森林土壤有机碳稳定性对外源氮输入的响应：有机-矿质-微生物交互作用

Stabilization of soil organic carbon (SOC) plays a crucial role in the global climate system. Many factors were considered to influence SOC stabilization in previous studies, mainly including molecular recalcitrance of organic carbon, microbial communities, and soil organic-mineral complexities which protects SOC from decomposition by microorganisms. In comparison with the former mechanism, roles of the latter two mechanisms, i.e., microbial communities and soil organic-mineral complexities, in the stabilization process of SOC remain unclear. Results of long-term investigations show that the subtropical area in southern China is one of the three severest nitrogen (N) deposition regions on the earth, and that the amount of atmospheric N deposition will keep rising in this region in the future. Atmospheric N deposition can change vegetational and edaphic properties, and consequently influence SOC stabilization in the long term scale. Therefore, this project is proposed to study effects of exogenous N input on SOC stabilization in a monsoon evergreen broad-leaved forest in south China, which is the climax in this region but sensitive to climate change. We would simulate atmospheric N deposition by anthropogenic spraying ammonia nitrate solution in different concentration levels (control, low N [35], medium N [70], and high N inputs [105 kg N ha-1 yr-1]) monthly. Funded by this project if available, we will investigate responses of SOC inputs (including litter and fine root), transfer (mainly microbial community structure and activity), storage forms (mainly SOC fractions and soil organic-mineral complexities), and outputs as gases and liquid. Furthermore, lab incubation using stable isotope tracing will be carried out to study relationship between microbial community structure and function, and relationship between shifts in microbial community structure and function and changes in SOC stabilization after three years exogenous N inputs. By doing this, we aim to explore mechanisms for the response of SOC stabilization to atmospheric N deposition, and provide empirical supports for assessing regional carbon cycling in the future.

土壤有机碳稳定性对全球气候系统具有重要的调节作用。影响土壤有机碳稳定性的因素主要包括分子惰性、分解者群落和土壤有机-矿质复合结构的保护。相比于前者，我们对微生物群落和有机-矿质复合结构在有机碳稳定过程中的作用了解仍然不够透彻。长期的观测数据表明，我国南亚热带是当前全球三大氮沉降区之一，大气氮沉降水平仍在逐年增加。氮沉降会改变森林生态系统植被和土壤属性，因此可能影响土壤有机碳稳定性。鉴于此，本项目以对气候变化敏感的南亚热带季风常绿阔叶林为研究对象，通过操纵不同水平外源氮输入(对照、低氮[35]、中氮[70]和高氮[105 kg N ha-1 yr-1]添加)处理，持续观测土壤有机碳输入、转化(微生物群落)、储存(形态与微结构)和输出过程的响应，结合室内培养同位素示踪实验，从土壤有机-矿质-微生物相互作用的角度探索森林土壤有机碳稳定性对氮沉降的响应机制，为评价区域碳循环提供科学依据。

土壤有机碳库对维持全球气候系统的稳定具有至关重要的作用。大气氮沉降会改变陆地生态系统碳循环，但其对土壤有机碳储存的影响仍然存在争议，机理尚不明确。本项目以鼎湖山季风常绿阔叶林为研究对象，通过野外施氮处理（对照、35 kg N ha-1 yr-1 [LN]、70 kg N ha-1 yr-1 [MN]和105 kg N ha-1 yr-1 [HN]）和室内培养（对照、尿素、铵态氮和硝态氮添加）实验，研究氮沉降对土壤有机碳含量的影响，从碳输入（凋落物产量、质量和分解）、储存（有机碳组分和形态）、输出（CO2排放和碳矿化速率）和土壤有机碳分解者群落（微生物群落结构和功能基因丰度）等多个方面探索其影响机理。结果表明：（1）氮添加趋于增加凋落物产量及总碳输入量，但对凋落物碳氮磷浓度和碳氮组分（溶解性和酸不溶组分碳氮含量）没有显著影响，不同树种凋落物分解过程对氮沉降的响应不同；（2）氮添加不会显著改变全土土壤有机碳及易变和惰性碳组分含量，也不会显著改变土壤有机碳化学成分，但会显著增加表层土壤小颗粒（< 50 μm和50-250 μm）团聚体有机碳（总有机碳、易氧化有机碳和不易氧化有机碳）浓度，却不会显著改变土壤团聚体颗粒有机碳易变指数；（3）相比于对照，LN处理显著增加湿季土壤CO2排放速率（Rs），而MN和HN处理显著降低湿季土壤Rs，干季处理效应不显著，表明低氮沉降促进土壤碳排放，而中高氮沉降抑制土壤碳排放，且作用效果主要发生在湿季；（4）氮添加没有显著改变土壤PLFAs组成，各微生物功能群丰度和比例没有显著变化，荧光定量PCR分析表明氮沉降可能显著增加氨氧化古菌amoA和nirK基因丰度，进一步使用16S rDNA和ITS测序分析表明氮沉降趋于改变土壤细菌群落结构，对真菌群落结构影响较小；（5）不同形态氮添加不会显著改变土壤微生物量、微生物呼吸和PLFAs组成，但会显著降低微生物呼吸的温度敏感性，且不同氮形态的影响没有显著差异。这些结果表明氮沉降可能增加森林土壤碳输入，减少碳输出且降低其温度敏感性，从而增加小颗粒团聚体土壤有机碳含量，但不改变土壤有机碳质量。