氮沉降增加背景下寒温性针叶林土壤有机碳转化与碳循环微生物的关联机制

Under the trend of nitrogen deposition elevation, it is still unclear whether the relationship between nitrogen deposition and soil carbon cycling microorganism and the mechanisms in regulating the soil organic carbon transformation processes that they underpin, as well as the in-depth study of the current problems of soil biochemistry. The objectives of this project is to explore soil organic carbon transformation and carbon cycling microorganism under the background of nitrogen deposition elevation, which remains poorly understood. In this project, we will take the N-limited boreal coniferous forest as the research object and conduct the multiform gradient nitrogen forms settlement control experiment using in situ, the aims are to study the evolution characteristics of soil microbial carbon cycling function genes (carbon fixation gene, activity, slow and resistant organic carbon degradation gene) under different morphological N input, combined with the analysis of microbial carbon metabolism and enzyme activity , we will analyse the impact of the N input on the direction and degree of soil microbial carbon cycling gene. We also study soil active , slow and resistant organic carbon and its content changes under different morphological N input, combined with the input (litter decomposition, fine root turnover) and output (soil respiration, leaching of dissolved organic carbon) of soil carbon sink, we will clarify the response mechanism of organic carbon fractions to nitrofen forms; and Further, by using Mantel test, we will analyze the interaction mechanism of soil organic carbon transformation and carbon cycling microorganism in boreal coniferous forest under nitrogen deposition elevation, and explore on the influence mechanism of N input on soil carbon sink ability of boreal coniferous forest. The results will provide new insight to the basic theory of soil carbon cycling in forest，and provide a scientific basis for the safety strategy of boreal coniferous forest.

氮沉降增加趋势下，土壤碳循环微生物如何与氮沉降关联，进而如何影响土壤碳转化是土壤生态过程研究的科学瓶颈。本项目针对N输入-碳循环微生物-土壤有机碳转化之间的关联机制不明确这一问题，以氮限制寒温性针叶林为研究对象，采用原位多形态梯度N沉降控制实验（3年），研究不同形态N输入下土壤碳循环微生物功能基因（固碳基因，活性、慢性和惰性有机碳降解基因）的演变特征，结合微生物碳源代谢功能与酶活性分析，阐明N输入影响碳循环功能基因的方向和程度；研究不同形态N输入下土壤活性、慢性和惰性有机碳含量的改变，结合土壤碳库输入（凋落物分解、细根周转）与输出（土壤呼吸、可溶性有机碳淋失）过程，阐明有机碳组分对N输入的响应与更新速率；利用Mantel检验，解析碳循环微生物与有机碳转化的关联机制，探讨N输入对寒温性针叶林土壤碳汇能力的影响机制，研究结果将丰富森林土壤碳循环的基础理论，为寒温性针叶林的安全战略提供科学依据。

本项目以寒温性针叶林长苞冷杉林为研究对象，开展了为期3年的原位多形态梯度氮添加控制实验，结合野外采样与室内分析，系统研究了长苞冷杉林土壤碳库输入输出过程对氮添加的响应，系统解析了氮输入-碳循环微生物功能基因-碳转化之间的关联机制。研究结果表明：（1）氮沉降在前期对凋落物分解有促进作用，而后期则表现为抑制作用。氮添加促进凋落C和p营养元素的释放，对N营养元素的释放表现为在前期抑制作用，后期表现为促进作用。不同形态氮添加对凋落分解和营养元素释放的作用排序为硝酸铵>硫酸铵>硝酸钠。从氮添加水平来看，中低氮对凋落分解与营养元素释放的作用大于高氮。（2）氮输入在早期促进了微生物呼吸和根系呼吸，进而增加了土壤呼吸，在实验后期，氮输入抑制了微生物呼吸，进而对减少了土壤呼吸，而对根系呼吸无显著影响。氮形态与水平对土壤呼吸的影响与凋落物分解相似。（3）中低高氮在前期显著促进0-0.5，0.5-1.0级细根的生长，后期各处理对0-0.5，0.5-1.0级细根的生长没有显著影响。氮添加使0~0.5mm径级细根死亡量显著降低。对0.5~1.0mm细根的影响表现为：前8个月显著降低了0.5-1.0径级细根的死亡量，后期没有显著影响。整个实验期间，低中高氮处理对1.0-1.5，1.5-2.0级细根的生长和死亡没有显著影响。（4）氮添加的第一年（2019年），显著增加了土壤有机质层可溶性有机碳的淋失，第二年（2020年）和第三年（2021年）显著减少了土壤有机质层可溶性有机碳淋失，对土壤矿物质层土壤可溶性有机碳淋失无显著影响。（5）氮添加显著降低了半纤维素降解基因、纤维素降解基因、木质素降解基因、淀粉相关酶基因和二氧化碳产生基因（apu,npit, chitinase，amyX, cellobiase）的丰度，进而降低活性、慢性有机碳的转化，最终增加活性、慢性有机碳的含量。研究结果可为我国双碳战略提供数据支撑，也可为寒温性针叶林的安全提供科学依据。