Assessment of allelopathic interference of Chenopodium album through its leachates,debris extracts,rhizosphere and amended soil

Abstract

A study conducted to assess the allelopathic interference of invasive weed Chenopodium album indicated that aqueous leachates of C. album plant parts (roots, whole plant, and leaves), debris extracts and rhizosphere soil deleteriously affected the germination and initial growth of two test plants – Cassia occidentalis (a weed) and Phaseolus aureus (a crop). Further, the plant growth in terms of height, biomass, chlorophyll and protein content was significantly lesser in soil amended with C. album debris. Aqueous leachates, debris extracts, rhizosphere soil and debris amended soil were found to contain significantly higher amounts of water-soluble phenolics – the known phytotoxins. These phytotoxins in root, above-ground parts and leaves were phenolic acids, namely gallic, chlorogenic, caffeic, vanillic, p-coumaric, syringic and ferulic. Among these, chlorogenic acid was present in the maximum amount in the leaves and roots. The study concludes that C. album releases phenolics into the soil and these are probably involved in the growth inhibitory effect of C. album.     

Soils and the conditional allelopathic effects of a tropical invader

Allelopathy may contribute to the formation of mono-dominant stands of exotic species, but the effects of allelochemicals can be highly conditional. We explored variation in the production of phenolics in leaves, accumulation of phenolics in soils, and the inhibitory effects of soils under an aggressive invader Prosopis juliflora across a range of invaded sites and potential mechanisms by which soils alter the effects of P. juliflora leaf litter. For eight sites in Northwest India we compared the concentration of total phenolics and the seedling growth of Brassica campestris in soils from beneath P. juliflora to that in soils collected away from P. juliflora canopies. We then explored these effects in detail in soils from two sites that differed substantially in texture by germinating seeds of B. campestris in these soils amended with P. juliflora macerated leaf leachate. Finally, we tested the effects of l-tryptophan in soils from these two sites on the seedling growth of B. campestris. Across all sites soil beneath P. juliflora contained higher levels of total phenolics and suppressed the growth of B. campestris than soil that was not under P. juliflora. We observed much variation among P. juliflora-invaded sites in the total phenolic levels of soils and the degree to which they suppressed B. campestris and the concentration of phenolics in soils significantly correlated with the root length of B. campestris when grown in these soils. Soil from two sites amended with P. juliflora macerated leachate suppressed seedling growth of B. campestris, with the effect being higher in sandy soil than sandy loam soil. In soil amended with leachate the strong suppression of B. campestris corresponded with much higher total phenolic and l-tryptophan concentrations. However, in other tests l-tryptophan did not affect B. campestris. Our results indicate that the allelopathic effects of P. juliflora can be highly conditional and that variation in soil texture might contribute to this conditionality.     

Nutrient and phytotoxic contributions of residue to soil in no-till continuous-corn ecosystems

Summary An understanding of no-till ecosystems is essential for increased acceptance of conservation tillage practices. The primary objective of the present research was to assess the nutrient contributions of leachates from decomposing corn residue to soil in continuous-corn no-till ecosystems. A secondary objective was to estimate the phytotoxic effects of these leachates on corn seedling growth. The effects of moisture, temperature, and resident and non-resident microflora on leachates recovered from decomposing surface-applied corn residue were also studied. Leachates were analyzed for organic C, total N, PO ₄ ^3– -P, acid and alkaline phosphatase activity, urease activity, and phytotoxic effects. Within the first 90 days of a 215-day field study 73%, 83%, and 60% of C, N, and P, respectively, were leached. In terms of nutrient concentration, this suggests two distinct phases of release of nutrients onto the soil: A high initial flux of nutrients that is followed by a low-concentration release. No phytotoxic effect of field leachates was observed. Acid and alkaline phosphatase activity was highest on days 39 and 47 whereas urease activity peaked on day 149. In laboratory studies, alterations in temperature or moisture had little effect on the leachate nutrient concentration, or phytotoxic or enzyme activity. Increasing amounts of organic C and N were extracted over time. No phytotoxic effects were expressed in the laboratory. Overall, it appears that the maximum leaching of nutrients occurs early in the decomposition process and that in no-till systems no phytotoxicity can be associated with decomposition of surface residues.Paper No. 11871 of the Purdue University Agricultural Experiment Station Series     

