Effect of soil moisture and temperature on the survival of phytophthora cinnamomi rands in soil

Soil moisture and temperature affect hyphal lysis and sporangial formation in Phytophthora cinnamomi. Hyphal lysis was most rapid in soils at, or below, field capacity when incubated at 25–27° C. Sporangia were not formed in soils below field capacity and only very slowly in waterlogged soil. Sporangia were not formed at temperatures below 15°C.     

Behaviour of Phytophthora cinnamomi rands in different soils and water regimes

The effect of different soils, nutrient states and water regimes on the growth, sporulation and lysis of mycelial inocula of Phytophthora cinnamomi has been examined. It has been observed that the requirements for chlamydospore and sporangium production in soils are relatively non-specific with respect to soil type, pH, percentage organic matter and the presence or absence of an additional food source. In contrast to chlamydospore production, however, production of sporangia in soil depends on a sufficiently low water suction pressure. In some soils a low percentage water content or a water content well below field capacity did not necessarily inhibit sporangium production. The pathogen was a good competitor for pieces of both fresh and rotting Castanea sativa radicles. Trichoderma viride appeared to play a significant role in soil by lysing hyphae of P. cinnamomi and inducing it to produce oospores.     

Evaluation of techniques for quantitative detection of Phytophthora cinnamomi

Three baiting techniques and a sieving technique were evaluated for the quantitative detection zof Phytophthora cinnamomi Rands. Baiting with seedlings of Lupinus angustifolius L. (blue lupins), cotyledons of Eucalyptus sieberi F. Muell. and pear fruits cv. Packham's Triumph, detected one chlamydospore introduced into 50 g of either sand or soil. Sieving detected one chlamydospore in 50 g sand and also in 5 g soil. In naturally-infested soil, diluted with a similar non-infested soil, P. cinnamomi was detected at dilutions of $\text{1}\text{8}$ and $\text{1}\text{128}$ by the sieving and the baiting techniques, respectively. In general, pears were the most satisfactory bait for the detection and isolation of P. cinnamomi. The potential of baiting techniques for estimating numbers of propagules of P. cinnamomi in naturally-infested soil is discussed.     

The relationship of inoculum density of Phytophthora cinnamomi chlamydospores in soil and infection of Eucalyptus marginata seedlings

Chlamydospores of Phytophthora cinnamomi were used as inoculum of known density (ID) in an investigation of the rate of jarrah seed germination and the level of infection of seedlings at the various ID's.Inoculum recovery by soil plating was about 2% of the added density. The germination of the jarrah seedlings was accelerated by the increase of the ID, especially at ID of 250 chlamydospores g^?1 soil. The nominal LD₅₀ value for jarrah seedlings per chlamydospore inoculum was calculated as 13.0 chlamydospores g^?1 soil and the number of viable chlamydospores in the rhizosphere at the highest ID was estimated as 9.1.     

有机质对土壤光谱特性的影响研究

为了探明土壤有机质的光谱特征及其影响作用,从而为有机质土壤铁氧化物的定量反演提供理论依据。利用去有机质前后土壤的光谱数据,研究了有机质对土壤反射率、土壤线参数、土壤铁氧化物定量反演的影响。研究结果表明,去除有机质后,能明显提高土壤反射率,变化最明显的为可见光橙黄光波段,即570～630 nm。相关性分析也显示橙黄光波段反射率的相对变化量或差值与有机质去除量之间的相关系数要比其他波段高,相关系数最大值在600 nm。因此,建议采用570～630 nm的光谱数据进行有机质的反演;土壤线斜率在去有机质后明显降低,截距显著增大,二者变化量与有机质去除量呈极显著相关关系,可用土壤线参数预测有机质含量。有机质对铁氧化物的反演具有明显影响,特别是有机质大于20 g kg-1的土壤,在进行反演时应考虑有机质对反演精度的影响,需采取有效地技术手段消减其影响作用,才能达到较好的效果。     

