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Volume 36 Issue 3
Jun.  2023
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The Allocation and Cycling Characteristics of Main Nutrients for Caragana intermedia With Different Stand Age on Alpine Sandy Land

  • Corresponding author: JIA Zhi-qing, jiazq369@caf.ac.cn
  • Received Date: 2022-11-02
    Accepted Date: 2022-12-22
  • Objective Understanding the allocation and cycling characteristics of the main nutrients in the process of artificial vegetation restoration on sandy land will help us fully understand the strategies of plant adaptation to the desert ecosystems, and provide theory for the vegetation management. Method This study was conducted in the shrub plantations of Caragana intermedia with different stand age of 6-, 9-, 11-, 17- and 31-year-old on alpine sandy land. The whole plant of average shrubs was completely harvested for analysing the main nutrients N, P and K concentration, accumulation, allocation and cycling characteristics. Result (1) In components, leaves and stem bark with the highest nutrient content, and stem wood with the lowest nutrient content. As the plantation age increased, N content in three root-diameter (coarse root: diameter > 5 mm, medium root: 2 mm < diameter ≤ 5 mm, fine root: diameter ≤ 2 mm) and P content in leaves increased significantly, whereas P and K contents in fine root, K contents in branches and medium root decreased significantly. The N contents of three root-diameters had a significant negative correlation with the P and K contents of fine root, and the N contents of medium and fine root had a significant negative correlation with the K contents of branches and medium root. (2) The nutrient accumulation of root was higher than aboveground components. The percentage of the nutrient accumulation of the root to the aboveground components firstly increased and then decreased as the plantation age increased. It peaked in 17-year-old plantation, and the percentage of N, P and K were 70%, 66% and 63%, respectively. (3) As the plantation age increased, the utilization coefficient of the nutrients decreased, while the recycling period and cycling coefficient increased. Utilization coefficient and cycling coefficient of K were higher and recycling period was shorter than N and P in all plantations. Conclusion With the development of C. intermediate plantation on alpine sandy land, more nutrients were allocated to the root system to adapt to the harsh environment. The nitrogen fixation process of C. intermediate will consume its own K and P, of which K with fast circulation rate and high mobility. Therefore, we suggested that K and P fertilizer should be added in the management and protection of C. intermedia shrub plantation.
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The Allocation and Cycling Characteristics of Main Nutrients for Caragana intermedia With Different Stand Age on Alpine Sandy Land

    Corresponding author: JIA Zhi-qing, jiazq369@caf.ac.cn
  • 1. Institute of Ecological Conservation and Restoration, Chinese Academy of Forestry, Beijing 100091,China
  • 2. Qinghai Gonghe Desert Ecosystem Research Station, Qinghai Gonghe 813005,Qinghai,China;
  • 3. Research Institute of Forestry, Chinese Academy of Forestry, Beijing 100091, China

Abstract:  Objective Understanding the allocation and cycling characteristics of the main nutrients in the process of artificial vegetation restoration on sandy land will help us fully understand the strategies of plant adaptation to the desert ecosystems, and provide theory for the vegetation management. Method This study was conducted in the shrub plantations of Caragana intermedia with different stand age of 6-, 9-, 11-, 17- and 31-year-old on alpine sandy land. The whole plant of average shrubs was completely harvested for analysing the main nutrients N, P and K concentration, accumulation, allocation and cycling characteristics. Result (1) In components, leaves and stem bark with the highest nutrient content, and stem wood with the lowest nutrient content. As the plantation age increased, N content in three root-diameter (coarse root: diameter > 5 mm, medium root: 2 mm < diameter ≤ 5 mm, fine root: diameter ≤ 2 mm) and P content in leaves increased significantly, whereas P and K contents in fine root, K contents in branches and medium root decreased significantly. The N contents of three root-diameters had a significant negative correlation with the P and K contents of fine root, and the N contents of medium and fine root had a significant negative correlation with the K contents of branches and medium root. (2) The nutrient accumulation of root was higher than aboveground components. The percentage of the nutrient accumulation of the root to the aboveground components firstly increased and then decreased as the plantation age increased. It peaked in 17-year-old plantation, and the percentage of N, P and K were 70%, 66% and 63%, respectively. (3) As the plantation age increased, the utilization coefficient of the nutrients decreased, while the recycling period and cycling coefficient increased. Utilization coefficient and cycling coefficient of K were higher and recycling period was shorter than N and P in all plantations. Conclusion With the development of C. intermediate plantation on alpine sandy land, more nutrients were allocated to the root system to adapt to the harsh environment. The nitrogen fixation process of C. intermediate will consume its own K and P, of which K with fast circulation rate and high mobility. Therefore, we suggested that K and P fertilizer should be added in the management and protection of C. intermedia shrub plantation.