Bacterial inoculants affecting nickel uptake by Alyssum murale from low, moderate and high Ni soils

Metal hyperaccumulator plants like Alyssum murale have a remarkable ability to hyperaccumulate Ni from soils containing mostly insoluble Ni. We have shown some rhizobacteria increase the phytoavailability of Ni in soils, thus enhancing Ni accumulation by A. murale. Nine bacterial strains, originally isolated from the rhizosphere of A. murale grown in serpentine Ni-rich soil, were examined for their ability to solubilize Ni in different soils and for their effect on Ni uptake into Alyssum. Microbacterium oxydans AY509223; Rhizobium galegae AY509213; Microbacterium oxydans AY509219; Clavibacter xyli AY509236; Acidovorax avenae AY512827; Microbacterium arabinogalactanolyticum AY509225; M. oxydans AY509222; M. arabinogalactanolyticum AY509226 and M. oxydans AY509221 were added to low, moderate and high Ni-contaminated soils. M. oxydans AY509223 significantly increased Ni extraction by 10 mM Sr(NO₃)₂ from the high and medium soils and had no effect on Ni extraction from the low Ni soils. The other eight bacterial isolates significantly increased Ni extraction from all soils. There were no significant effects of bacterial inoculation on fresh and dry weight of A . murale shoots grown in the low and high Ni soils compared to an unamended control. M. oxydans AY509223 significantly increased Ni uptake of A. murale grown in the low, medium, and high soils by 36.1%, 39.3%, and 27.7%, respectively, compared with uninoculated seeds. M. oxydans AY509223 increased foliar Ni from the same soils from 82.9, 261.3 and 2829.3 mg kg⁻¹ to 129.7, 430.7, and 3914.3 mg kg⁻¹, respectively, compared with uninoculated controls. These results show that bacteria are important for Ni hyperaccumulation and could potentially be developed as an inoculum for enhancing uptake during commercial phytoremediation or phytomining of Ni.     

紫茎泽兰叶片凋落物对入侵地4 种草本植物的化感作用

为了明确紫茎泽兰叶片凋落物对入侵地草本植物的化感作用, 研究了不同浓度紫茎泽兰叶片凋落物水提液对入侵地草本植物多年生黑麦草、白三叶、辣子草和紫花苜蓿种子萌发和幼苗生长的影响, 同时结合土培试验研究了叶片凋落物在入侵地土壤中的化感作用。结果表明, 除多年生黑麦草外, 水提液对其他3 种草本植物种子萌发均产生了显著的化感抑制作用, 且水提液的浓度越高抑制效果越强; 低浓度水提液对紫花苜蓿和辣子草的幼苗生长存在显著化感促进作用, 高浓度的水提液对除多年生黑麦草外的其他3 种植物幼苗的生长存在显著化感抑制作用, 水提液对多年生黑麦草幼苗生长的影响不显著; 土壤中按照50 g·kg^-1的比例添加叶片凋落物后, 显著抑制了白三叶的生长, 而添加活性炭后, 白三叶的单株生物量相对于未添加活性炭的处理增加71.25%, 进一步证实叶片凋落物在土壤中的化感抑制作用。这说明外来入侵植物紫茎泽兰可能通过其叶片凋落物在入侵地土壤中降解, 释放化感物质, 抑制伴生植物的种子萌发和幼苗生长, 为自身创造有利的生长环境, 实现其成功入侵和扩张。     

COMPETITIVE RELATIONSHIP OF CAULIFLOWER (BRASSICA OLERACEA L. VAR. BOTRYTIS) AND NETTLE-LEAVED GOOSEFOOT (CHENOPODIUM MURALE L.) AND THEIR DIFFERENTIAL RESPONSES TO POTASSIUM LEVEL

Growth and competitive abilities of cauliflower (Brassica oleracea L. var. botrytis cv. ‘White Cloud’) and nettle-leaved goosefoot (Chenopodium murale L.) were investigated under glasshouse conditions. The two species responded differently to a reduction in potassium (K) levels in the medium. Growth of C. murale was reduced while that of cauliflower increased with reduced K levels. At 0.125 mM K, average reduction in shoot dry weight of C. murale grown in pure and mixed stands was 35% and 58%, respectively. Average competition indices and relative competitive ability indices were higher for cauliflower than for C. murale. In a replacement series design, cauliflower showed higher shoot dry weight at high and low K levels in the medium than that of C. murale. Relative yield total (RYT) was > 1 indicating differences in species demand for K and would suggest possible control of the weed by managing the applied K level in the field.     

