Seasonality of organic phosphorus mineralization in the rhizosphere of the native savanna grass, Trachypogon plumosus

Savanna ecosystems have low primary productivity, strong seasonality, and acid soils with low phosphorus (P) content. Organic P (Po) comprises around 50% of the total soil P and is plant-available only after mineralization. Rhizosphere processes mediated by plants, microorganisms and arbuscular mycorrhiza (AM) are important for plant P nutrition. We studied P transformation rates, Po-fractionation, acid phosphomonoesterase activity (APA), AM status, dehydrogenase activity (DHA), and bacterial and fungal plate counts in the rhizosphere of the native dominant grass Trachypogon plumosus. We collected samples from three acid savanna soils differing in order and P content (Entisol, Vertisol and Ultisol) at Estación Experimental La Iguana (Northeastern Venezuela) during the dry, rainy and transitional seasons over a 2-year-period. Less available Po fractions (moderately labile, moderately and highly resistant) seem to be involved in short-term P-cycling transformations as they significantly varied with season. During the rainy season plant P content (576-1160 mg P kg⁻¹ dry weight) and APA (44-200 mg PNP kg⁻¹ dry soil) were higher, while microbial number and activity (DHA) were lower. The higher P availability in the Entisol (6-9 mg P kg⁻¹ dry soil) resulted in a better plant nutritional status and inhibited APA. T. plumosus seems to be highly dependent on AM symbiosis (45-71% AM colonized root length, 0.6-8 AM spores g⁻¹ dry soil), especially during the rainy season. Po mineralization processes, mediated by biological associations in the rhizosphere, are crucial for understanding seasonal P-cycling and fertility in acid savanna soils.     

Soil microbial biomass and activities in a Japanese Andisol as affected by controlled release and application depth of urea

This experiment was conducted in maize field plots to study the effects of controlled release and application depth of urea on soil microbial biomass and activities at two depths of surface soil of a Japanese Andisol from June to September, 2001. Three N amendment treatments and a Control were included in this experiment: deep application (8 cm) of controlled release urea; deep application (8 cm) of conventional urea; surface application of conventional urea; Control, without N application. Prior to this experiment, the field plots received the same N fertilizer treatments for two consecutive years under maize/barley rotation. Soil microbial biomass, dehydrogenase and nitrification activities exhibited great vertical and temporal variations during the maize growth season, and the microbial biomass was significantly correlated to soil water-filled pore space (p<0.01). N fertilization did not significantly affect the microbial biomass, but greatly increased the dehydrogenase and nitrification activities. The increase in the microbial activities following N fertilization was not attributed to the increase in microbial biomass but to the increase in intrinsic microbial activities. Controlled release urea was found to continuously affect the dehydrogenase activity over a shorter distance, while conventional urea could greatly increase the enzyme activity for a shorter period of time. Both controlled release and deep application of urea had potentials to reduce the nitrification activity and suggested that the nitrate production might be decreased in 0–10 cm surface soil. Deep application of urea increased aboveground N uptake by maize and then the recovery rate of N fertilizer, whereas controlled release of urea greatly increased grain yield and N uptake by grain.     

Comparison and improvement of methods for determining soil dehydrogenase activity by using triphenyltetrazolium chloride and iodonitrotetrazolium chloride

The triphenyltetrazolium chloride (TTC) method described by Thalmann (1968) and the iodonitrotetrazolium chloride (INT) method described by Spothelfer-Magaña and Thalmann (1992), used for measuring soil dehydrogenase activity, have been modified to overcome some methodical short-comings. Absorption maxima of 485 nm for triphenylformazan dissolved in acetone, 491 nm for iodonitrotetrazolium formazan (INTF) dissolved in tetrahydrofuran and 455 nm for INTF dissolved in N,N-dimethylformamide are recommended for measuring wavelengths. Extracting triphenylformazan twice with acetone is less toxic and proved to be at least as efficient as extraction with a mixture of 90% acetone and 10% carbon tetrachloride (Thalmann 1968 method). Tetrahydrofuran and dimethylformamide were equally good in extracting INTF from soils, but the former was less toxic. Anaerobic incubation resulted in the formation of higher amounts of triphenylformazan and INTF as well as reduced standard error. Both TTC and INT reduction showed high reproducibility and good differentiation of the microbial activity of six soils. For several reasons (more easily determined substrate dose depending on different soil types, better reduction, shorter incubation time), INT reduction seems to be a more suitable method of measuring soil microbial activity than TTC reduction.     

