experiment was established in a greenhouse using a Humic Cambisol in a randomized block design using Quartzo and Abalone as wheat cultivars and harvested 20 days after seedling. Pots with a horizontal mesh (25 µm) were used to isolate the soil from roots. At harvest, the soil under the mesh was sliced in five distances from the rhizoplane (0–3; 3–6; 6–10; 10–20; 20–30 mm). Organic P was the buffer to maintain P dynamics in the rhizosphere and there was phosphorus depletion in the first slice near the rhizoplane due to the root effect, regardless the cultivar. Quartzo showed high labile inorganic P, presumably due to the high amount of root hairs, which increased the acid phosphatase activity and consequently root P uptake. Quartzo was more efficient in changing rhizosphere regarding the P acquisition.     

Analysis of bacterial communities on alkaline phosphatase genes in soil supplied with organic matter

Abstract

We studied the effects of the application of organic matter (OM) and chemical fertilizer (CF) on soil alkaline phosphatase (ALP) activity and ALP-harboring bacterial communities in the rhizosphere and bulk soil in an experimental lettuce field in Hokkaido, Japan. The ALP activity was higher in soils with OM than in soils with CF, and activity was higher in the rhizosphere for OM than in the bulk soil. Biomass P and available P in the soil were positively related to the ALP activity of the soil. As a result, the P concentration of lettuce was higher in OM soil than in CF soil. We analyzed the ALP-harboring bacterial communities using polymerase chain reaction based denaturing gradient gel electrophoresis (DGGE) on the ALP genes. Numerous ALP genes were detected in the DGGE profile, regardless of sampling time, fertilizer treatment or sampled soil area, which indicated a large diversity in ALP-harboring bacteria in the soil. Several ALP gene fragments were closely related to the ALP genes of Mesorhizobium loti and Pseudomonas fluorescens. The community structures of the ALP-harboring bacteria were assessed using principal component analysis of the DGGE profiles. Fertilizer treatment and sampled soil area significantly affected the community structures of ALP-harboring bacteria. As the DGGE bands contributing to the principal component were different from sampling time, it is suggested that the major bacteria harboring the ALP gene shifted. Furthermore, there was, in part, a significant correlation between ALP activity and the community structure of the ALP-harboring bacteria. These results raise the possibility that different ALP-harboring bacteria release different amounts and/or activity of ALP, and that the structure of ALP-harboring bacterial communities may play a major role in determining overall soil ALP activity.     

Temporal dynamics of microbial communities on decomposing leaf litter of 10 plant species in relation to decomposition rate

Few empirical studies have examined how microbial communities on decomposing litters change in relation to litter chemistry or how microbial community composition is related to the rate of decomposition. We examined the relationships among microbial community composition, litter chemistry, and decomposition rates in a common garden experiment of the decomposition of leaf litters of 10 plant species. Microbial community composition, as measured by phospholipid fatty acids (PLFA), and 7 litter chemistry variables (%N, C:N, four carbon fractions, and lignin:N) were examined at 1, 2, and 8 months into decomposition. Both microbial and litter chemistry variables were reduced to a single axis each through nonmetric multidimensional scaling (NMS) to examine the relationship between microbes, litter chemistry, and decomposition rates. Although microbial communities were separated according initial litter chemistry and lability, individual measures of litter chemistry had limited ability to predict microbial community composition during decomposition. Decomposition rate constants were explained by litter chemistry of initial, 1-, 2- and, 8-month old litters (60–72% of the variance), and by microbial community composition at the 8-month collection date (67%). The results suggest that initial litter chemistry determines the rate of decomposition and microbial community composition early in decomposition while the composition of the microbial community plays a more important role in determining decomposition rate later in decomposition.     

Effect of salinity on growth and solute accumulation in two wheat lines differing in salt tolerance

Wheat (Triticum aestivum L.) line, Saline Agriculture Research Center line 1 (SARC), was selected in a salinity tolerance improvement program at the University of Agriculture, Faisalabad, Pakistan. In this study we compared SARC with Pothowar which is a common wheat cultivar grown in the same region, in order to study the mechanism of salinity tolerance in the SARC line. Two wheat lines were planted in pots and were subjected to salt stress by daily application of a 200 mM NaCI solution for 30 d during the vegetative growth stage. Dry weight of plant parts, leaf area, leaf water status, and solute concentrations in the cell sap of the leaf tissues were determined at 13 and 30 d after initiation of the stress treatment. Decrease in the plant dry weight and leaf area due to salt stress was more pronounced in Pothowar than in SARC, indicating that SARC was more tolerant to salinity. SARC maintained a higher turgor at low leaf water potentials and showed a higher capacity of osmotic adjustment compared to Pothowar. Major osmotic a that increased by salinity in order to maintain a lower osmotic potential in the two lines were Na⁺, Cl^-, K⁺, and glycinebetaine. Increase in the concentrations of Na⁺, Cl^-, and glycinebetaine was much higher in SARC than in Pothower. These results suggested that the SARC line had a physiological mechanism that conferred a higher salinity tolerance.     