Variations in soil microbial biomass and crop roots due to differing resource quality inputs in a tropical dryland agroecosystem

The influence of exogenous organic inputs on soil microbial biomass dynamics and crop root biomass was studied through two annual cycles in rice-barley rotation in a tropical dryland agroecosystem. The treatments involved addition of equivalent amount of N (80 kg N ha⁻¹) through chemical fertilizer and three organic inputs at the beginning of each annual cycle: Sesbania shoot (high-quality resource, C:N 16, lignin:N 3.2, polyphenol+lignin:N 4.2), wheat straw (low-quality resource, C:N 82, lignin:N 34.8, polyphenol+lignin:N 36.8) and Sesbania+wheat straw (high-and low-quality resources combined), besides control. The decomposition rates of various inputs and crop roots were determined in field conditions by mass loss method. Sesbania (decay constant, k=0.028) decomposed much faster than wheat straw (k=0.0025); decomposition rate of Sesbania+wheat straw was twice as fast compared to wheat straw. On average, soil microbial biomass levels were: rice period, Sesbania?Sesbania+wheat straw>wheat straw?fertilizer; barley period, Sesbania+wheat straw>Sesbania?wheat straw?fertilizer; summer fallow, Sesbania+wheat straw>Sesbania>wheat straw?fertilizer. Soil microbial biomass increased through rice and barley crop periods to summer fallow; however, in Sesbania shoot application a strong peak was obtained during rice crop period. In both crops soil microbial biomass C and N decreased distinctly from seedling to grain-forming stages, and then increased to the maximum at crop maturity. Crop roots, however, showed reverse trend through the cropping period, suggesting strong competition between microbial biomass and crop roots for available nutrients. It is concluded that both resource quality and crop roots had distinct effect on soil microbial biomass and combined application of Sesbania shoot and wheat straw was most effective in sustained build up of microbial biomass through the annual cycle.     

<Emphasis Type="Italic">Oryza sativa</Emphasis> straw restricts <Emphasis Type="Italic">Phalaris minor</Emphasis> growth: allelochemicals or soil resource manipulation?

Unharvested stubbles or harvested straw of rice (Oryza sativa L.) gets incorporated into soil and interferes with the seedling growth of crop plants. In this paper, we investigated whether rice straw, either through releasing allelochemicals and/or through manipulating soil properties, influences seedling growth of Phalaris minor Retz., a non-native weed largely restricted to wheat (Triticum aestivum L.) fields. One hundred twenty grams of soil was amended with rice straw (0.5, 1, 2, 4, or 6 g/pot) and its effect on fresh shoot biomass of P. minor was examined. Any modification of rice straw phytotoxicity through the use of washed rice straw, activated charcoal, soil sterilization, or nitrogen fertilization was also studied. We carried out chemical and microbial analysis of soils to examine the role of soil properties in influencing P. minor growth. Incorporation of rice straw into soil suppressed the growth of P. minor through modifying soil properties. A dose-dependent increase in total phenolics was observed in soil amended with rice straw. Activated carbon or washing of rice straw, however, could not ameliorate the phytotoxic effects of rice straw. Our results provide initial evidence that rice straw restricts P. minor growth by manipulating soil chemical and microbiological properties. Authors contributions IJ conceived of and supervised the study, and wrote the paper; SK carried out the work.     

