内蒙古温带3种优势锦鸡儿属灌木的氮吸收与固氮转移研究

Shrub encroachment of global grasslands in arid and semiarid areas was considered the results of global climate change and human activities. Most studies proved shrub encroachment was a kind of grassland degradation, however, still some studies proved it was helpful for grasslands restoration and plant biodiversity maintenance. To clarify the nutrient acquisition strategies by dominant Caragana shrubs and its nutrient supplies to coexisting plants, which was conductive for us to deeply understand the mechanisms of shrub encroachment, and its ecological significances in grasslands restoration, and provided us important theories and data support for grasslands management in arid and semiarid areas of North China. Across the typical steppe, desert steppe and steppe desert from east to west, three Caragana shrubs of Caragana microphylla, C. davazamcii and C. korshinskii were chosen as our target species. In the larger scale of Inner Mongolia temperate grasslands, isotope tracer techniques combined with geostatistics method were used to explore the nitrogen (N) acquisition abilities by dominant Caragana shrubs from both soil and atmospheric N2. Firstly, based on the soil isotope 15N distribution map around the dominant Caragana shrubs, we draw up the effective influencing sphere of symbiotic N fixation by Caragana shrubs. Secondly, with the isotope tracer techniques, within the effective influencing sphere we set the quadrat to unveil the N acquisition abilities of Caragana shrubs absorbing diverse N from soil and fixing symbiotic N from atmosphere, and to quantify the amount of symbiotic N transferred from dominant Caragana shrubs to its coexisting non-legumes.

全球干旱半干旱区草原灌木扩张是气候变化与人类活动作用的结果。目前主流观点认为灌木扩张是草地退化的表现，也有观点认为它有利于草地恢复和多样性维持，是草地退化最后屏障。因此厘清灌木养分获取及对伴生种的养分供应状况，有助于明晰灌木扩张的内在机制及其在草地恢复中的生态重要性，为我国草原可持续利用提供理论与数据支撑。本项目拟运用同位素示踪技术，结合地统计学方法，以内蒙古温带典型草原、荒漠化草原和草原化荒漠的小叶锦鸡儿（Caragana microphylla）、中间锦鸡儿（C. davazamcii）与柠条锦鸡儿（C. korshinskii）灌木为研究对象，在较大尺度上探究其从土壤和大气中获取氮素的能力。首先基于优势锦鸡儿灌木周围土壤15N同位素图谱，确定锦鸡儿灌木共生固氮的影响范围，然后在此范围内通过15N示踪法，量化其对土壤不同形态氮的获取能力与共生固氮能力，及其向周围伴生植物的固氮转移。

在本项目支持下，本研究自东向西跨内蒙古温带典型草原、荒漠化草原以及草原化荒漠，分别选择3种优势灌丛（小叶锦鸡儿（Caragana microphylla）、中间锦鸡儿（Caragana davazamcii）与柠条锦鸡儿（Caragana korshinskii））开展了为期三年（2018、2019以及2021年）的野外原位15N标记试验：①锦鸡儿灌木周围土壤15N同位素分布图谱制作；②量化锦鸡儿灌丛对不同形态土壤氮的获取能力；③估算锦鸡儿灌木的共生固氮及其固氮转移。项目执行期间，依托本项目共发表论文7篇。其中SCI论文5篇，中科院分区1区论文2篇，2区论文1篇，3区论文2篇。共培养博士生2名，硕士生6名。已毕业博士生1名，硕士生5名。在读博士生1名，硕士生1名。主要获得以下结果与结论：①半方差分析结合克里格插值法对三种锦鸡儿灌丛周围土壤δ15N进行空间统计分析，得到三种锦鸡儿灌木周围土壤15N同位素分布图谱。灌木中心的土壤δ15N受到豆科灌木生物固氮的影响显著低于其周围土壤的δ15N。结合后面部分生物固氮数据的分析可以看出，三种锦鸡儿灌木能够通过生物固氮向其周围临近共生的非豆科植物进行氮转移，进而影响周围5m以内土壤δ15N的空间分布。②同位素示踪实验说明三种锦鸡儿以土壤无机氮中的硝态氮吸收为主，且均能直接吸收土壤有机氮，对精氨酸吸收高于甘氨酸和脯氨酸；自内蒙古温带草原自东向西，随年均降水减少，三种锦鸡儿总氮吸收能力逐渐变小，小叶锦鸡儿氮吸收速率最高。③基于15N同位素稀释法和15N自然丰度法估算了三种锦鸡儿生物固氮，整株植物的%Ndfa为70-85%，固氮量为0.53-3.06 g N m-2，柠条锦鸡儿显著高于其它锦鸡儿。④基于15N自然丰度法估算了三种锦鸡儿生物固氮转移，其共生草本植物的固氮转移率约为41-56 %，固氮转移量为6.4-9.2 gN m-2，柠条锦鸡儿固氮转移量最高。⑤在内蒙古温带草原自东向西的三种锦鸡儿地理替代性分布中，随年均降雨量减少，三种锦鸡儿的生物固氮能力逐渐增强，生物固氮对三种锦鸡儿总氮吸收的贡献在增加，且生物固氮是柠条锦鸡儿的优势氮获取方式。