  • 林分养分的积累与分布是研究森林生态系统物流和能流的基础,养分循环是系统功能的主要表现之一,直接影响着生产力的高低和生态系统的稳定与持续,是生态系统的主要功能之一[1]。植物体养分元素的积累取决于生物量的积累以及各器官中养分元素的含量[2]。植物生物量分配是其更好的适应环境变化的策略,植物能够很好地调节各组分的相对生长,其各组分生物量的比例会随生境和个体大小等变化而变化[3]。当某些环境因子成为植物生长发育的限制性因素时,植物为了保证其正常生长,就会不断的调整营养物质的分配以及改变根冠比等指标来适应环境[4]。有关生物量在植物体不同组分分配的研究受到了生态学者们的广泛关注[5-6]。有些学者用异速生长理论预测了生物量在植物组分间的分配,认为叶、茎、根是同速生长[7]。还有一些学者发现,生物量的分配与营养元素的分配存在相关性,但大部分研究是关于乔木植物和草本植物的[8-10]。在干旱半干旱地区的沙化土地上,灌木是人工植被恢复的主要植被类型,锦鸡儿属(Caragana Fabr.)灌木是植被恢复采用的重要树种。有学者对锦鸡儿属植物的生物量分配及生物量建模[11-12]、生长指标及化学计量指标与基质关系[13]、干旱胁迫对幼苗C、N、P分配影响机理[14]等进行了研究,但关于锦鸡儿属植物在植被恢复过程中主要养分元素含量、积累、分配和循环方面的研究还鲜有报道。因此,对干旱半干旱沙化生态系统锦鸡儿属灌木主要养分元素含量、积累、分配和循环特征进行研究,了解其适应环境的策略,为脆弱沙化生态系统灌木人工林的可持续经营管理提供依据,还可为合理计算灌木人工林养分储量及评估其生长潜力提供技术支撑。

    共和盆地是青藏高原生态屏障重要构成部分,是青海省荒漠化与沙化土地的典型代表,与其他沙漠化地区相比,具有海拔高、气温低、无霜期短,自然环境条件比较严酷,生态系统脆弱的特性。中间锦鸡儿(Caragana intermedia Kuang et H. C. Fu)为多年生豆科锦鸡儿属灌木,是青海共和高寒沙地植被恢复的主要物种。目前,中间锦鸡儿灌木人工林在共和盆地沙地大面积分布。关于高寒沙地中间锦鸡儿的研究多集中在水分利用策略[15-16]、土壤改良效应[17-18]等方面。本研究拟通过对高寒沙地不同林龄中间锦鸡儿灌木人工林主要养分元素分配及循环特征进行系统研究,阐明各组分主要养分元素含量、积累、分配和循环特征随林龄的变化趋势,进而揭示中间锦鸡儿人工灌木林在恢复过程中的资源分配策略及其生态功能,以期为高寒沙地人工植被的经营管理及青藏高原生态屏障区生态保护和修复工程实施提供理论依据和决策参考。

    • 本研究在国家林业局青海共和荒漠生态系统定位观测站进行,该站位于青藏高原东北部的青海省海南州共和县沙珠玉乡(100°16′ E,36°16′ N),海拔2 871 m。气候类型属于高寒干旱荒漠和半干旱草原过渡区域。年均气温2.4 ℃,年均降水量246.3 mm,年均潜在蒸发量1 716.7 mm,无霜期年平均91 d。研究区内以人工固沙植被为主:主要物种有乔木青杨(Populus cathayana Rehd.)、小叶杨(Populus simonii Carr.),灌木主要为中间锦鸡儿、柠条锦鸡儿(Caragana korshinskii Kom.)、沙棘(Hippophae rhamnoides Linn.)、乌柳(Salix cheilophila Schneid.)和沙柳(Salix psammophila C. Wang et Chang Y. Yang)等。

