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Construction and verification of a critical N dilution curve for flue-cured tobacco under tobacco-rice rotation systems in southeast China

  •   【Objectives】  This study aims to construct a critical N dilution curve for flue-cured tobacco and use the same to rapidly diagnose and evaluate the N nutrition status of flue-cured tobacco.  【Methods】  Three field experiments were conducted in two locations for two years. Six N application rates (0, 45, 90, 135, 180, and 300 kg/hm2) were setup in each experiment. The shoot and leaf dry matter accumulation recorded on different days after transplanting were analyzed. The critical N dilution curve equation and dry matter accumulation equation of shoot and leaves were established using N concentration and dry matter accumulation in plants at different growth stages. Combined with the non-destructive determination of N concentration based on the visible spectrum platform of UAV, the N nutrition index (NNI) was calculated and used to determine the N nutrition status of flue-cured tobacco plants.  【Results】  N application (P < 0.05) increased shoot and leaf dry matter accumulation of flue-cured tobacco, with variation among the N treatments. The N concentration in shoot and leaves decreased across the growth stages of flue-cured tobacco. The power equation described the relationship between the critical N concentration and dry matter accumulation in the shoot and leaves: Nc=3.1941×DMleaves-0.411. Independent validation of the model showed that observations for the N deficient group were below the critical N dilution curve, while those of the sufficient N group were near the curve. The RMSEs of the simulated and actual critical N concentrations for the shoots and leaves were 0.55 and 0.44, and the corresponding n-RMSEs were 25% and 17%, showing the high stability of the model. The leaf NNI of flue-cured tobacco gradually increased with an increasing N application rate. The leaf NNI was higher than 1 when N application rate reached surplus level (135 kg/hm2).  【Conclusions】  The constructed leaf critical N dilution curve for flue-cured tobacco under tobacco-rice rotation system was Nc = 3.1941 × DMleaves–0.411, which is accurate with low n-RMSEs. This was verified by leaf NNIs that coincide with the N application levels, suggesting the curve could be used to rapidly diagnose the N status of flue-cured tobacco in the study area.
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Construction and verification of a critical N dilution curve for flue-cured tobacco under tobacco-rice rotation systems in southeast China

    Corresponding author: YAN Hui-feng, yanhuifeng@caas.cn
  • 1. Tobacco Research Institute of CAAS, Key Laboratory of Tobacco Biology and Processing, Ministry of Agriculture and Rural Affairs, Qingdao, Shandong 266101, China
  • 2. Jiangxi Institute of Tobacco Science, Nanchang, Jiangxi 330025, China
  • 3. China Tobacco Shandong Industrial Co., Ltd, Jinan, Shandong 250014, China

Abstract:   【Objectives】  This study aims to construct a critical N dilution curve for flue-cured tobacco and use the same to rapidly diagnose and evaluate the N nutrition status of flue-cured tobacco.  【Methods】  Three field experiments were conducted in two locations for two years. Six N application rates (0, 45, 90, 135, 180, and 300 kg/hm2) were setup in each experiment. The shoot and leaf dry matter accumulation recorded on different days after transplanting were analyzed. The critical N dilution curve equation and dry matter accumulation equation of shoot and leaves were established using N concentration and dry matter accumulation in plants at different growth stages. Combined with the non-destructive determination of N concentration based on the visible spectrum platform of UAV, the N nutrition index (NNI) was calculated and used to determine the N nutrition status of flue-cured tobacco plants.  【Results】  N application (P < 0.05) increased shoot and leaf dry matter accumulation of flue-cured tobacco, with variation among the N treatments. The N concentration in shoot and leaves decreased across the growth stages of flue-cured tobacco. The power equation described the relationship between the critical N concentration and dry matter accumulation in the shoot and leaves: Nc=3.1941×DMleaves-0.411. Independent validation of the model showed that observations for the N deficient group were below the critical N dilution curve, while those of the sufficient N group were near the curve. The RMSEs of the simulated and actual critical N concentrations for the shoots and leaves were 0.55 and 0.44, and the corresponding n-RMSEs were 25% and 17%, showing the high stability of the model. The leaf NNI of flue-cured tobacco gradually increased with an increasing N application rate. The leaf NNI was higher than 1 when N application rate reached surplus level (135 kg/hm2).  【Conclusions】  The constructed leaf critical N dilution curve for flue-cured tobacco under tobacco-rice rotation system was Nc = 3.1941 × DMleaves–0.411, which is accurate with low n-RMSEs. This was verified by leaf NNIs that coincide with the N application levels, suggesting the curve could be used to rapidly diagnose the N status of flue-cured tobacco in the study area.

