大兴安岭森林演替过程中土壤碳-氮耦合机制研究

Boreal forest in the Da Hinggan Mountains is the southern expansion of Siberia forest ecosystem and it situates within the cold-temperate climate zone of Northeast China. Different forest succession stages can differ in carbon turnover strategy and also their responses to nitrogen deposition. All these processes may affect carbon and nitrogen pools in forest soil, and hence, it will have profoundly impacts on the global climate change process. The proposal we prepared here aims to compare soil properties, soil carbon-nitrogen storages and soil carbon-nitrogen fluxes in different forest succession stages. These purposes achieved by the methods of spatial series instead of temporal series which are mainly through field measurements of carbon-nitrogen pools and fluxes, combined with respiration and decomposition studies. We hope to understand better why and how soil carbon-nitrogen fluxes differ between different forest succession stages. To explore why soil carbon storage varied among three succession stages, and trying to explain the phenomenon mentioned before by experiments regarding litter quality and quantity, tree growth and decomposition, soil carbon accumulation and mineralization, organic material fluxes and fine root turnover. Having an insight into effects of different tree species on soil carbon and nitrogen fluxes and pools is crucial for quantitatively evaluating regional carbon-nitrogen cycle patterns, and also this can provide data supporting and theoretic basis for better regulating and managing the boreal forest in the Da Hinggan Mountains of China.

大兴安岭森林作为我国重要的生态屏障，发挥着涵养水源、调节气候及保障国土安全等重要作用。不同演替阶段森林在碳周转及对氮沉降的响应存在较大差异，容易对土壤碳-氮库产生深刻影响，进而作用于气候变化进程。本项目以黑龙江漠河森林生态系统国家定位观测研究站为依托平台，通过对不同演替阶段森林土壤特性、凋落物分解与细根动态、土壤碳-氮储量以及土壤表面二氧化碳和氧化亚氮通量等方面研究，进一步确定土壤理化特征随演替发生而改变，分析森林演替过程中土壤碳-氮周转的变化规律，量化不同演替阶段森林土壤碳-氮通量的动态特征，及森林土壤碳通量对生态系统碳通量的贡献，深入探讨森林土壤碳-氮的耦合机制，为调控和管理大兴安岭林区北方森林生态系统生态过程提供数据支撑与理论依据。

本项目基于大兴安岭北部典型森林演替过程为研究对象，分析不同演替阶段森林土壤特性、碳-氮储量和土壤碳-氮通量的变化特征、差异及其耦合机制。结果表明：（1）土壤有机碳和全氮储量从演替前期的白桦林到演替顶级群落兴安落叶松林有着显著的增加趋势，而且这种积累主要发生在演替后期；演替初期不同林龄白桦林生态系统地上和地下碳储量均随林龄的增加而明显升高，老龄白桦林仍具有较强的碳汇作用；演替后期不同林下植被的兴安落叶松森林生态系统碳氮储量存在显著差异，林下植被对兴安落叶松林生态系统的碳和氮储量有较大影响。（2）在短期凋落物分解实验中发现，白桦林凋落物质量损失率显著小于兴安落叶松林。此外，非生长季凋落物质量损失率、C、P、K元素释放均以秋季为主，而N元素释放则以春季为主；白桦和兴安落叶松根系分解总体表现为夏秋季分解较快，冬春季分解较缓慢。（3）土壤温室气体排放随着森林演替进程呈现显著的变化，且季节动态差异较大。森林土壤在森林演替的不同阶段均表现出明显的二氧化碳和氧化亚氮的源，以及甲烷的汇；此外，林下植被和凋落物处理对温室气体有显著影响，去除林下植被和凋落物会增加白桦林和兴安落叶松林的CO2通量，去除凋落物后则有降低土壤N2O通量的趋势，去除林下植被也会对土壤N2O通量的升高有不同程度的影响。（4）兴安落叶松林土壤碳氮含量随土壤深度的增加呈现下降趋势，土壤碳含量和氮含量、微生物量碳含量和氮含量均存在显著正相关的线性耦合关系。此外，不同林龄白桦林生态系统碳氮通量的变化均主要集中在4-8月份。在一定范围内，土壤碳释放速率随着土壤氮释放量的增多而增加，超过一定范围后土壤氮释放量的继续增加，土壤碳释放受到抑制，而开始下降。以上研究结论为气候变化背景下增加森林生态系统碳储量的碳汇管理策略提供科学依据。