宁夏东部荒漠草原-灌丛地转变过程土壤氮素响应

选取近30年荒漠草原灌丛引入形成的典型草地-灌丛镶嵌体内部的荒漠草地、草地边缘、灌丛边缘、灌丛地为研究样地,对各样地及其微生境(植丛与空斑)相关土壤指标进行测定,以了解荒漠草地向灌丛地转变过程中土壤氮素的响应特征.结果 表明:随草地灌丛化转变,草本与灌丛生物量均增加,其中一年生草本随灌丛引入增加明显;土壤水分、全碳、全...     

基于参数优化的人工灌丛生态系统碳水通量模拟

荒漠草原是陆地生态系统中最为脆弱且受人类干扰较为严重的生态类型之一,精准模拟其碳水通量及对人为干扰的响应,不仅能够揭示其复杂的生态学过程,而且还可为人为生态修复和保护提供决策依据。生态模型能够有效地模拟陆地生态系统的碳水循环过程,但模型众多的参数及其取值的合理性限制了其普遍应用,故探索参数优化是提升生态模型应用的有效途径。利用PEST参数优化方法和涡度相关观测数据对Biome-BGC模型的生理生态参数进行优化,在评估参数优化效果的基础上模拟了1986-2018年宁夏盐池荒漠草原区人工灌丛生态系统的总初级生产力（Gross primary productivity,GPP）和蒸散（Evapotranspiration,ET）。结果表明：（1）参数优化可以改善Biome-BGC模型对荒漠草原区人工灌丛生态系统GPP和ET的模拟效果,参数优化后模拟的GPP和ET均更接近于观测值,其中月尺度的模拟效果更佳;（2）基于PEST的Biome-BGC模型参数优化方法具有较强的普适性,优化后的参数可推广应用于荒漠草原区人工灌丛生态系统长时间序列的GPP和ET模拟;（3）宁夏盐池荒漠草原区人工灌丛生态系统的GPP在1986-2018年呈缓慢上升趋势,增幅为1.47 g C m^-2 a^-1,但ET的年际变化率较大,且无显著变化趋势。     

宁夏东部荒漠草原灌丛引入对土壤水分动态及亏缺的影响

全球气候变化背景下,荒漠草原人工灌丛引入加速其灌丛化进程,对草原土壤水分产生重要影响。为了解宁夏东部荒漠草原灌丛引入过程中土壤水分动态及亏缺现状,选取了封育草地、放牧草地、不同年限(3a、12a、22a)和间距(40 m、6 m、2 m)灌丛柠条(Caragana korshinskii)地进行土壤水分测定,并利用土壤水分相对亏缺指数(compared soil water deficit index,CSWDI)、样地土壤水分相对亏缺指数(plot compared soil water deficit index,PCSWDI)对土壤水分亏缺进行定量分析。结果表明:灌丛引入过程中不同年限、间距灌丛地0—200 cm土层土壤含水量均显著低于封育草地与放牧地(P<0.05);各样地季节动态均表现为春季返潮、夏季消耗、秋季蓄积的季节规律,但不同年限、间距灌丛地表现为春季返潮微弱,土壤含水量仅为7.80%—10.90%,显著低于封育草地和放牧地(11.90%—16.09%);灌丛引入过程中各灌丛地0—100 cm有效储水量(-16.98—18.69 mm)均低于封育草地(34.67 ...     

宁夏东部荒漠草原灌丛引入下土壤水分空间异质性

结合地理信息系统(GIS),运用经典统计学和地统计学方法对宁夏东部荒漠草原人工灌丛引入下0～200 cm土层土壤水分空间异质性进行研究.结果表明: 0～200 cm土层土壤含水量为0.6%～19.0%,均值为4.4%,土壤水分含量较低,变异系数为49.5%～86.3%,属于中等变异.不同土层半变异函数的最佳理论模型分别为:0～60、80～120 cm各层土壤水分符合球状模型,60～80 cm符合指数模型,120～200 cm符合高斯模型.不同土层土壤含水量均呈不同程度的空间相关,0～40、60～80、120～200 cm各层土壤水分的块金系数C₀/(C₀+C)为26.1%～49.9%,为中等程度空间相关;40～60、80～100、100～120 cm各层土壤含水量的块金系数为15.5%～22.1%,具强烈空间相关.0～200 cm不同土层土壤水分含量变程不同,0～20 和 20～40 cm土层变程较大,为37.10～45.18 km,40～200 cm各层土壤含水量的变程较小,为3.58～8.66 km.荒漠草原人工灌丛引入过程中加速土壤水分利用和深层水分消耗,导致土壤水分的空间异质性和破碎化程度加强,且对深层次土壤水分作用更强.     

