活性氮增加对我国北方泥炭地土壤有机碳的影响及其酶学机理

Northern peatlands store a large amount of organic carbon in soils, and play a key role in global carbon cycling. Meanwhile, northern peatlands are extremely nutrient poor ecosystems, and thus are sensitive to changes in nutrient availability. In northern peatlands, enhanced nitrogen deposition and nitrogen-fixing tree expansion have increased the amount of reactive nitrogen, which would cause a marked influence on soil organic carbon pool. Although the effect of increased reactive nitrogen on soil organic carbon has been received widespread attention, the results of these studies are inconsistent, and the underlying mechanisms still remain unclear. Moreover, the previous studies have focused on enhanced nitrogen deposition, and litter is known about the effect of nitrogen-fixing tree expansion on soil organic carbon pool. In this proposed project, we will simultaneously investigate the effect of nitrogen deposition and nitrogen-fixing tree expansion on plant biomass, soil organic carbon pool, soil organic carbon chemical structure, soil organic carbon mineralization, soil microbial community structure, soil hydrolytic enzyme activity, and phenol oxidase enzyme activity in a peatland in the North Da Xing’an Ling Mountain in Northeast China, and then compare the differences in these selected soil properties between nitrogen deposition and nitrogen-fixing tree expansion. We attempt to examine the effect of increased reactive nitrogen on soil organic carbon pool and its stability, and reveal the enzymatic mechanisms controlling the effect of increased reactive nitrogen on soil carbon pool in northern peatlands. The results from this project will not only help to accurately assess future soil carbon budget and long-term carbon sequestration potential in northern peatlands, but also provide essential data for understanding coupled carbon and nitrogen cycling and its biochemical regulatory mechanism.

北方泥炭地是个巨大的土壤碳库，在全球碳循环中发挥着关键的作用。而且，北方泥炭地养分极为贫瘠，对养分有效性的变化极为敏感。氮沉降和固氮树种扩张等引起的活性氮增加将导致北方泥炭地土壤有机碳库发生变化，但研究结果目前存在很大的分歧，而且作用机理也不清楚。另外，这些研究都集中在氮沉降，很少关注固氮树种扩张现象。本项目拟在我国大兴安岭北部选取典型泥炭地，研究氮沉降和固氮树种扩张两种活性氮增加方式对植物生物量、土壤有机碳储量、土壤有机碳化学结构特征和有机碳矿化速率的影响，测定土壤微生物群落结构、水解酶和酚氧化酶活性等的变化，并对比氮沉降和固氮树种扩张两者之间的差异，阐明活性氮增加对土壤碳库及其稳定性的影响，揭示活性氮增加改变土壤碳库的酶学机理，这不仅为准确评估今后北方泥炭地碳收支动态变化和长期固碳能力提供科学依据，也有助于认知陆地生态系统碳氮循环的耦合过程及其生化调控机制。

北方泥炭地土壤中储存了大量的有机碳，在全球碳循环中发挥着关键的作用。而且，由于养分极为贫瘠，北方泥炭地土壤有机碳对养分有效性的变化极为敏感。目前，由于气候变暖、干旱以及人类活动的干扰等，北方泥炭地均出现大气氮沉降增加和固氮树种扩张现象，这必然会改变土壤有机碳动态，但已有研究结论存在较大的分歧，而且调控机理也不清楚。本项目以我国大兴安岭地区典型泥炭地作为研究对象，基于野外原位氮添加控制试验和自然梯度试验，研究了氮沉降和固氮树种扩张这两种活性氮增加方式对土壤有机碳库的影响及其酶学机理。本研究发现：.（1）氮沉降和固氮树种扩张均促进植物生长，增加了植物生物量，但降低了物种多样性。而且，物种多样性降低主要是由于苔藓和地衣的消失所引起的。然而，氮沉降和固氮树种扩张对维管束植物的影响存在差异。具体来说，氮沉降增加了落叶灌木和禾草的盖度和生物量，而固氮树种扩张则增加了落叶灌木生物量。减少了常绿灌木和禾草生物量。.（2）氮沉降和固氮树种扩张对土壤有机碳库及其稳定性的影响是一致的。氮沉降和固氮树种扩张不仅减少了土壤有机碳储量，而且降低了土壤有机碳中芳香碳组分的比例，导致有机碳芳香度呈现下降的趋势。这表明，固氮树种扩张和氮添加均将导致北方泥炭地土壤有机碳数量和稳定性呈现降低的现象。.（3）氮沉降和固氮树种扩张降低土壤有机碳库的酶学机理也基本一致。具体来说，由于土壤有机碳的芳香度降低，氮沉降和固氮树种扩张均提高了酚氧化酶活性，改变水解酶活性，导致土壤多酚含量降低，刺激微生物分解，导致土壤出现碳丢失现象。.本研究结果不仅为准确评估和预测全球变化背景下北方泥炭地土壤碳收支动态变化和碳汇功能提供科学依据，也有助于认知和理解生态系统碳氮循环的耦合过程及其生化调控机制。