A laboratory method for the evaluation of nutrient residues in soils

Summary A laboratory method used ion-exchange resins to extract phosphate ions from soils and to measure the rate at, and the extent to which, non-labile phosphate reserves in soils change into the isotopically exchangeable phosphate when the soils are allowed to recover. The method was tested on heavy and acid Indian soils, both unmanured and treated with NPK fertilisers.     

Chlorophyll compounds and nitrogen availability in West Indian soils

Summary The amounts of chlorophyll-type compounds extracted by 90 per cent aqueous acetone from 70 West Indian soils were inversely correlated with soil pH; chlorophyll compounds decomposed when soils were limed to pH 7. Chlorophyll compounds were only significantly correlated with nitrogen used by maize in pot experiments and with mineralizable nitrogen for soils in the pH 5.0 to 5.9 range. Chlorophyll may persist in acid soils because micro-organisms are less active or because the chlorophyll molecule is altered, possibly by the isomorphus replacement of magnesium by reduced iron or manganese.     

Turnover of organic nitrogen in soils and its availability to crops

The root development of barley seedlings grown for one week in an aerated nutrient solution was studied in the presence of dissolved organic matter from an aqueous chestnut leaf litter extract. In particular, the different effects of low and high molecular weight fractions (small molecules: molecular weight <1000; large molecules: >10,000) of the leaf litter extract were examined. In the presence of large molecules root growth was inhibited, an irregular root tip morphology was observed, and Ca and Mg concentrations in the shoots were lower than in control plants. These phytotoxic effects were not caused by the formation of an impermeable layer of large molecules on the root surfaces that lower accessibility for nutrient cations as inferred from voltammetric experiments. A germination assay using spruce seeds, however, indicated allelochemical effects of large molecules, which exhibit a higher aromaticity than the small molecules as indicated by spectroscopic characterisation. In the growth experiments with small molecules, no influence on the root development of barley was evident, but an increase of Ca and Mg in the shoots was detected. During these growth experiments, a large amount of the small molecules, mainly simple phenols and amino acids, disappeared from the nutrient solution. The loss of small molecules was most likely the effect of mineralisation.Abbreviations DOC dissolved organic carbon - DOM dissolved organic matter - LLE leaf litter extract - MW molecular weight - HMDE hanging mercury drop electrode     

Prediction of nitrogen availability and rice yield in lowland soils: Nitrogen mineralization parameters

Field experiments were conducted under flooded soil conditions using Maahas clay amended with urea and rice straw-sesbania mixtures during the wet and dry seasons. Parallel laboratory incubation tests were done. The objectives were 1) to determine N mineralization patterns and establish the relationship between mineralization parameters and either N availability or grain yield, and 2) to correlate the results of organic N mineralization studies in the laboratory with data from field experiments. The N mineralization patterns of flooded soils in the laboratory followed a logistic function. In laboratory studies, mineralization potential was positively correlated with extractable soil NH₄ ⁺-N at the end of the incubation period (cumulative available N). Likewise, mineralization potential calculated from laboratory studies was positively correlated with N uptake and grain yield from field studies. Extractable (NH₄ ⁺+NO₃ ^–)-N in the field correlated positively with extractable NH₄ ⁺-N in the laboratory. The extractable NH₄ ⁺-N from laboratory incubations at 14 days after transplanting, panicle initiation, and maturity was also highly and positively correlated with grain yield from field experiments.     

Soil nitrogen availability evaluations based on nitrogen mineralization potentials of soils and uptake of labeled and unlabeled nitrogen by plants

Summary Sudangrass [Sorghum sudanense (Piper) Stapf] was grown in a greenhouse pot experiment on 39 soils having a broad range of chemical and physical characteristics. Labelled N as sodium nitrate (9% excess N¹⁵) was applied at rates of 200 and 400 mg of N per pot (2kg of soil). After 6 weeks of growth, total N and N¹⁵ were determined on plant tops and roots and on the cropped soils. Maximum yield differed widely among the soils owing to variations in yield-limiting factors other than N. Despite the diversity of responses to N fertilizer, the experiment provided a meaningful basis for assessing soil nitrogen availability. Amounts of N taken up from soils were similar from pots receiving no fertilizer N and from pots receiving labeled N.Amounts of soil organic N mineralized during cropping plus the mineral N present initially in the soils correlated highly with amounts of soil N taken up by whole plants (tops and roots). Average recovery by whole plants of mineral N formed before and during the cropping period was about 85 per cent, a value corresponding closely to recovery of fertilizer N in this experiment. The similarity in recovery of N provided by soil and fertilizer suggests that mineral N from these sources comprised a common pool that behaved as an entity with respect to mineralization-immobilization relations or other reactions affecting N availability to plants.A-values, the amounts of soil N having an availability equivalent to that of applied fertilizer N, were similar for two levels of applied labeled N and for tops and whole plants. Moreover, A-values were similar to amounts of N mineralized before and during crop growth. This result is particularly significant, since amounts of N mineralized during crop growth were estimated from N mineralization potentials, taking into account the effects of temperature on the mineralization rate constant. Thus, the study provides preliminary evidence that the soil N mineralization potential offers a basis for reliably estimating amounts of soil N mineralized during selected periods of time under specified temperature regimes.     

