Input, dynamics and loss of reactive nitrogen in a central African tropical mountain forest and Eucalyptus plantation
Input, dynamics and loss of reactive nitrogen in a central African tropical mountain forest and Eucalyptus plantation
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中非热带山地森林和桉树人工林活性氮的输入、动态和损失
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发表时间：
2015
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通讯作者：
C. Landry
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作者：
C. Landry
除了土地利用变化和气候变化之外，氮（N）沉降是森林生态系统功能的另一个威胁。中部非洲拥有世界第二大面积的连续湿润热带森林。热带森林占初级生产的三分之一，对陆地碳汇做出了重大贡献。目前，中部非洲热带（山地）森林的实地研究严重缺乏。因此，本论文的总体目标是研究中非原始热带山林（Nyungwe）和附近桉树种植园的生物地球化学过程。
Nyungwe森林位于卢旺达西南部（南纬2°15' - 2°55'，东经29°00' - 29°30'），位于西边刚果盆地和东边尼罗河盆地的分水岭上，面积约970平方公里。地形完全是山地（海拔 1,600 – 2,950 米），气候为潮湿的热带气候。在这项研究中，选择了两个集水区，一个位于原始 Nyungwe 森林内，另一个位于 Nyungwe 国家公园缓冲区附近的桉树种植园内。在每种森林类型中，选择了三个实验地块（20 x 30 m）并进行永久标记。本研究重点关注氮输入、动态和损失，并具体研究了：1) 凋落物动态和落叶分解率，2) 通过径流沉积、凋落物渗滤、土壤溶液和河水的氮和碱阳离子通量，以及 3) 通过原位 15N 池稀释实验的土壤氮动态。
连续两年测量了 Nyungwe 原始森林的垃圾量，并在一年内测量了附近桉树种植园的垃圾量。在原始林中安装单一物种垃圾袋，在两个林分中安装混合物种垃圾袋，进行了为期 361 天的单一物种和混合物种凋落物分解实验。在 Nyungwe 原始森林和邻近的桉树种植园中对降雨、腐殖质渗透以及土壤溶液通量和成分进行了研究。这项研究随后通过在原始森林流域出口处使用 V 型切口（90°）和稳定同位素分析，对径流、腐殖质渗滤、土壤溶液和河水中硝酸盐的来源进行了调查。最后，在原始森林中进行了原位 15N 同位素稀释实验，使用“虚拟土壤核心”方法来量化 Nyungwe 原始森林土壤中的氮动态和路径。
总垃圾量约为Nyungwe 原始森林和桉树人工林分别为 4 t ha-1 yr-1 和 2 t ha-1 yr-1。原始森林中落叶层的贡献约为。 Nyungwe 和桉树种植园分别为 70% 和 79%。垃圾量在大旱季（七月至八月）和小旱季（十二月至一月）以及雨季（九月至十月）开始时达到高峰。原始森林中，Cleistanthus polystachyus 落叶物的初始凋落物衰减率最高（0.033 day-1），其次是森林凋落物混合物（0.016 day-1），Parinari excelsa 的初始落叶物衰减率最低（0.0094 day-1）。 Cleistanthus polystachyus、Carapa grandiflora 和桉树凋落物混合物的最终腐烂率相似（0.0014、0.0013 和 0.0017 day-1），并且低于森林凋落物混合物的最终腐烂率（0.0021 day-1）。腐烂率可能与凋落物特性有关，例如氮、木质素、钙和多酚含量。混合凋落物物种对初始腐烂率产生了负累加效应，而在原始林中观察到对最终腐烂率产生了正累加效应。总的来说，与混合物中各个垫料类型基于称重的预期质量损失相比，混合物种垫料显示出更多的质量损失。最后，林分类型仅影响森林凋落物混合物（PE+CP+CG）的最终腐烂率，桉树中的腐烂率低于原始森林，这被认为是由森林地面湿度降低引起的。
两年平均降雨量为 2520±23 mm yr-1，但原始森林的树冠拦截量 (43%) 高于桉树人工林 (30%)。年降雨输入NH4+-N、NO3--N、Na+、K+、Ca2+、Mg2+和Cl-分别为2.80、3.61、3.84、12.03、5.66、2.08和5.07 kg ha-1。 NH4+-N 和 NO3--N 通量在其他山地雨林观测到的范围内； NH4+ 部分被两个地点的树冠保留，而 NO3- 由原始森林树冠释放，但被桉树人工林树冠保留。两个树冠均释放阳离子（Na+、K+、Ca2+ 和 Mg2+），但除 Na+ 外，原始森林中的阳离子（Na+、K+、Ca2+ 和 Mg2+）释放量高于桉树人工林。在生根区，NH4+、NO3-等碱性阳离子被吸收，而NO3-则从表层土壤中淋失。 NH4+ 优先于 NO3- 被吸收。原始森林淋溶无机氮损失占总流失量的49%，而桉树人工林则比总流失量多60%，其中NO3-和NH4+分别占原始林总流失量的94%和6%，桉树人工林分别占79%和21%。通过溪流离开原始森林集水区的无机氮总量为 20.8 kg N ha-1 yr-1。同位素组成测量表明，径流中的NO3-主要来自大气沉降，而腐殖质渗滤、土壤溶液和河水中的NO3-主要来自土壤氮过程。河水中的18O-NO3-值在10.2～20.8‰之间，证实河水中NO3-的来源主要是土壤N，仅部分是大气NO3-。高氮矿化伴随着高硝化速率，产生的 NO3- 很容易流失到环境中。
15N 标记实验显示 NH4+ 库的 15N 标记后 15NO3- 非常快速地富集，表明 NH4+ 快速转移到 NO3-。研究的热带森林土壤显示出两种不同的 NH4+ 氧化途径：一种是自养硝化菌的缓慢氧化途径，另一种是与铁还原 (Feammox) 相结合的快速氧化途径。 Feammox 的总速率与硝化作用的大小相似，而且获得的 Feammox 速率近似于添加 NH4+ 和 Fe(III) 后热带森林土壤的浆液培养中获得的速率。森林土壤表现出高比例的硝化作用与 NH4+ 固定化特征，具有开放式氮循环特征，且具有高风险的 NO3- 损失。 Nyungwe原始森林土壤的特点是开放的氮循环，其中由Feammox产生的铵(NH4+)是主要的重要氮转化途径，植物对氮的吸收以NH4+为主。