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  • 与主要粮食作物不同,烟草以叶片为收获器官,产量和品质是实现烟草生产最大经济效益的两个决定性因素,而叶片的产量和品质分别受到氮肥用量的正负调控[1]。含氮量为烟叶品质的重要特征指标,实现最优品质的烟草叶片含氮量为2.0%~2.5%[2];高效实时的叶片含氮量诊断方法是建立烟草优质高效农业综合管理措施的重要基础。烟稻水旱轮作是我国南方烟区一种典型的种植模式[3],烟稻轮作区烤烟种植面积占我国烤烟种植面积的20%以上。不同于其他烟草轮作复种模式,水旱轮作季节间干湿交替变化显著地影响了系统的氮素循环,水稻季收获后土壤残留无机氮以NH4+-N为主,而旱季作物收获后残留的无机氮则以NO3-N 为主[4],因此烟稻轮作区在烤烟氮肥施用和氮素利用方面表现独有的特征[5],烟稻轮作区高温高降雨量的气候特征导致土壤中氮的转化非常活跃,因此氮肥管理一直是烟稻轮作区烤烟养分管理中的重点和难点[6]。由于缺乏明确的氮素营养诊断指标,难以准确判断烤烟植株氮素营养状况,生产中施氮不合理的现象常常发生,根据前期调研结果,我国烟稻轮作区烤烟氮肥施用量加权平均为N 127.5 kg/hm2,比全国平均水平高35%。进行植物氮素诊断是作物氮素养分精准管理的重要方法之一,其核心在于确定作物的临界氮浓度。所谓的临界氮浓度是指作物达到最大干物质所需要的最低氮浓度[7]。在作物生长的过程中,作物体内的临界氮浓度会随着生物量的增加而降低,两者间存在幂指数关系(N = a DM−b)[8]。多个不同作物上建立了地上部临界氮浓度稀释曲线模型,Greenwood 等[9]提出了C3、C4作物的临界氮稀释曲线通用模型分别为N = 5.17W−0.5和N = 4.11W−0.5,随后国内外学者又先后在油菜[10]、棉花[11]、玉米[12-13]、小麦[14-17]、马铃薯[18]、番茄[19-20]、水稻[21-23]、紫花苜蓿[24]等作物上建立了各自的参数模型,临界氮浓度稀释模型可能因气候、品种等的不同而出现差异[25],这在很大程度上限制了临界氮稀释模型的通用性[26]。在临界氮浓度稀释模型基础上Lemaire等[8]提出了氮营养指数的概念,定义为实际氮浓度与临界氮浓度的比值,氮亏缺指临界氮积累量与实际氮积累量之差。氮营养指数受品种、种植密度[27]、降雨量[28-30]、氮肥用量[17,27]、地膜覆盖[31]等多种因素的影响。氮营养指数为氮营养诊断重要指标,将临界氮浓度稀释曲线应用于作物氮营养无损检测[23, 32]。本研究通过两年的田间试验,构建烟稻轮作区烤烟临界氮素浓度稀释曲线,同时利用氮素营养指数评估烤烟氮素营养状况,以期为烤烟合理氮肥施用、氮素营养诊断和氮肥优化管理提供理论依据。