Effect of cover crop management on soil organic matter

Characterization of soil organic matter (SOM) is important for determining the overall quality of soils, and cover crop system may change SOM characteristics. The purpose of this study was to examine the effect of cover crops on the chemical and structural composition of SOM. We isolated humic substances (HS) from soils with the following cover crop treatments: (a) vetch (Vicia Villosa Roth.)/rye (Sesale cereale L.), (b) rye alone, and (c) check (no cover crops) that were treated with various nitrogen (N) fertilizer rates. CPMAS-TOSS (cross-polarization magic-angle-spinning and total sideband suppression) ¹³C NMR results indicated that humic acids (HA) from soils under rye only were more aromatic and less aliphatic in character than the other two cover crop systems without fertilizer N treatment. Based on the DRIFT (diffuse reflectance Fourier transform infrared) spectra peak O/R ratios, the intensities of oxygen-containing functional groups to aliphatic and aromatic (referred to as recalcitrant) groups, the highest ratio was found in the HA from the vetch/rye system with fertilizer N. The lowest ratio occurred at the vetch/rye system without fertilizer N treatment. The O/R ratio of fulvic acids (FA) can be ranked as: vetch/rye without fertilizer>vetch/rye with fertilizer>no cover crop without fertilizer>rye alone (with or without fertilizer) soils. Both organic carbon (OC) and light fraction (LF) contents were higher in soils under cover crop treatments with and without fertilizer N than soils with no cover crop. These chemical and spectroscopic data show that cover crops had a profound influence on the SOM and LF characteristics.     

耕作对土壤有机物和土壤团聚体稳定性的影响

Agricultural sustainability relates directly to maintaining or enhancing soil quality. Soil quality studies in Canada during the 1980‘s showed that loss of soil organic matter (SOM) and soil aggregate stability was standard features of non-sustainable land management in agroecosystems. In this study total soil organic carbon (SOC), particulate organic matter (POM), POM-C as a percentage of total SOC, and aggregate stability were determined for three cultivated fields and three adjacent grassland fields to assess the impact of conventional agricultural management on soil quality. POM was investigated using solid-state ^13C nuclear magnetic resonance (NMR) to determine any qualitative differences that may be attributed to cultivation. Results show a highly significant loss in total SOC, POM and aggregate stability in the cultivated fields as compared to the grassland fields and a significant loss of POM-C as a percentage of total SOC.Integrated results of the NMR spectra of the POM show a loss in carbohydrate-C and an increase in aromatic-C in the cultivated fields, which translates to a loss of biological lability in the organic matter. Conventional cultivation decreased the quantity and quality of SOM and caused a loss in aggregate stability resulting in an overall decline in soil quality.     

Effect of polar-dissolved organic matter fractions on the mobility of prometryne in soil

Purpose  

Using two fractions of dissolved organic matter (DOM) with different polarity, we carried out the experiments with standard batch equilibration, soil column leaching, and soil thin layer chromatography to investigate the behavior of the herbicide prometryne in soils. The purpose of the study was to: (1) separate DOM into hydrophilic matter and hydrophobic acid forms and characterize their chemical properties; (2) analyze interaction between the DOM fractions and prometryne in soils.     

开垦对黑土有机质化学结构的影响

基于傅里叶红外光谱研究东北黑土开垦对土壤有机质化学结构特征的影响,探明各粒级团聚体及密度组分中有机质化学结构变化特征,为开垦黑土农田土壤有机质结构变化提供理论参考。依托中国科学院海伦农业生态试验站长期定位试验研究平台,以试验开垦前自然草地土壤为对照,选取开垦8年、没有有机物料投入的耕层（0～20 cm）土壤为研究对象,采用湿筛法、密度分组将土壤有机质分成不同组分,用傅里叶红外光谱仪测定原土及各组分有机质红外光谱,分析开垦对土壤有机质化学结构的影响。结果表明：开垦前后,有机质红外结构特征峰相似,均以多糖C—O和醇酚—OH为主。全土芳香族C C含量较开垦前显著减少4.77%,有机质稳定性下降6.32%。在>0.25 mm团聚体中芳香族C C和羰基C O相对强度较开垦前分别显著降低29.91%和27.16%,醇酚—OH相对强度显著增加15.60%;0.25～0.053 mm团聚体中,多糖C—O相对强度增加4.57%,脂肪族—CH相对强度显著减少23.85%。<0.053 mm团聚体中,脂肪族—CH相对强度显著减少16.33%。有机质游离态轻组（LF）、闭蓄态组分（OF）中多糖C—...     