放牧强度对内蒙古荒漠草原土壤有机碳及其空间异质性的影响

放牧是内蒙古荒漠草原主要利用方式之一,研究不同放牧强度下土壤有机碳分布规律对退化草原恢复以及推广精准放牧技术具有重要的指导意义。基于不同放牧强度长期放牧样地（0、0.93、1.82、2.71羊单位hm^-2（a/2）^-1）,采用高样本数量的取样设计并结合地统计学分析方法,研究荒漠草原土壤有机碳及其空间异质性。结果表明：中度放牧会显著降低0-30 cm土层全氮含量（P<0.05）,全磷含量随放牧强度增强出现先降低后升高趋势;放牧样地土壤有机碳含量均显著低于对照样地（P<0.05）,不同放牧强度处理土壤有机碳含量没有显著差异;土壤有机碳密度受放牧影响在0-20 cm土层出现显著下降（P<0.05）,变化趋势同有机碳含量相似,碳氮比在重度放牧区0-10 cm土层显著降低（P<0.05）。土壤有机碳空间异质性和异质性斑块的破碎程度随放牧强度增加而增大;土壤有机碳含量与海拔高度在对照、轻度放牧和中度放牧区均呈极显著负相关（P<0.01）,在重度放牧区土壤有机碳含量和海拔无显著相关性;土壤有机碳含量与土壤全氮、全磷含量均呈极显著正相关（P<0.01）。综上所述,放牧降低土壤有机碳含量,提高土壤有机碳空间异质性,土壤有机碳含量的空间变异受海拔和土壤养分含量等因素的共同影响。     

人工灌丛化对荒漠草原生态系统碳储量的影响

宁夏盐池县从20世纪70年代开始在荒漠草原上人工种植柠条灌木用以防风固沙和生态恢复,这一人为措施极大地改变了区域生态系统的植被结构和碳循环,而定量评估人工灌丛化对荒漠草原生态系统碳储量的影响,不仅能够揭示人类活动的碳循环反馈机制,而且可为地方政府生态治理提供理论指导。结合Biome-BGC模型和Logistics生长模型模拟了1958—2017年间荒漠草原人工灌丛化前后的碳储量变化,定量分析了人工灌丛化对生态系统碳储量和组分的影响。结果表明:(1)结合Biome-BGC模型和Logistics生长模型可以较准确地模拟出荒漠草原人工灌丛化过程中生态系统碳储量的变化。(2)人工灌丛化会快速改变荒漠草原的碳储量累积特征,柠条灌木种植后的快速生长阶段极大增强了生态系统的总碳储量,导致生态系统的碳储量特征由草地型向灌木型转变。(3)人工灌丛化改变了生态系统各类型碳储量的组分结构,其对地上植被和枯落物碳储量的影响非常明显,灌丛化后生态系统的植被和枯落物碳分别增加了6倍和1.76倍;因植被碳向土壤碳转化过程较慢,故人工灌丛化对地下土壤碳储量的影响在短期内较为微弱。以上结果显示,荒漠草原人工灌丛化能显...     

