川西亚高山针叶林不同恢复阶段土壤的硝化和反硝化作用

 采用气压过程分离(Barometric process separation, BaPS)技术对川西亚高山针叶林不同恢复 阶段土壤的总硝化和反硝化作用速率进行了测定,结果表明：川西亚高山针叶林不同恢复阶段土壤的总硝化和反硝化速率差异不显著(p<0.05),不同恢复阶段土壤总硝化作用的 Q10值 差异不显著(p<0.05);总硝化作用速率与土壤含水量呈显著正相关(p<0.05),与土 壤pH值、 土壤有机质、全氮及C/N相关不显著;不同恢复阶段土壤反硝化速率均维持在一个较低的水 平,反硝化速率与土壤中的C/N显著正相关(p<0.05),与土壤含水量、pH值、有机质及全氮相关不显著。与反硝化作用相比,硝化作用对亚高山针叶林土壤氮损失的影响可能更大     

覆盖措施对雷竹林地土壤硝化和反硝化作用的影响

为探讨林地覆盖对雷竹林土壤硝化和反硝化作用的影响,以不覆盖雷竹林为对照,测定了林地覆盖期间(覆盖后30、60、90 d)雷竹林土壤基本理化性质,并用气压分离过程技术(Ba PS)测定了土壤硝化速率和反硝化速率。结果表明:覆盖措施和覆盖时间对雷竹林土壤硝化和反硝化作用均有显著影响,而且两者存在明显的交互作用;覆盖能促进雷竹林土壤反硝化作用,但长时间覆盖会抑制雷竹林土壤硝化作用;覆盖总体上会降低雷竹林土壤硝化速率、反硝化速率与土壤理化性质的相关性程度,并使土壤硝化和反硝化作用的主要环境影响因子趋于多样化和复杂化;覆盖雷竹林土壤硝化速率的主要环境影响因子是土壤含水量、p H值、铵态氮含量和总孔隙度,反硝化速率的主要环境影响因子是土壤p H值、含水量和总孔隙度。林地覆盖会显著影响雷竹林土壤的氮循环过程,可能会增加土壤氮素损失。     

氮硅添加对青藏高原高寒草甸土壤氮矿化的影响

为了解全球气候变化背景下氮沉降对土壤氮矿化的影响及硅添加对土壤氮矿化的促进作用,该试验设置不同浓度的氮肥单独添加(0、20、40、60 g·m~(–2),分别为对照CK、N20、N40、N60)以及与硅肥配施(硅酸4 g·m~(–2), Si4),测定不同处理下0–20、20–40、40–60cm土层土壤硝态氮含量、铵态氮含量、净硝化速率、净氨化速率以及净矿化速率。结果显示:(1)单独添加氮肥,各土层土壤硝态氮和铵态氮含量均随处理浓度的增加而增加, 0–20 cm土层N20、N40、N60处理下土壤硝态氮和铵态氮分别较CK增加63.48%、126.04%、247.03%和80.66%、152.52%、244.56%;随着土层深度增加,土壤硝态氮、铵态氮含量均有下降,20–40、40–60cm土层较0–20cm土层硝态氮含量分别平均减少53.90%、76.05%,铵态氮含量分别平均减少48.62%、68.23%。(2)土壤净硝化速率、净氨化速率及净矿化速率随着氮肥浓度增加均呈上升趋势。相同氮肥添加浓度下,土壤净硝化速率、净氨化速率和净矿化速率随着土层深度增加逐渐下降(除CK外)。(3)与单独添加氮肥比较,氮硅肥配施,土壤氮含量有显著提高,在0–20 cm土层硝态氮和铵态氮较CK分别增加98.78%、192.62%、330.16%和99.96%、195.82%、306.32%,20–40、40–60 cm土层也有类似趋势。同时,氮硅配施促进了土壤氮矿化行为,在0–20 cm土层, N60Si4处理下的土壤净硝化速率、净氨化速率较单独施氮时分别增加35.88%、27.41%。以上结果表明,与单独氮肥添加相比,氮硅配施不但能提高土壤氮含量,而且能促进土壤氮的矿化作用,对大气氮沉降有一定的缓解作用。     