  • 1.   材料与方法

      1.1.   试验地基本情况

    • 本研究由3部分田间试验组成,试验1于2017年在江西省安福县(114.351969E,27.341312 N)进行,试验2于2018年在江西省黎川县 (116.95895E,27.281882N)进行,试验3于2018年在江西省安福县 (114.373883 E,27.384067 N) 进行。试验1和试验3所在地区属中亚热带湿润性季风气候,其中烤烟生育期内(3—6月)月均平均气温分别为11.96℃、17.97℃、22.74℃、25.79℃,生育期日平均气温≥10℃的有效活动积温为694.9℃,占全年有效活动积温的22.2%;烤烟生育期内(3—6月)月均降雨量分别为181.8、221.5、211.3和254.6 mm,生育期内总降雨量占全年总降雨量(1573.2 mm)的55.3%。试验2所在地区属中亚热带湿润性季风气候,其中烤烟生育期内(3—6月)月均平均气温分别为12.47℃、18.44℃、23.05℃、26.11℃,生育期日平均气温≥10℃的有效活动积温为734.3℃,占全年有效活动积温的22.9%;烤烟生育期内(3—6月)月均降雨量分别为232.3、270.4、245.7和326.6 mm,生育期内总降雨量占全年总降雨量(1829.1 mm)的58.7%。试验地土壤均为水稻土,前茬作物为水稻。土壤基础肥力指标为见表1

      试验编号
      Experiment
      pH有机质 (g/kg)
      Soil organic matter
      碱解氮 (mg/kg)
      Alkaline N
      有效磷 (mg/kg)
      Available P
      速效钾 (mg/kg)
      Available K
      14.7920.47 92.0719.2567.04
      24.6833.60108.9715.3381.00
      36.7435.10228.8513.7176.49

      Table 1.  Basic physical and chemical properties of soils at the three field experimental sites

    • 1.2.   试验设计

    • 试验1用于烤烟临界氮浓度稀释曲线模型构建,试验2用于烤烟临界氮浓度稀释曲线模验证和烤烟干物质积累曲线构建,试验3用于氮营养诊断应用。各试验均设6个氮肥用量处理,分别为N·0、45、90、135、180 和300 kg/hm−2。不同处理的磷肥和钾肥用量相同,分别为P2O5 135 kg/hm−2和K2O 405 kg/hm−2。氮磷钾肥形态分别为硝酸铵钙(15.5-0-0)、钙镁磷肥(0-12-0)和硫酸钾(0-0-50)。氮、磷、钾肥均一次性基施,施肥前先起垄,垄高15 cm,肥料混匀后撒于垄顶,然后进行二次起垄,垄高30 cm。分别于2017年3月1日和2018年3月3日进行移栽,两个试验点的行距均为1.2 m,株距为0.45 m,试验品种均为云烟87。每小区面积48 m2,种植烤烟4行,3 次重复,完全随机区组排列。

    • 1.3.   烤烟氮浓度的测定

    • 于移栽后不同时期在每个小区选择有代表性植株3株进行取样,根据生育期进行取样,其中试验1 取样时间分别在移栽后29、34、47、55、59、65和 90 天,试验2 取样时期在移栽后44、50、65、86、93、100、107 天,将取回的植株用去离子水洗净,分为叶片和茎两部分,于105℃杀青30 min,60℃恒温烘干、称重。磨细过0.5 mm筛,采用H2SO4-H2O2法联合消煮,消煮液使用全自动凯氏定氮仪测定不同器官的含氮量。