    • 2017年7月中旬,在研究区选择2011、2008、2006、2000和1986年种植的中间锦鸡儿人工灌木林(即6、9、11、17和31年生)为研究对象,人工林均为线性条播,行间距为2 m。在每个人工林内随机设置3个20 m × 20 m的样方为3个重复。在每个样方内随机选取1 m长的灌丛为标准株,以1 m长的灌丛为中心向两侧分别延伸1 m(到2条中间锦鸡儿灌丛带的中间位置)为取样面积(1 m × 2 m)。取样时,采用直接收割法,将选出的标准灌丛从基部剪下,分为叶、枝、干、皮。然后按照取样面积挖取0~80 cm深度的根系,即中间锦鸡儿根系主要分布区[19],并将根系分为粗根(直径 > 5 mm)、中根(2 mm < 直径 ≤ 5 mm)和细根(直径 ≤ 2 mm)3个径级。将样品带回实验室,65 ℃恒温烘干至质量恒定,测干质量。

      取样完成后,在每个样方内随机设置6个相同的尼龙网枯落物收集框(1 m × 1 m),9、10月份收集枯落物,并将每个收集框的枯落物分别装袋。样品带回实验室置于65 ℃干燥箱中烘干至恒质量,称量干质量。

    • 将所有样品进行粉碎,并过0.25 mm筛,测定N、P和K含量。采用元素分析仪法测定N含量(元素分析仪:vario EL III, CHNOS Elemental Analyzer,Elementar Analysensysteme GmbH,Germany),采用HNO3消解-ICP法测定P和K含量(电感耦合等离子体发射光谱仪iCAP 6300 ICP-OES Spectrpmeter,Thermo Scientific,USA)。样品测试地点为中国科学院植物研究所植被与环境变化国家重点实验室。

    • 采用养分元素积累量、年存留量、年吸收量和年归还量计算养分利用系数、循环系数和周转期等参数来分析养分循环特征[8]。具体参数计算如下:

      贮存量:各组分养分元素积累量之和。

      存留量:林分年增长生物量与其养分含量的乘积。

      归还量:林分枯落物量与其养分含量乘积。

      吸收量 = 存留量 + 归还量

      利用系数 = 吸收量/贮存量

      循环系数 = 归还量/吸收量

      周转期 = 贮存量/归还量

    • 采用单因素方差分析和Duncan法分析不同林龄和不同组分养分含量之间的差异性,不同林龄枯落物量、养分含量及养分积累量之间的差异性。采用相关分析和冗余分析(RDA)法分析各组分N含量与P、K含量的相关分析。所有数据使用SPSS 19.0和CANOCO 4.5进行数据处理。

    2.   结果与分析
    • 中间锦鸡儿人工灌木林各组分生物量随着林龄的增加显著增加(p<0.001)(表1),各林龄根系生物量占比高于地上组分,为54.60%~71.57%,17年生人工灌木林根系生物量占比最高(71.57%)。各组分生物量中,粗根的生物量占比最高(42.12%~59.19%),其次为枝(18.43%~32.18%),叶(5.51%~14.25%)和中根(7.26%~12.52%)、细根(2.74%~5.59%)和干(2.34%~5.00%)生物量占比相对较低,皮的生物量占比最低(1.19%~2.68%),叶、中根和细根生物量占比随着林龄的增加而降低。

      林龄
      Age/a
      林分生物量 Stand biomass /(t·hm−2)
      叶 Leaves枝 Branches皮 Stem bark干 Stem wood粗根 Coarse root中根 Medium root细根 Fine root合计 Total
      61.07 ± 0.02 Ac1.40 ± 0.10 Ad0.13 ± 0.03 Aa0.24 ± 0.07 Aa3.32 ± 0.15 Ae0.94 ± 0.05 Ac0.42 ± 0.01 Ab7.51 ± 0.41 A
      91.24 ± 0.02 Bc2.42 ± 0.44 Ad0.19 ± 0.02 Aa0.41 ± 0.05 ABab4.22 ± 0.06 Ae1.03 ± 0.05 Ac0.51 ± 0.03 ABb10.02 ± 0.66 AB
      111.30 ± 0.02 Bd3.23 ± 0.05 Ae0.25 ± 0.02 Aa0.51 ± 0.02 Bb6.20 ± 0.10 Bf1.16 ± 0.05 Ac0.58 ± 0.02 Bb13.23 ± 0.26 B
      171.41 ± 0.02 Cc4.02 ± 0.24 Ad0.26 ± 0.03 Aa0.51 ± 0.16 Bab12.91 ± 0.58 Ce1.81 ± 0.14 Bc0.89 ± 0.08 Cb21.81 ± 1.08 C
      311.95 ± 0.11 Da11.39 ± 3.09 Bb0.95 ± 0.28 Ba1.77 ± 0.23 Ca15.79 ± 2.10 Dc2.57 ± 0.47 Ca0.97 ± 0.16 Ca35.40 ± 5.55 D
        注:不同大写字母表示同一组分不同林龄之间差异显著,不同小写字母表示同一林龄不同组分之间差异显著(p < 0.05)
        Notes: Values followed by different uppercase letters indicate a significant difference among the stand age; different lowercase letters indicate a significant difference among the components according to Duncan’s multiple range test (p < 0.05)