土壤有机质对有效磷及水提取磷含量的影响

为了解土壤有机质对有效磷与总磷、水提取磷与有效磷关系的影响,采用田间调查的方法,对洛阳市郊主要农田4种土壤耕作层土壤进行取样调查,并采用常规土壤测试方法,分别测定土样有机质、金磷、有效磷和水提取磷含量.根据有机质含量(高于或低于16.0 g·kg-1)把土样分成两组,对两组土样的总磷与有效磷、有效磷与水提取磷分别做相关关系分析.结果表明:与低有机质土样组相比,高有机质土样组有效磷与总磷相关方程的斜率较大,水提取磷与有效磷相关方程的斜率较小,这一趋势在以Olsen法和Mehlich-3法测定有效磷时均相同.表明在缺磷环境中,有机质增加条件下,虽然磷的生物有效性降低,但有助于在土壤溶液中维持相对较高的磷酸根浓度,这可能是由于磷的周转加快;随着土壤磷含量增加,有机质增加有助于更快提高磷的生物有效性,但土壤溶液中维持相对较低的磷酸根浓度,这可能有助于减轻农田磷的渗漏流失.     

Effect of barley plants on the decomposition of 14C-labelled soil organic matter

The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with ¹⁴C-labelled ryegrass for 1 year. The plants reduced the loss of ¹⁴CO₂, from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little ¹⁴C label being associated with the shoot. Chemical fractionation of the root showed that the ¹⁴C was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of ¹⁴CO₂, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.     

Effect of Collembola on mineralization of litter and soil organic matter

Although soil Collembola are known to contribute to soil carbon (C) cycling, their contribution to the mineralization of C sources that differ in bioavailability, such as soil organic C (SOC) and leaf litter, is unknown. Stable C isotopes are often used to quantify the effects of both soil C and litter C on C mineralization. Here, ¹³C-labeled litter was used to investigate the effects of Collembola (Folsomia candida) on the mineralization of both SOC and litter C in laboratory microcosms. The three microcosm treatments were soil alone (S); soil treated with δ¹³C-labeled litter (SL); and soil treated with δ¹³C-labeled litter and Collembola (SLC). The presence of Collembola did not significantly affect soil microbial biomass or litter mass loss and only had a small effect on CO₂ release during the first week of the experiment, when most of the CO₂ was derived from litter rather than from SOC. Later, during the experiment (days 21 and 63), when litter-derived labile C had been depleted and when numbers of Collembola had greatly increased, Collembola substantially increased the emission of SOC-derived CO₂. These results suggest that the effect of Collembola on soil organic C mineralization is negatively related to C availability.     

Effect of soil organic matter on peroxidase activity of wheat roots

Some soil organic matter fractions inhibited the peroxidase activity of wheat-root filtrates, but the effect was related to the enzyme hydrogen donor used. In the presence of o-dianisidine as the hydrogen donor, humic acid and the fractions obtained from it by water or acid refluxing inhibited the enzyme activity. The greatest effects were produced by those fractions which were insoluble after such treatments. The inhibitory effects were reversible and non-competitive, the Michaelis constant of the enzyme being only slightly affected, and independent of pH of the assay media. When guaiacol was used as the enzyme substrate the inhibitory effects of the humic acid and its fractions were variable and less marked.p-Coumaric and p-hydroxybenzoic acids inhibited peroxidase activity when o-dianisidine was used as substrate, but stimulated the enzyme in the presence of guaiacol. Polymaleic acid which is thought to have a similar structure to fulvic acid, inhibited peroxidase in the presence of both hydrogen donors. whereas fulvic acid was considerably less effective.     

Hyperparasitism of oospores of Phytophthora megasperma var. sojae

Hyperparasites of oospores of Phytophthora megasperma Drechs. var. sojae Hildb. were present in each of 15 field soils tested. Maximum numbers of oospores parasitized ranged from 42.5 to 87.5% for flooded soils, and from 25.5 to 73.0% for soils adjusted to 50% water holding capacity; the mean for all soils was 51.5%. The frequency of hyperparasitism was not correlated with the disease potential soils for Phytophthora root-rot of soybean as determined in seedling tests on flooded soil samples. Of eight isolated hyperparasitic fungi tested in steamed soil, the most efficient parasites were Hyphochytrium catenoides, Humicola fuscoatra, and Pythium monospermum, each of which parasitized at least 76% of oospores during 3 weeks. Hyphae were not parasitized by any of the eight fungi. Parasitism by H. catenoides in sterilized soil increased as soil temperature increased from 16° to 28°C. Parasitism by P. monospermum was maximum at 20°–24°C. Oospores of P. meyasperma var. sojae race 7 were more resistant to infection by hyperparasites than were oospores of races 1 and 3. Oospores produced in culture were slightly more susceptible to hyperparasitism in soils than were oospores produced in soybean seedlings.     