Transgenic Bt plants decompose less in soil than non-Bt plants

Bt plants are plants that have been genetically modified to express the insecticidal proteins (e.g. Cry1Ab, Cry1Ac, Cry3A) from subspecies of the bacterium, Bacillus thuringiensis (Bt), to kill lepidopteran pests that feed on corn, rice, tobacco, canola, and cotton and coleopteran pests that feed on potato. The biomass of these transgenic Bt plants (Bt+) was decomposed less in soil than the biomass of their near-isogenic non-Bt plant counterparts (Bt−). Soil was amended with 0.5, 1, or 2% (wt wt⁻¹) ground, dried (50 °C) leaves or stems of Bt corn plants; with 0.5% (wt wt⁻¹) ground, dried biomass of Bt rice, tobacco, canola, cotton, and potato plants; with biomass of the near-isogenic plants without the respective cry genes; or not amended. The gross metabolic activity of the soil was determined by CO₂ evolution. The amounts of C evolved as CO₂ were significantly lower from soil microcosms amended with biomass of Bt plants than of non-Bt plants. This difference occurred with stems and leaves from two hybrids of Bt corn, one of which had a higher C:N ratio than its near-isogenic non-Bt counterpart and the other which had essentially the same C:N ratio, even when glucose, nitrogen (NH₄NO₃), or glucose plus nitrogen were added with the biomass. The C:N ratios of the other Bt plants (including two other hybrids of Bt corn) and their near-isogenic non-Bt counterparts were also not related to their relative biodegradation. Bt corn had a significantly higher lignin content than near-isogenic non-Bt corn. However, the lignin content of the other Bt plants, which was significantly lower than that of both Bt and non-Bt corn, was generally not statistically significantly different, although 10-66% higher, from that of their respective non-Bt near-isolines. The numbers of culturable bacteria and fungi and the activity of representative enzymes involved in the degradation of plant biomass were not significantly different between soil amended with biomass of Bt or non-Bt corn. The degradation of the biomass of all Bt plants in the absence of soil but inoculated with a microbial suspension from the same soil was also significantly less than that of their respective inoculated non-Bt plants. The addition of streptomycin, cycloheximide, or both to the soil suspension did not alter the relative degradation of Bt+ and Bt− biomass, suggesting that differences in the soil microbiota were not responsible for the differential decomposition of Bt+ and Bt− biomass. All samples of soil amended with biomass of Bt plants were immunologically positive for the respective Cry proteins and toxic to the larvae of the tobacco hornworm (Manduca sexta), which was used as a representative lepidopteran in insect bioassays (no insecticidal assay was done for the Cry3A protein from potato). The ecological and environmental relevance of these findings is not clear.     

Dynamics of maize (Zea mays L.) leaf straw mineralization as affected by the presence of soil and the availability of nitrogen

An incubation experiment was carried out with maize (Zea mays L.) leaf straw to analyze the effects of mixing the residues with soil and N amendment on the decomposition process. In order to distinguish between soil effects and nitrogen effects for both the phyllospheric microorganisms already present on the surface of maize straw and soil microorganisms the N amendment was applied in two different placements: directly to the straw or to the soil. The experiment was performed in dynamic, automated microcosms for 22 days at 15 °C with 7 treatments: (1) untreated soil, (2) non-amended maize leaf straw without soil, (3) N amended maize leaf straw without soil, (4) soil mixed with maize leaf straw, (5) N amended soil, (6) N amended soil mixed with maize leaf straw, and (7) soil mixed with N amended maize leaf straw. ¹⁵NH₄¹⁵NO₃ (5 at%) was added. Gas emissions (CO₂, ¹³CO₂ and N₂O) were continuously recorded throughout the experiment. Microbial biomass C, biomass N, ergosterol, δ¹³C of soil organic C and of microbial biomass C as well as ¹⁵N in soil total N, mineral N and microbial biomass N were determined in soil samples at the end of the incubation. The CO₂ evolution rate showed a lag-phase of two days in the non-amended maize leaf straw treatment without soil, which was completely eliminated when mineral N was added. The addition of N generally increased the CO₂ evolution rate during the initial stages of maize leaf straw decomposition, but not the cumulative CO₂ production. The presence of soil caused roughly a 50% increase in cumulative CO₂ production within 22 days in the maize straw treatments due to a slower decrease of CO₂ evolution after the initial activity peak. Since there are no limitations of water or N, we suggest that soil provides a microbial community ensuring an effective succession of straw decomposing microorganisms. In the treatments where maize and soil was mixed, 75% of microbial biomass C was derived from maize. We concluded that this high contribution of maize using microbiota indicates a strong influence of organisms of phyllospheric origin to the microbial community in the soil after plant residues enter the soil.     

