氮肥调控剂对潮褐土中不同氮源氮素转化及油菜生长的影响

采用土壤培养和盆栽试验相结合的方法,研究了硝化抑制剂双氰胺(DCD)与纳米碳配合施用对尿素和碳酸氢铵在华北平原典型土壤潮褐土中转化的调控效果及其对油菜生长的影响.结果表明： 尿素和碳酸氢铵在施入土壤后的2周内,土壤无机氮的供应强度差别较大,2周以后则基本相似.2种氮源对油菜生长及氮素利用的影响在生育前期(播种后34 d)差异显著,但最终达到商品生物量收获时,氮源之间差异不大.DCD对尿素和碳酸氢铵在潮褐土中的转化表现出显著的硝化抑制作用,其抑制强度和有效抑制时间随DCD用量的增加而增强,且以对碳酸氢铵施入土壤后的硝化抑制效果更好.在本研究条件下,DCD用量以占肥料纯氮量的1.0%～1.5%相对较佳,可显著提高油菜产量,改善叶色,降低植株硝酸盐含量,提高氮肥利用率.纳米碳与DCD配合施用对土壤铵氧化有明显的协同抑制效果,且可以显著刺激油菜前期的生长发育和氮素利用,降低油菜硝酸盐含量.     

氮素调控剂对不同类型土壤氮素转化的影响

采用室内培养试验方法,比较了硝化抑制剂双氰胺（DCD）和3,5-二甲基吡唑（DMP）对华北平原两种主要土壤类型潮褐土和潮土中氮素转化的调控效果,并进一步研究了DMP与脲酶抑制剂氢醌（HQ）的配合施用对潮土中氮素转化的影响.结果表明： 在两种供试类型土壤上DMP对尿素氮的硝化抑制作用均强于DCD.与单施尿素相比,在氮素转化高峰时,DMP可使土壤中NH₄⁺-N含量显著提高149.5%～387.2%,NO₃^--N含量降低22.3%～55.3%;同一抑制剂对潮土中氮素转化的调控效应较潮褐土更为明显;与DMP单施相比,DMP和HQ配施表现出明显的对氮素转化的协同抑制效果.
     

南京郊区番茄地中氮肥的效应与去向

采用田间小区和微区试验,研究了施用化学氮肥对南京郊区菜地番茄产量、氮肥去向及氮素损失的影响.结果表明, 由于土壤和有机肥供氮充分,氮肥施用未增加番茄产量.用差值法计算得到的氮肥利用率在14.5%～22.5%之间.¹⁵N标记尿素微区试验表明,施入氮量的16.6%～28.8%被作物吸收,氮素总损失为34.2%～46.0%.施用化学氮肥增加了土壤剖面中的硝酸盐含量,番茄收获时,10%～10.2%的标记尿素被淋洗到40 cm以下土层.增施化学氮肥也显著增加了菜地土壤的反硝化损失和N₂O排放,其中反硝化损失占施入氮量的5.50%～6.01%;N₂O排放量占施入氮量的2.62%～4.92%.但番茄生长期间未检测到氨挥发.减少氮肥用量或施用包衣尿素可降低菜地施用氮肥的环境风险,特别是减少硝酸盐淋洗和硝化反硝化损失.     

施用缓/控释尿素对玉米苗期土壤生物学活性的影响

采用盆栽试验,模拟田间生态环境,研究了施用不同种缓/控释氮素底肥对玉米苗期土壤硝酸还原酶、脲酶活性及微生物量碳、氮的影响.结果表明,施用硝化抑制剂(双氰胺)和脲酶抑制剂(n-丁基硫代磷酰三胺)涂层大颗粒尿素肥料的土壤硝酸还原酶活性最高;施用大颗粒尿素,脲酶活性最强,微生物量碳、氮最高.施用醋酸酯淀粉包膜大颗粒尿素、包膜双氰胺涂层大颗粒尿素、丙烯酸树脂包膜双氰胺涂层大颗粒尿素与不施氮肥土壤脲酶活性较高;每种处理微生物量碳与氮变化完全一致.施用醋酸酯淀粉包膜硝化和脲酶抑制剂涂层大颗粒尿素肥料,土壤微生物量碳、氮最低.同种膜材料包膜抑制剂涂层大颗粒尿素制成的缓/控释氮肥,对土壤生物学活性的影响效果好于直接包膜大颗粒尿素;丙烯酸树脂包膜大颗粒尿素制成的缓/控释氮肥,对氮素的控释效果明显好于醋酸酯淀粉包膜.     

