模拟氮沉降对高山森林土壤碳积累与稳定性影响

The responses of alpine forest soil carbon(C) changes and stability to nitrogen(N) deposition are being paid more attention to. In this study, two 15N labeled N additions treatments with 8 kg N/ha/y and 50 kg N/ha/y respectively,were used to study the effects of N addition on alpine forest soil C. First,the 15N enrichments and proportion of 15N tracer recovered in the soil pools were calculated based on N mass and amount of 15N applied. The soil carbon changes due to N additions were estimated multiplied by soil C:N ratio by N enrichments in the soil. Moreover, two traditional methods, paired sites comparison method (comparison of soil carbon between the reference site and the fertilization sites at a point in time) and retrospective method (detection in soil carbon at the different points in time in the same fertilization site), were also used to estimate the changes in soil carbon.Second, the soil organic carbon (SOC) of each treatment was separated into labile and recalcitrant SOC, and the 13C and 14C fractionation (δ13C andΔ14C)were detected in these two fractions and the aboveground litter. The analysis of 13C and 14C was used to investigate the responses of stability of labile and recalcitrant SOC to N additions. Third, the soil leaching solution was collected to estimate the solution volume, dissolved organic carbon (DOC) and dissolved organic/inorganic nitrogen (DON/DIN) content in the solution, at the different soil depths in each treatment. The effects of N additions on the soil carbon and nitrogen leaching were analyzed, and the roles of DOC playing in accumulation of deeper SOC were also investigated. The results and conclusions of this study can provide some information in the responses of forest soil carbon sequestration and stability to N deposition.

高山森林土壤碳对大气氮沉降的响应是当前全球变化研究的重要方向。本项目利用15N标记的增氮实验（8 kg N/ha/y和50 kg N/ha/y两个水平）模拟氮沉降增加，首先测算15N在高山森林土壤中的积累量与分配比例，并结合土壤C:N，估算N添加下土壤碳变化。同时利用传统的配对样地方法（对比对照样方和施氮样方土壤碳储量）和连续观测方法（比较施氮前后土壤碳储量）评估土壤碳对N添加的响应。此外，通过土壤有机碳分组（易变碳和稳定性碳），分析其13C和14C组成（δ13C和Δ14C），研究不同有机碳组分的稳定性对N添加的差异性响应。最后，利用土壤淋溶实验，研究土壤碳、氮淋溶对N添加的响应，并结合15N研究，分析淋溶碳对土壤碳积累的作用。本研究为深入研究高山森林土壤碳氮循环提供重要基础，并对于认知森林土壤在N沉降下的碳汇作用具有重要价值。

本项目以我国西南典型的亚高山森林为研究对象，利用15N标记的氮肥添加模拟氮沉降，研究氮素在土壤中的分配及积累以及氮沉降对土壤碳的影响和机制。结果显示：短期内，氮素在土壤中的分配主要集中在表土（0-5 cm），且与氮沉降量呈正相关；进一步利用物理分组方法对土壤进行细分，发现氮素在表土较为活跃的团聚体中分配较多（比较化学性质更稳定的粘粉粒组分），表明团聚体至少在短期内对于氮素吸附与利用发挥着关键作用。同时，采用配对样地以及连续观测对比方法发现氮添加显著促进了表土团聚体的碳、氮含量，其增量与氮添加量正相关。连续两年的定期（每5-7天）采样和观测显示，氮添加增加了有机质层和矿质土壤层（表层20 cm）淋溶氮的损失，对淋溶碳的影响与土层和氮添加水平有关：低氮添加对不同土层淋溶碳影响不大，高氮添加显著降低了有机质层碳淋失，但对矿质层淋溶碳无明显影响。有机质层碳淋失的下降可能与高氮添加下微生物生物量（微生物碳氮指标）和纤维素酶活性降低有关。氮添加同时降低了蔗糖酶活性，说明氮添加可能通过抑制微生物分解作用并减小在此过程中的碳淋失损失而促进土壤碳积累。此外，氮添加显著改变土壤碳稳定同位素组成（更接近植物碳同位素组成）的证据表明氮添加同时也可能通过促进植物碳输入或其残留的增加而提高土壤碳积累。本研究的结果有力地支持了氮沉降对森林土壤碳积累的正向作用，为全球变化下森林土壤碳动态评估提供了不同的视角。同时本研究从碳淋溶揭示了氮沉降对土壤碳影响的多面性，有助于系统性理解碳氮的耦合关系。