模拟N沉降下不同林龄马尾松林凋落叶分解-土壤C、N化学计量特征

模拟N沉降对森林生态系统的影响是当今全球变化生态学研究的一个热点问题,土壤碳库对N沉降比较敏感,N沉降增加了凋落叶分解过程中外源N含量,间接影响凋落叶分解的化学过程并改变凋落叶分解速率,因此,研究模拟N沉降下凋落叶分解-土壤C-N关系对预测森林C吸存有重要意义。利用原位分解袋法研究了模拟N沉降下三峡库区不同林龄马尾松林(Pinus massoniana)凋落叶分解过程中凋落叶-土壤C、N化学计量响应及其关系;N沉降水平分对照(CK,0 g m~(-2)a~(-1))、低氮(LN,5 g m~(-2)a~(-1))、中氮(MN,10 g m~(-2)a~(-1))和高氮(HN,15 g m~(-2)a~(-1))。结果表明:分解540 d后,N沉降促进20年生和30年生马尾松林凋落叶分解,46年生马尾松林中仅低氮处理促进凋落叶分解,4种处理均是30年生分解最快,说明同一树种起始N含量低的凋落叶对N沉降呈正响应,N沉降处理促进起始N含量低的凋落叶分解,起始N含量高的凋落叶分解过程中易达到"N饱和"。N沉降抑制20年生和46年生凋落叶C释放(低于对照0.62%—6.69%),促进30年生C释放(高于对照0.28%—5.55%);30年生和46年生林分N固持量均高于对照(高于对照0.15%—21.34%),20年生则低于对照(5.70%—13.87%),说明模拟N沉降处理促进起始C含量低的凋落叶C释放和起始N含量低的凋落叶N固持。N沉降处理下仅30年生马尾松林土壤有机碳较对照增加,且土壤有机质与凋落叶C、N和分解速率呈正相关,与凋落叶C/N比呈显著负相关;土壤总氮与凋落叶分解速率、凋落叶N含量呈正相关,土壤有机碳/总氮比与凋落叶C、N含量呈正相关;对照处理中凋落叶分解指标对土壤养分影响顺序是分解速率凋落物C含量凋落物C/N比凋落物N含量,低、中、高氮处理中则是凋落物C含量分解速率凋落物N含量凋落物C/N比。研究表明低土壤养分含量马尾松林对N沉降呈正响应,N沉降促进低土壤养分马尾松林凋落叶分解并提高土壤肥力;凋落叶质量和土壤养分含量低的生态系统土壤C对N沉降响应更显著。

Relationship between leaf litter decomposition and soil C, N stoichiometry in different-aged Pinus massioniana stands exposed to simulated nitrogen deposition

The effect of simulated N deposition on forest ecology systems is a hot topic in global change ecology research. Soil C storage is sensitive to atmospheric N deposition, which helps to increase the exogenous N content in litter decomposition process and indirectly affects chemical process and the decomposition rate of litter decomposition. Therefore, it is of great importance to understand the relationship between litter decomposition and soil C and N in response to N deposition in order to accurately predict ecosystem C sequestration. In this study, we surveyed the relationship between leaf litter decomposition and soil C and N stoichiometry in different-aged P. massioniana stands in the Three Gorges Reservoir Region using an situ litter bag method. Four levels of simulated N content were implemented:control (CK, 0 g m^-2 a^-1), low N (LN, 5 g m^-2 a^-1), medium N (MN, 10 g m^-2 a^-1), and high N (HN, 15 g m^-2 a^-1). The results showed that simulated N deposition accelerated leaf litter decomposition in 20- and 30-year-old stands. In 46-year-old stands, however, only the litter in the LN treatment accelerated decomposition over 540 days of decomposition. The leaf litter decomposition rate was the highest in the 30-year-old stands for all four treatments, showing that leaf litter with lower initial N content from the same specie presented positively response to simulated N deposition. Specifically, N deposition promoted the decomposition of leaf litter with lower initial N content, while the decomposition of leaf litter with higher initial N content easily reached nitrogen saturation. Nitrogen deposition inhibited the litter carbon release from the litter in 20- and 46-year-old stands (0.62%-6.69% lower than the control), while it promoted carbon release in 30-year-old stands (0.28%-5.55% higher than in the control). The amount of immobilized N was 0.15%-21.34% higher in the leaf litter of the N deposition treatments than in that of the control for 30- and 46-year-old stands, while it was 5.70%-13.87% lower than in the control for 20-year-old stands. This indicates that simulated N deposition accelerated C release from the litter with lower initial C content and N immobilization of the litter with lower initial N content. Soil organic carbon increased in 30-year-old stands under simulated N deposition when compared to the control and was positively correlated with leaf litter C and N content and decomposition rate, while it was negatively correlated with the C/N ratio. Soil total nitrogen was positively correlated with the leaf litter decomposition rate and leaf litter N content, and the ratio of soil organic carbon to total nitrogen was positively correlated with leaf litter C and N content. In the control, the effect of leaf litter N content on soil nutrients was most significant, litter C:N ratio and C content followed, litter leaf decomposition rate was least, while in LN, MN and HN treatments, the effect of leaf litter C:N ratio on soil nutrients was most significant, litter N content and leaf litter decomposition rate followed, litter C content was least. In this study, P. massinonana stands with a low concentration of soil nutrients responded positively to N deposition, which accelerated leaf litter decomposition and improved soil fertility. These results suggest that soil carbon in ecosystems with low-quality leaf litter and a low concentration of soil nutrients would respond more to N amendment than it would in other ecosystems.