氮沉降对温带5个常见树种细根分解的影响

Atmospheric nitrogen (N) deposition has been considered as one major component of global change and is predicted to increase in the future. Fine root decomposition is a key process in terrestrial carbon and nutrient cycling, which may be potentially affected by the elevated atmospheric N deposition. Yet the effects of N deposition on fine root decomposition have rarely been examined. The objective of this project is to experimentally examine the effects of long term nitrogen deposition (beginning in 2009, in six applications per year) on fine root decomposition among five common temperate tree species (Quercus mongolica Fisch., Betula platyphylla Suk., Fraxinus mandshurica Rupr.,; Larix gmelinii Rupr. and Pinus koraiensis Sieb. et Zucc.) in five forests in northeastern China. Three simultaneous litter bag experiments will be used to determine whether the effects of N deposition on fine root decomposition (if it occurs) results primarily from direct limitation by increased soil nutrient availability or indirectly, from increased substrate N of fine root litter, or both. Furthermore, the response of the following extracellular enzyme activity to N deposition will be observed: 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. In addition to measuring enzyme activity of decomposing substrates, we will also 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 its microbial community. This study 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 context and perspective for empirical studies of the effects of N on decomposition.

氮沉降增加已经成为全球变化的重要现象之一。细根分解是森林生态系统碳和养分循环的重要环节，其分解过程受到大气氮沉降增加的潜在影响。然而，目前关于氮沉降增加对细根分解影响的研究还极其罕见。本项目拟在长期模拟氮沉降试验（2009年-现在）的5种典型温带森林内开展。选择每个林型的主要树种（蒙古栎、白桦、水曲柳、兴安落叶松和红松）为研究对象，从氮沉降增加可能通过提高土壤有效氮含量、凋落物基质氮含量或者两者同时提高来影响细根分解这三方面出发，综合探讨氮沉降对不同根序细根分解的影响。同时结合凋落物分解酶活性（纤维素降解酶、木质素降解酶、几丁质降解酶及磷酸酶）和微生物群落组成（细菌、真菌和放线菌的磷脂脂肪酸含量、革兰氏阳性菌及阴性菌含量）的测定，来阐明氮沉降对细根分解影响的机理，以期为更准确地预测生态系统碳、氮循环对氮沉降的响应提供重要科学依据。

氮沉降增加已经成为全球变化的重要现象之一。然而，目前关于氮沉降增加对细根分解影响的研究还极其罕见。按照项目计划任务书要求，我们在2016-2018年长期模拟氮沉降试验的5种典型温带森林内，选择了每个林型的主要树种（蒙古栎、白桦、水曲柳、兴安落叶松和红松）为研究对象，从氮沉降增加可能通过提高土壤有效氮含量、凋落物基质氮含量或者两者同时提高来影响细根分解这三方面出发，设计了三个独立相互补充的实验方案，并结合微生物群落组成及凋落物分解酶活性动态变化的测定，来分析氮沉降增加对细根分解影响的作用机理，并构建了不同根序细根分解对氮添加的响应与凋落物基质质量的关系。研究结果发现，经过为期前两年的分解，指数分解模型能够更好的解释根系分解质量残留率的变化。实验进行到第二年，5个温带森林树种根系初期的分解速率差异显著（P < 0.05），分解速率大小顺序为：胡桃楸>紫椴>黄菠萝>蒙古栎>山杨。同时，本研究的5个典型温带树种中，根系分解质量残留率随时间的延长总体呈现低级根[1+2]高于高级根[3+4]的趋势。低级根[1+2]的分解速率比高级根[3+4]低10%-28%左右。由于低级根和高级根功能上的不同，导致根系化学成分产生差异，并可能对细根分解速率产生了重要影响。本研究5个温带树种根系的初始化学成分浓度在不同根序之间存在较大差异（p<0.01）。然而，值得注意的是5个树种的[1+2]级根中酸不溶性物质（AUF）的浓度始终高于[3+4]级根；此外，[1+2]级根中的全非结构性碳水化合物（TNC）的浓度普遍低于[3+4]级根。这也是导致所有树种的[1+2]级根分解常数k值普遍小于[3+4]级根的主要原因。同时，研究发现氮添加均促进了初期分解速率，而抑制了后期的分解速率。这一规律与氮添加对木质素降解酶（多酚氧化酶和过氧化酶）的活性和微生物群落组成（细菌、真菌和放线菌的磷脂脂肪酸含量、真菌/细菌）含量变化密切相关。以上研究结果发表在PNAS，SBB和Forests等杂志。研究结果为更准确地预测生态系统碳、氮循环对氮沉降增加的响应提供了重要科学依据。