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The subsurface nutrient maintenance mechanism of forest productivity and community stability under global change has always been a key scientific issue and frontier hotspot to be solved in forest ecology research, but so far the understanding is still very limited
.
In particular, alpine coniferous forests with high latitude/high altitude distribution are affected by low temperature, seasonal frozen soil, low nutrient availability, short growing season and other characteristics, which make such coniferous forests exhibit typical nitrogen (N) restriction
.
However, as a top-level community formed by regional ecological succession, alpine coniferous forest has stable stand productivity and community structure
.
This raises an important question, under the scenario of long-term N restriction and no exogenous N addition, what nutrient acquisition strategies are used in alpine coniferous forests to maintain tree growth and stand productivity? Based on this cutting-edge basic scientific problem and the actual needs of forestry management, the project team of forest ecological process and regulation of Chengdu Institute of Biology, Chinese Academy of Sciences, took the typical alpine coniferous forest in southwest China as the research object, took the rhizosphere interface as the core, focused on (i) N nutrient supply characteristics--- (ii) N transformation process--- (iii) N nutrient absorption and other key links, focused on the underground nutrient maintenance strategy of forest productivity, and systematically revealed the mechanism of stable rhizosphere nutrient maintenance in alpine coniferous forest from the unique perspective of rhizosphere
。
Figure 1: Landscape of alpine coniferous forest in western Sichuan
(i) The unique soil N pool supply characteristics of alpine coniferous forests are described--- Through continuous sampling analysis in different seasons, the relative proportion and seasonal dynamic change characteristics of soil organic N bank in the total soil N pool were clarified, and the effective N of soil soluble organic N bank in soil total was clarified the dominant position in the repertoire, and found that the dominant contribution of organic N was further highlighted during the non-growing season (approximately 80% of the total effective N repertoire of soil) (Figure 2), It effectively ensures the nutrients required for the growth of forest trees in the area (Zhang.
.
.
Yin*, 2017,Soil Biology & Biochemistry）
。 Further stable isotope analysis confirmed that alpine coniferous forest plants have the ability to directly absorb organic N (Zou Tingting.
.
.
).
Huajun Yin*, 2017, Chinese Journal of Plant Ecology; Guo.
.
.
Yin*, 2020, Plant and Soil), and identified soil soluble organic N as one of the important sources of forest N nutrient supply in this area (approximately soil N 23%~44% of nutrient contribution) (Zhang.
.
.
Yin*, 2018a,Soil Biology & Biochemistry）
。
Fig.
2: Differences between soil soluble organic N and inorganic N content in the growing and non-growing seasons of artificial spruce forest
(ii) revealed the unique N element transformation mechanism of rhizosphere soil in alpine coniferous forests
Transformation mechanism I--- Rhizosphere soil NH4+ was achieved through root activity It is the dominant nutrient supply model
.
Using 15 N stable isotope labeling technology, the microbial process of rhizosphere N element conversion was used to explore how tree roots in this area differentially regulate soil NH4 + and the direction and magnitude
of the generation and retention process of NO 3-.
The results show that alpine coniferous forest trees promote the generation and retention of NH4+ in rhizosphere soil through root activity, and limit the production ofNO3-, so as to achieve rhizosphere efficiency NH4+ nutrient supply model (Zhu.
.
.
Yin*, 2021,Biogeochemistry）
。
Transformation mechanism II---ECM epitaxial hyphae, large amount of new carbon input induces more efficient supply of N nutrients
.
Combined with internal growth tube and isotopic technology, the in-situ differentiation and quantitative evaluation of the difference and contribution range of root/mycelial C input on soil nutrient transformation process were realized, and ECM was found The C input by the hyphae pathway in highly symbiotic alpine coniferous forests plays a dominant role in soil new C input (accounting for about ~65% of the total soil new C input).
The high excitation efficiency
of mycelial pathway C input in soil nutrient mineralization and decomposition was clarified.
Specifically, the C-excitation effect induced by the mycelial pathway was approximately twice as strong as that of the root pathway (Zhang.
.
.
Yin*, 2018b, Soil Biology & Biochemistry), and mycelial pathway C is imported to rhizosphere soil The contribution of N transformation process is about 4 times that of the root pathway (Zhang.
.
.
Yin*, 2019,Functional Ecology）
。
(iii) discovery of unique N-uptake complementarity in alpine taiga ( complementary) strategy – to distinguish and quantify root/mycelial pathways in situ against soil organic/inorganic states in situ The difference of N uptake and its relative contribution show that although soil inorganic N is the main N source of nutrient supply in alpine coniferous forests, epitaxial hyphae play an important role in soil organic N nutrient economy.
The contribution of mycelial pathways to soil organic N uptake in the non-growing season was further highlighted, increasing from 36% in the growing season to 53% in the non-growing season (Zhang.
.
.
Yin*, 2019, Soil Biology & Biochemistry), showing that alpine coniferous forests form different soil components N between the root system and the mycelial pathway Efficient complementary strategies for nutrient uptake (Figure 3).
Figure 3: Differences in root/hyphae pathways contribute to organic N and inorganic N uptake
Based on the comprehensive research results, the research team further focused on three key links directly related to forest nutrient acquisition strategies (N supply, N transformation and N absorption) from the unique perspective of rhizosphere, and explained the rhizosphere nutrient maintenance mechanism with stable structure and function in alpine coniferous forest communities "Code" and constructs a schematic diagram of the conceptual framework of subsurface nutrient acquisition strategies in alpine coniferous forests
.
Its core points are as follows: i) Unique nutrient supply characteristics: Alpine coniferous forest soil has a considerable soluble organic N pool, especially in the non-growing season, which alleviates the dependence
of forest tree growth on soil inorganic N in this area.
ii) Efficient rhizosphere N element conversion mechanism: A large number of new C input from epitaxial hyphae induced efficient soil N nutrient conversion and supply efficiency.
iii) Unique N nutrient uptake complementary strategy: The root/hyphal pathway formed a complementary pattern of N efficient nutrient uptake for different soil components during the growth and non-growing seasons (Figure 4）
。 In conclusion, an efficient N nutrient acquisition strategy was formed by the synergy of multiple rhizosphere nutrient processes, which maintained the structure and functional stability
of alpine coniferous forest communities.
The results of this study enrich and advance the scientific understanding of the rhizosphere ecological mechanism and theoretical system of nutrient maintenance in alpine forests, especially give new insights into the unique functions of ECM epitaxial hyphae, and provide important theoretical support
for the adaptive management of alpine forests in response to global climate change.
Fig.
4 Schematic diagram of the strategic framework of rhizosphere nutrient acquisition in alpine coniferous forests in southwest China (*GS: growing season; NGS: Non-growing season).
The results of these studies have recently been concluded as "How do nitrogen-limited alpine coniferous forests acquire nitrogen?" A rhizosphere perspective" was published in Forest Ecosystems
, a TOP journal in Forestry Region I.
The first author of the paper is Yin Huajun, a researcher at the Chengdu Institute of Biology, Chinese Academy of Sciences, and the corresponding author is Dr.
Zhang Ziliang (now a postdoctoral researcher at the University of Illinois at Urbana-Champaign).
In addition, researcher Zhu Biao of Peking University and Dr.
Bartosz Adamczyk of the Institute of Natural Resources of Finland participated in part of the work
.
The above research was jointly funded
by the Second Qinghai-Tibet Plateau Scientific Expedition, the National Natural Science Foundation of China, and the "Light of the West" cross-cutting team of the Chinese Academy of Sciences.
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