氮肥种类和油菜秆还田对水稻苗期碳氮累积的影响

为探讨不同氮肥种类和油菜秆还田对水稻苗期碳氮累积的影响, 设置了氮肥种类(尿素、碳酸氢铵和硫酸铵)和秸秆还田的双因素的盆栽试验, 测定了移栽后水稻苗期碳氮累积量和碳氮比。结果表明, 相比施用尿素, 硫酸铵显著提高了水稻地上部氮素累积量, 显著降低了不添加油菜秸秆条件下的水稻地上部和根碳氮比。添加油菜秆条件下, 施用硫酸铵较尿素显著提高了播后57 d时水稻地上部和根部碳素累积量。相比不添加油菜秆, 油菜秆还田显著抑制了播后57 d时水稻地上部和根氮吸收累积, 显著增加了播后71 d时水稻根部氮碳素累积和57 d时碳酸氢铵和硫酸铵处理下的水稻地上部和根部碳氮比。综合来看, 稻油轮作系统中油菜秆还田时配施硫酸铵能更好的促进水稻苗期碳氮的吸收累积。研究结果为南方稻油轮作系统中氮素养分管理提供了一定的技术支持。     

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.     

脲酶/硝化抑制剂在黑土和褐土中对尿素氮转化的调控效果

本试验研究脲酶/硝化抑制剂不同组合在黑土和褐土中对尿素水解和硝化作用的调控效果,旨在筛选出适合东北黑土、褐土的高效抑制剂组合。采用室内恒温、恒湿培养试验,以不施氮肥(CK)和施用普通尿素肥料(U)为对照,研究分别添加脲酶抑制剂N-丁基硫代磷酰三胺(NBPT)及其与硝化抑制剂双氰胺(DCD)、3,4-二甲基吡唑磷酸盐(DMPP)、2-氯-6(三氯甲基)-吡啶(CP)、2-氨基-4-氯-6-甲基嘧啶(AM)、3-甲基吡唑(MP)组合制成的6种高效稳定性尿素在黑土和褐土中的尿素水解和氨氧化特征。在培养125 d内分别取土壤样品15次,通过测定2种土壤中尿素态氮、铵态氮和硝态氮含量,及氨氧化作用强度,计算硝化抑制率,确定最适合2种土壤的抑制剂或组合。结果表明: 尿素在黑土和褐土中水解时间约7 d,添加NBPT以及其与不同硝化抑制剂组合均能将尿素水解时间延长21 d以上。与U处理相比,添加抑制剂可显著增加土壤NH₄⁺-N含量,降低NO₃^--N生成量,维持土壤中高NH₄⁺-N含量的时间更久。黑土中,添加硝化抑制剂的处理均能显著抑制土壤硝化作用,有效硝化抑制时间超过125 d;DMPP、CP与NBPT配施使黑土NH₄⁺-N含量提高1.6～1.8倍,培养125 d时其硝化抑制率分别为47.9%和24.1%。褐土中,U处理培养80 d左右基本完成硝化过程,而添加硝化抑制剂使硝化过程延长至少30 d;DCD、DMPP与NBPT配施使土壤NH₄⁺-N含量提高2.1～3.4倍,培养125 d时其硝化抑制率分别为25.3%和23.2%。因此,尿素与NBPT+DMPP和NBPT+DCD制成的高效稳定性尿素分别在黑土和褐土中施用效果最好,其次分别是NBPT+CP和NBPT+DMPP。     

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.     

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.     

