土壤水分和氮添加对3种质地紫色土氮矿化及土壤pH的影响

为正确认识土壤水分、质地和外源氮添加对紫色土氮矿化作用和土壤pH的影响,以西南地区典型的紫色土为研究对象,通过90d的室内恒温(25℃)好气培养,研究了3种质地(粘土、粉粘壤土和砂土)紫色土在不同含水量(55%,65%和75%田间持水量)和尿素氮添加水平(0mg/kg土和250mg/kg土.)条件下,土壤氮矿化作用和pH的变化。结果表明:前30d的累积矿化氮量可占培养期间(90d)的78.48%~91.55%,且各处理的土壤累积矿化氮量和净矿化速率均随着培养时间的延长而快速增加;第30~90d,土壤累积矿化氮量增长缓慢,净矿化速率迅速下降并趋于稳定。土壤累积矿化氮量和净矿化速率在各培养阶段均随土壤水分含量的增加而逐渐增大,其中75%WHC(75%田间持水量)和75%WHC+U(75%田间持水量+尿素)处理的矿化作用最强。土壤质地从一定程度上对土壤的矿化产生影响,但其影响并不显著。外源氮添加能促进土壤氮矿化,其净氮矿化量和净矿化速率在各培养阶段均极显著(p0.01)高于未加氮处理,分别为未施氮处理的1.68~4.56倍,0.60~6.47倍。外源氮添加使土壤pH显著下降,55%WHC+U、65%WHC+U和75%WHC+U处理分别下降了0.57,0.66,0.72个pH单位,土壤有酸化趋势。土壤pH值与土壤氮素净矿化速率呈极显著线性相关,净矿化速率对pH变化贡献巨大。总之,土壤含水量增加和外源氮添加均促进了土壤氮矿化,增加了土壤矿质氮含量,同时外源氮添加也加速了土壤pH下降,土壤有酸化趋势。     

长期不同化肥氮用量对设施菜地土壤氮素矿化和硝化作用的影响

为揭示长期施用不同化肥氮对设施菜地土壤供氮能力的影响，选取连续种植15年不同化肥氮用量下的设施菜地土壤，采用好气密闭培养法研究长期不同化肥氮用量对设施菜地土壤氮素矿化和硝化作用的影响。供试土壤为5个氮施用水平，分别为：不施化肥氮（CK），常规化肥氮（100% N），常规化肥氮上减氮20%（80% N）、40%（60% N）、60%（40% N）。结果表明：与初始矿质氮量相比，培养结束后CK、40% N、60% N、80% N和100% N处理土壤矿质氮变化量分别为38.9、44.7、20.6、-32.7、-87.6 mg/kg；CK、40% N和60% N处理土壤矿质氮变化量分别占各自土壤全氮量的2.7%、2.5%和1.0%，80% N和100% N处理土壤矿质氮量与初始矿质氮量相比下降1.3%和3.1%；CK、40%、60% N、80% N和100% N处理土壤硝化速率分别为19.3、11.2、4.9、5.2、1.2 mg/（kg·d）。长期高量化肥氮（80% N和100% N）投入下，设施菜地土壤氮素矿化和硝化速率显著降低，土壤供氮能力下降，土壤pH降低可能是导致土壤矿化和硝化作用受到抑制的原因之一。鉴于此，设施蔬菜种植体系在现有施氮水平上应减少化肥氮投入，科学优化施肥，维持土壤的供氮能力，确保设施土壤的可持续利用。     