荒漠草原典型植物群落土壤活性有机碳组分特征及其与酶活性的关系

以宁夏荒漠草原典型植物柠条(Caragana korshinskii)、沙蒿(Artemisia ordosica)、短花针茅(Stipa breviflora)和蒙古冰草(Agropyron mongolicum)群落为研究对象,分析不同植物群落不同土层深度(0~5、5~10和10~15cm)土壤活性有机碳组分土壤微生物量碳(MBC)、可溶性有机碳(DOC)和易氧化有机碳(EOC)特征及其与土壤酶(蔗糖酶、脲酶、碱性磷酸酶和过氧化氢酶)活性之间的关系。结果表明:(1)4种典型植物群落土壤SOC、MBC、EOC含量均随土层深度的增加而减少,且表层(0~5cm)土壤显著高于亚表层(5~10cm)和深层(10~15cm)土壤(P0.05),而土壤DOC含量随土层深度的增加呈先增加后减少的趋势。在同一土层深度,灌木(柠条和沙蒿)群落土壤活性有机碳组分含量高于禾本科植物(短花针茅和蒙古冰草)。(2)4种典型植物群落土壤酶(蔗糖酶、脲酶、磷酸酶和过氧化氢酶)活性整体上随土层深度的增加而降低,局部土层深度表现出波动性;同一土层不同植被群落土壤酶活性未表现出一定的变化规律。(3)4种典型群落土壤活性有机碳各组分除DOC外,其余均与SOC呈显著正相关关系,与土壤酶活性、微生物量熵以及有机碳活度具有一定的相关关系,表明土壤活性有机碳不仅依赖于总有机碳,也与土壤酶活性密切相关。     

荒漠草原灌丛转变土壤有机碳矿化及其对降水梯度的响应

为探究灌丛引入对荒漠草原土壤有机碳稳定性的影响,选取近40年的典型草原-灌丛镶嵌体(荒漠草地,草地边缘,灌丛边缘、灌丛地)为研究对象,开展荒漠草原向灌丛人为转变过程野外原位观测结合室内培养的土壤有机碳矿化及其对降水变化(+40%、+20%、CK、-20%、-40%)的响应特征研究,主要结果如下:荒漠草原人为灌丛转变的过程中土壤有机碳、全氮和全磷含量显著降低;土壤有机碳累积矿化量、碳矿化速率 、碳矿化及微生物代谢熵均随着转变过程呈上升趋势,且灌丛地较荒漠草地分别增加了26.6%、27%、25.8%和26.8%;土壤潜在可矿化有机碳量随转变波动上升,灌丛地显著高出荒漠草地31.55%;转变样地在不同降水梯度下土壤有机碳矿化速率呈下降趋势,但灌丛地、灌丛边缘土壤有机碳矿化始终高于荒漠草地和草地边缘地,其中灌丛地与灌丛边缘有机碳矿化对降水降低响应更加敏感和积极,结果表明荒漠草原向灌丛转变过程增强了土壤有机碳微生物矿化的同时,还提高了其应对干旱的能力,这种能力或许是以更高的微生物代谢熵和二氧化碳通量为代价。     

平衡与非平衡生态学在锡林河流域典型草原放牧系统中的应用

为认识放牧系统的复杂性和稳定性 ,产生了放牧系统的平衡生态学和非平衡生态学原理。放牧系统的平衡生态学原理假定 :一旦干扰在系统中发生 ,系统将偏离平衡态 ;而当干扰解除后 ,系统将自动返回原来的状态或在新的领域实现平衡。在对内蒙古锡林河流域典型草原放牧系统动态的研究中 ,来自平衡生态学的 Clem ents- Duksterhuis演替理论提供了一个基本的研究框架。尽管已经证实对退化不太严重的典型草原放牧系统 ,平衡生态学原理是适用的 ,但是对于这一地区严重退化的放牧系统的动态 ,它显然并不能给予合理的解释。事实上许多放牧系统动态遵循非平衡生态学原理。在非平衡放牧系统中 ,稳定的状态是不会实现的 ,因为在这样的系统中 ,非生物变量对于植被的动态似乎起着决定性的影响 ,从而也决定着草食动物的种群动态。状态与过渡模型基于非平衡生态学原理 ,它能够解释过度放牧下典型草原生态系统的崩溃或灌丛化 ,因此它适于该地区严重退化的典型草原放牧系统的动态。鉴于内蒙古锡林河流域典型草原放牧系统普遍严重退化的现实 ,未来该地退化放牧系统的研究应更多地应用非平衡生态学原理 ,并且严重退化的草原生态系统的恢复试验 ,特别是灌丛化草原的重建也应置于它的指导之下     