氮硅添加对青藏高原高寒草甸土壤氮矿化的影响

为了解全球气候变化背景下氮沉降对土壤氮矿化的影响及硅添加对土壤氮矿化的促进作用, 该试验设置不同浓度的氮肥单独添加(0、20、40、60 g·m ^-2, 分别为对照CK、N20、N40、N60)以及与硅肥配施(硅酸4 g·m ^-2, Si4), 测定不同处理下0-20、20-40、40-60 cm土层土壤硝态氮含量、铵态氮含量、净硝化速率、净氨化速率以及净矿化速率。结果显示: (1)单独添加氮肥, 各土层土壤硝态氮和铵态氮含量均随处理浓度的增加而增加, 0-20 cm土层N20、N40、N60处理下土壤硝态氮和铵态氮分别较CK增加63.48%、126.04%、247.03%和80.66%、152.52%、244.56%; 随着土层深度增加, 土壤硝态氮、铵态氮含量均有下降, 20-40、40-60 cm土层较0-20 cm土层硝态氮含量分别平均减少53.90%、76.05%, 铵态氮含量分别平均减少48.62%、68.23%。(2)土壤净硝化速率、净氨化速率及净矿化速率随着氮肥浓度增加均呈上升趋势。相同氮肥添加浓度下, 土壤净硝化速率、净氨化速率和净矿化速率随着土层深度增加逐渐下降(除CK外)。(3)与单独添加氮肥比较, 氮硅肥配施, 土壤氮含量有显著提高, 在0-20 cm土层硝态氮和铵态氮较CK分别增加98.78%、192.62%、330.16%和99.96%、195.82%、306.32%, 20-40、40-60 cm土层也有类似趋势。同时, 氮硅配施促进了土壤氮矿化行为, 在0-20 cm土层, N60Si4处理下的土壤净硝化速率、净氨化速率较单独施氮时分别增加35.88%、27.41%。以上结果表明, 与单独氮肥添加相比, 氮硅配施不但能提高土壤氮含量, 而且能促进土壤氮的矿化作用, 对大气氮沉降有一定的缓解作用。     

不同硝化抑制剂对红壤氮素硝化作用及玉米产量和氮素利用率的影响

本研究分析添加不同种硝化抑制剂及其组合的高效稳定性氯化铵氮肥对红壤硝化作用、玉米产量和氮肥利用率的影响,旨在筛选出适合酸性红壤的高效稳定性氯化铵态氮肥。在氯化铵中分别添加硝化抑制剂2-氯-6-三甲基吡啶(CP)、3,4-二甲基吡唑磷酸盐(DMPP)和双氰胺(DCD)及其组合,制成6种高效稳定性氯化铵态氮肥,以不施氮肥(CK)和施氯化铵(N)为对照,进行等氮量玉米盆栽试验。结果表明: 与N处理相比,CP+DMPP和DMPP+DCD处理红壤中铵态氮含量提高56%～62%,显著高于CP、DMPP和DCD处理;土壤表观硝化率显著降低33%～34%。添加硝化抑制剂及其组合的6个处理均显著提高了玉米生物量和氮肥吸收利用率。与N处理相比,单独添加硝化抑制剂处理生物量均显著高于硝化抑制剂组合处理,平均提高1.3倍;添加DCD处理效果最显著,玉米籽粒产量、吸氮量和氮肥吸收利用率分别显著提高4.1、6.3和4.4倍。为了达到既能低成本又能提高产量和氮肥利用率的效果,在红壤上添加硝化抑制剂DCD是最佳选择。     