植烟黄壤氮素矿化动态模拟研究

研究不同温度和水分条件下植烟土壤有机氮的矿化动态,为田间土壤氮素矿化的预测提供依据。采用好气间歇淋洗方法,探求不同温度培养模式[恒温20℃,恒温35℃,变温(5、10、15、20、25、30、35、30、25、20℃)],不同温度(5~40℃)与不同土壤含水量(风干土~53%)交互作用下的土壤有机氮矿化动态,并建立回归方程。运用田间土壤氮素矿化数据,进行模型验证。结果显示,变温培养下土壤氮素矿化动态与恒温培养显著不同,变温下土壤矿化氮的累积动态以积温模型的拟合效果最好;指数模型能够较好描述土壤有机氮矿化对土壤水分含量的反应。在土壤氮素矿化积温模型和水分函数的基础上,建立了变化温度与水分条件下的土壤氮素矿化模型。田间实测矿化数据验证了此模型的可行性。因此,可以利用有效积温和土壤含水量来估测田间土壤氮素矿化量。     

半干旱区全膜覆土穴播对小麦土壤氮素矿化、 无机氮及产量的影响

研究全膜覆土穴播对半干旱区小麦土壤氮素矿化及无机氮的影响,为优化田间氮肥管理措施提供理论依据。于2014 ～ 2017 年在甘肃省中部半干旱区设置定位试验,设全膜覆土穴播(MS)、传统地膜覆盖穴播(M)、露地穴播(CK)3 个处理,研究各处理对春小麦生育期土壤氮素矿化、无机氮及产量的影响。结果表明,MS 较M 处理增加了耕层0 ～ 20 cm 土壤水分含量,提高了0 ～ 15 cm 土壤氮素矿化速率,显著增加了土壤和矿质氮净矿化量,较传统地膜覆盖(M)分别显著提高29.02% 和27.81%。MS 处理增加了0 ～ 20 cm 土层土壤无机氮含量,且以为主,降低了40 ～ 200 cm 深层土壤和无机氮含量, 含量变化不明显。从小麦生长阶段看,小麦生长早期T2 ～ T4(14 ～ 62 d)阶段矿质氮表现为净固持,小麦生长中后期T5 ～ T7(63 ～ 116 d)阶段表现为净矿化,小麦全生育期表现为净矿化,MS、M、CK 处理矿质氮净矿化量分别为23.9、18.7、8.5 mg/kg;在小麦生长中期T5(63 ～ 75 d)阶段MS 处理土壤氮素净矿化速率[1.61 mg/(kg·d)]达到最大,较M[1.39 mg/(kg·d)]和CK[0.27 mg/(kg·d)]分别显著提高15.83% 和496.30%。从小麦产量及水分利用效率看,MS 较M 分别显著增加8.81% ～ 59.45% 和5.91% ～ 22.15%。因此,在本试验条件下,全膜覆土穴播改善了土壤的水热条件,提高了土壤氮素的矿化速率,增加了土壤氮素的有效性,提高了土壤氮素供应能力,显著提高了产量和水分利用效率。     

温度和水分对华中地区菜地土壤氮素矿化的影响

为研究华中地区菜地土壤的矿化特征和矿化规律,拟定菜地土壤合理的氮肥施用量,本文以华中地区两种典型菜地土壤——黄棕壤和潮土为研究对象,利用室内连续培养试验研究了温度、水分对菜地土壤矿化的影响。结果表明,黄棕壤的矿化速率和氨化速率均随着温度的升高而升高;硝化速率在15%和25%含水量下随温度的升高而升高,而在35%含水量下随温度的升高而降低。潮土矿化速率在15%含水量时随温度的升高而升高,而在25%和35%含水量下随温度的升高先增加后减小;硝化速率在15%和35%含水量时随温度的升高而增加,25%含水量时随温度的升高先增加后降低;氨化作用随温度的升高而降低。黄棕壤的矿化量在含水量为25%、温度35℃时高达34.9 mg.kg 1;潮土的矿化量在含水量为25%、温度为25℃时最高,为63.9mg.kg 1。不同温度下潮土矿化量均大于黄棕壤。黄棕壤的氨化速率随含水量的增加而增加,硝化速率随含水量的增加而降低,矿化速率则在含水量25%时最大。潮土的氨化、硝化和矿化作用随水分变化不明显。本研究还发现,25%的含水量是黄棕壤微生物活性的水分临界点,潮土的水分临界点不明显。通过对土壤氮素的矿化速率与水分含量和温度之间的函数关系模拟发现,黄棕壤模拟效果好于潮土。     

