不同退化程度高寒草甸生态系统碳通量

徐干君; 柴永煜; 颜亮; 吴胜义; 李勇; 康恩泽; 汪晓东; 杨澳; 康晓明; 张骁栋

doi:10.11829/j.issn.1001-0629.2021-0676

不同退化程度高寒草甸生态系统碳通量

徐干君¹,
柴永煜¹,
颜亮^2,3,
吴胜义¹,
李勇^2,3,
康恩泽^2,3,
汪晓东^2,3,
杨澳^2,3,
康晓明^2,3,
张骁栋^2,3

1.
国家林业和草原局西北调查规划院, 陕西西安 710048
2.
湿地生态功能与恢复北京市重点实验室 / 中国林业科学研究院生态保护与修复研究所, 北京 100091
3.
四川若尔盖高寒湿地生态系统定位观测研究站, 四川阿坝 624500

基金项目: 国家林业和草原局西北调查规划院科技创新项目(GLXD-2021-QT-20)，国家自然科学基金(31770511)

摘要: 高寒草甸是青藏高原的主要植被类型之一。受气候变化和人类活动的影响，高寒草甸面临不同程度的退化，生态系统碳通量也随之改变。当前，高寒草甸退化过程中的碳通量组分变化特征仍不明晰，对于深入理解高寒草甸碳源汇格局对全球变化的响应产生阻碍。本研究以4个退化梯度(对照样地、轻度退化、中度退化和重度退化)的高寒草甸为研究对象，利用静态箱法测定了夏季不同退化程度的高寒草甸生态系统净交换(NEE)、生态系统呼吸(ER)、总生态系统生产力(GEP)和甲烷(CH₄)通量以及相关环境因子。结果表明：1)夏季不同退化程度的高寒草甸为显著的CO₂汇，NEE范围为−1 437.55～−98.45 mg·(m²·h)⁻¹ (CO₂)，NEE、ER和GEP随着高寒草甸退化而显著减弱(P < 0.05)；同时高寒草甸呈微弱的CH₄汇，平均吸收速率为0.029 mg·(m²·h)⁻¹ (CH₄)，CH₄通量随退化加剧无显著变化(P > 0.05)。2)土壤有机碳含量和植被盖度是影响NEE、GEP和ER的主要环境因素。本研究发现高寒草甸碳汇从轻度退化到中度退化急剧减弱了约55%，说明加强预防高寒草甸从轻度到中度退化的演替，将有效维持高寒草甸的碳汇功能。

关键词:

English

Dynamics of carbon flux of alpine meadows along a degradation gradient

1.
Northwest Surveying and Planning Institute of National Forestry and Grassland Administration, Xi’an 710048, Shaanxi, China
2.
Beijing Key Laboratory of Wetland Services and Restoration / Institute of Ecological Protection and Restoration, Chinese Academy of Forestry, Beijing 100091, China
3.
Sichuan Zoige Wetland Ecosystem Research Station, Aba 624500, Sichuan, China

Received Date: 2021-11-11
Accepted Date: 2022-03-30
Available Online: 2022-06-12
Publish Date: 2022-09-14

Abstract: Alpine meadow is one of the main vegetation types in the Qinghai-Tibet Plateau. Alpine meadows are variably degraded under the influence of climate change and human activities. The carbon flux in alpine meadows changes with the degradation. However, the characteristics of carbon flux components during degradation in alpine meadows are still unclear, hindering our understanding of the transition between carbon sink and source in responding to global changes. In this study, four degradation degrees of alpine meadows, including enclosure as control check, lightly degraded, moderately degraded, and severely degraded meadows were studied. The net ecosystem exchange (NEE), ecosystem respiration (ER), gross ecosystem productivity (GEP), methane (CH₄) fluxes, and related environmental factors were measured during the growing season by the static chamber method. Results showed that: 1) Alpine meadows with different degrees of degradation in summer were significant CO₂ sinks, with NEE ranging from −1437.55 to −98.45 mg·(m²·h)⁻¹ (CO₂); NEE, ER, and GEP decreased significantly with the degradation of alpine meado(CH₄)w (P < 0.05). The alpine meadow presented a weak CH₄ sink with a mean rate of 0.029 mg·(m²·h)⁻¹ (CH₄), showing no significant difference among degradation degrees (P > 0.05). 2) Soil organic carbon content and vegetation coverage were the two main environmental factors that affected NEE, GEP, and ER. Our study found that the capacity of carbon sequestration of alpine meadows decreased by 55% from lightly degraded to moderately degraded in the growing season, indicating that more attention should be paid to the succession of alpine meadows from light to moderate degradation, which is crucial to maintaining the carbon sequestration function of alpine meadow.