Photodissolution of soil organic matter

Sunlight has been shown to enhance loss of organic matter from aquatic sediments and terrestrial plant litter, so we tested for similar reactions in mineral soil horizons. Losses of up to a third of particulate organic carbon occurred after continuous exposure to full-strength sunlight for dozens of hours, with similar amounts appearing as photodissolved organic carbon. Nitrogen dissolved similarly, appearing partly as ammonium. Modified experiments with interruption of irradiation to include extended dark incubation periods increased loss of total organic carbon, implying remineralization by some combination of light and microbes. These photodissolution reactions respond strongly to water content, with reaction extent under air-dry to fully wet conditions increasing by a factor of 3–4 fold. Light limitation was explored using lamp intensity and soil depth experiments. Reaction extent varied linearly with lamp intensity. Depth experiments indicate that attenuation of reaction occurs within the top tens to hundreds of micrometers of soil depth. Our data allow only order-of-magnitude extrapolations to field conditions, but suggest that this type of reaction could induce loss of 10–20% of soil organic carbon in the top 10 cm horizon over a century. It may therefore have contributed to historical losses of soil carbon via agriculture, and should be considered in soil management on similar time scales.     

不同形态氮肥对玉米产量和土壤浸提性有机质的影响

田间条件下,研究了不同形态氮肥(尿素、NH4+-N和NO3--N)对玉米产量、根际和非根际土壤氮和浸提性有机质的影响.结果表明,施氮处理的产量和吸氮量明显高于不施肥处理;施氮处理中,NO3--N和尿素处理开花前吸氮量显著高于NH4+-N处理,产量也略高于NH4+-N处理,但未达到显著水平;不同氮形态处理之间的土壤NH4+-N、NO3--N和浸提性有机碳(EOC)、氮(EON)没有差异;抽雄期EOC最高,与根系生长发育一致,而EON苗期相对最高.可见,在基础肥力较高的黑土上,不同形态氮肥对玉米产量、土壤养分影响不明显.     

Effect of earthworm cast formation on the stabilization of organic matter in fine soil fractions

To study the effects of earthworm casting on organic matter dynamics, control soil and casts were added as a surface layer (Horizon I) to perspex cylinders containing a ‘base’ soil depleted in organic C (Horizon II). Three treatments with different Horizon I were used; a control containing uningested soil and oak litter (Quercus petraea (Mattuschka) Lieblein), cast derived from the same substrates, and a third (Ew+cast) where cast as well as endogeic (Aporrectodea caliginosa Savigny, Allolobophora chlorotica Savigny) and anecic (Lumbricus terrestris L.) earthworms were included. These were monitored over a 2 year period. Moisture fluctuations were reduced and higher amounts of organic matter and C were present in Horizon I with the cast treatment after 2 years. In addition, the proportion of clay C (and to a lesser extent light fraction) in Horizon I decreased significantly in the control and cast treatments while there was a corresponding increase in the proportions of silt C. Overall, earthworm activity had a limited effect on C distribution in the particle size fractions studied, though the dynamics of organic matter/particle associations may require more than 2 years before clear patterns emerge.     

土壤有机质对疏水性有机污染物的非线性吸附及其影响因素

土壤有机质(SOM)是土壤的重要组成部分,它对疏水性有机污染物(HOCs)的吸附作用对污染物在环境中的迁移转化有着至关重要的影响。近年来的一些研究表明,HOCs在SOM上的吸附行为并不严格地符合传统的线性分配模型。本文综述了近年来一些学者提出的SOM对HOCs的非线性吸附作用的机制及其影响因素,认为SOM的组成和结构均对HOCs的吸附有着重要的影响。SOM吸附在土壤矿物表面后,其结构或各组分的比例发生了改变,从而对其本身的吸附作用产生影响。溶液的化学环境(主要是阳离子)会对SOM的结构和吸附作用产生影响,特别是当溶液中存在一些过渡金属(如,铜或银)的离子时,共吸附的金属离子可能会与多环芳烃类分子形成阳离子-π键的作用,从而促进多环芳烃类物质在SOM上的吸附。