黄土高原北部生长季土壤氮素矿化对植被和地形的响应

氮素矿化是陆地生态系统氮循环的重要过程,对氮素有效性有着重要影响。本文在黄土高原北部六道沟小流域选取退耕年限相近的油松和柠条坡地,用原位培养法测定生长季节(4—10月)不同坡位冠层下和冠层外0~10 cm和10~20 cm土层土壤氮素矿化速率,以确定该区氮素矿化的季节动态特征和主要影响因素。结果表明,研究区生长季土壤矿质氮以铵态氮为主,其含量在0~10 cm和10~20 cm土层分别占矿质氮总量的61%和70%,并随生长季的推移而升高。油松林上坡位和中坡位土壤铵态氮显著高于下坡位土壤,柠条林不同坡位铵态氮差异不显著。土壤硝态氮和矿质氮不受坡位的影响,但与林型和采样位置有关,冠层下硝态氮在油松林与冠层外相近,在柠条林则高于冠层外。生长季土壤氮素矿化在0~10 cm土层由硝化作用引起,在10~20 cm土层则由硝化和铵化作用共同引起。铵化速率在生长季初期较高,中期较低,并受坡位、林型和采样位置的影响。土壤硝化和矿化速率在油松林不受采样位置影响,但是在柠条林则以冠层下较高。硝化和矿化速率在冠层下以下坡位土壤最高,在冠层外则以下坡位土壤最低。柠条林促进了冠层下土壤氮素的硝化和矿化过程,有利于矿质氮的积累;油松林对矿质氮和氮素矿化的影响不受采样位置影响。     

秸秆和生物质炭添加对陇中黄土高原旱作农田土壤氮素矿化的影响

通过采集2014年设置于甘肃省定西市李家堡镇的不同碳源配施氮素田间定位试验土壤进行120天的室内培养试验,利用Stanford间歇淋洗培养法研究了无碳素和氮素添加(N0)、只施氮素(N100)、秸秆配施氮素(SN100)和生物质炭配施氮素(BN100) 4种施肥方式对陇中黄土高原旱作农田土壤氮素矿化的影响。结果表明:秸秆和生物质炭配施氮素提升了表层土壤氮素矿化量,分别比只施氮素显著提升16.5%和15.4%;土壤氮素矿化呈现先快速增加而后迅速下降,降速逐渐转为慢速直至稳定的趋势,硝化速率,氨化速率分别在7,15天时达到最大,45,30天时趋于稳定。秸秆和生物质炭配施氮素均可显著提升上层土壤氮素矿化速率,硝化速率和氨化速率则处于一种相对平衡的状态。土壤各有机氮组分在培养前后的变化量与土壤氮素矿化量之间的冗余分析结果表明,对土壤可矿化氮贡献最大的是氨基酸态氮,酸解未知氮次之;矿质态氮与未酸解氮呈负相关,与各酸解有机氮组分均呈正相关。此外,相比于不施氮,在施氮条件下添加秸秆和生物质炭增加了表层土壤各有机氮组分的变化量,说明添加秸秆和生物质炭可通过改变各有机氮组分占全氮的比例来增加易矿化氮的含量,从而促进有机氮组分的矿化,以提升土壤供氮潜力。     

基于SUGIHARA模型的树脂包衣控释肥在土壤中释放规律的研究

包衣控释肥在土壤中释放规律的评价及预测对其推荐施肥有重要作用。通过室内恒温、不同含水量的土培和田间裸地试验,研究了土壤温度和含水量对控释肥(树脂包衣尿素,简称L1)氮素释放的影响,并用SUGIHARA模型预测L1在不同施肥时期的氮素释放规律。结果表明:温度是影响L1释放的主要因素;土壤含水量高于田间持水量的35%后,L1的释放不受含水量变化的影响;SUGIHARA模型适合对L1的氮素释放进行拟合,其中溶出速率常数K由温度决定,不同施肥期对应的K值不同,二者有很好的相关性。经检验,SUGIHARA模型可用于预测不同施肥时期的树脂包衣控释肥在土壤中氮素释放。     