Keywords:

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高寒草原是西藏自治区主要植被类型之一，约占全区草地面积的40%^[1]。高寒草原不仅是优质的种质资源库，也是当地畜牧业发展的重要基础，更为域内外居民提供了诸多服务功能，如牧草供给^[2]、养分循环^[3-5]和固碳释氧^[6-7]等。以紫花针茅(Stipa purpurea)为优势物种的高寒草原主要位于藏北羌塘高原(那曲市和阿里地区)，该区域冻融作用强烈、风蚀强度高、土壤粗骨性强、植被稀疏且多呈斑块状分布，生态环境极其脆弱^{[1, 6, 8-9]}。近年来，由于气候变化和过度放牧等干扰，致使原本脆弱的高寒草原生态系统呈现出草地退化和土地沙化等问题，并不断向荒漠化方向发展^{[1, 10]}。为遏制草地退化趋势，促进退化生态系统恢复与重建，区域内实施了大批退牧还草工程，其中围栏封育是运用最广的草原管理措施之一^[11-12]。

氮(N)是高寒草原生态系统重要的限制性因子，草原管理方式会对高寒草地土壤－植被间N循环产生直接或间接的影响。一方面，围栏封育排除了牲畜和野生动物的践踏作用，提升土壤孔隙度和入渗率，改善土壤结构并增强微生物活性，进而调控土壤N转化过程^[13-15]。例如，周天阳等^[13]研究发现，6年的围栏封育促进高寒草地土壤硝化作用，显著提升了土壤NO₃⁻含量，而对土壤全N和NH₄⁺含量的影响不显著。苏洪烨等^[14]发现了围栏封育措施下0－10 cm土层NO₃⁻、速效N和全N含量均显著高于传统放牧草地，而NH₄⁺含量在不同的管理方式下差异并不显著。然而，也有研究认为围栏封育限制了大型哺乳动物排泄物(粪便、尿液等)返还土壤，会降低土壤N营养的有效性^[16]。另一方面，围栏封育降低啃食作用，促进植被高度、盖度和生物量恢复，同时根系在土壤剖面中分布格局也会发生变化^[17-19]。但禁牧草地牧草采食率降低会限制植物补偿生长能力，而受营养物质向幼嫩器官分配能力的减弱以及稀释效应的共同作用，植物N利用效率也随之下降^[17-18]。尽管前人已分别对不同管理方式下高寒草地土壤或植被N营养特征开展了单独研究，但在土壤－植被N传递和利用效率方面仍缺乏报道。

植物N吸收偏好策略是寒区生态系统物种共生于同一群落的适应机制之一，群落中物种为了避免对稀缺资源的竞争从而形成了养分利用的生态位分化，即不同物种表现出对NO₃⁻、NH₄⁺或有机N的吸收偏好^[20]。然而，近期的研究发现，植物N吸收策略也受到外界环境因子和植物根系特征的共同调控。森林和农田生态系统的数据表明，植物可以适应土壤中不同N类型比例的快速变化，转向吸收土壤中含量较多的营养形态^[21-22]。围栏封育导致的草原土壤有效N形态及其比例潜在变化可能决定着草本植物N吸收偏好也会发生动态调整^[23]，但目前尚缺乏数据验证。此外，不同根系构型物种及其生物量剖面分布格局对围栏封育的差异性响应规律^[23-25]，会进一步加剧植物N吸收策略的复杂性。因此，目前亟须开展不同草地管理方式对高寒植物N吸收速率和偏好的研究，以期揭示高寒草原典型物种对生态工程实施的营养适应性，并为生态管理和工程优化提供基础数据。