氮沉降对草地土壤及团聚体元素有效性的影响

深入理解土壤及团聚体元素有效性对氮沉降的响应机制,是研究全球变化背景下土壤养分供应及生态系统结构和功能的关键.本研究综合评述了草地生态系统土壤表土及团聚体内元素分布及其对氮沉降的响应机制.总体而言,草地表土内碳、氮、磷、硫有效性研究较多,且研究结果因氮添加形态、添加时间及生态系统类型而异.氮沉降通过改变碳、氮、磷、硫等生源要素的转化过程及其在土壤团聚体内的再分配,而影响这些元素的生物有效性.然而,氮沉降影响草地土壤交换性盐基及有效态微量元素的研究较少.氮沉降促进土壤酸化,导致各团聚体内钙、镁差异性流失,其中大粒径团聚体内盐基元素更易流失;酸化还有助于提高团聚体内铁、锰、铜、锌有效性.土壤小粒径团聚体内的养分对外界环境变化响应不敏感.当前研究的不足之处在于,较少关注氮沉降对土壤团聚作用及团聚体元素有效性的影响.今后应加强团聚体元素有效性与土壤酶活性耦合变化关系的研究,并分析氮沉降背景下土壤物理结构和化学组成的变化对植物群落的反馈作用.     

Deep placement: A method of nitrogen fertilizer application compatible with algal nitrogen fixation in wetland rice soils

Summary The effect of different methods of nitrogen fertilizer application on the algal flora and biological nitrogen fixation (Acetylene-reducing activity) in a wetland rice soil was studied in pot and field experiments. Broadcast application of urea inhibited nitrogen fixation and favored the growth of green algae. In contrast, deep placement of urea supergranules (1–2 g urea granules) did not suppress the growth of N₂-fixing blue-green algae and permitted acetylene-reducing activity on the soil surface to continue virtually uninhibited.     

A laboratory test for screening iron chelates for use in alkaline soils

Summary A soil incubation test is described for screening Fe-chelates for use under alkaline conditions. Ten grams of air-dry soil were mixed with 4 ml of an aqueous solution containing the appropriate amount of iron of the test chelate adding to the soil 24 ppm of iron (soil air-dry weight basis). The moistened soil was incubated in stoppered plastic vials at 24°C. On day 4 from the start of incubation, and at weekly intervals thereafter, the contents of triplicate vials were extracted with 16 ml of DTPA solution prepared according to the method of Lindsay and Norvell. The Fe extracted in this way was considered to be a measure of the stability of the chelates tested in comparison to Sequestrene 138.     

Modification of the acid permanganate method for obtaining an index of soil nitrogen availability

Summary The acid permanganate method proposed as a chemical index of soil nitrogen (N) availability simply measures the mineralizable N. A modification of the method has been proposed that allows the estimation of soil mineral N simultaneously. The N values obtained by the modified method correlated highly significantly with the mineral N of incubation test (r=0.80,P<0.01) and plant uptake (r=0.69,P<0.01).     

Phosphorus availability indices in calcareous lebanese soils

Summary Three indices of available P were evaluated with 20 mainly calcareous Lebanese soils using wheat (Triticum aestivum L.) and tomato (Lycopersicum esculentum L.) in the greenhouse. Both the NaHCO₃-and anion exchange resin-extractable P were significantly correlated with P uptake. The relationship for NaOH–Na₂C₂O₄ was not significant. Langmuir isotherm and actual sorption parameters were also poorly related to uptake. In general, soil properties were not significantly related to extractable P or crop uptake. Sequential inclusion of selected soil properties with test values only slightly improved the multiple correlation coefficients.Contribution from the Department of Soils, Irrigation and Mechanization, American University of Beirut, Beirut, Lebanon. Journal No. 523B. Sci. Paper Series No.297.     