华中地区两种典型菜地土壤中氮素的矿化特征研究

以华中地区两种典型的菜地土壤——黄棕壤和潮土为研究对象,利用室内连续培养试验研究了菜地土壤氮素的矿化规律和矿化特征.结果表明,黄棕壤和潮土氮素的矿化以硝态氮量较多,铵态氮较少.两种土壤的矿化速率随培养时间的延长而降低,培养13周以后黄棕壤的矿化速率为N 0.13 mg/(kg.d),潮土为N 0.32 mg/(kg.d).随着培养时间的延长,黄棕壤和潮土的累计氮矿化量缓慢增长.培养结束时黄棕壤矿化量为N 68.65 mg/kg,潮土矿化量为N109.37 mg/kg,分别占土壤全氮量的24.52％和21.45％.黄棕壤和潮土氮素的矿化势分别为N 74.63 mg/kg和123.45mg,/kg,分别占土壤全氮量的26.65％和24.21％.     

贵州省主要植烟黄壤氮素矿化潜力研究

采用Stanford间歇淋洗好气培养法研究了贵州省9个主要烤烟产区的943份植烟黄壤样品的氮素矿化势,并抽取典型代表样品93份测定了矿化率常数.结果表明,贵州省主要植烟黄壤的氮矿化速率常数平均为0.031 mg kg-1d-1,氮素矿化势平均为92.8 mg kg-1,变幅在10.2 ～280.5 mg kg-1.不同植烟区域氮素矿化势差异较大,安顺、毕节、贵阳、六盘水、黔东南、黔南、黔西南、铜仁和遵义的土壤矿化势分别为103.5、90.6、71.0、116.2、91.1、89.0、79.6、84.6和99.6 mg kg-1.贵州省主要植烟黄壤的潜在供氮能力以中部地区较低,向四周辐射潜在供氮能力增强,六盘水和黔东南部分烟区潜在供氮能力过高.土壤有机质含量与植烟黄壤潜在供氮能力呈显著正相关,可采用指数函数模型对植烟黄壤氮素矿化势进行初步估测.在宏观上把握贵州省主要植烟黄壤的潜在供氮能力,可以为贵州省烤烟种植的合理布局提供依据.     

低氮和干旱胁迫对富士和秦冠生长及氮素利用的影响

【目的】以富士（Fuji）、 秦冠（Qinguan）嫁接在平邑甜茶（Malus hupehensis Rehd.）上的当年生盆栽苗为试验材料,采用砂培方法,研究了缺氮胁迫和干旱对富士和秦冠生长情况、 光合参数、 植株各部位氮磷钾含量及氮素利用效率的影响,分析比较了低氮干旱条件下富士和秦冠生长及氮素利用的差异,以期为果树生产高效肥水利用提供理论指导。【方法】试验共设四个处理: 正常氮正常水（ZZ）、 低氮正常水（DZ）、 正常氮干旱（ZG）、 低氮干旱（DG）。氮素和水分均设置两个水平,分别为正常氮（6 mmol/L NO-3-N）、 低氮（0.3 mmol/LNO-3-N）、 正常供水（保持盆中砂子相对含水量为饱和含水量的80%~85%）、 干旱处理（保持盆中砂子相对含水量为饱和含水量的60%~65%）。【结果】富士和秦冠的生物量（茎和叶）、 株高茎粗等生长指标以及光合速率、 气孔导度、 蒸腾速率均为正常氮正常水（ZZ）＞低氮正常水（DZ）＞正常氮干旱（ZG）＞低氮干旱（DG）,并且相对应处理下秦冠的以上指标均高于富士;正常供水下,缺氮处理使富士、 秦冠的根冠比比正常氮处理均有所增加,富士提高了2.05%,秦冠提高了22.40%。富士和秦冠的氮、 磷、 钾含量均表现出正常氮正常水（ZZ）＞低氮正常水（DZ）＞正常氮干旱（ZG）＞低氮干旱（DG）; 氮、 钾元素含量在植株各部位的分布顺序依次是叶＞根＞茎,磷元素则是根＞叶＞茎;光合氮素利用效率（PNUE）和氮素利用效率表现为秦冠处理之间差异极显著,富士处理之间差异不显著;秦冠的PNUE和NUE明显高于富士,在低氮正常水（DZ）处理下,秦冠氮肥利用率比富士高42.07%,在低氮干旱（DG）处理下高64.14%;低氮胁迫下富士和秦冠的NUE显著提高,并且秦冠提高的幅度高于富士。【结论】施用氮肥能够显著提高富士与秦冠的干物质量,同等水肥条件下,秦冠生长优于富士;水分亏缺会减少叶片对氮的吸收,干旱条件下适度增施氮肥,可提高果树的抗旱能力;低氮干旱胁迫下秦冠的生长指标、 光合指标及氮素利用效率指标均优于富士,表现出较强的抗低氮干旱胁迫的能力。     