A microcosm experiment to evaluate the influence of location and quality of plant residues on residue decomposition and genetic structure of soil microbial communities

The effects of location (soil surface vs. incorporated in soil) and nature of plant residues on degradation processes and indigenous microbial communities were studied by means of soil microcosms incubation in which the different soil zones influenced by decomposition i.e. residues, soil adjacent to residues (detritusphere) and distant soil unaffected by decomposition (bulk soil) were considered. Plant material decomposition, organic carbon assimilation by the soil microbial biomass and soil inorganic N dynamics were studied with ¹³C labelled wheat straw and young rye. The genetic structure of the community in each soil zone were compared between residue locations and type by applying B- and F-ARISA (for bacterial- and fungal-automated ribosomal intergenic spacer analysis) directly to DNA extracts from these different zones at 50% decomposition of each residue. Both location and biochemical quality affected residue decomposition in soil: 21% of incorporated ¹³C wheat straw and 23% left at the soil surface remained undecomposed at the end of incubation, the corresponding values for ¹³C rye being 1% and 8%. Residue decomposition induced a gradient of microbial activity with more labelled C incorporated into the microbial biomass of the detritusphere. The sphere of influence of the decomposing residues on the dynamics of soluble organic C and inorganic N in the different soil zones showed particular patterns which were influenced by both residue location and quality. Residue degradation stimulated particular genetic structure of microbial community with a gradient from residue to bulk soil, and more pronounced spatial heterogeneity for fungal than for bacterial communities. The initial residue quality strongly affected the resulting spatial heterogeneity of bacteria, with a significance between-zone discrimination for rye but weak discrimination between the detritusphere and bulk soil, for wheat straw. Comparison of the different detrituspheres and residue zones (corresponding to different residue type and location), indicated that the genetic structure of the bacterial and fungal communities were specific to a residue type for detritusphere and to its location for residue, leading to conclude that the detritusphere and residue corresponded to distinct trophic and functional niches for microorganisms.     

Nutrients in forest litter treated with naphthalene and simulated throughfall: A field microcosm study

The effects of naphthalene (arthropod exclusion) and simulated throughfall (N, P, K, Ca and Mg) additions on the decomposition and mineralization of dogwood (Cornus florida L.) litter were studied by using a field microcosm approach in a southeastern United States deciduous forest. Treatments without microarthropods decayed more slowly than litter with microarthropods. Simulated throughfall additions alone had no effect on litter decay rates. Fauna, simulated throughfall, and fauna plus simulated throughfall treatments increased the nutrient concentrations of decomposing litter; the treatment with both microarthropods and simulated throughfall generally exhibited the highest nutrient concentrations. Simulated throughfall also significantly increased microarthropod densities in litter. Litter immobilization of elements in throughfall was insignificant in litter with microarthropods; naphthalene-treated litter immobilized up to 8% of the elements contained in simulated throughfall.     

A microcosm study on the influence of pH and the host-plant on the soil persistence of two alfalfa-nodulating rhizobia with different saprophytic and symbiotic characteristics

The acid tolerance of Sinorhizobium meliloti in culture media and in soils is considered a useful criteria to select for strains with improved survival in agricultural acidic soils. Using a glass tube system with gamma-irradiated soil at different pH values, we analysed the survival of two different alfalfa-nodulating rhizobia: S. meliloti (pH_limit for growth 5.6–6.0) and the acid-tolerant Rhizobium sp. LPU83, closely related to the strain Rhizobium sp. Or191 (pH_limit for growth below 5.0). Although the acid-tolerant rhizobia showed a slightly better survival during the first months in acid soil (pH=5.6), none of the strains could be detected 2 months after inoculation (bacterial counts were below 10³ colony-forming units (cfu)/30 g of soil). The inclusion of two alfalfa plants/glass tube with soil, however, supported the persistence of both types of rhizobia at pH 5.6 for over 2 months with counts higher than 9×10⁶ cfu/30 g of soil. Remarkably, in the presence of alfalfa the cell densities reached by S. meliloti were higher than those reached by strain LPU83, which started to decline 1 week after inoculation. Although more acid-sensitive in the culture medium than the Or191-like rhizobia, in the presence of the host plant the S. meliloti strains showed to be better adapted to the free-living condition, irrespective of the pH of the soil.     

