华北温带森林植物根际土壤硝酸根浓度和同位素组成研究

Nitrate is one of primary available nitrogen sources for both soil microorganisms and plants. Studies regarding the concentrations and isotopic compositions of nitrate in the rhizosphere soil are key to accurately understand the microbial turnover of soil nitrate and the ability of plant nitrate utilization. However, the variation characteristics and mechanisms of nitrate in the rhizosphere soil remain poorly understood and unclear. This project, by combining the methods of stable isotopes and soil ecology, chooses dominant trees and shrubs in Baxian mountain in Jizhou district, Tianjin city. The concentrations and nitrogen and oxygen isotopic compositions of nitrate in roots, rhizosphere and non-rhizosphere soils of different plant species are detailedly analyzed, as well as the variations of above-mentioned indicators along with the growing season and soil depths. The objective of the study is to detailedly quantify the differences of the concentrations and isotopic compositions of nitrate between rhizosphere and non-rhizosphere soils, in addition to the differences of that in the rhizosphere soil between plant species, the growing season and soil depths. Furthermore, the mechanisms of the bioavailability and isotopic values of nitrate in the rhizosphere soil will be discussed. Meanwhile, the effects of the variation of nitrate in the rhizosphere soil on evaluating the ability of plant nitrate absorption and reduction based on nitrate dynamics in non-rhizosphere soil and roots will be revealed. This project aims to improve the understanding of interfacial process of nitrogen cycle in plant-soil systems as well as its response to environmental changes.

硝酸根是土壤微生物和植物可利用氮的主要形式之一。研究根际土壤硝酸根的浓度和同位素组成有助于更准确理解土壤硝酸根的微生物周转和植物硝酸根利用，但至今研究稀少，其变化特征和机制并不清楚。本项研究结合稳定同位素和土壤生态学的方法原理，选取天津蓟州区八仙山森林乔木和灌木优势种，分析不同植物种根组织内、根际土壤和非根际土壤的硝酸根浓度和氮、氧同位素组成，以及这些指标随生长季节和土层深度的变化特征。详细量化森林植物根际和非根际土壤之间硝酸根浓度和同位素组成的差异，以及根际土壤硝酸根浓度和同位素组成的植物种间、生长季节间和土层间的差异。探讨根际土壤硝酸根可利用性和同位素特征值变异的机制，揭示根际土壤硝酸根指标变化对过去采用非根际土壤和植物根内硝酸根指标诊断植物硝酸根吸收和还原能力的影响。本项研究对详细理解植物-土壤界面氮循环的过程机制及其如何响应环境变化具有的科学意义。

根际是根系与土壤的微区域，是植物吸收氮素的活跃界面。准确理解植物根际土壤硝酸根浓度和同位素组成对土壤硝酸根可利用性及植物硝酸根利用的研究具有重要意义。目前，相关研究稀少，其变化特征和过程机制亟待阐明。本项目运用氮自然稳定同位素方法原理，以我国不同纬度带不同类型森林优势种乔木和灌木为研究对象，分析植物根际与非根际土壤硝酸根浓度和氮同位素组成及其影响机制。主要结论如下：1）不同植物根际土壤NO3-浓度及其δ15N值差异明显，变化范围分别为0-29.6 mg·kg-1和-15.2-55.3‰；根际土壤NO3-浓度有随纬度降低而降低的趋势，且不同采样点的同一植物种根际土壤NO3-浓度及其δ15N值也有显著差异。土壤中NO3-的来源及土壤硝化、反硝化微生物过程均会对NO3-浓度及其δ15Nδ值产生显著的影响。2）绝大多数植物根际土壤NO3-浓度高于非根际土壤，而根际土壤NO3-的δ15N值低于非根际土壤；且根际与非根际土壤NO3-浓度及同位素组成的差异性有明显的纬度变异性。那么，以往以非根际土壤为氮源评估植物氮利用可能会低估植物对硝酸根的利用能力。本项目详细解析了不同森林植物根际土壤硝酸根浓度及同位素组成的变化特征，揭示了根际土壤硝酸根对植物硝酸根利用的重要意义，对植物-土壤氮循环及植物氮利用的研究提供了研究思路，同时，该项目也促进了氮稳定同位素在土壤氮有效性及植物氮利用研究上的应用。