Pasture Vegetation Elemental Analysis by Laser-Induced Breakdown Spectroscopy

Laser-induced breakdown spectroscopy (LIBS) is a new technique for the analysis of plant material. This study investigates the application of LIBS to pasture-based plant samples. The LIBS measurements were obtained from pelletized pasture samples (100 samples) that had been also analyzed by inductively coupled plasma–optical emission spectroscopy (ICP-OES) following microwave digestion for calibration and comparison purposes. Comparisons for elements sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), boron (B), phosphorus (P), and sulfur (S) showed that LIBS could be used for almost all the standard profile total elements with concentrations down to low mg/kg levels (observed error of Na: 0.024 percent, K: 0.18 percent, Mg: 0.016 percent, Ca: 0.073 percent, P: 0.017 percent, Mn: 31 mg/kg, Fe: 150 mg/kg, Zn: 6.6 mg/kg, and B: 1.1 mg/kg). Elemental analysis at less than mg/kg levels was not possible using LIBS. The elements S and Cu were particularly difficult to analyze with reliability using LIBS at the concentration levels found in the plant samples. Replacing microwave digestion and subsequent ICP analysis with a direct analysis of dried plant samples using LIBS has the potential to improve the productivity and reduce the cost of testing.     

Faculty positions:Center for Agricultural Resources Research,Chinese Academy of Sciences

正The Center for Agricultural Resources Research(CARR),the Institute of Genetics and Developmental Biology(IGDB),Chinese Academy of Sciences,invites applicants for several research group leader positions.CARR is one of the research organizations in Chinese Academy of Sciences(CAS).We seek nominations and applications from individuals who have expertise and a record of accomplishment in research areas related to ecology,agro-hydrology,     

Biodiversity and ecogeography of wild Lactuca spp. in some European countries

Staff members of the Department of Botany of Palacký University in Olomouc and Gene Bank Department – Workplace Olomouc, Research Institute of Crop Production in Prague, Czech Republic, conducted an expedition in seven European countries (Austria, Czech Republic, France, Germany, Italy, the Netherlands, Switzerland) in August/September 1999 to collect wild Lactuca spp. germplasm and study its geographic distribution, ecology and biodiversity. During the mission, more than 600 locations were visited resulting in the collection of 602 seed samples (accessions) of wild Lactuca species and 13 seed samples of related genera (Chondrilla and Mycelis). Lactuca serriola f. serriola, L. serriola f. integrifolia, L. saligna and L. viminea subsp. chondrilliflora were prevalent in southern Europe (Italy, France), however, only L. serriola was common in central and western Europe (Austria, Czech Republic, Germany, Netherlands, Switzerland). The greatest diversity of Lactuca species was found in France, where also the most seed samples (165) were collected. The most characteristic habitats with a high density of Lactuca spp. populations were observed along roads and highways, grassy ditches, ruderal communities, and dust-heaps. Natural infections by powdery mildew (Erysiphe cichoracearum) and downy mildew (Bremia lactucae) on some wild Lactuca spp. were observed. Recent observations concerning the geographic distribution, population structure, habitats, and natural occurrence of diseases of Lactuca spp. are discussed. This assemblage of genetic resources of Lactuca spp. can serve as the basis of future studies of species diversification, spatial population structure, plant microevolution, domestication processes, and genetic variability of host-parasite interactions.     