Changes in mineral nitrogen, phosphorus availability and salt-extractable aluminium following the application of green manure residues in two weathered soils of South Vietnam

Phosphorus deficiency and aluminium toxicity in weathered soils can be amended by applying organic residues. Nitrogen mineralization, changes in P-availability and changes in salt-extractable Al following the incorporation of residues of various green manures (Flemingia congesta, Mucuna pruriens, Pueraria phaseoloides, Tithonia diversifolia) were quantified in a field core incubation experiment. Dried residues were added at an application rate of 45 kg P ha⁻¹ to two soils representative for the main soil groups of the South Vietnamese uplands, set up in incubation cores in an experimental field near Ho Chi Minh City, Vietnam.Decomposition of the residues proceeded at high rates. Mineralized nitrogen from the residues was recovered mainly as ammonium during the first 2 weeks of incubation. Nitrogen release from Tithonia residues with the highest lignin content and lignin:N ratio occurred more gradually compared to the three legumes. Resin-extractable P was significantly increased by residue treatments. Largest and sustained increases in resin-extractable P (0.67 and 2.06 mg P kg⁻¹ in the two soils) were observed in samples amended with Tithonia, which was related to the large P-content (0.37%) and small C:P ratio (110) of the residues. The P-concentration in the residues, rather than the total amount of P applied through the residues, affected the increase in P-availability. The increase in resin-extractable P was correlated to the P-content (R=0.64) and C:P ratio (R=−0.65) of the residues. Salt-extractable Al-concentrations were considerably reduced by the organic amendments, up to 70 and 50% in the two soils. At the rate of 45 kg P ha⁻¹, no significant differences between the residue treatments to reduce soil acidity were observed.As such, the application of high quality residues that are rich in P, in particular T. diversifolia, may enhance crop production by creating favourable soil conditions during the initial stages of plant development of the main crop.     

Assessment of allelopathic effects of residues of Flaveria bidentis (L.) Kuntze on wheat seedlings

Flaveria bidentis (L.) Kuntze, an invasive plant, poses a serious threat to the structure and function of the native ecosystem in Hebei Provience, China. However, little is known about the allelopathic activity of the residues of this plant species. In this study, the impact of the phytotoxicity of unburnt (UR) and burnt (BR) residues of the exotic invasive weed on the growth of wheat (Triticum aestivvm L.) was assessed. Extracts prepared from both UR and BR reduced the shoot length, root length and dry weight of wheat significantly compared with the control, thereby indicating the presence of some water-soluble allelochemicals in F. bidentis residues. Growth studies conducted in soil amended with UR and BR extracts or residues also revealed phytotoxic effects towards wheat. A significant amount of phenolics was detected in the residue extracts and in residue-incorporated soil. The phenolic content rose with increasing residue concentration, thereby showing their direct involvement in the observed growth inhibition. The inhibition of wheat growth by the burnt residue and its water extracts was similar to that of unburnt residue, so burning it is not a good way to eliminate the allelopathic effects of the residue of F. bidentis on native plants.     

Nickel-tolerant Brevibacillus brevis and arbuscular mycorrhizal fungus can reduce metal acquisition and nickel toxicity effects in plant growing in nickel supplemented soil