      Table 1.  Biomass of C. intermedia plantations of different stand age (means ± SD, n = 3)

      表2表明:各组分N含量为13.03~39.59 g·kg−1,P含量为0.86~2.25 g·kg−1,K含量为3.48~21.28 g·kg−1,N和K含量整体表现为叶 > 皮 > 细根 > 中根 > 枝 > 粗根 > 干的趋势,P含量为叶 > 皮 > 枝 > 粗根 > 中根 > 细根 > 干的趋势。

      林龄 Age/a组分 Components养分含量 Nutrient concentration /(g·kg−1)
      NPK
      6 叶 Leaves 37.70 ± 1.31 Af 1.74 ± 0.16 Ad 19.20 ± 0.22 Ae
      枝 Branches 24.67 ± 1.19 Ac 1.38 ± 0.09 Bc 8.67 ± 0.08 Cc
      皮 Stem bark 32.24 ± 0.74 Ae 1.43 ± 0.08 Ac 10.18 ± 0.30 Ad
      干 Stem wood 13.08 ± 1.93 Aa 0.91 ± 0.16 Aa 4.35 ± 0.42 Aa
      粗根 Coarse root 22.70 ± 0.78 ABb 1.18 ± 0.01 Ab 7.55 ± 0.09 CDb
      中根 Medium root 25.05 ± 0.59 Ac 1.17 ± 0.04 Bb 8.93 ± 0.03 Bc
      细根 Fine root 27.22 ± 0.65 Ad 1.09 ± 0.03 Db 9.65 ± 0.04 Dd
      9 叶 Leaves 37.48 ± 0.94 Ae 1.91 ± 0.18 ABc 20.26 ± 0.44 Af
      枝 Branches 22.64 ± 1.18 Ab 1.11 ± 0.03 Aab 8.41 ± 0.06 BCc
      皮 Stem bark 29.80 ± 1.35 Ad 1.30 ± 0.11 Ab 10.09 ± 0.23 Ae
      干 Stem wood 14.28 ± 0.38 Aa 0.99 ± 0.16 Aa 4.50 ± 0.27 Aa
      粗根 Coarse root 22.35 ± 0.64 Ab 1.11 ± 0.09 Aab 7.28 ± 0.09 Bb
      中根 Medium root 26.25 ± 0.79 Bc 1.10 ± 0.04 Bab 8.87 ± 0.03 Bcd
      细根 Fine root 28.87 ± 0.51 Bd 1.02 ± 0.02 Ca 9.49 ± 0.05 CDde
      11 叶 Leaves 38.48 ± 1.72 Ae 2.25 ± 0.16 Cd 19.65 ± 0.43 Ae
      枝 Branches 24.97 ± 0.46 Ab 1.38 ± 0.09 Bc 7.90 ± 0.13 Ac
      皮 Stem bark 29.86 ± 2.23 Ad 1.29 ± 0.06 Ac 9.70 ± 0.08 Ad
      干 Stem wood 13.54 ± 1.43 Aa 0.90 ± 0.08 Aa 3.61 ± 0.56 Aa
      粗根 Coarse root 23.95 ± 0.23 BCb 1.10 ± 0.01 Ab 6.96 ± 0.02 Ab
      中根 Medium root 27.58 ± 0.31 Cc 0.95 ± 0.07 Aa 8.99 ± 0.06 Bd
      细根 Fine root 31.40 ± 0.52 Cd 0.92 ± 0.03 Ba 9.06 ± 0.10 Bd
      17 叶 Leaves 39.59 ± 1.56 Ae 2.19 ± 0.07 BCe 19.22 ± 0.61 Ae
      枝 Branches 25.14 ± 1.51 Ab 1.35 ± 0.09 Bd 8.23 ± 0.06 BCc
      皮 Stem bark 30.05 ± 1.68 Ac 1.37 ± 0.11 Ad 9.91 ± 0.32 Ad
      干 Stem wood 13.96 ± 1.30 Aa 0.91 ± 0.11 Aa 4.23 ± 0.26 Aa
      粗根 Coarse root 24.39 ± 0.82 Cb 1.15 ± 0.04 Ac 7.36 ± 0.03 BCb
      中根 Medium root 29.04 ± 0.24 Dc 1.14 ± 0.01 Bbc 8.47 ± 0.05 Ac
      细根 Fine root 32.99 ± 1.08 Dd 1.01 ± 0.04 Cab 9.39 ± 0.07 Cd
      31 叶 Leaves 38.06 ± 0.80 Ae 2.16 ± 0.15 BCe 21.28 ± 1.06 Ad
      枝 Branches 25.36 ± 1.09 Ab 1.27 ± 0.09 Bc 7.72 ± 0.07 Ab
      皮 Stem bark 31.44 ± 0.81 Ac 1.40 ± 0.08 Ad 9.36 ± 0.15 Ac
      干 Stem wood 13.03 ± 1.56 Aa 0.93 ± 0.03 Aa 3.48 ± 0.77 Aa
      粗根 Coarse root 25.33 ± 1.30 Cb 1.11 ± 0.01 Ab 7.61 ± 0.10 Db
      中根 Medium root 30.21 ± 0.58 Ec 1.12 ± 0.02 Bb 8.63 ± 0.10 Abc
      细根 Fine root 33.43 ± 0.65 Dd 0.86 ± 0.04 Aa 8.72 ± 0.06 Abc
        注:不同大写字母表示同一组分不同林龄之间的养分含量差异显著,不同小写字母表示同一林龄不同组分之间的养分含量差异显著(p < 0.05)
        Notes:Values followed by different uppercase letters indicate a significant difference among the stand ages; different lowercase letters indicate a significant difference among the components A Ccording to Duncan’s multiple range test (p < 0.05)