Macroinvertebrates in North American tallgrass prairie soils: effects of fire, mowing, and fertilization on density and biomass

The responses of tallgrass prairie plant communities and ecosystem processes to fire and grazing are well characterized. However, responses of invertebrate consumer groups, and particularly soil-dwelling organisms, to these disturbances are not well known. At Konza Prairie Biological Station, we sampled soil macroinvertebrates in 1994 and 1999 as part of a long-term experiment designed to examine the effects and interactions of annual fire, mowing, and fertilization (N and P) on prairie soil communities and processes. For nearly all taxa, in both years, responses were characterized by significant treatment interactions, but some general patterns were evident. Introduced European earthworms (Aporrectodea spp. and Octolasion spp.) were most abundant in plots where fire was excluded, and the proportion of the total earthworm community consisting of introduced earthworms was greater in unburned, unmowed, and fertilized plots. Nymphs of two Cicada genera were collected (Cicadetta spp. and Tibicen spp.). Cicadetta nymphs were more abundant in burned plots, but mowing reduced their abundance. Tibicen nymphs were collected almost exclusively from unburned plots. Treatment effects on herbivorous beetle larvae (Scarabaeidae, Elateridae, and Curculionidae) were variable, but nutrient additions (N or P) usually resulted in greater densities, whereas mowing usually resulted in lower densities. Our results suggest that departures from historical disturbance regimes (i.e. frequent fire and grazing) may render soils more susceptible to increased numbers of European earthworms, and that interactions between fire, aboveground biomass removal, and vegetation responses affect the structure and composition of invertebrate communities in tallgrass prairie soils.     

Properties of anaerobically digested and composted municipal solid waste assessed by linking soil mesofauna dynamics and nitrogen modelling

We have studied the effect of anaerobically digested (ADMSW) and composted municipal solid waste (CMSW) on mineralization and foodweb dynamics to verify the hypothesis that ADMSW would immobilize N right after addition to soil in contrast to addition with CMSW. Another hypothesis was that the mesofauna (enchytraeids and microarthropods) would stimulate N release from the decomposer community. We measured excretion of -N and urea-N from the mesofauna and hypothesized that enchytraeids would release urea. ADMSW and CMSW were amended to pots with sandy loam and barley. The pots were divided into treatments with or without mesofauna. Mesofauna, plant N and biomass, soil N and ergosterol (fungal biomass) were measured over a 113-day period of four equidistant samplings. Soil respiration, N mineralization and N release by the mesofauna were modelled from concurrent studies. ADMSW- and CMSW-treated soils initially (<20 days) immobilized N. The amendments did not increase plant growth substantially, and this was probably due to N-limitation in the early stages of plant growth. Enchytraeid abundance was about three times higher in ADMSW- than CMSW-treated soils, indicating that ADMSW contained more labile compounds, bacteria, and microfauna. The mesofauna did not affect N-content, but the cumulated -N excreted by the mesofauna was estimated to be substantial and about one-fifth of total plant N in ADMSW. An explanation for this discrepancy might be that in the absence of the mesofauna, other members of the detrivore and microbivore community performed the mesofauna’s function. Neither enchytraeids nor microarthropods excreted urea.     