The growth of clover (Trifolium repens ) and its uptake of N, P and Ni were studied following inoculation of soil with Rhizobium trifolii, and combinations of two Ni-adapted indigenous bacterial isolates (one of them was Brevibacillus brevis) and an arbuscular mycorrhizal (AM) fungus (Glomus mosseae). Plant growth was measured in a pot experiment containing soil spiked with 30 (Ni I), 90 (Ni II) or 270 (Ni III) mg kg⁻¹ Ni-sulphate (corresponding to 11.7, 27.6 and 65.8 mg kg⁻¹ available Ni on a dry soil basis). Single inoculation with the most Ni-tolerant bacterial isolate (Brevibacillus brevis) was particularly effective in increasing shoot and root biomass at the three levels of Ni contamination in comparison with the other indigenous bacterial inoculated or control plants. Single colonisation of G. mosseae enhanced by 3 fold (Ni I), by 2.4 fold (Ni II) and by 2.2 fold (Ni III) T. repens dry weight and P-content of the shoots increased by 9.8 fold (Ni I), by 9.9 fold (Ni II) and by 5.1 fold (Ni III) concomitantly with a reduction in Ni concentration in the shoot compared with non-treated plants. Coinoculation of G. mosseae and the Ni-tolerant bacterial strain (B. brevis) achieved the highest plant dry biomass (shoot and root) and N and P content and the lowest Ni shoot concentration. Dual inoculation with the most Ni-tolerant autochthonous microorganisms (B. brevis and G. mosseae) increased shoot and root plant biomass and subtantially reduced the specific absorption rate (defined as the amount of metal absorbed per unit of root biomass) for nickel in comparison with plants grown in soil inoculated only with G. mosseae. B. brevis increased nodule number that was highly depressed in Ni I added soil or supressed in Ni II and Ni III supplemented soil. These results suggest that selected bacterial inoculation improved the mycorrhizal benefit in nutrients uptake and in decreasing Ni toxicity. Inoculation of adapted beneficial microorganisms (as autochthonous B. brevis and G. mosseae) may be used as a tool to enhance plant performance in soil contaminated with Ni.     

Crude extract of Astragalus mongholicus root inhibits crop seed germination and soil nitrifying activity

Astragalus mongholicus has been of medicinal use within the traditional Chinese system for centuries. However, little information is available on its allelopathic effects on other crop plants and soil biochemical properties. Field experiment showed that the extracted residues of A. mongholicus root inhibited seed germination of wheat. Inhibition of seed germination was further confirmed in laboratory using the same crude extract. When the crude extract was applied to soil at various rates and incubated for 30 days, soil urease activity and denitrifying enzyme activity were significantly increased while soil nitrification rate was significantly decreased at 10% amendment rate as compared to the control. Soil respiration rate was significantly increased by the crude extract when measured at the start of incubation but returned to basal levels after 30 days of incubation. The crude extract supplemented to NB medium significantly decreased the colony numbers of Agrobacterium tumefaciens C58, Paraccocus denitrificans and soil bacteria. The stimulating effects of crude extract observed in the amended soil was attributed to the easily-available carbohydrates in the extract, which might served as external energy sources for heterotrophic microbial activities. It was concluded that A. mongholicus contained some compounds that inhibited seed germination, soil nitrification and bacterial growth in general. Possible links between allelochemicals responsible for the inhibitory effects observed in the present study and the medically bioactive compounds are discussed based on information reported in other fields. Further work is needed to specify and verify the allelochemicals produced by this herbal plant.     

Effects of chitin amendment of soil on microorganisms,nematodes, and growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.)

Effects of soil amendment with crabshell chitin on the growth of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.), and on populations of soil bacteria, fungi, and plant-parasitic and free-living nematodes were investigated in a pot trial. Five soil samples were collected from Te Puke (Paengaroa Shallow Sand, a Typic Hapludand) and five from Hamilton (Bruntwood silt loam, an Aquic Hapludand), New Zealand. Subsamples of each soil were either amended with chitin or unamended and planted with white clover and ryegrass. The ryegrass shoot weight in amended soil was greater (P<0.01), most probably due to N mineralised from chitin. A significantly lower (P<0.01) root: shoot ratio of ryegrass in the amended soil also suggested improved N availability, and therefore less root mass was needed to support a given shoot mass. A reduction in nodulation was observed in 12-day-old white clover seedlings (P<0.05) and also in 6-week-old seedlings (P<0.01). The shoot weight of white clover was significantly lower (P<0.05) in amended soil, possibly due to phytotoxic effects of chitin. Chitin increased (P<0.01) the populations of bacteria and fungi by 13-fold and 2.5-fold, respectively. The cyst nematode of white clover, Heterodera trifolii, was significantly reduced in chitin-amended soil, possibly due to increased levels of chitinase produced by rhizosphere microorganisms. Two other plant-parasitic nematodes, Pratylenchus spp. and Tylenchus spp., were also reduced in ryegrass roots and in soil as a result of the chitin amendment. However, the total number of free-living nematodes increased 5.4-fold in amended soil.     