      Table 2.  Nutrient concentrations of C. intermedia plantations of different stand age (means ± SD, n = 3)

      各径级根系N含量和叶片P含量随林龄增加显著增加(p < 0.05);细根P含量以及枝、细根和中根的K含量随林龄增加显著降低(p < 0.05)。

      从各组分养分含量随林龄增加的RDA排序图也可以看出:粗根、中根、细根的N含量及叶的P含量与林龄呈明显的正相关,细根、中根和枝的K含量及细根的P含量与林龄呈明显的负相关(图1)。各组分N含量与P和K含量相关分析和RDA分析结果(表3图1)表明:粗根、中根和细根的N含量与叶的P含量呈极显著正相关(p < 0.01),与细根的P和K含量呈极显著负相关(p < 0.01);中根和细根的N含量与枝和中根的K含量呈极显著负相关(p < 0.01);粗根N含量与枝的K含量呈显著负相关(p < 0.05);中根的N含量与皮的K含量呈显著负相关(p < 0.05)。

      Figure 1.  Ordination diagram of RDA on the nutrient concentrations of different components with stand age

      叶N
      Leaves N
      枝N
      Branches N
      皮N
      Stem bark N
      干N
      Stem wood N
      粗根N
      Coarse root N
      中根N
      Medium root N
      细根N
      Fine root N
      叶P Leaves P 0.442 0.301 −0.486 0.043 0.651** 0.756** 0.680**
      枝P Branches P 0.350 0.691** 0.094 0.008 0.177 0.033 0.119
      皮P Stem bark P 0.134 0.291 0.515* −0.544* −0.013 −0.043 0.029
      干P Stem wood P 0.271 −0.240 −0.266 0.184 0.062 −0.028 −0.078
      粗根P Coarse root P −0.058 0.397 0.166 −0.186 −0.247 −0.312 −0.201
      中根P Medium root P 0.022 −0.026 0.425 0.173 −0.014 −0.063 −0.202
      细根P Fine root P −0.079 −0.181 0.203 0.109 −0.724** −0.761** −0.664**
      叶K Leaves K 0.058 −0.160 −0.018 −0.139 0.433 0.397 0.236
      枝K Branches K −0.062 −0.440 −0.032 0.004 −0.620* −0.767** −0.806**
      皮K Stem bark K −0.375 −0.211 −0.278 −0.111 −0.424 −0.580* −0.502
      干K Stem wood K −0.112 −0.192 −0.136 0.385 −0.217 −0.354 −0.381
      粗根K Coarse root K 0.019 −0.062 0.392 −0.118 −0.268 0.166 −0.097
      中根K Medium root K −0.254 −0.281 0.048 −0.010 −0.429 −0.653** −0.637**
      细根K Fine root K −0.246 −0.378 0.139 0.140 −0.715** −0.759** −0.729**
        注:*代表显著水平p < 0.05;**代表显著水平p < 0.01
        Notes: *. Correlation is significant at the 0.05 level; **Correlation is significant at the 0.01 level