稳定性铵态氮肥在黑土和褐土中的氮素转化特征

以稳定性氯化铵为氮源,采用室内培养的方法,研究0.20、0.50、1.00 g N·kg^-1干土3种浓度的稳定性铵态氮肥在黑土、褐土中的氮素转化特征.结果表明: 在褐土中,随着氯化铵添加量的增加,土壤中发生硝化作用的时间逐渐推迟,添加0.20、0.50 g N·kg^-1干土处理开始发生明显硝化反应的时间分别为第3、7天,在高浓度氮量(1.00 g N·kg^-1干土)添加下硝化作用受到明显抑制;在黑土中,各浓度氮量添加处理开始发生硝化反应的时间相同,均为第3天,且随着添加量的增加,硝化作用潜势逐渐减弱.只加铵态氮肥的处理中,添加0.20 g N·kg^-1干土的氯化铵氮肥在褐土和黑土中的硝化反应时间分别可维持3周和2周左右;添加0.50 g N·kg^-1干土的氯化铵氮肥在褐土和黑土中的硝化反应时间分别可维持4周和3周左右.与单施氯化铵相比,黑土和褐土在0.20、0.50 g N·kg^-1干土添加浓度下,按纯氮量的1.0%添加3,4-二甲基吡唑磷酸盐(DMPP)、4.0%添加二氰二胺(DCD)均能显著抑制硝化作用,降低硝态氮的含量,抑制硝化作用潜势.综上,在褐土中,随着氯化铵添加浓度增加,土壤硝化作用受到抑制效果大于黑土.在0.20、0.50 g N·kg^-1干土外源铵态氮时,添加抑制剂可以显著抑制铵态氮的硝化作用.因此室内硝化抑制剂培养试验时,建议铵态氮添加量不超过1.00 g N·kg^-1干土,以0.50 g N·kg^-1干土效果最好.     

硝化抑制剂DCD、 DMPP对褐土氮总矿化速率和硝化速率的影响

采用15N库稀释-原位培养法研究了硝化抑制剂DCD、DMPP对华北盐碱性褐土氮总矿化速率和硝化速率的影响.试验在山西省运城市种植玉米的盐碱性土壤上进行,设单施尿素、尿素+DCD、尿素+DMPP 3个处理.结果表明:施肥后2周,DCD、DMPP分别使氮总矿化速率和氮总硝化速率减少了25.5％、7.3％和60.3％、59.1％,DCD对氮总矿化速率的影响显著高于DMPP,两者对氮总硝化速率的影响无显著差异;而在施肥后7周,不同硝化抑制剂对氮总硝化速率的影响存在差异.施肥后2周,3个处理的土壤氮总矿化速率和硝化速率分别是施肥前的7.2 ～10.0倍和5.5 ～21.5倍;NH4+和NO3-消耗速率分别是施肥前的9.1 ～12.2倍和5.1 ～8.4倍,这是由氮肥对土壤的激发效应所致.硝化抑制剂使氮肥更多地以NH4+形式保持在土壤中,减少了NO3-的积累.土壤氮总矿化速率和总硝化速率受硝化抑制剂的抑制是N2O减排的主要原因.     

植被类型与坡位对喀斯特土壤氮转化速率的影响

土壤氮素转化对于植物氮素营养具有重要作用,尤其是对于受氮素限制的喀斯特退化生态系统。选取植被恢复过程中4种典型喀斯特植被类型(草丛、灌丛、次生林、原生林)和3个坡位(上、中、下坡位)表层土壤(0—15cm)为对象,利用室内培养的方法,研究不同植被类型和坡位下土壤氮素养分与氮转化速率(氮净矿化率、净硝化率和净氨化率)的特征及其影响因素。结果表明,植被类型对土壤硝态氮含量、无机氮含量、氮净矿化率、净硝化率和净氨化率均有显著影响(P0.01),即随着植被的正向演替(草丛—灌丛—次生林—原生林),土壤硝态氮含量、无机氮含量、土壤氮净矿化速率和净硝化速率整体上呈增加趋势,而坡位以及坡位与植被类型的交互作用对上述土壤氮素指标无显著影响(P0.05)。冗余分析结果表明凋落物氮含量、凋落物C∶N比和硝态氮含量对土壤氮转化速率有显著影响,其中凋落物氮含量是影响土壤氮转化速率的主要因子(F=35.634,P=0.002)。可见,尽管坡位影响喀斯特水土再分配过程,但植被类型决定的凋落物质量(如凋落物氮含量等)对喀斯特土壤氮素转化速率的作用更为重要。因此,在喀斯特退化生态系统植被恢复初期,应注重植被群落的优化配置(如引入豆科植物)和土壤质量的改善(如降低土壤C∶N),促进土壤氮素转化及氮素的有效供给。     

