天山林区不同类型群落土壤氮素对冻融过程的动态响应

季节性冻融过程对北方温带森林土壤氮素的转化与流失具有重要影响,但不同类型群落对冻融过程响应的差异尚不明确。通过在林地、草地、灌丛上设置系列监测样地,采用原位培养的方法,利用林冠遮挡形成的自然雪被厚度差异,监测分析了冻融期天山林区不同群落表层土壤(0—15 cm)的氮素动态及净氮矿化速率间的差异。结果表明:(1)不同类型群落土壤的铵态氮(NH+4-N)含量、微生物量氮(MBN)含量基本与土壤(5 cm)温度呈正相关,深冻期林地土壤铵态氮含量低于其他群落类型而硝态氮含量高于其他群落类型;(2)硝态氮(NO-3-N)为天山林区季节性冻融期间土壤矿质氮的主体,占比达78.4%。灌丛土壤硝态氮流失风险较大,融化末期较融化初期灌丛土壤硝态氮含量下降了64.6%;(3)冻融时期对整体氮素矿化速率影响显著,群落类型对氨化速率影响显著;(4)天山林区土壤氮素在冻结期主要以氮固持为主。通过揭示不同类型群落土壤氮素对冻融格局的响应,能够助益于对北方林区冬季土壤氮素循环的认识。

Dynamic response of soil nitrogen to freeze-thaw processes in different cenotypes in the forests of the Tianshan Mountains

Seasonal freeze-thaw regimes are the main factors influencing nitrogen (N) transformations in the temperate forests of northern China and are one of the key processes in the soil N cycle. However, the response to freeze-thaw cycles in different cenotypes is unclear. In this study, we conducted experiments to understand the effect of soil freeze-thawing on N transformations in soils (0-15 cm) from different cenotypes (woodland, grassland, and shrubs) in the forests of the Tianshan Mountains by simulating extreme soil freeze-thawing in different natural snow covers formed by canopy shading. The results showed that the content of ammonium nitrogen, NH₄⁺-N, and microbial biomass nitrogen (MBN) in the soil of the different cenotypes was positively correlated with soil temperature (at a depth of 5 cm), whereas the content of NH₄⁺-N in forest soil was lower than that in the other communities in the deep-freezing period. The content of nitrate nitrogen, NO₃^--N, was higher in forest soil than in the other community types. NO₃^--N was the major form of soil mineral N during the seasonal freeze-thaw period in the forests of the Tianshan Mountains; it accounted for 78.4% of soil mineral N. NO₃^--N content decreased by 64.6% from the early-thawing to late-thawing period in shrub soil, indicating a high risk of soil NO₃^--N loss. The N mineralization rate was significantly affected by the freeze-thaw period, and the cenotypes significantly influenced the ammonification rate. The main process involving soil N in the forests of the Tianshan Mountains during the freezing period was N immobilization. Investigating the response of soil N to freeze-thaw patterns in different communities would help us understand the soil N cycle in the northern forest regions.