硝化抑制剂对不同旱地农田土壤N2O排放的影响

通过室内培养法,研究了硝化抑制剂双氰胺(DCD)和3,4-二甲基吡唑磷酸盐(DMPP)对施加尿素的沈阳草甸棕壤、运城褐土、美国明尼苏达州粉砂壤土的N2O排放、氮素转化速率和微生物群落结构的影响.结果表明:抑制剂DCD和DMPP对草甸棕壤的N2O减排率为54.1％ ～75.9％,但对速效氮含量影响不显著,约24％的硝化潜势被DCD所抑制,而在高含水量下DMPP却对硝化潜势无抑制作用;在褐土中,DMPP抑制效果显著,其在两种含水量下的N2O减排率为85.5％和66.7％、对硝化作用潜势抑制率为97.2％和96.4％,但DCD只在低含水量下有少许抑制效果(24.6％ ～57.5％),而在高含水量下则失效;DMPP对粉砂壤土在两种含水量下的N2O减排率为42.9％和53.1％,而DCD在高含水量下未能减排N2O;在草甸棕壤和褐土中,施氮肥有效促进氨氧化细菌(AOB)的生长繁殖,DCD与DMPP使AOB amoA数量减少了4.1％ ～63.5％,有显著抑制作用,而对氨氧化古菌(AOA)和反硝化菌则影响不大;与AOB相比,AOA在数量上占优势,但AOB amoA基因丰度与硝化潜势显著正相关,表明AOB在硝化过程中起了更重要的作用.     

速效氮肥配施双氰胺对减控小白菜镉积累的作用

本文以小白菜为试材,研究了镉(Cd)污染土壤中配施双氰胺(DCD)对不同速效氮肥(硫酸铵,简称全铵;硝酸钠,简称全硝;11硫酸铵和硝酸钠,简称硝铵;尿素)下小白菜生长及Cd含量的影响。结果表明: 与对照相比,Cd污染土壤中配施DCD使全铵、硝铵和尿素处理下的小白菜可食部分鲜重分别增加583.3%、41.5%和206.8%,耐受系数(TI)和光合作用速率均大幅度提高;全硝处理则无明显变化。配施DCD使各氮肥处理下小白菜叶片的丙二醛、过氧化氢含量和超氧自由基产生速率均不同程度地下降,且以全铵和尿素处理的下降幅度最大。与对照相比,全铵、硝铵和尿素处理的小白菜叶片的Cd含量分别减少58.3%、34.0%和44.5%;叶柄的Cd含量分别减少61.8%、29.4%和55.6%。全硝处理下小白菜叶片和叶柄的Cd含量与对照相比无明显差异。配施DCD不同程度降低了各处理根际土壤的酸化可能是造成上述结果的主要原因。在缓冲能力较弱的Cd污染土壤中,施用全铵、硝铵和尿素氮肥时配施DCD可以缓解Cd污染土壤对小白菜生长的胁迫,并有效减少Cd在植物体内的积累。     

几种硝化抑制剂对土壤和小白菜硝酸盐含量及产量的影响

通过田间试验,对湖南长沙地区的红菜园土和冲积菜园土两种土壤条件下氢醌 (HQ)、双氰胺 (DCD)和硫脲 (TU) 3种硝化抑制剂对土壤和小白菜硝酸盐含量及小白菜产量的影响进行研究.结果表明,3种硝化抑制剂在试验的各个时期均不同程度地降低了土壤和小白菜的硝酸盐含量,其中以双氰胺的效果最好.但不同供试土壤条件下的试验结果不完全一致.3种硝化抑制剂均可不同程度提高小白菜产量,其中双氰胺效果最好,且在红菜园土的增产效果比冲积菜园土显著.     

脲酶抑制剂/硝化抑制剂对植稻土壤中尿素N行为的影响

采用自制根盒试验,主要研究了脲酶抑制剂氢醌（HQ）,硝化抑制剂双氰胺（DCD）及二者组合对离水稻根际不同距离处NH4^--N和NO3^-N分布的影响,结果表明,DCD及其与HQ组合均能显著促进稻株地上部分生长,始终显著降低水稻根际与近根际土中NH4^ -N含量直至施肥后60d,施肥后20d时,DCD及其与HQ组合可使非根际土中NH4^ -N含量显著增加,随后,却出现相反现象,施肥后20d时,距根际不同距离的土壤中,配施DCD或DCD＋HQ处理均能显著降低NO3^-N含量,随后,近根际和非根际仍保持上述现象直至施肥后40d;同未施DCD处理相比,根际土壤却较早出现NO3^--N含量高峰,正好与水稻N营养需求时期相一致,因此,DCD及其与HQ组合可减少水稻根际环境下尿素N损失潜势,通过不种稻土壤和距根际3cm处的土壤中尿素无机氮形态分布的差异,充分显示了研究水稻根际土壤氮素转化及相关抑制剂对其影响时,以取离根际3cm外的土壤作为非根际明显优于不种稻土壤。     