Next to land use change and climate change, nitrogen (N) deposition is another threat for forest ecosystem functioning. Central Africa contains the second largest area of contiguous moist tropical forests of the world. Tropical forests account for one third of primary production contributing significantly to the terrestrial carbon sink. Currently, there is a huge lack of field-based research in tropical (mountain) forests in Central Africa. Hence, the general objective of this thesis was to investigate biogeochemical processes in a central African pristine tropical mountain forest (Nyungwe) and a nearby Eucalyptus plantation.
Nyungwe forest is located in southwestern Rwanda (2°15' – 2°55' S, 29°00'– 29°30' E) in a watershed dividing the Congo basin to the west and the Nile basin to the east and covers an area of approximately 970 km2. The topography is entirely mountainous (1,600 – 2,950 m above sea level) while the climate is humid tropical. For this study, two catchments were selected, one inside pristine Nyungwe forest and another one in a nearby Eucalyptus plantation in the buffer zone of the Nyungwe national park. In each forest type, three experimental plots (20 x 30 m) were selected and marked permanently. This study focused on N input, dynamics and losses, and specifically investigated: 1) litterfall dynamics and leaf litter decomposition rates, 2) N and base cation fluxes via throughfall deposition, litter percolation, soil solution and river water, and 3) soil N dynamics via an in situ15N pool dilution experiment.
Litterfall was measured in the Nyungwe pristine forest during two consecutive years and in the nearby Eucalyptus plantation during one year. A 361-days litter decomposition experiment with single and mixed-species leaf litter was carried out with single-species litterbags installed in the pristine forest and mixed-species litterbags in both forest stands. Throughfall, humus percolation and soil solution fluxes and composition were investigated in the Nyungwe pristine forest and in the neighboring Eucalyptus plantation. This study was followed by an investigation on the origin of nitrate in throughfall, humus percolation, soil solution and the river water through use of a V-notch (90°) at the outlet of the pristine forest catchment and stable isotope analyses. Finally, an in situ 15N isotope dilution experiment was carried out in the pristine forest stand, using the ‘virtual soil core’ approach to quantify N dynamics and pathways in the Nyungwe pristine forest soil.