本研究以西藏申扎县高寒高原紫花针茅和昆仑蒿(Artemisia nanschanica)为研究对象，采用¹⁵N稳定同位素示踪技术(¹⁵N-NH₄⁺和¹⁵N-NO₃⁻)，研究围栏封育和自由放牧草地植物N吸收速率、吸收偏好及其土壤剖面差异，阐明草地生态工程影响下的高寒植物N营养策略变化，并为后续退化草地保护与恢复提供科学依据。

1. 材料与方法

1.1 研究区概况

2013年在西藏申扎高寒草原与湿地观测试验站(30° 57′ N, 88° 42′ E, 4 750 m)针对退化高寒草原开展围栏封育，试验利用网围栏隔离牲畜和野生动物，防止动物对草地采食。样地区域地势平坦、退化程度类似、土壤母质一致、植被均匀。优势物种为紫花针茅，伴生种包括昆仑蒿、矮火绒草(Leontopodium nanum)等。本研究筛选紫花针茅和昆仑蒿两个物种为研究对象，主要原因是这两个物种在该区域最为常见，且具备不同的根系构型特征。土壤类型为高寒草原土，表层土壤(0－15 cm)砂砾、粉粒、粘粒的含量分别为91.23%、7.24%和1.53%。土壤碳、N和磷浓度分别为11.12、1.03和0.52 g·kg⁻¹。年均降水量为300 mm，年均温为0 ℃。

1.2 试验设计

2017年8月在围栏内外分别挑选配对的3块15 m × 15 m的样方开展试验(3 次重复)，并在每个样方内筛选14个(10 cm × 10 cm)含有紫花针茅或昆仑蒿微区斑块备用，其中12个微区斑块用于标记¹⁵N (2种N类型 × 2个物种 × 3个标记深度)，2个未标记¹⁵N微区斑块用于土壤采样(2个物种)。共随机建立84 个微区斑块(每个物种42 个)，微区内紫花针茅或昆仑蒿的盖度和生物量在群落内占比均在90%以上。¹⁵N同位素标记试验在2017年8月初开展(生长旺季)。为避免微区之间的干扰，微区间距大于50 cm。将(¹⁵NH₄)₂SO₄ (¹⁵N, 98%)和Na¹⁵NO₃ (¹⁵N, 98%)两种同位素试剂折合成相等的¹⁵N量，溶于蒸馏水中，然后进行标记试验。在每个微区利用网格(2.5 cm × 2.5 cm)和针筒在网格中心位置，注射同位素标记试剂，确保试剂在微区中的均匀分布。每个网格中心注射1针(1 mL)，每个微区注射16针(16 mL，3.06 mg ¹⁵N)。¹⁵N同位素注射深度分别为2.5、7.5和12.5 cm，为紫花针茅和昆仑蒿根系主要分布层次。¹⁵N同位素总共标记了72个微区(2 类草地管理方式 × 2 种N类型 × 2 个物种 × 3 个标记深度 × 3 次重复)。为降低示踪¹⁵N在土壤中的快速转化带来的试验误差，在同位素标记后的2 h后分别采集紫花针茅或昆仑蒿植物样品。植物冠层样品用剪刀齐地面收获采集。根系样品利用铁锹挖掘法分3层采集(0－5、5－10和10－15 cm)，然后用纯净水清洗干净。个别微区内存在极少量其他伴生物种，均予以去除，仅保留目标物种紫花针茅或昆仑蒿用于后续分析。将紫花针茅或昆仑蒿样品静置于0.5 mmol·L⁻¹ CaCl₂溶液30 min以去除植株表面吸附的同位素^[26]，然后分别对根系和冠层烘干称重(天平精度：0.001 g)。称重后3层植物根系样品混合为该物种总根系样品，研磨后测定根系¹⁵N吸收量。同时，植物冠层样品研磨后测定冠层¹⁵N吸收量。在余下的12个未标记¹⁵N的微区(2类草地管理方式 × 2个物种 × 3次重复)采集土壤样品(0－15 cm)分析土壤基本理化性质。土壤鲜样采集后置于4 ℃保存，分析土壤NH₄⁺和NO₃⁻浓度。土壤容重样品采用环刀法采集。