Medicinal and aromatic plant materials as nitrification inhibitors for augmenting yield and nitrogen uptake of Japanese mint (Mentha arvensis L. Var. Piperascens)

Pot experiments were conducted to evaluate the relative performance of medicinal and aromatic plant materials and dicyandiamide (DCD) as nitrification inhibitors to regulate transformation of N from urea. Their effect on the efficiencies of use of N by Japanese mint (Mentha arvensis cv. Hy 77) was tested. Urea was coated with these materials viz., Mentha spicata, Artemisia annua or DCD at the rate of 5% (w/w) of fertilizer urea using an appropriate coating technique. Nimin (tetranortriterpenoids, an ethanol extract of neem (Azadirachta indica Juss) coating was done at the rate of 1% w/w of urea. Fertilizer nitrogen was applied at 100 and 200 mg kg(-1) soil. These natural coating materials significantly increased the herb and essential oil yields of the crop at both rates of fertilizer nitrogen compared to urea alone and were found to be as effective as DCD in retarding NO3- formation in soil. Herb yield increased by 6-81% when compared to uncoated urea. The increase in essential oil yield ranged between 3% and 68% due to coating. The effectiveness of the nitrification-inhibitor--coated urea, however, varied with the soils used and the rate of fertilizer nitrogen applied. The results suggest that the natural products could be potential nitrification inhibitors for increasing fertilizer N use efficiency.     

深松与包膜尿素对玉米田土壤氮素转化及利用的影响

耕作方式和氮肥施用是影响土壤中氮肥转化、利用效率和作物产量的重要因素。通过夏玉米田的2a(2011—2012)定位试验,研究了两种耕作方式(深松、旋耕)配合不同尿素类型(包膜尿素、普通尿素)的施用对玉米田土壤硝态氮和铵态氮含量、脲酶活性、硝化细菌和反硝化细菌数量、玉米产量以及氮肥农学效率的影响。研究结果表明:相同耕作方式下,包膜尿素处理土壤中脲酶活性较稳定,且增加了旱田土壤亚硝酸细菌数量而降低了反硝化细菌数量,有利于土壤硝态氮含量的提高,尤其是作物生长的中后期;包膜尿素处理的产量比普通尿素提高7.25%—10.82%,同时提高氮肥农学效率。深松处理增加了土壤中的反硝化细菌数量,配合施用包膜尿素进一步提高了土壤脲酶活性,增加了亚硝酸细菌数量;旋耕与包膜尿素配合施用在一段时期内能显著增加土壤硝态氮含量,减少反硝化细菌数量。深松配合包膜尿素处理能够显著的增加玉米产量,2a分别比旋耕配合包膜尿素增加1.41%和10.62%。因此,深松措施配合施用包膜尿素能够增强土壤脲酶活性,增加亚硝酸细菌数量,提高氮素转化速率,增加作物产量和氮肥农学效率,其稳产效果在干旱年份尤为显著。     

不同种类氮肥对土壤释放N2O的影响

用培养试验模拟研究了在正常水分 ( 2 2 % )、干旱 ( 1 2 % )和高含水量 ( 32 % )条件下 ,普通碳酸氢铵 (普碳 )、尿素及新型肥料长效碳酸氢铵 (长碳 )对土壤释放N2 O的影响 ;同时考察了土壤NO-3 的形成时间和形成量 .结果表明 ,农田中施加的无机氮肥是大气中N2 O的重要来源 ,而长碳与普碳和尿素相比 ,不但可以明显延后N2 O释放高峰期出现时间 ,而且大多数情况下可以显著减少其释放量 (P <0 .0 1 ) .在 5个月的监测期内 ,与普碳和尿素相比其减少N2 O释放的比例分别为 80 .2 3和 88.41 % ( 1 2 %含水量 ) ,40 .0 0和2 7.59% ( 2 2 %含水量 ) ,无减少作用和 45.88% ( 32 %含水量 ) .本研究结果提示长碳具有作为农田生态系统N2 O减排措施的巨大潜力 ,同时暗示在农业中用长碳代替目前普遍应用的普碳 ,可以减少地下水中NO-3 引起的污染 .     