红壤区不同产量籼稻品种苗期根表和根际土壤硝化特征

以不同产量籼稻品种中旱22(ZH,高产品种)和禾盛10号(HS,低产品种)为材料,采用根际培养箱(三室)--速冻切片技术研究了红壤水稻土种植条件下,水稻苗期生长、氮素积累和氮素利用率(NUE)、根孔隙度(POR)、根际土壤矿质态氮含量和硝化强度.结果表明,ZH苗期生长、氮素吸收及NUE均显著优于HS,且ZH单株不定根数量及根系通气组织发育程度(用POR表示)均显著高于HS.ZH根际和土体土壤中铵(NH+4)含量始终低于HS,而硝(NO-3)含量则始终高于HS,但二者根表土壤NH+4和NO-3含量均无显著差异.ZH和HS硝化强度最大发生部位均是在距根表2 mm的根际土壤,分别为:0.48 μmol kg-1h-1和0.31 μmol kg-1h-1.随着距根表越远,硝化强度就越弱,直至距根表10～20 mm处土壤硝化强度就接近于土体土壤.ZH根际土壤硝化强度始终显著高于HS,但二者根表和土体土壤硝化强度均无显著差异.与不种水稻的CK相比,根际土壤硝化强度提高了约2～3.5倍.尽管红壤水稻土硝化作用很弱,但红壤区水稻根际硝化作用与水稻苗期生长和氮素营养密切相关.     

高效稳定性硫酸铵氮肥在黑土中的施用效果

为筛选高效稳定性氮肥,采用盆栽试验,通过监测施用不同处理的稳定性硫酸铵对黑土铵态氮和硝态氮含量、表观硝化率、硝化抑制率、玉米生长指标、产量和氮素效率等指标的影响,研究添加不同生化抑制剂配方的稳定性硫酸铵态氮肥在吉林黑土玉米栽培中的施用效果。本试验以不施氮肥(CK)和施硫酸铵(N)为对照,在硫酸铵中分别添加硝化抑制剂3,4-二甲基吡唑磷酸盐(DMPP)、2-氯-6-三甲基吡啶(CP),氮保护剂(N-GD)和肥料增效剂(HFJ)及其组合,制成9种稳定性硫酸铵氮肥。结果表明: 与单施硫酸铵氮肥处理相比,在黑土中添加DMPP和DMPP组合显著影响土壤中铵态氮和硝态氮含量及土壤表观硝化率,铵态氮含量提高1.4～2.0倍,硝态氮含量降低13.6%～17.9%,土壤表观硝化率降低55.3%～59.8%;添加DMPP、DMPP+HFJ和DMPP+N-GD组合硝化抑制率最高,达到16.5%以上;添加DMPP+HFJ+N-GD和HFJ的硫酸铵处理玉米叶片叶绿素含量增加最显著,增加4.5～5.3倍;硫酸铵添加硝化抑制剂和肥料增效剂对株高无显著影响;添加HFJ的硫酸铵处理玉米生物量、籽粒产量、经济系数、收获指数、氮肥农学利用率、氮素吸收利用率、肥料贡献率和氮肥偏生产力增加最显著,分别增加1.2、2.5、0.7、0.6、2.7、2.1、1.3和2.5倍。添加HFJ和DMPP、DMPP+HFJ、DMPP+N-GD处理的硫酸铵处理在黑土中施用效果最好,但是DMPP成本较高,因此,兼顾成本和氮肥利用率,建议稳定性硫酸铵态氮肥生化抑制剂首选氮肥增效剂HFJ,其次选择DMPP+HFJ或者DMPP+N-GD。     