      Table 3.  Correlation analysis between N concentrations and P, K concentrations of different components

    • 根据中间锦鸡儿人工灌木林各组分生物量和养分元素含量可以计算出养分积累量及其分配。由表4可知:各组分养分积累量随着林龄的增加而增加。各组分N积累量为3.09~399.99 kg·hm−2,P积累量为0.19~17.53 kg·hm−2,K积累量为1.03~120.24 kg·hm−2,整体为粗根 > 枝 > 叶 > 中根 > 细根 > 皮 > 干。

      (kg·hm−2)
      养分
      Nutrients
      林龄
      Age /a

      Leaves

      Branches

      Stem bark

      Stem wood
      粗根
      Coarse root
      中根
      Medium root
      细根
      Fine root
      合计
      Total
      N
      640.2234.464.193.0975.4523.5411.43192.38
      946.4854.875.665.8194.4127.0314.72248.98
      1150.1580.657.376.86148.5132.0818.11343.73
      1755.69100.997.817.16314.9152.6629.47568.70
      3174.35288.7329.7623.07399.9977.7532.54926.19
      P61.861.930.190.223.911.100.469.66
      92.372.690.250.404.671.140.5212.04
      112.934.470.320.466.801.100.5316.60
      173.085.420.360.4714.892.060.9027.18
      314.2214.421.321.6517.532.870.8442.86
      K
      620.4812.101.321.0325.098.394.0572.48
      925.1220.391.921.8330.759.144.8493.98
      1125.6125.512.391.8343.1310.465.22114.15
      1727.0433.072.582.1795.0015.358.39183.60
      3141.5787.918.866.17120.2422.208.49295.43

      Table 4.  Nutrient accumulation of C. intermedia plantations of different stand age

      叶养分积累量占总养分积累量的百分比随林龄的增加而降低(图2)。根系养分积累量占总养分积累量的百分比随林龄的增加先增加后降低,在17年生人工林达到峰值,N、P和K积累量百分比分别为70%、66%和63%。

      Figure 2.  Distribution of nutrients accumulation in different components of C. intermedia plantations of different stand age

    • 不同林龄人工灌木林枯落物量为1.06~2.05 t·hm−2,随着林龄的增加显著增加 (p < 0.05)。枯落物N含量为20.57~22.72 g·kg−1,其中,31年生人工灌木林显著高于其他林龄;P含量为1.01~1.20 g·kg−1,K含量为13.41~15.80 g·kg−1。枯落物N积累量为22.53~46.56 kg·hm−2,P积累量为1.25~2.46 kg·hm−2,K积累量为16.79~27.48 kg·hm−2,3种养分元素积累量随着林龄的增加显著增加 (p < 0.05)(表5)。

      林龄
      Age/a
      枯落物量
      Litter accumulation /(t·hm−2)
      养分含量 Nutrient concentration/(g·kg−1)养分积累量 Nutrients accumulation/(kg·hm−2)
      NPKNPK
      61.06 ± 0.10 a21.22 ± 0.42 a1.18 ± 0.04 c15.80 ± 0.24 e22.53 ± 1.91 a1.26 ± 1.26 a16.79 ± 1.67 a
      91.24 ± 0.06 b21.13 ± 0.56 a1.01 ± 0.03 a15.36 ± 0.20 d26.18 ± 1.66 b1.25 ± 1.25 a19.04 ± 0.88 b
      111.34 ± 0.02 b21.14 ± 0.19 a1.01 ± 0.01 a14.13 ± 0.17 b28.32 ± 0.51 bc1.35 ± 1.35 a18.93 ± 0.20 b
      171.48 ± 0.04 c20.57 ± 0.10 a1.08 ± 0.01 b15.01 ± 0.04 c30.41 ± 0.77 c1.59 ± 1.59 b22.20 ± 0.68 c
      312.05 ± 0.05 d22.72 ± 0.19 b1.20 ± 0.02 c13.41 ± 0.09 a46.56 ± 0.77 d2.46 ± 2.46 c27.48 ± 0.53 d
        注:不同小写字母表示同一林龄不同组分之间的养分含量差异显著(p < 0.05)
        Notes: Values followed by different lowercase letters indicate a significant difference among the components according to Duncan’s multiple range test (p < 0.05)