内蒙古半干旱草原灌丛化过程中小叶锦鸡儿引起的土壤碳、氮资源空间异质性分布

过度放牧下内蒙古半干旱草原,由于小叶锦鸡儿多度增加导致植被灌丛化,这已经成为该地草原退化时的普遍现象。草原灌丛化过程中,灌丛内凋落物的累积使得养分循环区域化,草原土壤有机碳(SOC)、土壤全氮(STN)空间异质性增强,导致灌丛沃岛形成。半干旱草原灌丛化过程机制的假说认为:灌丛斑块扩展与其引起的土壤空间异质性的增强之间存在着正反馈作用,正是这种反馈作用促使草地向灌丛地的转变。小叶锦鸡儿通过克隆生长形成不同大小的斑块,它们对应于其发育的不同阶段。因此,将不同大小的灌丛沃岛划分为小灌丛组与大灌丛组,它们代表着灌丛沃岛发育的早期与晚期两个阶段。结果表明:内蒙古半干旱草原灌丛化过程中,小叶锦鸡儿灌丛斑块引起了土壤有机碳(SOC)与土壤全氮(STN)的空间异质性,表现在水平方向上,对于0～5cm,5～10cm,10～20cm3个土壤层次,由灌丛斑块内部向外部SOC与STN均趋于降低;而在垂直方向上,对于灌丛斑块内部、边缘和外部3个位置,由0～10cm到10～20cm再到20～30cm,随着土壤深度增加SOC与STN均趋于降低。随着小叶锦鸡儿灌丛斑块扩展,SOC与STN空间异质性的分布不断增强,表现在灌丛斑块内部相对于外部(或边缘)对SOC与STN富集程度均显著增加。草原灌丛化过程SOC与STN空间异质性变化在0～5cm,5～10cm,10～20cm3个土壤层次中,以表层0～5cm的表现最为强烈。表层0～5cm土壤空间异质性的形成与灌丛自身凋落物的生产以及对灌丛外凋落物的截留有关。由此推断:退化草原灌丛化的过程中,小叶锦鸡儿灌丛斑块引起SOC与STN空间异质性的变化是一个自我增强过程,灌丛斑块扩展与其导致的SOC与STN空间异质性增强之间存在着正反馈作用。因此,以上关于半干旱草原灌丛化机制的假说对我国内蒙古半干旱草原灌丛化也是适用的。     

Prediction of long-term fertilizer nitrogen requirements of maize in the tropics using a nitrogen balance model

A simple N balance model was used to calculate fertilizer requirement for a target N uptake by maize. Nitrogen uptake from soil sources and target uptake of N with fertilizer N additions were obtained from fertilizer trials in Africa and Latin America. Most experiments had data for only one cropping period, although some from Latin America had data for four to six crops. The transfer coefficient of fertilizer N to the crop was adjusted to realize maximum recovery of fertilizer N under best methods of fertilizer application. The time constants of transfer of soil N to the crop were allowed to vary and were affected mainly by soil texture. Where 4 to 6 cropping periods were available good agreement between actual and predicted fertilizer N requirements was obtained. With this approach long-term fertilizer N requirements for 14 sites were predicted using first cropping period N uptake. This study showed that pools of organic N in more coarse-textured soils were usually smaller and declined more rapidly than in fine-textured soils. Labile organic N pools declined with time under all simulations, but approached equilibrium within 10 croppings seasons. Equilibrium N uptake from the soil organic N pool was predicted to be 31 kg ha^–1 for the more coarse-textured soils and 36 kg ha^–1 for the fine-textured soils. Long-term projections of fertilizer requirements using input data of the field experiments were reasonable, and effects of legume green manures and other amendments could be clearly evaluated.     

Identification of anthropogenic parameters for a regional nitrogen balance model via field investigation of six ecosystems in China

To evaluate the impact of human behavior (with regard food consumption, waste disposal and farming method) on nitrogen flow, a field investigation was conducted in six typical ecosystems in China. A number of parameters for regional nitrogen balance models were identified during the investigation. The results show that the average per-capita daily protein intake is 107 g. While there is an insignificant difference in total protein intake among the different ecosystems, protein intake from all food groups, except for eggs, is significantly different (P ≤ 0.05). Differences in diet, along with those in socio-economic conditions, reflect differences in the characteristics of the ecosystems. Regarding per-capita annual potential nitrogen loading from human excrement, a considerable difference exists between the urban rich and the rural poor. In urban areas, approximately 1.02 kg N is returned to farmlands and 5.49 kg N is directly discharged into rivers. In rural regions, on the other hand, approximately 4.33 kg N is returned to farmlands and 1.60 kg N is directly discharged into rivers. Furthermore, urea and mixed fertilizers constitute the most common chemical fertilizers in the study area. Fertilizer diversification is practiced in a range of agricultural lands, paddy-fields and irrigated plains. In the oasis and paddy-field agricultural systems, many of the agricultural by-products (e.g., straw) are burned or mixed with base-fertilizers and plowed into the soil. In irrigated agricultural systems, over 70% of agricultural by-products are recycled as livestock feed. In most instances, livestock excrement is directly reduced in the pasturelands or reused in the fields as manure. Occasionally, as in the case of large-scale breeding, excrements are usually abandoned.     