变性梯度凝胶电泳法研究我国不同土壤氨氧化细菌群落组成及活性

实验选用了我国3种不同土壤研究土壤硝化活性、硝化细菌数量,并使用变性梯度凝胶电泳(DGGE)的方法研究了不同土壤中氨氧化细菌(AOB)区系变化。通过28d的土壤培养实验研究发现,潮土具有最强的硝化势,几乎100%的铵态氮转化为硝态氮;而红壤中的硝化势最弱,只有4.9%的铵态氮转化为硝态氮。对这3种土壤硝化细菌进行计数发现,3种土壤氨氧化菌数量差异显著,而3种土壤亚硝酸氧化菌(NOB)处于一个数量级。采用氨氧化菌功能基因amoA(氨单加氧酶ammoniamonooxygenase)特异PCR结合DGGE的方法对土壤氨氧化菌区系进行分析。红壤有4个氨氧化菌种属,与潮土和黄泥土没有共同的氨氧化菌种属。4个种属中两个是与潮土和黄泥土亲源性比较远的,特有的氨氧化菌种属,这两个种属与已知的Nitrosospira属的cluster3bz97838和Nitrosospira属的cluster3aAF353263亲源性比较近。潮土有5个氨氧化菌种属,潮土与黄泥土有两个共同的氨氧化菌种属,这两个种属中的一个是潮土和黄泥土特有的,与其他氨氧化菌种属亲源性比较远的氨氧化菌种属,这个种属与已知的Nitrosospira属的cluster3bZ97849亲源性比较近。黄泥土有4个氨氧化菌种属,除了与潮土共有的一个种属是两种土壤特有的氨氧化菌种属外,黄泥土还有一个与其他氨氧化菌种属亲源性比较远的,黄泥土特有的种属,与Nitrosospira属的cluster3aAF353263亲源性很近。3种土壤中分离到的硝化细菌表现出不同的硝化能力。实验结果表明,以amoA基因为目标的PCR-DGGE是比以16SrDNA为目标的PCR-DGGE更有效的研究氨氧化菌种群的方法;3种土壤的氨氧化菌种群差异显著,尤其是红壤的氨氧化菌种群与另外两种土壤差异明显,这种差异可能与红壤的低pH条件对氨氧化菌种群的长期选择有关;3种土壤中的硝化活性与土壤中的硝化细菌数量没有显著相关,可能由于3种土壤差异显著的土壤环境对硝化活性的影响造成。因此在对不同土壤硝化细菌进行研究时不仅需要对硝化细菌数量进行研究,还需要研究不同土壤中硝化细菌的种属及不同土壤环境条件对硝化细菌硝化活性的影响。     

Leaching of nitrogen from subtropical soils as affected by nitrification potential and base cations

A soil column method was used to determine the effect of nitrification on leaching of nitrate and ammonium from three acid subtropical soils after application of ammonium bicarbonate. Three soils, designated QF, GB and SU, derived from Quaternary red earth, granite and tertiary red sandstone, were collected from forest land, brush land and upland field, ranged in nitrification potential and cation exchange capacity. The results indicated that nitrate leaching increased with the soil nitrification potential. The soils with higher nitrification potential had a higher nitrate peak concentration and required a shorter time to reach it. In soils QF and GB with low cation exchange capacity, and a low content of exchangeable base cations, there were not sufficient base cations to accompany the nitrate leached with the result that ammonium and hydrogen ions were leached from the soil, and pH changes occurred in different layers of the soil column.     