Species composition of arbuscular mycorrhizal fungi in two mountain meadows with differing management types and levels of plant biodiversity

Species composition of arbuscular mycorrhizal fungi (AMF) was analysed in two differently managed mountain grasslands in Thuringia (Germany). Arbuscular mycorrhizal fungi were studied in the roots of 18 dominant plant species from a total of 56 (32%). Additionally, spores of AMF were isolated from soil samples. Arbuscular mycorrhizal fungi species composition was analysed based on 96 sequences of the internal transcribed spacer of the nuclear ribosomal DNA, 72 originated from mycorrhizal roots, and 24 originated from AMF spores. Phylogenetic analyses revealed a total of 19 AMF species representing all genera of the Glomeromycota except Scutellospora and Pacispora. Despite a different farming intensity, resulting in remarkable differences concerning their plant species diversity (27 against 43 plant species), the diversity of AMF was found to be similar with 11 species on the intensively farmed meadow and ten species on the extensively farmed one. Nevertheless, species composition between both sites was clearly different. It thus seems likely that the AMF species composition, but not necessarily the species number, is related to above ground plant biodiversity in the system under study.     

Rapid effects of plant species diversity and identity on soil microbial communities in experimental grassland ecosystems

Changes in plant community structure, including the loss of plant diversity may affect soil microbial communities. To test this hypothesis, plant diversity and composition were experimentally varied in grassland plots cultivated with monocultures or mixtures of 2, 3 or 4 species. We tested the effects of monocultures versus mixtures and of plant species composition on culturable soil bacterial activity, number of substrates used and catabolic diversity, microbial biomass N, microbial respiration, and root biomass. These properties were all measured 10 months after seeding the experiment. Soil bacterial activity, number of substrates used and catabolic diversity were measured in the different plant communities using BIOLOG GN and GP microplates, which are redox-based tests measuring capacity of soil culturable bacteria to use a variety of organic substrates. Microbial biomass N, microbial respiration, and root biomass were insensitive to plant diversity. Culturable soil microbial activity, substrates used and diversity declined with declining plant diversity. Their activity, number of substrates used and diversity were significantly higher in plots with 3 and 4 plant species than in monocultures and in plots with 2 species. There was also an effect of plant species composition. Culturable soil microbial activity and diversity was higher in the four-species plant community than in any of the plant monocultures suggesting that the effect of plant diversity could not be explained by the presence of a particular plant species. Our results showed that changes in plant diversity and composition in grassland ecosystems lead to a rapid response of bacterial activity and diversity.     

Phosphomonoesterase production and persistence and composition of bacterial communities during plant material decomposition in soils with different pH values

The aim of this work was to study the synthesis and persistence of acid and alkaline phosphomonoesterases in three soils with different pH values amended with ryegrass residues. The organic input increased soil respiration, as estimated by CO₂-C evolution in all soils. The ATP content of the three soils showed a 3-7-fold increase between 7 and 10 d in the different soils since the amendment. The dsDNA content of the three amended soils also peaked between 7 and 10 d, increasing by 2.5-3.5 times in the different soils. The bacterial species richness increased in the amended as compared to the control soils during the early stages (7-10 d) of organic matter decomposition, as indicated by the decreasing values of the Sørensen similarity index between the treatments in this period. Soil amendment increased the alkaline phosphomonoesterase activity by 6, 8 and 15 times in the Vallombrosa acidic, Romola neutral and Vicarello alkaline soil, respectively, whereas the acid phosphomonoesterase activity showed a 6-, 2- and 10-fold increase in the Vallobrosa acidic, Romola neutral and Vicarello alkaline soil, respectively. Phosphatase activities peaked between 4 and 10 d, depending on the activity and the soil considered, but activity of alkaline phosphomonoesterase was higher in alkaline soils and persisted longer than the acid phosphomonoesterase activities; the opposite occurred in the acid soil. During a 180 d decomposition period, both acid and alkaline phosphomonoesterase activities were related to dsDNA and ATP contents in all soils. Peaks of phosphmonoesterase activity coincided with the changes in the composition of the bacterial microflora, as detected by 16S-rDNA analysis, although no relationship between bacterial community composition and persistence of the phosphomonoesterase activities could be shown. It was concluded that acid and alkaline phosphomonoesterases are produced in greater amounts during plant residue decomposition, and that in acid soils acid phosphomonoesterase activity predominates and in neutral and alkaline soils alkaline phosphomonoesterase activity predominates. However, the persistence of newly produced enzymes is determinated by other factors such as soil texture, organic matter content and formation of soil colloid-enzyme complexes.     