    • 1.4.   模型拟合

      1.4.1.   烤烟临界氮浓度稀释曲线
    • 参考Lemaire等[8]提出的方法计算临界氮浓度稀释曲线,步骤如下:1)对每次取样时不同氮肥用量处理的地上部干物质积累量采用LSD法进行方差分析,根据差异显著性(P < 0.05)将氮肥用量处理分为两组,分别为生长受氮素营养限制组和与生长不受氮素营养限制组。2)对生长受氮素营养限制组,其地上部和叶片的干物质积累量与氮浓度值的关系以线性方程拟合。3)对于生长不受氮素营养限制组,用其地上部和叶片干物质积累量的平均值代表最大干物质积累量。4)每次取样的临界氮浓度值由线性方程与以最大干物质为横坐标的垂线相交的交点纵坐标决定。5)将每次取样的临界氮浓度值使用临界氮浓度稀释曲线模型进行拟合,方程为:

      式中,Nc为烤烟地上部分的临界氮浓度值(%);DM为烤烟地上部干物质积累量(t/hm2);a、b为参数,a为地上干物质为1 t/hm2时临界氮浓度值,b为统计参数。

      模型的验证采用国际通用的回归估计标准误差均方根误差RMSE和标准化均方根误差n-RMSE以及通过模拟值与实测值之间直方图来检测模型的拟合度和可靠性。其中模拟值为根据试验2 不受氮素营养限制组的地上部和叶片生物量,通过临界氮稀释曲线计算获得,实测值为试验2 不受氮素营养限制组的地上部和叶片氮浓度的实测值。RMSE和n-RMSE 的计算公式分别为:

      式中,Si和Ai分别表示模拟值和实测值,n 为样本量,M 为实测值的平均值。

    • 1.4.2.   烤烟干物质积累曲线
    • 烤烟干物质积累的逐日增加量曲线呈“S”形,符合Logistic增长曲线变化规律[33],其曲线方程为:

      式中:DM为移栽后天数;e为自然对数底;c、r、k为待定参数,其中 k 为地上部干物质积累量所能达到的上限值。本研究中对每次取样的生长不受氮素营养限制组的地上部干物质积累量的平均值为样本,进行方程拟合。

    • 1.4.3.   烤烟氮营养指数
    • 氮营养指数通常被用来定量植物体内的氮素状况[8],其计算公式为:

      式中:NNI(nitrogen nutrition index)为氮营养指数;Na为烤烟地上部氮浓度的实测值(%);Nc为根据临界氮浓度稀释曲线求得的临界氮浓度值(%)。NNI = 1,表征植株氮平衡;NNI > 1,表明植株氮富裕;NNI < 1 表征植株氮亏缺。

    • 1.5.   数据处理

    • 采用Microsoft Excel 2016软件进行数据整理和作图,采用SPSS19软件进行模型拟合,采用SAS9.2软件进行单因素方差分析和显著性检验。

    2.   结果

      2.1.   施氮量对烤烟地上部干物质积累的影响

    • 表2 可知,随着施氮量的增加,烤烟地上部生物量均显著增加。依据临界氮浓度稀释曲线理论,在地上部干物质积累量较小时,没有氮营养限制;而在地上部干物质积累量较大时,根据地上部生物量及其差别将氮肥用量处理分为两组。