1.3 样品测定与计算

N同位素比值测定利用DELTA V Advantage同位素比率质谱仪测定。植物全N含量利用Flash 2000 HT元素分析仪测定。样品在元素分析仪中高温燃烧后生成N₂，质谱仪通过检测N₂的¹⁵N与¹⁴N比率，并与国际标准物(大气N₂)比对后计算出样品的δ¹⁵N比率值。测定精度：δ¹⁵N：± 0.2‰。土壤NH₄⁺和NO₃⁻浓度分别由靛酚蓝比色法和紫外分光光度法测定。土壤容重由烘干法测定。

植物N吸收速率利用稀释法计算。NH₄⁺或NO₃⁻吸收速率[µg·(g·h)⁻¹] = APE × 植物干重(g) × 植物N含量(%)/根系干重(g)/标记时间(h)/(土壤背景N含量/¹⁵N标记量)，其中APE为¹⁵N的原子百分超，APE = 标记组¹⁵N的原子百分含量–对照组¹⁵N的原子百分含量^{[20, 26]}。对照组¹⁵N的原子百分含量常为固定值(0.3663%)^[27]。

1.4 统计分析

草地管理方式(围栏封育、自由放牧)对土壤理化性质影响利用配对样本T检验进行分析。草地管理方式(围栏封育、自由放牧)对紫花针茅和昆仑蒿N浓度、生物量和N储量的影响利用配对样本T检验进行分析。草地管理方式(围栏、自由放牧)、N形态(NH₄⁺、NO₃⁻)和标记深度(2.5、7.5和12.5 cm)对紫花针茅和昆仑蒿N吸收速率的影响利用三因素方差分析(three-way ANOVA)进行分析。数据在进行方差分析前对数据进行数转换以满足数据正态性和方差齐性要求。统计分析与图形绘制分别利用SPSS 16.0和Sigmaplot 11.0软件完成。

2. 结果与分析

2.1 围栏封育对土壤理化性质的影响

与自由放牧相比，围栏封育对紫花针茅斑块的土壤NH₄⁺、NO₃⁻以及无机N浓度影响不显著(P > 0.05) (图1a, b, c)，主要是由于土壤NH₄⁺、NO₃⁻以及无机N浓度较为稳定，对于紫花针茅群落生存的土壤环境来说，围栏封育和自由放牧两种草地管理方式的变化，对于土壤NH₄⁺、NO₃⁻以及无机N浓度数值差异并不能产生显著效果；围栏封育后昆仑蒿斑块土壤的NO₃⁻和无机N浓度有显著提升(P < 0.01)，分别增加了277%和222% (图1b, c)；围栏封育后，紫花针茅和昆仑蒿斑块的土壤容重均有所下降，但差异性未达到显著性水平(P > 0.05) (图1d)。

图 1 围栏封育对高寒草原土壤理化性质的影响

NS表示不显著(P > 0.05)；**表示显著(P < 0.01)；***表示极显著(P < 0.001)。图2、图3同。

Figure 1. Effects of fencing on the physicochemical properties of alpine steppe soils

NS indicates no significant difference (P > 0.05); ** and *** indicate significant differences at the 0.01 and 0.001 levels, respectively. This is applicable for Figure 2 and Figure 3 as well.