Soil microbial biomass and activity under a potato crop fertilised with N with and without C

Summary A range of soil microbiological parameters were measured at intervals throughout the growing season of a potato crop. Treatments applied to the soil at sowing were zero N fertilisation of N fertilisation at 120 kg N ha^–1, either alone or supplemented with straw or sucrose at 1200 kg C ha^–1. C and N flushes determined by fumigation-incubation and fumigation-extraction, and substrate-induced respiration, were measured as indicators of microbial biomass. Microbial activity was measured as respiration (CO₂ production) and dehydrogenase activity (formazan production). The greatest effects were obtained from the addition of N plus sucrose. Both biomass size and activity were significantly stimulated for up to 25 days after incorporation, with the magnitude of the effects consistently diminishing over time. By 125 days after planting, there was no detectable legacy from any of the treatmentson any of the biomass parameters that were measured, and all values had reverted to those prevalent at planting. There was no consistent effect from adding N, either alone or supplemented with straw, on any of the biomass parameters. There was no evidence for crop-induced stimulation of the biomass. The experiment demonstrates that biomass is only influenced where the quantity, quality, and rate of incorporation of C into the soil is appropriate, in this case, only by adding C as a pulse of sucrose.     

导入外源甜菜碱醛脱氢酶基因BADH对小麦盐旱抗性的影响

测定并比较了转BADH基因小麦品系99T6及其受体石4185,在盐、旱逆境下植株体内的脯氨酸含量、MDA含量、叶绿素含量、质膜透性及根活力,探讨了外源BADH基因导入对小麦抗性的改良作用。     

黄瓜叶片水浸提液对黄瓜、 黑籽南瓜种子萌发的影响

以黄瓜、 黑籽南瓜种子为试材, 研究不同浓度黄瓜叶片水浸提液对其种子萌发及生理特性的影响。 结果表明, 随着黄瓜叶片水浸提液浓度的增加, 黄瓜发芽率、 发芽指数、 α-淀粉酶活性及脱氢酶活性等均降低, 抑制了黄瓜根系、 胚轴的生长; 而一定浓度的黄瓜叶片水浸提液提高了黑籽南瓜 α-淀粉酶和脱氢酶活性, 促进黑籽南瓜种子萌发, 高浓度浸提液抑制黑籽南瓜种子萌发。 通过化感效应值可以得出, 黄瓜叶片水浸提液对黄瓜种子萌发的化感效应为负效应, 而对黑籽南瓜则一定浓度范围为正效应, 高浓度为负效应。 黄瓜叶片水浸提液抑制黄瓜种子萌发, 而对黑籽南瓜种子萌发表现为 “低促高抑”。     

柄扁桃果实抑制物对小麦种子萌发时2种抗氧化酶活性的影响

[目的]探讨柄扁桃果实提取物对小麦种子萌发过程中脱氢酶和过氧化氢酶活性的影响。[方法]通过分光光度计法和高锰酸钾滴定法分别测定小麦种子萌发过程中脱氢酶和过氧化氢酶的活性。[结果]柄扁桃内果皮粗提物在0.1g/ml时,小麦种子内脱氢酶含量最高,而果肉粗提物在0.1～0.15g/ml时,小麦种子内脱氢酶含量最高;内果皮粗提物在0.05g/ml时,小麦种子内过氧化氢酶含量最高,而果肉粗提物在0.10g/ml时,小麦种子内过氧化氢酶活性含量最高。[结论]柄扁桃的果肉和内果皮提取液对小麦种子萌发过程中的脱氢酶和过氧化氢酶含量均有影响,因此可以通过选择合适浓度的柄扁桃果实提取物来培养小麦种子。     