Impact of allelochemical exuded from allelopathic rice on soil microbial community

Allelopathic rice releases allelochemicals from its roots to paddy soils at early growth stages to inhibit neighboring weeds. However, little is currently known about the effects of allelochemicals on soil microbes. In this study, we show that allelopathic rice can have great impact on the population and community structure of soil microbes. Allelopathic rice PI312777 seedlings reduced the culturable microbial population and total PLFA when compared to non-allelopathic rice Liaojing-9. Similar results were observed when, instead of growing seedlings, soils were incubated with plant root exudates. This result demonstrates that the composition of root exudates from the rice varieties tested contributes to the soil microbial community. Further experiments showed that the microbial community was affected by the allelochemical 5,4′-dihydroxy-3′,5′-dimethoxy-7-O-β-glucopyranosylflavone exuded from allelopathic rice roots, through immediately hydrolyzing glucose with stimulation on soil bacteria and aglycone (5,7,4′-trihydroxy-3′,5′-dimethoxyflavone) with inhibition on soil fungi. This result indicates that the flavone O-glycoside can provide carbon and interact with soil microbes. PC analysis of the fatty acid data clearly separated the allelopathic PI312777 and the non-allelopathic Liaojing-9 variety (PC1 = 46.4%, PC2 = 20.3%). Similarly, the first principal component (PC1 = 37.4%) together with the second principal component (PC2 = 17.3%) explained 54.7% of the variation between the allelopathic and non-allelopathic root exudates. Furthermore, the canonical correlation between allelopathic root exudates and the flavone O-glycoside was statistically significant (Canonical R = 0.889, χ² (25) = 69.72, p = 0.0041). Although the data generated in this study were not completely consistent between culturable microbes and PLFA profile, it is a fact that variation in soil microbial populations and community structures could be distinguished by the allelopathic and non-allelopathic rice varieties tested. Our results suggest that individual components of rice root exudates, such as allelochemicals from allelopathic rice, can modify the soil microbial community.     

Water-soluble organic materials in paddy soil ecosystem

Abstract

Column experiments were conducted to analyze the effect of the temperature on the amounts of organic materials in the leachate, especially organic acids and methane, from samples of the plow layer soil amended with rice straw. Total amount of inorganic carbon in the leachate during the 30-d period of incubation in relation to the temperature was 18°C < 25°C ≤ 30°C > 37°C > 45°C. Total amount of organic carbon in the leachate was signiicantly larger under 45°C incubation than that at other temperatures.

Acetic acid was the dominant organic acid in the leachate regardless of the temperature. Butylic and propionic acids were also present in large amounts in the early and the late period of incubation of temperatures ranging between 18 and 37°C, while only acetic acid was the dominant organic acid during the 30-d period of incubation at 45°C.

The total amount of methane in leachate during the 30-d period of incubation was very small at 18°C, while very large at 25, 30, and 37°C. It decreased nearly to one half at 45°C compared with that at 30°C. Based on the values of δ¹³CH₄ in the leachate, 3 different stages were recognized in the predominant processes of methane production in the submerged paddy soil amended with rice straw: the stage when methane production from CO₂-B₂ was predominant followed by the stages of methane production from acetic acid and from CO₂-H₂ in this order. The second stage coincided with the time of decrease of the organic acid contents in the leachate. Under 45°C incubation, methane production from CO₂-H₂ was predominant throughout the 30-d period of incubation.     