Responses of soil inorganic nitrogen to increased temperature and plant removal during the growing season in a Sibiraea angustata scrub ecosystem of eastern Qinghai-Xizang Plateau北大核心CSCD

Aims Little information has been available on the soil nitrogen transformation process of alpine scrubland under global warming and changing climate. This study aimed at clarifying seasonal dynamics of the soil nitrate and ammonium contents and their responses to increased temperature under different plant treatments. Methods We conducted a field experiment including two plant treatments (removal- or unremoval-plant) subjected to two temperature conditions (increased temperature or control) in Sibiraea angustata scrub ecosystem on the eastern Qinghai-Xizang Plateau. The contents of soil nitrate and ammonium were measured at the early, middle and late growing seasons. Important findings The results showed that soil nitrate and ammonium contents exhibited obvious seasonal dynamics. Throughout the entire growing season, the soil nitrate contents increased firstly and then decreased, while the soil ammonium contents increased continually. Particularly, in the early and middle growing season, the soil nitrate contents were significantly higher than those of ammonium, regardless of increased temperature and plant treatments; however, in the late growing season, the soil nitrate contents were significantly lower than those of ammonium. These results implied that soil nitrification was the major process of soil nitrogen transformation in the early and middle growing season; soil ammonification contributed mostly to soil nitrogen transformation in the late growing season. Furthermore, different responses of soil nitrate and ammonium contents to increased temperature and plant removal treatments were observed at the different stages in the growing season. The effects of increased temperature on soil nitrate contents mainly occurred in the middle and late growing season, but the effects varied with plant treatments. Increased temperature only significantly increased soil ammonium contents in the unremoval-plant plots during the middle growing season. The effects of plant treatments on soil nitrate contents only occurred in the control plots (controlled temperature). Plant removal only increased soil nitrate contents in the early and middle growing season, but significantly decreased soil nitrate contents in the late growing season. Plant removal significantly decreased soil ammonium contents in the increased temperature plots during the middle growing season. Probably, in the early and middle growing season, scrub vegetation mainly absorbed soil nitrate and the absorption process was not affected by increased temperature. These results would increase our understanding of the soil nitrogen cycling process in these alpine scrub ecosystems under global warming and changing climate. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.     

氯嘧磺隆与尿素对土壤微生物生物量碳、氮及无机氮的影响

通过室内培养试验,研究了不同浓度氯嘧磺隆(20、200、2000 μg·kg-1土)单一施用及与尿素(120 mg· kg-1土)配合施用情况下,土壤微生物生物量碳、氮和土壤铵态氮、硝态氮随时间的动态变化规律.结果表明:各浓度氯嘧磺隆单独处理在整个培养期(60 d)中对微生物生物量碳、氮均有抑制作用,且浓度越高,后期抑制作用越强;各浓度氯嘧磺隆处理在培养前期对硝态氮、铵态氮没有明显影响,中期(15 d)能显著提高土壤中铵态氮的含量,后期(30 d后)显著提高了土壤中硝态氮的含量.尿素单独施用及与氯嘧磺隆配施均能在短时间内增加微生物生物量碳、氮,但随后配施处理的促进作用减弱;尿素单独和配施均能持久增加土壤中铵态氮、硝态氮含量.     

Root-hair infection and nodulation of four grain legumes as affected by the form and the application time of nitrogen fertilizer

The effects of application of combined nitrogen fertilizer (ammonium nitrate or urea) on root-hair infection and nodulation of four grain legumes were studied. Young roots of each legume were inoculated with their compatible rhizobia. The application of the two forms of combined N either at the early stages of plant growth and/or at the time of nodule formation depressed root-hair curling, infection and nodulation. Infection of hairs on the primary roots was more sensitive to the N fertilizer than hair infection of secondary roots in bothVicia faba andPisum sativum. The nodule number and total fresh mass of the four legumes were drastically affected by fertilizer application. The combined N added both at early and at later stages significantly reduced the nodulation ofV. faba, Phaseolus vulgaris andVigna sinensis. The inhibitory effect of urea on nodulation ofP. sativum was only observed when the fertilizer was applied at the late stages of plant growth. It is concluded that, although the nodulation of the four legumes was suppressed by combined N, the initial events ofRhizobium-legume symbiosis (infection of roots and nodule initiation) are more sensitive to combined N than the stages after nodule formation.