长期氮素添加对内蒙古典型草原优势物种根系分解的影响

Atmospheric nitrogen (N) deposition is one of the important global change drivers, with consequences on ecosystem biogeochemical cycling. Root decomposition represents a large pool of carbon and nutrient cycling in grassland ecosystems, which may be potentially affected by the elevated atmospheric N deposition. Presently, most studies about the effects of N deposition on litter decomposition have focused on above-ground litter decomposition, but neglected root decomposition. The objective of this project is designed to experimentally examine the effects of long term nitrogen deposition (beginning in 2008, in 12 applications per year) on root decomposition among different plant functional group assemblages (Leymus chinensis, Stipa grandis, Allium tenuissimum, and Chenopodium aristatum.) in an steppe. Three simultaneous litterbag experiments will be carried out to determine whether the effects of N deposition on root decomposition (if it occurs) results primarily from direct limitation by increased soil nutrient availability or indirectly, from increased substrate N of root litter, or both. Furthermore, the response of the following extracellular enzyme activity to N deposition will be examined: two enzymes involved in cellulose decomposition; two enzymes involved in decomposition of lignin; an enzyme involved in chitin decomposition; and an enzyme involved in mineralizing ester-bonded phosphate. Furhermore, we will measure the response of microbial community composition (phospholipid fatty acids of bacteria, actinomycete and fungi of microbial community and the relative abundances of Gram-negative and Gram-positive bacteria) to N deposition in order to determine whether treatments were affecting decomposition by altering soil microbial community. This project will help us to determine the relationships between the response of root decomposition to N addition and root litter quality (especially lignin concentrations). Moreover, it will inform modeling efforts that aim to determine the effects of elevated atmospheric N deposition on regional and global carbon and N cycling, providing much-needed perspective for understanding of the effects of N on decomposition.

氮沉降增加是重要的全球变化因子，影响着生源要素的生物地球化学循环。根系分解是草原生态系统地下养分和有机质输入的主要途径，该过程受到氮沉降增加的潜在影响。然而，目前关于氮沉降对凋落物分解影响的研究多集中于地上凋落物分解，而忽视了根系分解过程。本项目拟依托长期模拟氮沉降实验平台（2008年至今），以典型草原不同植物功能群的优势物种（羊草、大针茅、细叶葱和刺穗藜）为研究对象，通过3个相互补充的独立实验来研究长期氮添加对草原根系分解的影响：1）土壤有效氮增加；2）根系基质变化；3）两者共同变化对根系分解的影响。同时，结合对根系分解过程中微生物群落组成（细菌、真菌、放线菌、革兰氏阳性及阴性菌的磷脂脂肪酸含量）和分解酶（纤维素降解酶、木质素降解酶、几丁质降解酶及磷酸酶）活性的分析，解析氮沉降增加对根系分解的影响机理，并为构建根系分解对氮沉降的响应与根系初始基质（尤其是木质素）含量的关系提供数据支撑。

氮沉降增加已经成为全球变化的重要现象之一。目前，在氮沉降研究中，国内外学者均较为关注地上凋落物分解的研究，而忽视了地下凋落物—根系的分解。越来越多的研究表明，细根的生长、死亡和分解可能是地下养分和有机质输入的主要途径，土壤碳库的大部分碳可能主要来源于细根。严格按照项目任务书要求，以长期模拟氮沉降试验（2008年-现在）的典型草原生态系统为研究对象，选择了不同功能群的4个优势物种（羊草、大针茅、细叶葱和刺穗藜），通过3个相互补充的独立实验系统研究了长期氮添加对草原根系分解的影响：1）土壤有效氮增加；2）凋落物基质变化；3）两者共同变化对根系分解的影响。经过2018和2019连续两年的分解实验后，我们发现四个典型草原优势种的根系分解速率和养分释放速率均存在显著差异（P < 0.05），氮添加均促进了细根的分解速率（P < 0.01）。然而氮促进程度在不同直径间差异显著（P < 0.01）。我们发现氮添加对<0.5 mm的根系分解速率促进程度表现为：细叶葱>羊草>大针茅>刺穗藜；对0.5-2.0 mm的根系，促进程度分解表现为：羊草>细叶葱>大针茅>刺穗藜；对 > 2.0 mm的根系促进程度分解表现为：大针茅>细叶葱>羊草>刺穗藜。氮沉降对根系分解速率增加的强度与凋落物基质含量密切相关，其中与木质素含量呈正相关，与初始氮产量呈正相关，而与根系的初始木质素含量不相关。同时，结合根系分解过程中微生物群落组成（细菌、真菌、放线菌、革兰氏阳性及阴性菌的磷脂脂肪酸含量）和分解酶（纤维素降解酶、木质素降解酶、几丁质降解酶及磷酸酶）活性的动态测定，发现氮沉降增加对根系分解的影响很大程度上受凋落物分解酶的调控。相关研究结果为更准确地预测草地生态系统土壤碳循环和碳储存对氮沉降增加的响应提供了重要参考依据。