      Table 5.  Litter accumulation, nutrient concentration and accumulation of C. intermedia plantations of different stand age (means ± SD, n=3)

    • 表6可以看出:3种养分元素的贮存量和归还量随着林龄增加而增加,并表现出N > K > P的趋势。N元素的利用系数为0.08~0.32,循环系数为0.37~0.60,周转期为8.54~19.89 a;P元素的利用系数为0.09~0.33,循环系数为0.39~0.63,周转期为7.67~17.41 a;K元素的利用系数为0.13~0.40,循环系数为0.58~0.74,周转期为在4.32~10.75 a。3种养分元素的利用系数随着林龄增加而降低,循环系数和周转期随着林龄增加而增加。各林龄中间锦鸡儿K元素的利用系数和循环系数明显大于N元素和P元素,周转期明显小于N元素和P元素。

      养分 Nutrients项目 Ttem林龄 Age/a
      69111731
      N 贮存量 Storage/(kg·hm−2) 192.38 248.98 343.73 568.70 926.19
      存留量 Retention/(kg·hm−2·a−1) 38.48 31.12 34.37 35.54 30.87
      归还量 Return/(kg·hm−2·a−1) 22.53 26.18 28.32 30.41 46.56
      吸收量 Absorption/(kg·hm−2·a−1) 61.01 57.31 62.69 65.95 77.43
      利用系数 Utilization coefficient 0.32 0.23 0.18 0.12 0.08
      循环系数 Cycling coefficient 0.37 0.46 0.45 0.46 0.60
      周转期 Recycling period/a 8.54 9.51 12.14 18.70 19.89
      P 贮存量 Storage/(kg·hm−2) 9.66 12.04 16.60 27.18 42.86
      存留量 Retention/(kg·hm−2·a−1) 1.93 1.51 1.66 1.70 1.43
      归还量 Return/(kg·hm−2·a−1) 1.26 1.25 1.35 1.59 2.46
      吸收量 Absorption/(kg·hm−2·a−1) 3.19 2.75 3.01 3.29 3.89
      利用系数 Utilization coefficient 0.33 0.23 0.18 0.12 0.09
      循环系数 Cycling coefficient 0.39 0.45 0.45 0.48 0.63
      周转期 Recycling period/a 7.67 9.65 12.28 17.09 17.41
      K 贮存量 Storage/(kg·hm−2) 72.48 93.98 114.15 183.60 295.43
      存留量 Retention/(kg·hm−2·a−1) 12.08 10.44 10.38 10.80 9.53
      归还量 Return/(kg·hm−2·a−1) 16.79 19.04 18.93 22.20 27.48
      吸收量 Absorption/(kg·hm−2·a−1) 28.87 29.48 29.31 33.00 37.01
      利用系数 Utilization coefficient 0.40 0.31 0.26 0.18 0.13
      循环系数 Cycling coefficient 0.58 0.65 0.65 0.67 0.74
      周转期 Recycling period/a 4.32 4.94 6.03 8.27 10.75

      Table 6.  Cycling characteristics of nutrients in C. intermedia plantations of different stand age

    3.   讨论
    • 叶是植物代谢活动最旺盛的同化器官,是进行光合作用、合成有机物的重要场所,需要更多的养分来维持其生理功能[6]。干是养分和水分的运输通道,生理生化活动较弱,养分被消耗和转移的较多,导致其养分含量远低于叶[6]在本研究中,不同林龄中间锦鸡儿人工灌木林各组分中,叶的养分含量最高,干的养分含量最少,这与其他学者对沙枣(Elaeagnus angustifolia Linn.)、白榆(Ulmus pumila L.)和新疆杨(Populus alba Linn. Var. pyramdalis Bunge)等农田防护林各组分养分分布特征的研究结果一致[20],这说明在干旱贫瘠的沙地生态系统中,植物在有限的养分供应条件下维持较高的叶片养分含量,进而完成其正常的生活史[21]