Yield and crop parameters of wetland rice as influenced by soil and fertilizer nitrogen

Summary The behavior of soil N, fertilizer N and plant N was studied in a greenhouse experiment with 2 plant densities of rice (IR 36) under flooded conditions. Increasing plant density from 25 hills m² to 50 hills m² increased tiller number and panicle number but had no influence on grain yield. The yield of grain was linearly related to N content of the above ground dry matter at harvest (r²=.96) and thus the effect of manipulating the N supply on yield was directly related to N uptake.Mixing of (NH₄)₂SO₄ with the soil volume before transplanting resulted in increases in N in the aboveground dry matter equal to 87% of the applied N. When (NH₄)₂SO₄ was broadcast into the flood water at 4 stages of growth beginning 25 DAT, the corresponding increase was 77% of the applied N. When (NH₄)₂SO₄ was split between shallow mixing before transplanting and a broadcast application of 32 DAT, the corresponding increase was 42%. Thus several applications of fertilizer N increased grain production per unit of applied N.Inorganic N extractable by KCl was a useful but not an infailible guide to the behavior of the soil and fertilizer inorganic N.     

稻鸭共作系统的稻田氮素渗漏和径流特征

通过田间试验,分析了稻鸭共作（MRD）、常规栽培(MR)和淹灌单作（CK）3种稻作方式的稻田氮素渗漏和径流特征.结果表明：与MR处理相比,稻鸭共作系统稻田渗漏液氮素浓度尤其是NO₃^--N浓度显著减少,与CK处理相比,稻鸭共作稻田渗漏液氮素浓度有增加趋势;MRD处理在施肥7～9 d后,田面水中的NH₄⁺-N和NO₃^--N浓度高于MR处理,由于MRD处理不用搁田且田埂较高,田间排水量显著减少,氮素径流损失反而显著少于MR处理;与MR处理相比,MRD处理增加了鸭粪的氮素投入,减少了氮素渗漏和径流损失,减少了化学氮肥用量,增加了水稻地上部吸氮量;MRD系统的氮素输入总量和氮素输出总量均减少,且减少量基本相等.     

Estimation of nitrogen fixation and transfer from alfalfa to associated grasses in mixed swards under field conditions

Fixation and transfer of nitrogen (N) from alfalfa (Medicago sativa L.) to different grass species including timothy (Phleum pratense L.) and bromegrass (Bromus inermis Leyss) were studied under field conditions, using the¹⁵N dilution technique.The percentage of alfalfa N derived from fixation (%NF) increased throughout the growing seasons and ranged from 62 to 83%. Nitrogen transfer (NT) from alfalfa to associated grasses was evident and contributed 26,46 and 38% of the total annual N yield of associated grasses or represented absolute amounts of 5, 20 and 19 kg N ha^–1 during the first, second and third year, respectively. The gradual and consistent percentage of NT that occurred before first harvest indicated that this transfer is a result of a direct excretion of N compounds from alfalfa root systems. Decomposition of root and nodule debris seems to contribute to the NT from alfalfa to associated grasses in the later cuts. All grass species benefitted similarly from alfalfa, although earlier maturing species with greater competitive ability were slightly more responsive.Contribution No. 1159 from the Plant Research Centre     

Effects of cattle slurry and mineral N fertilizer applications on various components of the nitrogen balance of mown grassland