Respiration pattern and microbial use of field-grown transgenic Bt-maize residues

A 49-day incubation experiment was carried out with the addition of field-grown maize stem and leaf residues to soil at three different temperatures (5, 15, and 25 °C). The aim was to study the effects of two transgenic Bt-maize varieties in comparison to their two parental non-Bt varieties on the mineralization of the residues, on their incorporation into the microbial biomass and on changes in the microbial community structure. The stem and leaf residues of Novelis-Bt contained 3.9 μg g⁻¹ dry weight of the Bt toxin Cry1Ab and those of Valmont-Bt only 0.8 μg g⁻¹. The residues of the two parental non-Bt varieties Nobilis and Prelude contained higher concentrations of ergosterol (+220%) and glucosamine (+190%) and had a larger fungal C-to-bacterial C ratio (+240%) than the two Bt varieties. After adding the Bt residues, an initial peak in respiration of an extra 700 μg CO₂-C g⁻¹ soil or 4% of the added amount was observed in comparison to the two non-Bt varieties at all three temperatures. On average of the four varieties, 19-38% of the maize C added was mineralized during the 49-day incubation at the three different temperatures. The overall mean increase in total maize-derived CO₂ evolution corresponded to a Q₁₀ value of 1.4 for both temperature steps, i.e. from 5 to 15 °C and from 15 to 25 °C. The addition of maize residues led to a strong increase in all microbial properties analyzed. The highest contents were always measured at 5 °C and the lowest at 25 °C. The variety-specific contents of microbial biomass C, biomass N, ATP and adenylates increased in the order Novelis-Bt ? Prelude<Valmont-Bt ? Nobilis. The mineralization of Novelis-Bt residues with the highest Bt concentration and lowest N concentration and their incorporation into the microbial biomass was significantly reduced compared to the parental non-Bt variety Nobilis. These negative effects increased considerably from 5 to 25 °C. The transgenic Bt variety Valmont did not show further significant effects except for the initial peak in respiration at any temperature.     

Response of soil enzymes to Linear Alkylbenzene Sulfonate (LAS) addition in soil microcosms

Soil enzymatic activities (phosphatases, arylsulphatase and dehydrogenase) were measured in microcosm systems designed for the study of the impact of a commercial mixture of Linear Alkylbenzene Sulphonate (LAS) homologues on a xerofluvent agricultural soil. The soil microcosms consisted of glass columns filled with 800 g of dry soil which were fed with sterile commercial LAS solutions at concentrations of 10 or 50 mg l⁻¹ for periods of time up to 21 days. A soil microcosm fed with sterile distilled water was included in this study and considered as control. Our results showed that the continuous application of the anionic surfactant to soil increased the values of the enzymes acid and alkaline phosphatases and arylsulphatase. On the contrary, the dehydrogenase activity was decreased by the continuous application of 10 or 50 mg l⁻¹ LAS when compared with control microcosms. In addition, a statistically negative correlation was found between this enzymatic activity in the upper portion of the soil columns amended with LAS and the viable counts of heterotrophic aerobic microorganisms. Moreover, in order to test the influence of LAS on nutrient availability and, consequently, on bacteria populations and soil biological activities, phosphate concentration was regularly determined in the microcosm leachates. The phosphate concentration tested in the leachate of the microcosm continuously amended with 50 mg l⁻¹ LAS solution was significantly lower than the concentrations detected in the leachate of the microcosms continuously amended with 10 mg l⁻¹ LAS throughout the experiment.     

硝化作用驱动下红壤渗漏液的酸化

土壤渗漏液pH对于亚热带酸性土壤的物质迁移和溶液中物质形态具有重要影响。为了研究亚热带酸性土壤硝化作用释放H+与渗漏液pH的关系,以具有不同硝化强度的3个红壤样本为供试材料,分别加入铵态氮0、150和300 mg kg-1,进行112 d的室内土柱模拟淋溶实验。结果表明:酸性土壤的渗漏液并不一定呈酸性。土壤渗漏液pH取决于硝化作用产生H+的速率与土壤酸缓冲能力的综合作用。当硝化作用使渗漏液中NO3-浓度升高至一定程度时,渗漏液pH突然下降,这一临界NO3-浓度与土壤盐基饱和度及加入土壤的铵态氮量呈线性正相关(p0.05)。所以,硝化作用最强的旱地土壤,由于其盐基饱和度达81%,渗漏液始终保持中性;而硝化作用不强、盐基饱和度为21%的灌丛土壤,其渗漏液pH可降至4.0以下。