      试验编号
      Experiment
      移栽后天数
      Days after transplanting
      氮肥施用量 N application rate (kg/hm2)
      04590135180300
      1290.81±0.04 b1.00±0.17 b1.15±0.07 b1.53±0.41 a1.63±0.39 a1.25±0.21 ab
      341.26±0.12 b1.71±0.36 b1.78±0.07 b1.92±0.32 b2.65±0.58 a3.30±0.18 a
      4717.65±0.97 a18.59±1.94 a16.66±1.97 a16.49±1.13 a16.94±3.79 a12.92±5.87 a
      5525.45±1.30 c30.68±2.28 ab28.40±1.26 b32.40±3.51 a30.10±0.29 ab32.40±2.33 a
      5935.28±9.13 b52.72±5.28 ab58.91±5.29 a58.81±6.32 a62.09±2.71 a57.66±3.92 a
      6569.76±17.90 c101.07±4.35 b105.71±8.77 b108.35±11.39 b131.91±13.66 a122.26±10.77 a
      90104.10±17.36 c118.73±18.88 c145.51±7.56 b161.34±3.04 ab154.56±23.21 b190.40±6.37 a
      24415.07±6.09 b15.87±2.96 b26.53±4.81 a19.53±1.35 ab15.34±3.80 b21.11±5.09 a
      5028.10±0.61 c55.06±5.04 b60.97±5.58 a58.71±4.52 ab59.54±10.98 a48.08±14.38 b
      6536.76±3.47 d121.88±8.86 b118.65±1.38 c121.36±12.79 b124.16±2.59 b137.08±22.81 a
      8665.11±14.45 c109.11±11.07 c156.92±12.37 b158.67±16.90 b184.6±30.62 a193.86±10.95 a
      9345.16±7.44 d128.89±8.19 c139.98±31.02 c182.91±7.54 b182.09±14.44 b225.90±34.20 a
      10047.72±5.19 c138.33±14.82 b137.50±13.05 b213.52±13.47 a206.68±16.66 a215.28±15.54 a
      10727.75±0.12 d100.69±20.39 c191.46±34.13 ab166.19±25.42 b224.43±35.63 a183.74±16.52 b
      注(Note):表中数字表示为平均值 ±SD,小写字母表示同一取样时期不同处理间 LSD 检验 5% 水平差异显著 The data in the table are Means ± SD, and the small letters indicate significant difference in LSD test at 5 % level between different treatments at different stages.

      Table 2.  Shoot biomass at different stages under different N fertilizer application rate

      试验1 中,移栽后29、34和47天时地上部积累量较小(小于0.5 t/hm2),所有处理均为不受氮限制组;移栽后55和59天,生长受氮限制组为 N·0、45、90 kg/hm2 3个处理,其余3 个处理为生长不受氮限制组;移栽后65天,生长受氮限制组为 N·0、45、90和135 kg/hm2 4 个处理,其余两个处理为生长不受氮限制组;移栽后90天,生长受氮限制组为 N 0、45、90、135和180 kg/hm2 5 个处理,N·300 kg/hm2为生长不受氮限制组。

      试验2 中,移栽后44天时地上部积累量较小(小于0.5 t/hm2),所有处理均为不受氮限制组;移栽后50天,生长受氮限制组为N·0、45 kg/hm2 2个处理,其余为4 个处理生长不受氮限制组;移栽后65、86和93天,生长受氮限制组为N·0、45、90、135和180 kg/hm2五个处理,N·300 kg/hm2为生长不受氮限制组;移栽后100天,生长受氮限制组为N·0、45、90 kg/hm2 3个处理,其余3个处理为生长不受氮限制组;移栽后107天,生长受氮限制组为N·0、45、90和135 kg/hm2 4个处理,其余2 个处理为生长不受氮限制组。

    • 2.2.   烤烟临界氮素稀释曲线的构建

    • 随着作物的生长,烤烟地上部干物质积累量不断增加,地上部氮浓度逐步下降。以试验一的地上部生物量和氮浓度、叶片生物量和氮浓度进行拟合,得出每个时期的地上部临界氮浓度和叶片临界氮浓度,随地上部和叶片生物量的增加,烤烟临界氮浓度呈逐渐下降的趋势(图1)。对临界氮浓度进行方程拟合建立烤烟临界氮浓度稀释曲线,结果显示拟合方程均达到了极显著水平,地上部临界氮浓度稀释曲线方程和叶片临界氮浓度稀释曲线方程决定系数分别为0.712和0.736。