It is essential to establish more accurate N balances for different soil-plant systems in order to improve N use efficiency. In this study the N balance was studied in a poorly drained clayey loam soil under natural grassland supplied with either calcium ammonium nitrate or cattle slurry at two application rates. The aim was to determine the efficiency of the N applied and the factors which affect this efficiency. Mineralization-immobilization of N was calculated by balance between the quantified inputs and outputs of N. As N inputs increased, output via herbage yield was accompanied by an increase in apparent immobilization of N in the soil and by larger losses of N by denitrification. The difference between cattle slurry and N fertilizer was that the slurry behaved as a slow release fertilizer, its supply of mineral N being greater in the periods of time when fertilizer was applied a long time ago. Denitrification losses (up to 17% of the N applied) are suggested to be the main factor to mitigate in order to increase N use efficiency. A decrease in net mineralization (up to 136 kg N ha^-1 year^-1) was observed which was related to the mineral N application rate. There was evidence to suggest that this decrease was due both to the immobilization of the N applied and to a decrease in the rate of gross mineralization when mineral N was applied. Microbial biomass determinations could not explain the changes in the mineralization-immobilization equilibrium of N because of the great coefficients of variation for this determination (mean value of 18%). Nevertheless, it contributed to verify and explain some of the changes observed in this equilibrium.     

Kinetic parameters of nitrogen mineralization rates of leguminous crops incorporated into soil

Summary Fresh leguminous plant residues were incorporated into soil columns and incubated at 23°C for up to 20 weeks. The N released from specific fractions (foliage, stems, and roots) of each residue were monitored at specific time intervals. Relationships between organic carbon, total nitrogen, CN ratio, lipids, and lignin content of the plant materials and the cumulative amount of N mineralized in soil were investigated. Statistical analyses indicated that the rates of N mineralized were not significantly correlated with the organic C nor lipid content of the residues. However, the cumulative amount of N released was significantly correlated with the total N content of the plant material (r=0.93^***). The percentage of organic N of the legumes mineralized in soil ranged from 15.9 to 76.0%. The relationship between the percentage of N released and the CN ratio of the plant material showed an inverse cuvilinear response (r= 0.88^***). It was also evident that the composition of lignin in the residue influenced N mine-ralization rates of the leguminous organs incorporated into soil.There was a curvilinear relationship between the cumulative amount of N released from the residues and time of incubation. Nitrogen mineralization rates were described by first-order kinetics to estimate the N mineralization potential (N₀), mineralization rate constant (k), and the time of incubation required to mineralize one-half of N₀ (t_1/2). The kinetic parameters were calculated by both the linear least squares (LLS) and nonlinear least squares (NLLS) transformations. The N₀ values among the crop residues varied from –35 to 510 g Ng^–1 soil. Statistical analyses revealed that the N₀ values obtained by both LLS and NLLS methods were significantly correlated (r=0.93^***). The mineralization rate constants (k) of the residues ranged from 0.045 to 0.325 week^–1. The time of incubation required to mineralize one-half the nitrogen mineralization potential (t_1/2) of the legumes incorporated into soil ranged from 2.1 to 15.4 weeks.     

减量施氮对冬小麦-夏玉米种植体系中氮利用与平衡的影响

研究了冬小麦-夏玉米种植体系中减量施N对作物N利用与平衡的影响,结果表明,与原有高量施N处理(N240和N360)相比,在冬小麦季减半施N未引起产量和吸N量的变化。但在原有低量施N处理(N120)下减半施N显著降低了小麦产量和吸N量;在夏玉米季,在上季减半施N的基础上停止施N后作物产量和吸N量均比原固定施N处理显著下降,N平衡计算结果表明,减量施N条件下0～1m土壤N残留和表观损失的数量均显著低于原有施N量处理,作物N利用率显著提高,但在1～2m层次中累积的硝态氮却不因减量施N而下降,说明这一土层的硝态氮可能难以被作物吸收利用,由此可见,在前茬高施N量下减少氮肥用量有利于提高作物的氮肥利用率、减少N残留与表观损失。     

Biological N2 Fixation in wetland rice fields: Estimation and contribution to nitrogen balance