淹水条件下控释氮肥对污染红壤中重金属有效性的影响

采用淹水培养方法研究了不同氮水平(100、200和400 mg/kg,分别记为1、2、3)下普通尿素(PU)、硫包膜尿素(SCU)、树脂包膜尿素(PCU)和硫加树脂双层包膜尿素(SPCU)对污染红壤中Cd、Pb、Cu、Zn有效性的影响.结果表明,不同包膜尿素对土壤pH值和水溶性SO42-含量有较大影响.各施氮处理红壤pH值随着施氮量的增加(除5d时PU和60 d时SCU)而增加,不同包膜尿素对土壤中水溶性SO42-含量有较大影响,在同一施氮水平下不同包膜尿素处理间土壤pH值和土壤中水溶性SO42-含量差异较大.60 d培养期间PU、SCU、PCU和SPCU处理pH值比对照分别升高0.17-0.38、0.08-0.27、0.07-0.36和0.10-0.21;水溶性SO42-含量PU、SCU和PCU处理比对照分别升高39.5％-157.3％、40.9％-94.5％和7.55％-55.8％,而SPCU处理降低5.67％-90.7％.不同尿素类型和氮肥的施用量对红壤Cd、Pb、Cu和Zn有效性的影响均存在显著差异.60 d培养期间红壤有效态Cd含量以树脂包膜尿素100 mg N/kg下最低,其有效态Cd含量比对照显著降低20.7％-69.8％;有效态Pb、Cu和Zn含量以普通尿素400 mg N/kg下最低,其有效态Pb、Cu和Zn含量比对照分别显著降低17.0％-54.2％、18.5％-34.6％和15.6％-59.5％.随施氮量提高,PU处理有效态Cd含量先升高后降低,有效态Pb、Cu和Zn含量逐渐降低;SCU处理有效态Pb含量逐渐降低,有效态Cd、Cu和Zn含量变化规律不一致;PCU处理有效态Cd含量逐渐升高,有效态Pb、Cu和Zn含量变化规律不一致;SPCU处理有效态Cd、Pb、Cu和Zn含量逐渐降低.有效态Pb和Zn含量与pH值和水溶性SO42-含量呈显著负相关,有效态Cd与水溶性SO42-含量呈显著正相关.在多重金属污染红壤中,可考虑不同控释氮肥的配合使用,降低土壤中重金属的有效性.     

添加氮素对沙质草地土壤氮素有效性的影响

通过氮素添加(20g.m-2.a-1)试验,研究了科尔沁沙地东南部沙质草地生态系统土壤氮矿化及有效氮的季节变化。对2006年生长季的观测发现,添加氮素显著提高了沙质草地生长季土壤铵态氮、硝态氮、矿质氮的含量以及9月1日至10月15日的净氮矿化速率与硝化速率;添加氮素导致土壤有效氮的季节变异增大,净氮矿化(1.29～11.60mg.kg-1.30d-1)与硝化(-4.15～11.20mg.kg-1.30d-1)速率随时间呈上升趋势,铵态氮含量逐渐降低,硝态氮与矿质氮(6.49～20.66mg.kg-1)含量的变化呈"V"型,最小值出现在生物量生长高峰期的7月中旬。该沙质草地土壤氮的有效性较低,施氮肥可明显提高土壤供氮能力。     

Root carbon inputs to the rhizosphere stimulate extracellular enzyme activity and increase nitrogen availability in temperate forest soils

The exudation of carbon (C) by tree roots stimulates microbial activity and the production of extracellular enzymes in the rhizosphere. Here, we investigated whether the strength of rhizosphere processes differed between temperate forest trees that vary in soil organic matter (SOM) chemistry and associate with either ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM) fungi. We measured rates of root exudation, microbial and extracellular enzyme activity, and nitrogen (N) availability in samples of rhizosphere and bulk soil influenced by four temperate forest tree species (i.e., to estimate a rhizosphere effect). Although not significantly different between species, root exudation ranged from 0.36 to 1.10 g C m^?2 day^?1, representing a small but important transfer of C to rhizosphere microbes. The magnitude of the rhizosphere effects could not be easily characterized by mycorrhizal associations or SOM chemistry. Ash had the lowest rhizosphere effects and beech had the highest rhizosphere effects, representing one AM and one ECM species, respectively. Hemlock and sugar maple had equivalent rhizosphere effects on enzyme activity. However, the form of N produced in the rhizosphere varied with mycorrhizal association. Enhanced enzyme activity primarily increased amino acid availability in ECM rhizospheres and increased inorganic N availability in AM rhizospheres. These results show that the exudation of C by roots can enhance extracellular enzyme activity and soil-N cycling. This work suggests that global changes that alter belowground C allocation have the potential to impact the form and amount of N to support primary production in ECM and AM stands.     

室内饲养灰飞虱的一种简便方法

介绍一种以海绵为介质培育稻苗,进行灰飞虱Laodelphax striatellus Falln大批量饲养的方法。水稻种子经24h浸种后,平铺在垫有海绵的塑料盒中。在26℃下,10d左右稻苗可长至6～8cm高,此时可以用于饲养灰飞虱。在一般饲养条件下稻苗能保持较好的长势,并且一批稻苗可持续30d左右。灰飞虱的单头饲养可采用单苗单管饲养法,该方法饲养的灰飞虱若虫存活率高、产卵量大。21～27℃、光周期12∶12条件下,灰飞虱世代历期约为20～35d,各龄若虫的存活率均在80%以上。