原位根系和凋落物输入对中亚热带森林不同层次土壤碳平衡过程的影响

Soil organic carbon is an important carbon sink in terrestrial ecosystem. Recent studies showed that the increase of exogenous carbon input caused by global change could increase surface soil carbon storage, but soil carbon storage in deep soil increased more slowly, or even decreased. When exogenous carbon is added to the soil, it will stimulate the mineralization of soil original organic carbon (called priming), and soil microorganisms can also assimilate the exogenous carbon and form new soil organic carbon. The balance between the “old” carbon mineralization and “new” carbon formation controls the change of carbon storage. The two processes are affected by the quality and quantity of the exogenous carbon and soil nitrogen availability. Furthermore, once the exogenous carbon and nitrogen are added into soil, the composition and quantity of the carbon and nitrogen sources will change along the soil profiles. Thus, it’s urgent to explore how the type and quantity of the exogenous carbon and exogenous nitrogen will affect carbon balance for soil from different depths. In this study, root and litter addition experiments are going to carry out at the field in subtropical mountain forest in China. First, four levels of 13C-labeled root/litter will be added to in-situ soils to test how root and litter input will affect soil carbon mineralization and sequestration for soil from different depths, and explore the relationship between the addition levels and carbon balance. Second, nitrogen will be added together with root and litter to explore the nitrogen effect on priming and carbon sequestration. Third, the components of the new formed carbon and their distribution on soil particles will be detected to analyze the stability of the new formed carbon. Through this study, we’ll gain a better understanding of the carbon dynamics for soil from different depths, and reduce the uncertainties in predicting how soil organic carbon pool may respond to future global change.

土壤是陆地生态系统最大的碳库。全球变化引起的外源碳输入量的增加可引起表层土壤碳储量增大，但深层土壤碳储量的增加幅度减弱，甚至出现下降。这与外源碳输入对不同层次土壤碳矿化产生的激发和对新碳固持的平衡过程有关。激发作用强度和土壤的固碳能力受外源碳输入的质量、数量及土壤氮有效性影响，且外源碳、氮输入后，其组成和含量会随层次加深而改变，因此有必要探明不同形式、不同量的外源碳及氮输入对剖面土壤碳平衡过程的影响，以准确评估土壤碳库的源汇功能。为此，本研究拟结合同位素示踪技术，在亚热带森林开展原位根系和地表凋落物输入试验：（1）研究不同量的根系和凋落物输入对不同层次土壤碳矿化和固持的影响，探明外源碳输入与碳平衡的关系；（2）明确氮添加对不同层次土壤产生的激发作用和碳固持的影响及其机理；（3）分析“新”合成有机碳的稳定特征，探明土壤长期固碳能力。本研究结果可为精准模拟全球变化下剖面土壤的响应提供数据支持。

土壤是陆地生态系统最大的碳库。外源碳输入土壤后一方面可被土壤固持，形成新的土壤有机碳，增加土壤碳含量；另一方面可刺激土壤原有有机碳的矿化，产生激发效应，降低土壤碳含量，两者共同决定土壤碳平衡过程。激发作用强度和土壤的固碳能力受外源碳输入的数量、质量及土壤氮有效性的影响，且影响新合成碳在土壤层次和碳组分中的分布，进而影响其稳定性。因此探明不同形式、不同量的外源碳及氮输入对剖面土壤碳平衡过程的影响，对准确评估土壤碳库的源汇功能具有重要的指导意义。本研究利用同位素示踪技术，开展野外原位凋落物输入和室内活性碳源添加试验：（1）研究不同数量和类型凋落物输入对不同层次土壤碳矿化和固持的影响；（2）分析氮添加对不同层次土壤产生的激发作用和碳固持的影响及其机理；（3）分析“新”合成有机碳的组分特征，探索其稳定特征。与室内培养实验发现的正激发效应不同，野外原位凋落物输入可降低土壤原有有机碳的矿化。外源碳输入土壤后，有40%的外源碳可固持在土壤中，抵消激发作用带来的负效应，形成正的碳平衡过程。凋落物碳在土壤中的固持比例随凋落物添加量的升高而降低，凋落物类型（凋落物叶和细根）影响凋落物碳在土壤中的固持比例。在野外和原位添加试验中，均未发现氮添加对激发效应大小和碳固持效率的影响。室内培养实验发现深层土壤的激发作用高于表层土壤，而对外源碳的固碳效率高于表层土壤；而野外原位实验未发现外源碳、氮输入对深层土壤碳矿化和固持的显著影响。相对于土壤原有有机碳，新形成的土壤有机碳更多的分布在轻组和颗粒态有机碳中，其稳定性较低。以上结果表明增加外源碳输入是提高本研究区土壤碳汇的潜在手段，不同数量、类型凋落物输入可潜在影响碳平衡过程。本研究结果可为精确模拟亚热带山地森林土壤碳循环提供数据支持。