The conversion of amino acids in soils

Summary In an investigation on the conversion of amino acids in percolated soils, it was found that during the breakdown of glutamic acid to ammonia micro-organisms developed in the soil capable of denitrifying nitrite and nitrate to gaseous nitrogen. The enrichment of a soil with these micro-organisms was studied.Drying of the enriched soil had a deleterious effect on the activity of these micro-organisms.The interaction between denitrification and soil nitrification processes was studied in soil subjected to various percolation treatments. When the denitrifying micro-organisms and their metabolites were present in the soil the amount of nitrogen lost by denitrification depended on the availability of nitrite and nitrate. When this was supplied externally, in glutamate—nitrite or glutamate—nitrate mixtures, considerable reduction occurred. Losses were less severe where nitrite and nitrate entered the system internall y by nitrification of the ammonia produced from the breakdown of the amino acid. In fresh soils there were indications that the amount of nitrification occurring during amino-acid breakdown was the important factor.All the data appeared to be consistent with the hypothesis that during the conversion of amino acids in soils a delicate balance is established between nitrification and denitrification reactions by different types of soil micro-organisms.     

Fungi of a forest soil nitrifying at low pH values

Abstract No autotrophic nitrifying organisms were found in a podzolic brown earth forming nitrate. 350 fungi and aerobic heterotrophic bacteria were isolated from this soil and examined for their nitrifying abilities. About one quarter of the isolates produced 0.05–0.90 mg N·1⁻¹ nitrite or nitrate in peptone solution, soil extract mixture or sterilised soil. The nitrification rate of the most active fungus, Verticillium lecanii , was highest at pH 3.5 in defined media. The results support the significance of heterotrophic nitrification in acid soils.     

Nitrification and occurrence of Nitrobacter by MPN-PCR in low and high nitrifying coniferous forest soils

The nitrification of three coniferous forest soils was investigated: a podzol (Fontainebleau, France) with low N deposition and no nitrate accumulation, an acid mull (Haldon, UK) with low N deposition and moderate nitrate accumulation, and a podzol (Wekerom, The Netherlands) with high N deposition and high nitrate accumulation. Twenty-one months in situ lysimeter experiments and short-term and long-term in vitro incubations were performed to respectively establish the status of NO ₃ ^- accumulation and potential nitrification of these soils. These complementary approaches allowed to conclude that the absence of NO ₃ ^- accumulation in Fontainebleau soil was effectively due to a lack of nitrifing activity in the whole profile while the high NO ₃ ^- accumulation observed in the Wekerom soil essentially resulted from an active nitrifying activity in the Oh horizon of this soil. For the Haldon soil, the inadequacy between the lysimeter and the short-term in vitro experiment was discussed.Surprisingly, relatively high densities of the nitrite-oxidiser Nitrobacter (enumerated by PCR-MPN technique) were found in all the studied horizons of the three soils. Moreover, a long-term incubation experiment showed that inhibition of nitrification in the Oh Fontainebleau soil could be removed after submitting the soil samples to constant conditions during 82 days. This demonstrates that in these soils, ammonium-oxidisers (as well as nitrite-oxidisers) are present and that extrinsic factors were involved in the inhibition of nitrification.     

Relationships among microclimate,edaphic conditions,vegetation distribution and soil nitrogen dynamics on the Bogong High Plains,Australia

We examined microclimatic conditions and soil nitrogen (N) dynamics of different alpine plant community types on the Bogong High Plains in Victoria, Australia. Three community types are predominant in the High Plains region, namely grassland, heathland and woodland and together they form so‐called inverted treelines, with grassland in valley floors below the treeline. Outdoor temperature loggers were deployed in the three vegetation types to establish differences among microclimatic conditions. We incubated soils to determine rates of N production and collected additional soil samples for analysis of soil properties and soluble N. Temperature data showed that only grassland communities experienced sub‐zero temperatures in winter. Temperature and soil moisture influenced indices of N mineralization and N nitrification in this alpine ecosystem. Rates of N mineralization were significantly faster than nitrification that only produced consequential amounts of nitrate in summer. This information, together with considerably lower pools of nitrate than ammonium and organic N in the soil, implies that ammonium is the dominant form of soluble N in the ecosystem whereas nitrate most likely only has minor importance for plant nutrition. The results of this study provide insight into ecological processes of this alpine ecosystem and demonstrate the vulnerability of the system to altered climatic and edaphic conditions in the course of climate change.     