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2.2 围栏封育对典型物种生物量、N浓度和N储量的影响

围栏封育促进了紫花针茅冠层生物量的累积，提高了92.32 g·m⁻² (P < 0.001) (图2a)。围栏封育对紫花针茅0－5 cm根系生物量没有显著影响(P > 0.05)，而对5－10和10－15 cm根系生物量有显著提升作用(P < 0.001)，分别增加了56.12和31.74 g·m⁻²。围栏封育改变了紫花针茅根系生物量的剖面分布比例。紫花针茅0－5 cm根系占比从自由放牧的84.40%显著下降到围栏封育的70.19% (P < 0.001)，而5－10和10－15 cm根系生物量占比均显著提高(P < 0.001) (图2b)。

图 2 围栏封育对高寒草原典型物种生物量分配的影响

Figure 2. Effects of fencing on biomass allocation in two typical species in an alpine steppe

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围栏封育显著提升昆仑蒿的冠层生物量，增加了52.26 g·m⁻² (P < 0.001) (图2c)。围栏封育显著提高了昆仑蒿0－5和5－10 cm根系生物量(P < 0.01)，而对10－15 cm根系生物量影响不显著(P > 0.05) (图2c)。围栏封育显著降低昆仑蒿表层根系生物量占比(0－5 cm) (P < 0.01)，但对深层根系生物量占比无显著影响(10－15 cm) (P > 0.05) (图2d)。

围栏封育对紫花针茅根系N浓度影响不显著(P > 0.05)，而显著降低昆仑蒿的根系N浓度(P < 0.01)，均值降低了0.57 mg·g⁻¹ (图3a)。与植物根系响应趋势不同的是，围栏封育显著降低紫花针茅冠层N浓度(自由放牧0.86 mg·g⁻¹，围栏封育0.64 mg·g⁻¹) (P < 0.01)，而对昆仑蒿冠层N浓度无显著性影响(P > 0.05) (图3b)。两种草地管理方式下，紫花针茅根系和冠层N储量无显著差异(P > 0.05) (图3c, d)。但围栏封育草地昆仑蒿根系和冠层的N储量均显著高于自由放牧草地(P < 0.01)，分别提升0.74和0.85 g·m⁻² (图3c, d)。

图 3 围栏封育对高寒草原典型物种N浓度和N储量的影响

Figure 3. Effects of fencing on N concentration and N stock in two typical species in an alpine steppe

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2.3 围栏封育对典型物种N吸收速率的影响

方差分析结果显示，草地管理方式、N类型和标记深度对紫花针茅和昆仑蒿N吸收速率均有显著影响(P < 0.01) (图4)。围栏封育促使紫花针茅NH₄⁺平均吸收速率由0.96 µg·(g·h)⁻¹提升至5.74 µg·(g·h)⁻¹，NO₃⁻平均吸收速率由27.46 µg·(g·h)⁻¹提升至36.80 µg·(g·h)⁻¹。然而，围栏封育对昆仑蒿NH₄⁺吸收速率的影响较弱，而NO₃⁻平均吸收速率由38.48 µg·(g·h)⁻¹提升至141.29 µg·(g·h)⁻¹。

图 4 围栏封育对典型物种N吸收速率的影响

GM为草地管理方式，N为不同形态氮，D为标记深度，NS为不显著。

Figure 4. Effects of fencing on the rate of N uptake by two typical species in an alpine steppe

GM, grassland management; N, different chemical forms of nitrogen; D, marking depth; NS, not significant.

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两种草地管理利用方式下植物NO₃⁻吸收速率均高于NH₄⁺ (P < 0.001) (图4)。高寒草原紫花针茅NO₃⁻和NH₄⁺平均吸收速率分别为32.13和3.35 µg·(g·h)⁻¹，而昆仑蒿NO₃⁻和NH₄⁺平均吸收速率分别为89.88和1.99 µg·(g·h)⁻¹。

¹⁵N标记在表层土壤的紫花针茅N吸收速率高于标记在深层土壤(P < 0.01) (图4)。围栏封育草地，¹⁵N标记在表层土壤昆仑蒿吸收速率高于其他土壤层次。但在自由放牧草地，¹⁵N标记在5－10 cm土壤，昆仑蒿的N吸收速率高于其他土壤层次。