Total litterfall amounted to ca. 4 and 2 t ha-1 yr-1 in the Nyungwe pristine forest and Eucalyptus plantation, respectively. The contribution of leaf litter in the pristine forest was ca. 70 and 79% inNyungwe and the Eucalyptus plantation, respectively. Litterfall peaked in the major (July - August) and minor (December -January) dry seasons and at the onset of the rainy season (September - October). In the pristine forest, the initial leaf litter decay rate was highest for Cleistanthus polystachyus leaf litter (0.033 day-1), followed by the forest litter mixture (0.016 day-1),and it was lowest for Parinari excelsa (0.0094 day-1). The final decay rates of Cleistanthus polystachyus, Carapa grandiflora and Eucalyptus litter mixture were similar (0.0014, 0.0013 and 0.0017 day-1) and lower than the final decay rate of forest litter mixture (0.0021 day-1). Decay rates could be related to litter properties such as N, lignin, Ca and polyphenol content. Mixing litter species caused a negative additive effect on the initial decay rate, while a positive additive effect was observed on the final decay rate in the pristine forest stand. Taken together, mixed-species litter showed increased mass loss compared to the expected weighed-based mass loss from the individual litter types in the mixture. Finally, stand type only affected the final decay rate of the forest litter mixture (PE+CP+CG) that was lower in the Eucalyptus than in the pristine forest and is suggested to be caused by reduced forest floor humidity.
The average incident rainfall over two years was 2520±23 mm yr-1, but the canopy interception was higher in the pristine forest (43%) than in the Eucalyptus plantation (30%). The annual input of NH4+-N, NO3--N, Na+, K+, Ca2+, Mg2+ and Cl- via rainfall was 2.80, 3.61, 3.84, 12.03, 5.66, 2.08 and 5.07 kg ha-1, respectively. Fluxes of NH4+-N and NO3--N were within the range observed for other mountain rain forests; with NH4+ partly retained by the canopy at both sites, and NO3- released by the pristine forest canopy but retained by the Eucalyptus plantation canopy. Cations (Na+, K+, Ca2+ and Mg2+) were released by both canopies but to a larger extent in the pristine forest than in the Eucalyptus plantation except for Na+. In the rooting zone, NH4+, NO3- and other base cations were absorbed while NO3-was leaching from the top soil. NH4+ was preferentially absorbed above NO3-. Inorganic N losses by leaching were 49% of the total thoughfall input in pristine forest while in the Eucalyptus plantation 60% more than the total thoughfall input was lost, for which NO3-and NH4+represented 94 and 6 % of the total loss in the pristine forest, respectively, and 79 and 21% in the Eucalyptus plantation respectively. The total amount of inorganic N leaving the pristine forest catchment by stream water was 20.8 kg N ha-1 yr-1. Isotope composition measurements showed that NO3- in throughfall was mainly from atmospheric deposition while in humus percolation, soil solution and river water it was mainly originated from soil N processes. 18O-NO3- values in the river water ranged between 10.2 and 20.8‰, confirming that the source of NO3- in the river water was mainly soil N and only partly atmospheric NO3-. High N mineralization is followed by high nitrification rates, with the produced NO3- readily lost to the environment.
The 15N labeling experiment showed very rapid 15NO3- enrichment following 15N labeling of the NH4+ pool indicating a fast transfer of NH4+ to NO3-. The investigated tropical forest soil showed two distinct NH4+ oxidations pathways: a slow one by autotrophic nitrifiers and a fast one coupled to iron reduction (Feammox). The gross rate of Feammox was of similar magnitude as nitrification, moreover the obtained Feammox rate approximate that obtained in slurry incubation of tropical forests soils after addition of NH4+ and Fe(III). The forest soil showed a high ratio of nitrification to NH4+immobilization characteristic of an open N cycle with high risk of NO3- losses. Nyungwe pristine forest soil is characterized by an open N cycle in which ammonium (NH4+.) produced by Feammox is a major important N transformation pathway and plant N uptake is dominated by NH4+.