刺梨半乳糖内酯脱氢酶的酶促反应条件优化

为进一步探明刺梨半乳糖内酯脱氢酶(GalLDH)的作用机理及其对维生素C(AsA)合成效率的影响,以高AsA果实刺梨为材料,研究了AsA合成关键酶--GalLDH酶促反应的动力学参数.结果表明,刺梨GalLDH反应的最适温度在30℃左右,最适pH值为8.3～9.4,以半乳糖内酯作为反应底物的Km为0.031 mM,最大反应速率Vmax=0.026 U/s.当以古洛糖内酯作为反应底物时,GalLDH也能表现约70%的活性.     

Plant growth and nutrition in pine (Pinus thunbergii) seedlings and dehydrogenase and phosphatase activity of ectomycorrhizal root tips inoculated with seven individual ectomycorrhizal fungal species at high and low nitrogen conditions

We investigated the functions and ecological traits of seven individual ectomycorrhizal (ECM) fungal species derived from a Pinus thunbergii-dominated (nitrogen-poor) and a Robinia pseudoacacia-dominated (nitrogen-rich) area. P. thunbergii seedlings were inoculated with seven individual ECM fungal species and cultured under high nitrogen (nitrogen group) and low nitrogen (control group) conditions. Control seedlings were not inoculated with any ECM fungus. Seedlings harvested were examined for the numbers of non-mycorrhizal and ECM root tips, growth, nutrient status (nitrogen, phosphorus and carbon) and the activity of dehydrogenase and phosphomonoesterase in the root tips. The relationships among the growth, nutrient status and the enzymatic activity in the seedlings inoculated with each ECM fungal species were also analyzed by canonical discriminants. As the results, inoculation of ECM fungi made the plant growth significantly higher than those of non-inoculated seedlings. Plant growth significantly correlated with the phosphorus content, N:P ratio and phosphomonoesterase activity, especially in the nitrogen group. Dehydrogenase and phosphomonoesterase activities in the seedlings were higher when the ECM fungi from a nitrogen-rich area were inoculated than when those from a nitrogen-poor area were inoculated. The canonical discriminant analyses separated seedlings by inocula: no fungus, ECM fungi from a nitrogen-poor area, and those from a nitrogen-rich area. These results suggested the possibility that nitrogen deposition caused P shortage and the P absorbing ability was higher in the ECM fungi in nitrogen-rich forests than those in nitrogen-poor forests.     

Drying and rewetting effects on C and N mineralization and microbial activity in surface and subsurface California grassland soils

Rewetting a dry soil has long been known to cause a burst of respiration (the “Birch Effect”). Hypothesized mechanisms for this involve: (1) release of cellular materials as a result of the rapid increase in water potential stress and (2) stimulating C-supply to microbes via physical processes. The balance of these factors is still not well understood, particularly in the contexts of multiple dry/wet cycles and of how resource and stress patterns vary through the soil profile. We evaluated the effects of multiple dry/wet cycles on surface and subsurface soils from a California annual grassland. Treatments included 4, 6, and 12 cycles that varied the length of the drying period between rewetting events. Respiration was monitored after each wetting event while extractable C and N, microbial biomass, and microbial activity were assayed initially, after the first rewetting event, and at the end of the experiment. Initially, microbial biomass and activity (respiration, dehydrogenase, and N mineralization) in subsurface soils were ca. 10% and 20% of surface soil levels. After multiple cycles, however, subsurface soil microbial biomass and activity were enhanced by up to 8-fold, even in comparison to the constantly moist treatment. By comparison, surface soil microbial biomass and activity were either moderately (i.e. 1.5 times increase) or not affected by wetting and drying. Drying and rewetting led to a cascade of responses (soluble C release, biomass growth, and enhanced activity) that mobilized and metabolized otherwise unavailable soil carbon, particularly in subsurface soils.