      Figure 1.  Shoot and leaf N dilution curve of flue-cured tobacco

    • 2.3.   烤烟临界氮素稀释曲线的验证

    • 利用试验二的数据进行烤烟氮素浓度稀释曲线的验证,由图2可知,试验二氮限制组的数据均在临界氮素稀释曲线以下,而非氮限制组的数据均在临界氮素稀释曲线附近。不同时期曲线反映出的合理施氮量存在差异,根据表2的结果可知,试验二合理临界氮浓度对应的氮肥施用量在N 135~180 kg/hm2之间。根据模型拟合的临界氮浓度和实测临界氮浓度显著线性相关,模型拟合均方根误差RMSE分别为0.55和0.44,标准化均方根误差n-RMSE分别为25%和17%,稳定度较高,表明本研究建立的烤烟地上部临界氮浓度稀释曲线和叶片临界氮浓度稀释曲线模型可用于烟稻种植区烤烟临界氮素浓度计算。

      Figure 2.  Validation of critical N dilution curve with independent data

    • 2.4.   烤烟干物质积累曲线的构建

    • 本研究通过建立以烤烟干物质积累曲线方程,为进一步计算氮营养指数提供依据。以试验二的地上部和叶片生物量进行干物质积累曲线拟合,得到烤烟生长过程中地上部干物质积累量和叶片干物质积累量Logistic增长曲线。曲线方程分别为DWshoot=242/(1+279324e−0.195t)和DWleaves=86/(1+2983749e−0.257t),R2分别为0.902和0.939。

      Figure 3.  Curve for shoot and leaf dry matter accumulation of flue-cured tobacco at logarithm

    • 2.5.   利用烤烟氮营养指数进行氮营养诊断

    • 氮营养指数通常被用来定量评估植株体内的氮素状况。对2018年试验三移栽后83天时不同氮肥用量下的氮营养指数进行了评价。根据前期研究结果,该时期为利用无损诊断技术进行氮浓度诊断的最佳时期[34]。其中临界氮浓度根据烤烟干物质积累曲线和烤烟临界氮素稀释曲线计算获得,实际氮浓度利用本课题组前期建立的基于无人机可见光谱平台的烤烟叶片氮浓度无损估算模型计算[34]。该试验点的氮营养指数分布情况如图4所示,其中左下图为根据叶片临界氮稀释曲线模型计算获得的叶片氮营养指数分布图,右下图为根据地上部临界氮稀释曲线模型计算获得的地上部氮营养指数分布图;可知,随着施氮水平的提高,叶片氮营养指数逐步升高;在氮肥施用量为N·45 kg/hm2时,不同重复间叶片氮营养指数存在差异,而随着氮肥施用量增加,不同重复间氮营养指数差异减少,说明随着施氮水平提高,烟草叶片接近最适氮营养;在氮肥施用量达到N·135 kg/hm2,叶片氮营养指数可在1以上,处于氮盈余状态;而地上部氮营养指数在不同氮肥用量处理下均大于1,一直处于氮盈余状态。

