高浓度臭氧背景下城市森林凋落物的分解动态及其机理

The continuously increasing atmospheric O3 concentration is considered one of the serious environmental problems in the present world, particularly in cities of developing countries. With the development of urbanization and the construction of urban forest，more attention is focused on the potential impact of elevated O3 pollution on the urban forest ecosystem. However, little information is known for the effects of elevated O3 on litter decomposition of urban forest, which plays an important role in maintaining soil fertilization, sustainable development of urban forest ecosystem, and global forest carbon balance and nutrient cycling. Based on the complexity and uncertainty of the processes and mechanisms of forest litter decomposition, and the results of our experiments on the relationship between urban trees and climate change in recent years, we will further carry out the studies in the effects of elevated O3 on the dynamics and mechanisms of litter decomposition of urban forest (Shenyang city, China) by the OTCs simulation and litter-bag method and isotope technique. Litter quality and dynamic decomposition of urban main tree species are evaluated by measurements of dry mass loss, C/N rate, lignin concentration, the discrimination of C and N isotopes during litter decomposition and many other biochemical parameters, by which the dynamics of litter decomposition of urban forest would be known. Influencing factors such as temperature and precipitation will be estimated by the integrated analysis. The main objectives are to provide the scientific references or new theoretical proofs for the litter management and the material cycle of forest ecosystem in response to elevated O3, as well as the sustainable development and management of urban forest under global climate change.

目前,全球近地大气臭氧浓度升高已成为世界范围内的重要环境问题。城市环境臭氧浓度的增加也日趋引起人们的关注。城市森林凋落物对于维持城市森林土壤肥力、实现城市森林可持续发展及评估全球森林碳平衡和物质循环具有重要作用。本项目针对森林凋落物分解过程及其机制研究中存在的诸多不确定性和复杂性，基于我们多年来对城市森林和气候变化相互关系的研究，拟以我国北方典型"森林城市"沈阳市城区和城郊森林主要树种为研究对象，采用OTC模拟手段，利用凋落袋法和稳定同位素技术，研究高浓度臭氧对城市森林凋落物质量及其分解过程中干物质损失、碳氮比及木质素含量等多项生理生化指标的影响，阐明和揭示高浓度臭氧对城市森林凋落物分解的影响规律及其机理，并通过影响凋落物分解的多个生态因子耦合和相关关系分析，探讨全球高臭氧浓度背景下城市森林凋落物的生态管理对策，为城市森林的可持续发展和经营及全球森林生态系统物质循环提供新的理论依据和参考。

全球近地大气O3浓度升高及其引发的诸多城市O3污染浓度超标已成不争事实。城市森林凋落物对于维持城市森林土壤肥力、实现城市森林可持续发展及评估全球森林碳平衡和物质循环具有重要作用。项目以我国北方典型“森林城市”沈阳市区和城郊森林主要树种为研究对象，通过OTC模拟系统，重点开展了高浓度O3对城市森林不同树种凋落叶质量及其分解规律的影响研究，阐明了高浓度O3对城市森林凋落物分解的影响机理。研究结果表明：.1) 不同浓度的O3熏蒸对城市不同树种凋落叶的影响存在较大差异。80 ppb的 O3处理使得银杏凋落叶干重残留率比对照有所升高，表明O3熏蒸抑制了凋落物的分解。与对照相比，120 ppb的高浓度O3显著提高了银杏凋落叶中N、K的含量 (P＜0.05)，但降低了总酚、缩合单宁和可溶性糖含量(P＜0.05)。2) 分解后期，80 ppb的高浓度O3熏蒸对10和30年生银杏凋落叶的分解具有促进作用，分解速率与对照相比分别增加了19.4%和10.6%。120 ppb的高浓度O3降低了银杏凋落叶木质素及总酚残留率，表明高浓度O3提高了银杏凋落叶营养物质的释放和周转，但抑制了蒙古栎凋落叶C、N和木质素的释放。3) 160 ppb的高浓度O3显著抑制了市区树木凋落叶的分解(P＜0.05)，但对城郊树木凋落叶的分解无显著影响。4) 高浓度O3对单一或混合凋落叶分解尤其是阔叶类型树种的凋落叶的影响较为显著。两种阔叶树种（银杏和蒙古栎）混合凋落叶的干重剩余率最低，表明其分解速率较快。分解末期，蒙古栎和油松混合凋落叶的干重剩余率最大，表明O3抑制了针阔混合凋落叶的分解。5) 高浓度O3显著降低了凋落物分解下的土壤微生物含量(P＜0.05)，微生物革兰氏阳性菌与阴性菌的比值升高，真菌与细菌的比值降低，前者比后者对O3更为敏感。O3对土壤微生物及主要土壤酶活产生了抑制作用，这些变化可能是高浓度O3抑制银杏凋落叶分解的重要原因之一。本项目研究可为我国应对气候变化背景下大气O3污染的树种选择，城市森林凋落物管理及丰富全球森林生态系统物质循环理论提供科学依据和参考。