3. 讨论

围栏封育对高寒草原植物N吸收速率具有显著提升作用。禁牧减少了家畜和野生动物的肆虐啃食、践踏以及人为活动的干扰，防止了土壤板结，土壤容重下降和孔隙度逐步增加，提高了水分和养分迁移效率^{[3, 14]}。同时，高寒草原在围栏封育后，在0－10 cm土层，紫花针茅和昆仑蒿植物根系生物量占比显著提升，也有利于植物对N素吸收。藏北高寒草原紫花针茅属于密从型根系构型，围栏封育后该物种具备较高的比根长和比表面积，根系在土壤中的延伸性以及与土壤的接触面积提高，会提升其N素的吸收效率^[24]。该发现与围栏封育会提高高寒草甸优势牧草高山嵩草(Kobresia pygmaea) N吸收效率的研究结论基本一致^[23]。但与之不同的是，¹⁵N标记土壤深度为0－5 cm时，紫花针茅N吸收速率在围栏封育和自由放牧草地之间无显著差异，可能原因是放牧导致根系分布发生了表层化趋势(表层根系占比高)，从而造成表层根系生物量在两种草地管理方式下无显著差异。此外，高寒草甸的研究结果揭示，围栏封育会促进伴生物种二裂委陵菜(Potentilla bifurca)对NH₄⁺的吸收效率，而降低其对NO₃⁻的吸收速率^[23]。本研究发现围栏封育对伴生物种昆仑蒿NH₄⁺的吸收速率影响较为微弱，但会显著提高其对NO₃⁻的吸收速率。一方面原因可能是，围栏封育后昆仑蒿斑块硝化作用增强^[13-14]，导致土壤NO₃⁻含量显著提升，从而有利于昆仑蒿对NO₃⁻的吸收。另一方面，NH₄⁺易吸附在带负电荷的土壤胶体表面，移动性弱，会在昆仑蒿粗壮根系周围形成养分吸收的耗竭区^[28]。

两种草地管理方式下，紫花针茅和昆仑蒿均对NO₃⁻具有吸收偏好。有研究认为，在常年低温，N矿化受到强烈限制的地区(如极地或高山地区)，同一植物群落中物种对N素类型的吸收偏好具有多元性^{[20, 29]}，体现出物种对N素营养吸收类型的生态位分化，对维持贫瘠环境下植物群落物种多样性和稳定性具有重要的意义^[20]。例如：在北极苔原的研究表明，苔属(Carex)植物主要吸收NO₃⁻，而杜香属(Ledum)和桦木属(Betula)植物主要吸收NH₄^{+ [20]}。然而，不同草地管理方式下高寒草原的研究发现，紫花针茅和昆仑蒿均偏好吸收NO₃⁻，表明这两个物种在长期的进化和群落建构过程中，已形成了偏好吸收NO₃⁻的生理生态适应特性。主要原因有：1)受高寒草原碱性土壤性质(pH > 8)以及极端干燥气候的影响，硝化作用较强烈，导致两种草地管理方式下土壤NO₃⁻含量均显著高于NH₄⁺，而植物往往偏好吸收土壤中占主导的N形态^[21-22]。2) Michaelis-Menten吸收动力学表明，NO₃⁻在土壤中的扩散速率较高，导致根系对NO₃⁻的吸收能力强于NH₄^{+ [28, 30]}。3)尽管NH₄⁺属于经济型N源，其吸收和同化所需的能量要远低于NO₃⁻，但过量吸收NH₄⁺会引起植物铵盐自毒^[30-31]。

