氮化合物形态对内蒙古典型草原生态系统凋落物分解影响的化学计量机制

The rapidly-increasing atmospheric nitrogen (N) deposition will generate profound impact on the productivity of terrestrial ecosystems and alter many ecological processes. It has been generally accepted that N deposition will enhance ecosystem productivity and at the same time accelerate the loss of biodiversity. These changes will drastically affect the quality and quantity of litter, and thus its decomposition rate, resulting in alterations of the plant-soil relationships and their feedbacks. Results from many studies in different terrestrial ecosystem across the world have demonstrated that the effects of N deposition on litter decomposition can be varying. For example, positive, negative and neutral effects of litter decomposition under N enrichment have been reported. These inconclusive findings might be attributable to the amount and forms of N compounds added. Nitrogen input to ecosystems through dry or wet deposition can contain a complex of different N compounds, with each having different impact on the soil physico-chemical properties and on the quality and quantity of dead litter. In this proposed study, we will conduct a large-scale field experiment in the Hulun Buir grassland to examine the effects of five forms of N compounds (e.g., NH4NO3, (NH4)2SO4, NH4CO3, (NH2)2CO and the slow-release (NH2)2CO) on the litter decomposition rates of five dominant plant species. Our objectives are to evaluate the influence of different N compounds on the litter decomposition rates, the stoichiometric dynamics of plant and soil carbon, nitrogen and phosphorus as well as the changes of belowground microbial community composition in order to elucidate the underlying mechanisms of litter decomposition in grassland ecosystems in the face of N deposition.

大气氮沉降会增加生态系统生产力和降低生物多样性, 同时也影响地上凋落物质量和分解速率，进而改变植物-土壤之间的物质循环过程。有关氮素添加对凋落物分解的研究很多，但是得出的结论并不一致，表现为氮素添加对草原凋落物的分解有促进作用或抑制作用或没有影响。这些不同与氮素添加量和添加的含氮化合物形态有直接关系。自然氮沉降是多种氮化物的复合体，且不同形态的氮素化合物对土壤理化性质的影响不同，生态系统对不同类型氮素添加的敏感度不同，因而其对凋落物分解的影响结果也会不同。本实验以大气氮沉降较低、受外界干扰较小的呼伦贝尔草原为研究对象，选择硝酸铵、硫酸铵、碳酸氢铵、尿素和缓释尿素四种含氮化合物（五种形态），采用野外控制实验方法，研究不同形态的氮素化合物对额尔古纳草原优势植物凋落物分解速率、凋落物和土壤C：N：P计量化学动态变化特征和土壤微生物群落组成的影响，以此揭示氮沉降影响凋落物分解的机理。

大气氮沉降会增加生态系统生产力和降低生物多样性，同时也影响地上凋落物质量和分解速率。本研究依托内蒙古额尔古纳不同氮素化合物添加实验平台，以呼伦贝尔草甸草原三种优势植物糙隐子草（Cleistogenes squarrosa）、披针叶黄华（Thermopsis lanceolata）和羊草（Leymus chinensis）的凋落物为研究对象，采用凋落物分解袋法，研究了不同氮添加处理对三种凋落物为期4年的分解。研究发现氮素添加形态显著影响凋落物分解速率及其控制因子。糙隐子草、披针叶黄华和羊草三种凋落物经过1445天的分解后，硝酸铵，缓释尿素，尿素，碳酸氢铵，硫酸铵添加处理下，豆科C3植物披针叶黄华分解速度最快，禾草C4植物糙隐子草分解速度次之，禾草C3植物羊草分解速度最慢，豆科植物分解速率显著高于禾本科，同一凋落物分速率k值氮形态间差异显著。同时，不同氮素化合物均改变了分解过程中凋落物的养分含量。三种凋落物的氮残留量、磷残留量和碳磷比均随分解时间降低，氮磷比则呈现出升高趋势。分解的1086天后，三种凋落物氮含量和碳氮比均表现为氮形态间差异不显著，但氮添加量间差异显著，氮添加量越高，氮含量越高，碳氮比越小；以上结果充分验证了优势物种植物凋落物、不同氮化合物及不同氮添加量对凋落物分解的影响机理不同。上述研究有助于准确评估氮沉降持续增加的背景下，我国草原生态系统养分循环过程的响应方式及机理。