Abstract:

【Objective】This paper elaborated upon the changes in soil total nitrogen and mineral nitrogen ($\mathrm{NH}_{4}{ }^{+}$-N and $\mathrm{NO}_{3}{ }^{-}$-N) contents, the nitrogen form characteristics and their spatial patterns in lightly, moderately and severely burned areas, which comprehensively reflected the characteristics of soil nitrogen nutrition habitats during the early restoration period of burned forest areas in the Greater khingan Mountains. This study aimed to provide a reference for the renovation and ecological restoration of burned coniferous forests in cold temperate zones.【Method】In the Ledum palustre-Rhododendron simsii-Larix gmelinii forest, fixed sample plots were set up with the grid method, and the intensity of the forest fire was controlled by artificial ignition. Soil samples were collected by auger boring before and one year after burning. The soil total nitrogen, $\mathrm{NH}_{4}{ }^{+}$-N and $\mathrm{NO}_{3}{ }^{-}$-N contents were measured, the change rate of each index and the ratio of mineral nitrogen forms were calculated, and the spatial patterns were drawn.【Result】A year after the burning: (1) Soil total nitrogen content changed, and its change rate was divided into two stages. In the lightly and moderately burned areas, soil total nitrogen content increased. Its spatial pattern was not obviously correlated with the fire intensity spatial pattern, but its change rate spatial pattern was. In the severely burned area, the soil total nitrogen content decreased. The spatial and change rate spatial patterns were both negatively correlated with the fire intensity spatial pattern. (2) Soil $\mathrm{NH}_{4}{ }^{+}$-N content increased in all burned areas; its spatial and change rate spatial patterns were not obviously correlated with the fire intensity spatial pattern. (3) Soil $\mathrm{NO}_{3}{ }^{-}$-N content increased significantly; its spatial and change rate spatial patterns were both positively correlated with the fire intensity spatial pattern. (4) The soil nitrogen form showed evident $\mathrm{NH}_{4}{ }^{+}$-N dominance, but its dominance (the content ratio of $\mathrm{NH}_{4}{ }^{+}$-N to $\mathrm{NO}_{3}{ }^{-}$-N) was reduced. The ratio spatial pattern and change rate spatial pattern were negatively correlated with the spatial pattern of fire intensity. 【Conclusion】During the early restoration period of burned areas in the Larix gmelinii forest, the change in soil nitrogen nutrition habitat did not constitute an obstacle to the restoration of native vegetation types; however, the decline of soil total nitrogen in the severely burned area may bring about the fall-off in nitrogen supply capacity in the long-term.

Key words: Larix gmelinii, forest fire, soil nitrogen, spatial pattern