Faculty positions: Center for Agricultural Resources Research,Chinese Academy of Sciences

正The Center for Agricultural Resources Research(CARR),the Institute of Genetics and Developmental Biology(IGDB),Chinese Academy of Sciences,invites applicants for several research group leader positions.CARR is one of the research organizations in Chinese Academy of Sciences(CAS).We seek nominations and applications from individuals who have expertise and a record of accomplishment in research areas related to ecology,agro-hydrology,     

Faculty positions: Center for Agricultural Resources Research,Chinese Academy of Sciences

正The Center for Agricultural Resources Research(CARR),the Institute of Genetics and Developmental Biology(IGDB),Chinese Academy of Sciences,invites applicants for several research group leader positions.CARR is one of the research organizations in Chinese Academy of Sciences(CAS).We seek nominations and applications from individuals who have expertise and a record of accomplishment in research areas related to ecology,agro-hydrology,     

Variation in quality parameters between and within 14 Nordic tree fruit and berry species

Abstract

A 3-year study was carried out to investigate quality parameters in 14 tree fruit and berry species grown in southern Norway. The species were blueberry, apple, aronia, sour cherry, sweet cherry, red raspberry, strawberry, blackcurrant, gooseberry, red currant and elderberry, harvested along with wild bilberry, cloudberry and lingonberry. Significant differences between species were identified for all quality parameters. The coefficient of variation between species was lowest for pH (12.5%), dry matter (18.9%) and soluble solids (25.3%), followed by titratable acids (59.3%), total phenolics (83.8%), antioxidant capacity FRAP (85.7%) and antiradical power by the DPPH-assay (97.8%), total monomeric anthocyanins (132%) and ascorbic acid (137%). Average coefficient of variation within species were lower and ranged from 4 (pH) to 62% (ascorbic acid). Only the FRAP values were significantly affected by harvesting year with lower levels in 2004 than in 2005 and 2006. There were significant interactions between species and harvesting year for dry matter, soluble solids, pH, ascorbic acid and FRAP. The results indicate generic ranges in composition within species independent upon growing location and climate, and the composition of the tree fruits and berries is not likely to deviate from these ranges. It is concluded that desirable composition of tree fruits and berries and their products should primarily be achieved by selection among species rather than searching fors broadened variation within individual species.     

Potassium Fixation and Release Characteristics in Rhizosphere and Nonrhizosphere Soils for a Rapeseed–Rice Cropping Sequence

Potassium (K) fixation and release in soil are important factors in the long-term sustainability of a cropping system. Changes in K concentration and characteristics of K fixation and release in rhizosphere and nonrhizosphere soils in the rapeseed (Brassica napus L.)–rice (Oryza sativa L.) rotation were investigated using a rhizobox system. The concentrations of different forms of K in both rhizosphere and nonrhizosphere soils decreased with plants compared to without plants, regardless of K fertilizer application. Potassium uptake by crops mainly came from the rhizosphere soil. In the treatment without K fertilizer (–K), the main form of K supplied by the soil to the crops was 1.0 mol L^?1 nitric acid (HNO₃) nonextractable K, followed by nonexchangeable K, and then exchangeable K. In the treatment with K fertilizer (+K), the main K forms supplied by the soil to the crops were exchangeable K and nonexchangeable K. The amount and rate of K fixation after one cycle of the rapeseed–rice rotation was greater in rhizosphere soil than in nonrhizosphere soil. The amount and rate of K fixation of soil in the +K treatment were significantly less than in the –K treatment. The cumulative amounts of K released with 1.0 mol L^?1 ammonium acetate (NH₄OAc) and 1.0 mol L^?1 HNO₃ extraction increased with the increasing numbers of extractions, but the K-releasing power of soil by successive extraction decreased gradually and finally became almost constant. The release of K was less in rhizosphere soil than in nonrhizosphere soil. The release of K in the +K treatment was similar to that in the –K treatment in rhizosphere soil, but the K release in nonrhizosphere soil was greater with the +K than the –K treatment. Overall, the information obtained in this study will be helpful in formulating more precise K fertilizer recommendations for certain soils.