土壤碳氮转化及微生物功能对氮沉降形态与竹林经营方式的响应机制

The subtropical region in China is becoming one of the areas with one of the highest nitrogen (N) deposition rates in the world. In subtropical China, Moso bamboo forests occupy a very large area， are considered the largest plantation type， and are being affected by the high rate of N deposition in the subtropical region. However, how Moso bamboo forests with different management intensities respond to different forms of N deposition has been little studied. The environmental implication of the high rate of N deposition to Moso bamboo forests is also poorly understood. Past research on N deposition has been focused on the effect of the deposition of inorganic N but the effect of organic form of N deposition on Moso bamboo forests has not been studied. This research will investigate the effect of inorganic and organic forms of N deposition (simulated) on both extensively and intensively managed Moso bamboo forests. We will use 15N isotope tracing, 13C NMR (nuclear magnetic resonance) and molecular microbial ecology techniques to study 1) the fate of the deposited N, 2) the effect of N deposition on soil N and carbon (C) transformations, and soil organic C stability, 3) the effect of the different forms of N deposition on greenhouse gas emissions, 4) the relationship between soil C and N transformations and greenhouse gas emissions, 5) the effect of the form of N deposition on microbial population structure and function, 6) the relationship between soil N and C transformations, as well as soil organic C stability and microbial population composition/structure, and 7) the effect of N deposition on bamboo growth, all in comparison between extensively and intensively managed bamboo forests. We aim to improve our understanding of the impact of N deposition on the global warming potential and the feedback mechanism for future climate change in subtropical bamboo forests. This research will provide opportunities to develop strategies to mitigate the negative environmental impact of increased N deposition in subtropical China.

我国亚热带地区将成为未来全球大气氮沉降最严重的区域之一。毛竹林作为亚热带面积最大的人工林，在不同经营方式下对不同形态的氮沉降如何响应尚缺乏系统研究，因为以往的研究局限于关注无机氮沉降对毛竹林的影响。本研究以粗放和集约经营的毛竹林为研究对象，用模拟无机和有机氮沉降的形式，应用15N同位素示踪、有机碳核磁共振分析及微生物功能基因测定来研究沉降的无机和有机氮在生态系统中的去向；明确不同形态氮素沉降对粗放和集约经营毛竹林土壤碳氮转化过程和有机碳稳定性的影响；探讨不同形态氮沉降对温室气体排放通量的影响，解析不同氮沉降形态导致的碳氮转化过程与温室气体排放通量的关联；揭示土壤微生物群落与功能对不同形态氮素沉降的响应机制，探讨氮素转化和去向及碳稳定性与介导上述过程中土壤微生物功能类群的耦合规律。研究结果将为阐明亚热带地区粗放和集约经营毛竹林响应不同形态氮沉降的生物地球化学机制提供理论基础。

大气氮沉降增加深刻影响着森林土壤碳氮循环。我国亚热带地区是全球大气氮沉降最严重的区域之一，而毛竹林作为该地区面积最大的人工林，其土壤碳氮循环如何响应不同形态（无机氮和有机氮）和方式（林冠和林下）的氮沉降尚不清楚。本项目依托浙江农林大学安吉毛竹林氮沉降模拟试验基地，设置林冠、林下有机氮和无机氮沉降处理，研究了氮沉降形态和方式对毛竹林土壤温室气体排放、碳氮循环微生物群落结构与功能、凋落物降解等的影响效应。研究发现：1）氮沉降显著促进了土壤N2O和CO2排放，且在林冠和林下两种氮沉降方式下，无机氮沉降的促进作用皆强于有机氮；2）除林下有机氮沉降未显著改变土壤中纤维二糖水解酶基因的丰度外，其他氮沉降处理均显著降低了该基因的丰度，同时，林冠无机和有机氮沉降均提高了土壤中酚氧化酶基因的丰度，而林下氮沉降未改变该基因丰度；3）林冠氮沉降未显著改变土壤中氨氧化和反硝化功能基因的丰度，而林下无机氮和有机氮沉降均显著降低了土壤中这些氮循环相关功能基因的丰度；4）氮沉降未显著改变土壤细菌的多样性和群落组成，但提高了其共发生网络的复杂度；5）除林冠有机氮沉降显著提高毛竹凋落叶的产量外，林冠和林下氮沉降均未改变凋落叶产量，同时林冠较林下氮沉降显著提高了毛竹凋落叶的降解速率，而无机氮和有机氮之间无显著差异。综上，本项目研究结果揭示了不同形态和方式的氮沉降对毛竹林土壤碳氮循环的影响差异。由于以往在森林生态系统中模拟氮沉降大多采用林下喷施无机氮的方式，本项目研究为更为准确评估氮沉降增加背景下的土壤碳氮循环过程提供了新的见解。