火干扰对兴安落叶松林土壤氮循环的影响机理

Nitrogen (N) is one of the most limiting nutrients in northern terrestrial ecosystems and it always has important influence on the structure and function of forest ecosystem with combining carbon cycling. Larch forest is representative type of boreal forest ecosystem, it has the largest area and it is also the most sensitive to global climate change. Fire disturbance is also the largest disturbance in the boreal forest ecosystem. Few studies have examined the direct effects of severe fire on soil N cycling, nor the long-term consequences.For scientific evaluation on postfire dynamic variation character of the nitrogen cycle and disturbance mechanism of forest soil nitrogen cycle in larch forest. We take a wildfire chronosequence to examine the long-term dynamics of net and gross nitrogen(N) transformation, mineralization, immobilization and the relations with vegetation recovery following severe buring. We will established paired, adjacent unburned sites for each burn year. We will test direct effects of fire by comparing burned and unburned sites for each year, and we want to assess long-term changes over time using the differences between each burned and unburned pair. The main research including postfire loss, transformation and its influencing factors of litter layer N in larch forest; the dynamic and influencing mechanism in different vegetation recovery stages after fire disturbance; How fire and restoration treatments influence N immobilization and transformation process of soil microbial biomass; How fire and restoration treatments influence the space-time characteristics of N ammonification, nitrification, mineralization and its response to fire disturbance; Comparing dynamic and immobilization mechanism of soil N pool under natural and artificial recovery treatments after fire disturbance. Based on the above research, we will explore postfire mechanism of N cycling transformation and control in boreal forest ecosystem. The results may aslo provide scientific basis for futher understanding the role of forest fire in boreal forest ecosystem and data basis for the restoration of forest ecosystem after severe fire disturbance. Our research may have long-term implications for the recovery of ecosystem N and associated rates of net primary productivity in boreal forest ecosystems.

氮（N）是北方陆地生态系统中最重要的养分元素之一，且常常与碳循环耦合而对森林生态系统产生重要影响。以兴安落叶松为优势群落的北方森林生态系统在全球森林生态系统中占有重要地位。火干扰是北方森林最重要的干扰因子之一，为科学评价火后兴安落叶松林氮循环动态特征及火干扰对北方森林土壤氮循环的影响机制，选择火后不同时期的火烧迹地以及火后造林地进行采样、连续监测，研究火干扰后兴安落叶松林凋落物层氮损失、转化及控制因素；火干扰后不同植被恢复阶段土壤氮库动态及影响因素；火干扰对兴安落叶松林土壤微生物生物量氮固持与转化的影响；兴安落叶松林土壤有机氮氨化、硝化和矿化时空特征及其对火干扰的响应；比较火干扰后自然与人工恢复模式下土壤氮库的动态及固持机制。探索火干扰后氮素循环在北方森林土壤中各个过程的转化控制机制，为进一步科学认识林火在北方森林生态系统中的作用提供科学依据，为重度火干扰后森林生态系统恢复提供数据基础。

氮元素是限制北方针叶林生态系统树木生长的重要养分元素之一，研究火干扰对森林生态系统土壤氮元素有效性的影响及其火后变化规律对于揭示火后森林生态系统的演替过程和火后森林生态系统恢复机制具有重要的生态学意义。本研究通过在不同时期的兴安落叶松林火烧迹地及火后造林地设置固定监测样地，研究重度火干扰对兴安落叶松林土壤氮的损失、转化与固持等各个过程的影响，揭示火后生态系统恢复过程中植被在土壤氮转化利用与固持中的作用，探索火干扰后氮素循环在北方森林土壤中各个过程的转化控制机制，为进一步科学认识林火干扰在北方森林生态系统中的作用提供科学依据，主要结果如下：.（1）火干扰后 29年 内，随着火干扰年限的增加，针叶和阔叶凋落物分解系数与对照样地相比先降低后增高；小枝凋落物分解系数与对照样地相比持续增高，且随着火干扰年限的增加，分解系数增加幅度逐渐降低。火干扰对针叶和阔叶凋落物分解先抑制后促进，对小枝凋落物分解始终起促进作用。.（2）火后3年、9年、28年上层土壤无机氮含量分别降低了约6%、21%、23%。火后3年样地上层土壤矿化速率（Rmin）显著升高，火后9年样地上层土壤Rmin下降了约26%，火后28年上层土壤Rmin基本恢复到火烧前的水平。.（3）与人工造林方式相比，火后自然恢复方式更加有利于土壤氮的有效性的恢复。自然恢复上层土壤NH4+-N、NO3--N和无机氮含量分别比人工恢复方式高约70%、29%、67%。自然恢复方式上层土壤Rmin是人工恢复方式上层土壤Rmin的3.6倍。.（4）自然恢复方式有利于森林生物多样性的恢复，火后自然恢复方式有利于豆科植物的恢复，火后自然恢复条件下豆科植物的恢复和土壤MBN的增加可能是引起土壤Rmin增加的主要原因。.