This paper 1) reviews improvements and new approaches in methodologies for estimating biological N₂ fixation (BNF) in wetland soils, 2) summarizes earlier quantitative estimates and recent data, and 3) discusses the contribution of BNF to N balance in wetland-rice culture.Measuring acetylene reducing activity (ARA) is still the most popular method for assessing BNF in rice fields. Recent studies confirm that ARA measurements present a number of problems that may render quantitative extrapolations questionable. On the other hand, few comparative measures show ARA's potential as a quantitative estimate. Methods for measuring photodependent and associative ARA in field studies have been standardized, and major progress has been made in sampling procedures. Standardized ARA measurements have shown significant differences in associative N₂ fixation among rice varieties.The ¹⁵N dilution method is suitable for measuring the percentage of N derived from the atmosphere (% Ndfa) in legumes and rice. In particular, the ¹⁵N dilution technique, using available soil N as control, appears to be a promising method for screening rice varieties for ability to utilize biologically fixed N. Attempts to adapt the ¹⁵N dilution method to aquatic N₂ fixers (Azolla and blue-green algae [BGA]) encountered difficulties due to the rapid change in ¹⁵N enrichment of the water.Differences in natural ¹⁵N abundance have been used to show differences among plant organs and species or varieties in rice and Azolla, and to estimate Ndfa by Azolla, but the method appears to be semi-quantitative.Recent pot experiments using stabilized ¹⁵N-labelled soil or balances in pots covered with black cloth indicate a contribution of 10–30 kg N ha^-1 crop^-1 by heterotrophic BNF in flooded planted soil with no or little N fertilizer used.Associative BNF extrapolated from ARA and ¹⁵N incorporation range from 1 to 7 kg N ha^-1 crop^-1. Straw application increases heterotrophic and photodependent BNF. Pot experiments show N gains of 2–4 mg N g^-1 straw added at 10 tons ha^-1.N₂ fixation by BGA has been almost exclusively estimated by ARA and biomass measurements. Estimates by ARA range from a few to 80 kg N ha^-1 crop^-1 (average 27 kg). Recent extensive measurements show extrapolated values of about 20 kg N ha^-1 crop^-1 in no-N plots, 8 kg in plots with broadcast urea, and 12 kg in plots with deep-placed urea.Most information on N₂ fixed by Azolla and legume green manure comes from N accumulation measurements and determination of % Ndfa. Recent trials in an international network show standing crops of Azolla averaging 30–40 kg N ha^-1 and the accumulation of 50–90 kg N ha^-1 for two crops of Azolla grown before and after transplanting rice. Estimates of % Ndfa in Azolla by ¹⁵N dilution and delta ¹⁵N methods range from 51 to 99%. Assuming 50–80% Ndfa in legume green manures, one crop can provide 50–100 kg N ha^-1 in 50 days. Few balance studies in microplots or pots report extrapolated N gains of 150–250 kg N ha^-1 crop^-1.N balances in long-term fertility experiments range from 19 to 98 kg N ha^-1 crop^-1 (average 50 kg N) in fields with no N fertilizer applied. The problems encountered with ARA and ¹⁵N methods have revived interest in N balance studies in pots. Balances are usually highest in flooded planted pots exposed to light and receiving no N fertilizer; extrapolated values range from 16 to 70 kg N ha^-1 crop^-1 (average 38 kg N). A compilation of balance experiments with rice soil shows an average balance of about 30 kg N ha^-1 crop^-1 in soils where no inorganic fertilizer N was applied.Biological N₂ fixation by individual systems can be estimated more or less accurately, but total BNF in a rice field has not yet been estimated by measuring simultaneously the activities of the various components in situ. As a result, it is not clear if the activities of the different N₂-fixing systems are independent or related. A method to estimate in situ the contribution of N₂ fixed to rice nutrition is still not available. Dynamics of BNF during the crop cycle is known for indigenous agents but the pattern of fixed N availability to rice is known only for a few green manure crops.     

解磷菌株B25的筛选、鉴定及其解磷能力

从安徽省铜陵市铜官山尾矿库木贼根际分离筛选出多株解磷细菌,经过多次筛选纯化获得一株解磷能力较好的菌株B25.采用透射电镜观察和DNA分子技术,确定此菌株属于芽孢杆菌属.研究了解磷菌株B25在培养168 h内的解磷能力、溶液pH值以及菌株生长量的变化情况,并比较了B25在不同条件下的解磷能力.结果表明：解磷菌株B25的解磷能力与溶液pH值之间存在微弱的相关性,在碳源为葡萄糖、初始pH值为7.0、培养温度为30 ℃时解磷效果较好.