      Figure 4.  Effects of N application rate on NNI of flue-cured tobacco

    3.   讨论

      3.1.   氮素稀释曲线模型的适用性

    • 根据Greenwood等[9]的研究结果,C3和C4植物分别具有各自通用的临界氮浓度模型(C3,Nc=5.7DM−0.5;C4,Nc=4.1DM−0.5),但在针对各种作物试剂应用时,发现通用模型并不能完全适用于所有的作物,不同作物氮素稀释曲线模型参数需要调整,因此国内外学者针对不同作物的临界氮素稀释曲线开展了大量研究。区域的差异会对临界氮稀释曲线的参数产生差异,国内构建的冬油菜[10]、小麦[16]、玉米[13]氮素稀释曲线与欧洲和美洲构建的氮素稀释曲线模型参数a值和b值均差异较大[35-37],因此,氮素稀释曲线模型的适用范围应将区域差异考虑在内。试验年度差异对模型构建稳定性的影响较小,小麦、冬油菜上的研究结果表明临界氮浓度稀释模型在不同年份之间保持稳定[10, 14]。因此,在本研究中用于模型验证的试验二,虽然与试验一在不同的年度进行,但模型验证中的差异主要来源与地区气候条件的变化。与黄淮烟区构建的烤烟氮稀释曲线模型相比[38],本研究构建模型参数的a值和b值均显著小于黄淮烟区。模型参数中的a值表示地上干物质为 1 t/hm2时临界氮浓度值,东南烟稻种植区的a值为3.37,而黄淮烟区的a值为4.81,这意味着形成相同生物量黄淮烟区往往需要较高的临界氮浓度,而在东南烟稻种植区临界氮浓度则相对较低,这可能与两个区域的烤烟生长特性和土壤供氮能力密切相关。虽然东南烟稻种植区土壤有机质含量和碱解氮含量较高,但东南烟稻种植区烤烟生育前期温度较低,土壤供氮能力(土壤Nmin含量)较低,烤烟生长较缓慢[5];而黄淮烟区生长季温度高,烤烟生长快速,需要提供充足且持续的氮素供应[39],因此可能导致形成相同的生物量黄淮烟区烤烟临界浓度较高。模型参数中的b值表示相对干物质积累速率和相对氮含量积累速率的比值,一般认为不同作物的b值均在0.5左右,区域b值的差异显著小于a值,但本研究中东南烟稻种植区的b值(0.428)与黄淮烟区的b值(0.6)存在较大的差异,本研究中的b值更接近于通用模型提供的b值[9]和其它作物在不同区域构建的b值,因此,不同烟区间b值的差异原因需进一步深入探索。两个临界氮稀释曲线模型相比,利用叶片临界氮稀释曲线可很好的将不同氮肥用量实验叶片氮营养指数区分开,模型的重现性和指导意义更强,而地上部临界氮稀释曲线重现性较差,不适用于进行氮营养诊断。同时,根据两个模型的比较结果分析,如何实现从茎向叶片氮素的快速转移,是提高烟稻种植区氮肥利用率的核心。

    • 3.2.   氮营养指数的应用

    • 基于临界氮浓度稀释模型推导的氮营养指数(NNI)不仅可以实时诊断作物不同生长阶段氮素营养状况,还可以量化作物氮胁迫强度。本文通过计算获得的与试验一相近地块不同试验年度下不同氮处理的NNI值发现,除了施肥量影响作物的养分营养状况外,干物质积累量及生育期对氮营养指数同样存在显著影响,在本研究中基于地上部氮浓度稀释模型计算的氮营养指数普遍大于1,而基于叶片氮浓度稀释模型计算的氮营养指数在不同处理间存在差异,这可能与本研究中应用的干物质积累曲线的局限性有关,本研究构建的干物质积累曲线为基于生长时期的Logistic方程,而地上部生物量的变化特征与积温密切相关[40],积温条件影响了烤烟生育期进而影响干物质积累,其中地上部干物质积累受积温影响更大,而叶片干物质积累受积温影响较小[41]。年度间移栽时间和温度条件的差异导致了应用氮营养指数的差异;而叶片生物量的变化特征与移栽时间密切相关[42],应用叶片氮浓度稀释模型与实际可能更相符。因此进一步构建基于积温的干物质积累曲线对应用氮浓度稀释模型具有重要意义。根据叶片临界氮稀释曲线模型应用的结果,东南烟稻轮作区合理氮肥用量应为135 kg N·hm-2,这与通过其他方法确定的东南烟稻轮作区烤烟合理氮肥用量基本一致[43]

    4.   结论
    • 在东南烟稻种植区,叶片临界氮浓度稀释曲线较地上部临界氮浓度稀释曲线更适用于氮营养诊断,东南烟稻种植区烤烟叶片的临界氮素稀释曲线为Nc=3.1941×DMleaves−0.411,该方程决定系数显著、模型稳定性高,可以预测烤烟的临界叶片氮浓度;结合叶片干物质积累曲线和叶片氮浓度无损检测技术,计算叶片氮营养指数,可实现评价该区域烤烟氮素营养状况。

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