模拟氮沉降对华北地区典型灌丛养分循环的影响

Human activities are rapidly increasing the atmospheric nitrogen deposition across the globe, which have been influencing shrublands, including nutrient cycle. Shrublands are mostly distributed in nutrient poor sites and the plant growth is severely limited by soil nutrient availability. Such nutrient-poor ecosystems are usually considered relatively vulnerable to environmental changes such as N deposition and climate change. Our previous study carried out in shrublands found that simulated short-term N deposition improved soil nutrient status, increased leaf N content and decreased nitrogen resorption efficiency(NRE), which will change the quality(e.g. C:N) of litter and its decomposition rate. In addition, the changes of soil nutrient availability and other physical and chemical properties caused by N addition can affect plant's nutrient uptake, distribution and stoichiometry, and also can affect soil microbial community, and then affect the decomposition of litter. The changes of above ecological processes and leaf functional traits ultimately will affect nutrient cycling processes of ecosystems. At present, it is not clear that the mechanism of the effect of N deposition on nutrient cycle of shrublands. We regard Vitex negundo var. heterophylla and Spiraea salicifolia shrublands as research objects, systemically research the effects of nitrogen addition on soil microbial, plant nutrient uptake and distribution, leaf nutrient resorption and ecological stoichiometry, and litter decomposition, reveal the status of plant nutrient limitation, adaptation strategies of plants and the response mechanism of N, P cycle to atmospheric N deposition in northern shrublands, and provide scientific basis and beneficial reference for the management of vulnerable ecosystems in China.

人类活动导致大气氮(N)沉降不断增强，正深刻影响着生态系统(包括养分循环过程)。灌丛通常为N限制生态系统，易受N沉降的影响。前期研究发现，模拟短期N沉降改善了灌丛土壤养分状况，使植物叶片N含量增加，N重吸收率(NRE)下降。这会改变凋落物的质量(如C:N比)并影响其分解速率。此外，N添加引起的土壤养分有效性及其他理化性质的变化，除了会影响植物对养分的吸收与分配，还会影响土壤微生物群落，进而影响凋落物的分解。上述生态过程及叶功能性状的改变最终会影响生态系统的养分循环过程。目前，有关N沉降对灌丛的养分循环影响机制尚不清楚。本项目以荆条和绣线菊灌丛为研究对象，系统研究氮添加对土壤微生物、植物的养分吸收与分配、叶片的养分重吸收和化学计量特征及凋落物分解的影响，揭示北方灌丛植物养分限制状况及其适应策略以及N、P循环对大气N沉降的响应机制，并为我国脆弱生态系统的管理利用提供科学依据和有益借鉴。

大气氮(N)沉降会显著增加土壤的可利用N水平，可能会对植物的N和P吸收策略产生潜在影响。N沉降会改变凋落物的质量(如C:N比)并影响其分解速率。此外，N沉降还会影响土壤微生物群落，进而影响凋落物的分解。上述生态过程最终会影响生态系统的养分循环过程。灌丛通常为N限制生态系统，生态系统脆弱，更易受环境变化的影响。目前，有关N沉降对灌丛的养分循环影响机制尚不清楚。研究了N添加对华北地区荆条和绣线菊灌丛凋落物分解、植物叶片养分重吸收和土壤微生物群落结构的影响。主要研究结果如下：(1)荆条凋落物分解速度大于绣线菊，这可能与二者养分含量和C/N比值的差异有关。(2)N添加对温带灌丛凋落物分解没有影响。N添加对凋落物分解的中性影响可能主要是由于样地气温低、受P限制以及外源无机N的相反作用，即外源N抑制木质素降解，促进易分解凋落物组分的分解。(3)灌木的N(NRE)和P(PRE)吸收率高于禾草状植物，这表明灌木可能具有较强的从土壤中获取养分的能力，而禾草状植物则依赖于通过重吸收获得更多的养分。(4)N添加增加了灌木和所有物种叶片N浓度，但对禾草状植物的影响不显著。N添加对植物叶片P浓度、NRE和PRE无影响。这些结果表明，随着土壤可利用N的增加，灌木表现出开放的养分适应能力，即增加对N的吸收，不改变内部养分循环，与之相比，禾草状植物的N获取策略没有改变。在P缺乏条件下，灌木和禾草状植物很可能倾向于完全利用土壤和衰老叶片中的P，导致N添加对叶片PRE无影响。(5)灌木和禾草状植物N重吸收潜力(NRP)对N添加的响应各不相同，这可能会改变温带灌丛群落结构和养分循环。(6)N添加对细菌物种多样性没有显著影响。这些结果不仅为评估温带灌丛生物地球化学循环对N沉降的响应提供了基础数据，也为我国脆弱生态系统的管理提供了科学依据和有益参考。