植物根系对表层土壤N吸收效率显著高于深层土壤N吸收速率显著高于深层土壤。高寒草地植物根系在土壤剖面上一般呈现“T”型分布特征^[32]，即根系主要分布于表层土壤，并随土壤深度逐渐降低，从而导致根系N吸收速率在土壤剖面上存在一定的差异。本研究中两种草地管理方式下，优势禾草紫花针茅N吸收速率均表现出表层土壤高于深层土壤的空间规律。类似的，在高寒沼泽湿地的研究也发现，木里苔草(Care xmuliensis)和毛果苔草(C. xlasiocarpa)具有不同的空间N吸收模式，其中0－5 cm处土壤NO₃⁻和甘氨酸含量以及根系N吸收速率均高于深层土壤^[25]。但对轴根型的昆仑蒿而言，自由放牧草地较高的土壤紧实度限制了其根系延伸至更深层土壤，导致次表层5－10 cm (其细根主要分布层次)具备较高的N吸收速率。也有研究发现，高寒物种鹅绒委陵菜(Potentilla anserina) N吸收速率在土壤剖面上无明显的空间分化^[25]，可能原因是该物种采用游击型资源捕获策略，能够通过横向空间的扩展吸收养分^[33]。由此可见，高寒植物N吸收策略在土壤剖面上呈现出物种特异性，主要受不同物种根系分布格局、构型特征及其养分获取方式等影响。

本研究是基于4年短期的围栏封育试验研究，目前无法量化不同围封年限对高寒草原植物N素吸收策略的影响。有监测数据表明，短期封育消除了放牧干扰影响，有效改善群落结构，种间资源竞争相对平稳^[34]。然而，随着围栏封育年限的延长，受土壤养分等资源限制作用增强的影响，群落地上生物量增幅减弱，而根系生物量显著提升^[35]，可能调控不同物种N吸收效率与竞争强度。此外，长期围栏封育还可能通过增加枯落物及半腐殖分解、改变土壤结构和孔隙率等因素影响土壤N转化过程^{[14, 35]}，土壤有效N形态比例的变化可能决定着植物N吸收偏好的转换。未来应加强不同封育年限序列上群落生物量与结构、土壤养分有效性、以及植物营养吸收效率的监测，以期为高寒退化草原恢复过程中最佳围栏封育年限的确定以及生态工程的优化管理提供科学依据。

4. 结论

与自由放牧相比，围栏封育显著提升了高寒草原典型物种N吸收速率，但对昆仑蒿NH₄⁺吸收速率影响较为微弱。自由放牧和围栏封育草地管理方式下，紫花针茅和昆仑蒿均偏好吸收NO₃⁻，表现出与土壤主导N形态相契合的特点。两种草地管理方式下，高寒草原典型物种表层根系N吸收速率均高于深层根系，与根系生物量的土壤剖面分布格局基本一致。

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图 1 2021年玛沁县月降水量和月平均气温

数据来源：中国气象数据网。

Figure 1. Monthly precipitation and mean temperature of Maqin County in 2021

Data source：China Meteorological Data Service Centre.

下载: 全尺寸图片幻灯片

图 2 不同退化程度高寒草甸生态系统净交换、生态系统呼吸、总生态系统生产力和甲烷通量

不同小写字母表示不同退化程度高寒草甸碳通量差异显著(P < 0.05)。CK：对照； LD：轻度退化；MD：中度退化；SD：重度退化。

Figure 2. Net ecosystem exchange, ecosystem respiration, gross ecosystem productivity, and methane flux in alpine meadows among different degradation degrees

Different lowercase letters indicate significant differences among different degradation degrees at the 0.05 level. CK: control check; LD: lightly degraded; MD: moderately degraded; SD: severely degraded.

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图 3 二氧化碳通量组分的相关性分析

Figure 3. Correlation tests among the components of CO₂ fluxes

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图 4 高寒草甸碳通量组分与环境因子的相关性热图

VC：植被覆盖度；AGB：植物地上生物量；SOC：土壤有机碳；SWC：土壤含水量；ST：表层土壤温度；NEE：生态系统净交换；ER：生态系统呼吸；GEP：总生态系统生产力；CH₄：甲烷通量；***、**、*和+分别表示0.001、0.01、0.05和0.1水平差异显著。

Figure 4. Heatmap for the correlation between CO₂ flux components and environmental factors

VC: vegetation cover; AGB: aboveground biomass; SOC: soil organic carbon; SWC: soil water content; ST: surface soil temperature; NEE: net ecosystem exchange; ER: ecosystem respiration; GEP: gross ecosystem productivity; CH₄: methane flux; ***, **, *, and + indicate significance at 0.001, 0.01, 0.05, and 0.1 levels，respectively.