Effect of earthworms on decomposition processes in raw humus forest soil: A microcosm study

Summary The earthworms Lumbricus rubellus (Hoffmeister) and Dendrobaena octaedra (Savigny) were studied in the laboratory to determine their effects on decomposition and nutrient cycling in coniferous forest soil. CO₂ evolution was monitored, and pH, PO ₄ ^3– –P, NH ₄ ⁺ –N, NO ₃ ^– –N, total N, and total C in the leaching waters were measured. After three destructive samplings, numbers of animals, mass loss, pH, and KCl-extractable nutrients were analysed.The earthworms clearly enhanced the mass loss of the substrate, especially that of litter. L. rubellus stimulated microbial respiration by 15–18%, whereas D. octaedra stimulated it only slightly. The worms significantly raised the pH of the leaching waters and the humus; L. rubellus raised the value by 0.2–0.6 pH units and D. octaedra by 0.1–0.4 units. Both worms increased N mineralization. Although the biomass of both worms decreased during the experiment, the N released from decomposing tissues did not explain the increase in N leached in the presence of earthworms. The worms influenced the level of PO ₄ ^3– –P only slightly.     

不同镉积累类型小麦根际土壤低分子量有机酸与镉的生物积累

采用土壤培养方法研究低镉积累小麦烟 86103和高镉积累小麦莱州 953不同生育期土壤低分子量有机酸含量与组成 ,及其与镉生物积累的关系。结果表明 ,不同镉积累类型小麦在高镉土壤中其根系镉含量差异不显著 ,但地上部镉浓度烟 86103显著低于莱州 953,而在低镉土壤中根系和地上部镉浓度烟 86 10 3均显著低于莱州 95 3。根际土壤有机酸 (柠檬酸、酒石酸、乙酸和丙酸 )含量及有机酸总量均为低镉品种烟 86103显著低于高镉品种莱州953。两个品种不同生育期有机酸含量均表现为柠檬酸 酒石酸 丙酸 乙酸 ,且各有机酸含量占有机酸总量的百分数表现稳定。小麦镉的生物积累与有机酸种类没有特异性关系 ,但与有机酸的总量有关。根际不同有机酸的水平对于土壤镉的复合或螯合溶解 ,在引起两品种地上部镉生物积累的差异方面起重要作用。     

Non-cellulosic neutral sugar contribution to mineral associated organic matter in top- and subsoil horizons of two acid forest soils

We investigated the polysaccharide composition of bulk and mineral-bound (density fractions >2 g cm⁻³) organic matter in topsoil and subsoil horizons of a Podzol and a Cambisol. Total sugar contents were generally higher in the Cambisol than in the Podzol. For most horizons of both soils, the sugars were enriched in the mineral-bound organic matter fraction. This fraction showed a monosaccharide distribution typical for microbial sugars, whereas in bulk soil horizons higher contributions of plant-derived sugars were observed. A strong relationship with the ¹⁴C activity of the dense fraction suggests that microbial-derived polysaccharides are most likely stabilised preferentially by mineral interactions compared to plant-derived polysaccharides.     

The effect of collembolan grazing on fungal activity in differently managed upland pastures: A microcosm study

Laboratory microcosms containing litter from three tussock grasslands were used to assess the impact of grazing by a collembolan, Onychiurus procampatus, on the abundance, nutrient release, and respiration of the saprotrophic fungus, Phoma exigua. The fungal biomass and respiration rate were significantly reduced only when Collembola were present in excess of mean field densities but perhaps more typical of spatial aggregations in the soil. A high efficiency of nutrient immobilization by P. exigua was demonstrated but nutrient release was not significantly affected by the fauna. Problems associated with the use of microcosms in the simulation of field conditions are discussed.     

Non-target effects of entomopathogenic nematodes on soil microbial community and nutrient cycling processes: A microcosm study

Under microcosm conditions, changes in the soil microbial biomass, respiration rates, and nitrogen pools as indicators of potential non-target effects of entomopathogenic nematodes on soil, were evaluated. Two tests were conducted using soil collected from the field with no history of entomopathogenic nematode application. Treatments consisted of applications of Steinernema carpocapsae All strain in the presence or absence of the wax moth Galleria mellonella larva as a target insect host, compared with the untreated control (soil only). In the second experiment an insecticide treatment (trichlorfon) was added. Microbial biomass (total nitrogen), and mineral nitrogen (NH₄-N, NO₃-N) were measured using standard methods up to 32 days and soil respiration up to 64 days in both experiments. No negative effect was detected in the soil microbial biomass, respiration and nitrogen pools after application of S. carpocapsae. However, a significant increase in ammonium was measured during almost the entire period of the test in the nematode plus larva treatment, not shown in the other treatments. This high level of ammonium in the nematode plus larva treatment showed that entomopathogenic nematodes can indirectly affect system-level processes in soil and adds evidence on the importance of indirect interactions affecting functions in soil food webs. The application of the insecticide trichlorfon significantly suppressed the microbial biomass and nitrification process.