Soil nitrogen availability and nitrification in Mediterranean shrublands of varying fire history and successional stage

The short-term effect of a single fire, and the long-term effect of recent fire history and successional stage on total and mineral N concentration, net nitrogen mineralization, and nitrification were evaluated in soils from a steep semi-arid shrubland chronosequence in southeast Spain. A single fire significantly increased soil mineral N availability and net nitrification. Increasing fire frequency in the last few decades was. associated with a sharp decrease in surface soil organic matter and total N concentrations and pools, and with changes in the long-term N dynamic patterns. The surface-soil extractable NH₄ ⁺:NO₃ ^– ratio increased throughout the chronosequence. All net mineralized N in laboratory incubations from all sites was converted to nitrate, suggesting that allelochemic inhibition of net nitrification is probably not important in this system. Net nitrification in samples during incubation increased through the sere. The maximum rate of net nitrification (k_max) increased through the first three stages of the sere. A linear relationship was found between total soil N and N mineralization, and both k_max and net nitrification for the first three stages of the sere, suggesting that total N and ammonification are likely to be the control mechanisms of nitrification within the sere. The oldest site exhibited the lowest specific k_max and the highest, potential soil respiration rate suggesting that a lower N quality and increasing competition for ammonium might also limit nitrification at least in the long-unburned garrigue site.     

Community profiling and gene expression of fungal assimilatory nitrate reductases in agricultural soil

Although fungi contribute significantly to the microbial biomass in terrestrial ecosystems, little is known about their contribution to biogeochemical nitrogen cycles. Agricultural soils usually contain comparably high amounts of inorganic nitrogen, mainly in the form of nitrate. Many studies focused on bacterial and archaeal turnover of nitrate by nitrification, denitrification and assimilation, whereas the fungal role remained largely neglected. To enable research on the fungal contribution to the biogeochemical nitrogen cycle tools for monitoring the presence and expression of fungal assimilatory nitrate reductase genes were developed. To the ∼100 currently available fungal full-length gene sequences, another 109 partial sequences were added by amplification from individual culture isolates, representing all major orders occurring in agricultural soils. The extended database led to the discovery of new horizontal gene transfer events within the fungal kingdom. The newly developed PCR primers were used to study gene pools and gene expression of fungal nitrate reductases in agricultural soils. The availability of the extended database allowed affiliation of many sequences to known species, genera or families. Energy supply by a carbon source seems to be the major regulator of nitrate reductase gene expression for fungi in agricultural soils, which is in good agreement with the high energy demand of complete reduction of nitrate to ammonium.     

Links between ammonia oxidizer species composition, functional diversity and nitrification kinetics in grassland soils

Molecular approaches have revealed considerable diversity and uncultured novelty in natural prokaryotic populations, but not direct links between the new genotypes detected and ecosystem processes. Here we describe the influence of the structure of communities of ammonia-oxidizing bacteria on nitrogen cycling in microcosms containing natural and managed grasslands and amended with artificial sheep urine, a major factor determining local ammonia concentrations in these environments. Nitrification kinetics were assessed by analysis of changes in urea, ammonia, nitrite and nitrate concentrations and ammonia oxidizer communities were characterized by analysis of 16S rRNA genes amplified from extracted DNA using ammonia oxidizer-specific primers. In natural soils, ammonia oxidizer community structure determined the delay preceding nitrification, which depended on the relative abundance of two Nitrosospira clusters, termed 3a and 3b. In batch cultures, pure culture and enrichment culture representatives of Nitrosospira 3a were sensitive to high ammonia concentration, while Nitrosospira cluster 3b representatives and Nitrosomonas europaea were tolerant. Delays in nitrification occurred in natural soils dominated by Nitrosospira cluster 3a and resulted from the time required for growth of low concentrations of Nitrosospira cluster 3b. In microcosms dominated by Nitrosospira cluster 3b and Nitrosomonas, no substantial delays were observed. In managed soils, no delays in nitrification were detected, regardless of initial ammonia oxidizer community structure, most probably resulting from higher ammonia oxidizer cell concentrations. The data therefore demonstrate a direct link between bacterial community structure, physiological diversity and ecosystem function.