    • 本研究中,各径级根系的N含量随着林龄的增加显著增加,细根的P、K含量以及枝和中根的K含量显著降低。中间锦鸡儿为豆科灌木,根部大量的根瘤菌具有高固氮活性,可以固定空气中的游离态氮[22],而豆科植物形成或维持共生系统需要消耗大量的P,根瘤菌固氮酶在固氮过程中也需要大量的P [23],K也参与了调节寄主细胞膜渗透及一系列同化过程,促进植物生长,提高光合效率,保证豆科植物的结瘤和固氮酶活性[24]。本研究对各组分N含量和P、K含量进行了相关分析和RDA分析,结果也表明各径级根系N含量与细根的P和K含量及枝和中根的K含量呈显著负相关。这说明中间锦鸡儿根系的固氮过程会消耗自身的K和P,且主要消耗细根的K和P、其次是枝和中根的K。有学者对晋西北小叶锦鸡儿(Caragana microphylla Lam.)灌丛营养特征的研究也得出相似的结果,即根系的N含量随着林龄的增加显著增加,而P和K含量随着林龄的增加显著降低[25]。Inagaki等[26]的研究也表明,豆科植物比非豆科植物需要更多的P,给金合欢(Acacia mangium Willd.)人工林施用P肥后,细根产量和根瘤都增加了。

      再吸收是植物用来保存养分的一种策略。在本研究中,枯落物的主要成分是叶,但枯落物的养分含量明显低于叶的养分含量,这是因为衰老组织的养分元素向存活组织运输迁移导致的[8]。说明中间锦鸡儿具有养分元素转移和再吸收的机能,在贫瘠脆弱的沙化生态系统中具有很好的适应性和养分元素利用的竞争策略,降低了对环境供给养分的依赖。有学者对山西太岳山不同林龄华北落叶松(Larix principis-ruppechtii Mayr.)人工林的研究也得到相似的结果,人工林凋落物C、N、P、K 4种养分元素含量低于叶[27]。Peri等[28]Nothofagus antarctica (G.Forster) Oersted的研究结果也表明,衰老前叶片的养分吸收可达到生长期最大浓度的50%。

    • 本研究6年生中间锦鸡儿生物量和养分积累量均高于黄土高原地区相似林龄(5年生)柠条人工林[29],并且随着林龄的增长,各组分养分积累量与生物量均呈现增加的趋势。中间锦鸡儿人工灌木林各组分中,粗根的生物量及养分积累量最高,干和皮的生物量及养分积累量较低,地下组分的养分积累量高于地上组分(9年生K除外)。根系养分积累量占总养分积累量的百分比随林龄的增加先增加后降低,在17年生人工林达到峰值,N、P和K积累量百分比分别为70%、66%和63%,说明这个阶段是中间锦鸡儿人工灌木林根系快速发展阶段。其他学者对柠条锦鸡儿人工林生长过程中生物量积累的研究也表明,在成熟林和过熟林中,根系生物量占总生物量的比值较高,达到58.7%[5]

    • 养分利用系数的大小反应植物对该养分贮存能力的大小[30]。本研究中,3种养分元素的利用系数随着林龄的增加而降低,这说明随着林龄的增加,中间锦鸡儿对养分的需求变小,贮存能力也有所降低。有学者对不同林龄马尾松(Pinus massoniana Lamb.)人工林的研究发现,随着林龄的增加,马尾松自身养分利用效率降低[8]。还有学者对黄土高原不同乔灌树种人工纯林生态系统的养分循环研究表明,林龄较大的旱柳(Salix matsudana Koidz.)对养分的需求较小,贮存能力小[29]。中间锦鸡儿人工灌木林3种养分的循环系数随着林龄的增加而增加,31年生人工林的养分循环系数最高,具有低存留、高归还的特点。各林龄中间锦鸡儿K元素的利用系数和循环系数明显大于N元素和P元素,周转期明显小于N元素和P元素,说明K元素的循环速率较快,在系统中存留量的比例小,流动性大。有些学者对黄土高原不同乔灌树种人工纯林的研究发现,白桦(Betula platyphylla Suk.)、刺槐和油松(Pinus tabulaeformis Carr.)林K元素的利用系数和循环系数高于N和P[29]

    4.   结论
    • 高寒沙地中间锦鸡儿人工灌木林随着林龄的增加,(1)各径级根系N含量和叶片P含量显著增加,细根的P、K含量以及枝和中根的K含量显著降低;(2)根系养分积累量大于地上组分,其占总养分积累量的百分比先增加后降低,在17年生达到峰值;(3)3种养分元素的利用系数降低,循环系数和周转期增加,K元素的利用系数和循环系数明显大于N元素和P元素,周转期明显小于N元素和P元素。综上所述,高寒沙地中间锦鸡儿人工灌木林在生长过程中将更多的养分分配给根系来适应严酷的自然环境,其固氮过程会消耗自身的K和P,其中,K元素的循环速率较快,流动性大。因此,建议对中间锦鸡儿人工灌木林的管护过程中适当添加K肥和P肥。

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