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表 1 样点地理位置与植被覆盖

Table 1 Location and plant cover of sampling sites

退化梯度 Degradation degree	纬度 Latitude	经度 Longitude	海拔 Altitude/m	植被盖度 Vegetation coverage/%	优势物种 Dominant plant
对照 Control	34°28′47.28″ N	100°11′59.64″ E	3 723.98	76.67 ± 1.20	垂穗披碱草、早熟禾 Elymus nutans、Poa annua
轻度退化 Lightly degraded	34°18′55.8″ N	100°34′35.76″ E	4 057.28	74.67 ± 1.45	高山嵩草、矮生嵩草、钉柱委陵菜 Kobresia pygmaea、Kobresia humilis、Potentilla saundersiana
中度退化 Moderately degraded	34°20′3.12″ N	100°29′47.04″ E	3 962.70	66.00 ± 2.00	薹草、田葛缕子、紫花针茅 Carex spp.、Carum buriaticum、Stipa purpurea
重度退化 Severely degraded	34°20′18.24″ N	100°34′15.24″ E	4 049.41	29.67 ± 0.88	黄帚橐吾、单子麻黄、铺散亚菊 Ligularia virgaurea、Ephedra monosperma、Ajania khartensis

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表 2 不同退化程度高寒草甸土壤和植被特征

Table 2 Soil and vegetation characteristics in alpine meadows with different degradation degrees

退化程度 Degradation degree	土层 Soil layer/cm	土壤有机碳 Soil organic carbon/(g·kg⁻¹)	土壤含水量 Soil water content/%	土壤pH Soil pH	表层土壤温度 Surface soil temperature/℃	地上生物量 Aboveground biomass/(g·m⁻²)
对照 Control	0－10	35.23 ± 2.71	19.47 ± 1.05	7.47 ± 0.07	12.37 ± 0.28	190.05 ± 20.09
	10－30	22.18 ± 1.72	27.37 ± 9.92	8.17 ± 0.09
	30－50	18.27 ± 1.63	15.50 ± 0.57	8.33 ± 0.07
轻度退化 Lightly degraded	0－10	63.39 ± 4.67	31.70 ± 1.25	6.40 ± 0.06	8.43 ± 0.19	143.49 ± 22.15
	10－30	25.33 ± 0.59	20.03 ± 1.07	6.83 ± 0.07
	30－50	15.01 ± 1.78	16.60 ± 0.25	7.30 ± 0.36
中度退化 Moderately degraded	0－10	27.08 ± 1.44	24.83 ± 0.27	8.20 ± 0.06	14.20 ± 0.80	195.68 ± 40.57
	10－30	15.39 ± 2.03	20.50 ± 0.25	8.47 ± 0.03
	30－50	7.53 ± 1.30	15.00 ± 1.50	8.63 ± 0.03
重度退化 Severely degraded	0－10	16.55 ± 2.15	15.00 ± 1.19	8.20 ± 0.15	8.37 ± 0.38	76.66 ± 23.12
	10－30	7.54 ± 1.35	13.13 ± 1.61	8.47 ± 0.13
	30－50	4.85 ± 0.91	11.87 ± 0.87	8.67 ± 0.09

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1. 材料与方法
1.1 研究区概况
1.2 试验设计
1.3 样品测定与计算
1.4 统计分析
2. 结果与分析
2.1 围栏封育对土壤理化性质的影响
2.2 围栏封育对典型物种生物量、N浓度和N储量的影响
2.3 围栏封育对典型物种N吸收速率的影响
3. 讨论
4. 结论
参考文献

1. 材料与方法
1.1 研究区概况
1.2 试验设计
1.3 样品测定与计算
1.4 统计分析
2. 结果与分析
2.1 围栏封育对土壤理化性质的影响
2.2 围栏封育对典型物种生物量、N浓度和N储量的影响
2.3 围栏封育对典型物种N吸收速率的影响
3. 讨论
4. 结论
参考文献

参考文献(43)

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