Objective  The variation characteristics of soil nitrogen of Pinus tabuliformis natural secondary forest in Pingquan County, Hebei Province of northern China were analyzed in different years after fire, and the influencing factors of soil nitrogen were explored in combination with stand factors, site factors and fuel factors, so as to provide a scientific basis for nutrient circulation and vegetation restoration after fire.

  Method  Study sites were burnt areas located in Liuxi Town of Pingquan County. Soil samples were collected in 2015(the year after the fire, 0 year), 2016 (1 year after the fire, 1 year) and 2021 (6 years after the fire, 6 years), respectively. We analyzed the tendency of content of soil total nitrogen (TN), alkali-hydrolysable nitrogen (AN), ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) under different years (0 year, 1 year and 6 years), different fire intensities (CK, control test plots which is unburnt area; L, low intensity burnt area; M, moderate intensity burnt area; H, heavy intensity burnt area), and different soil layers (0−10 cm and 10−20 cm). Correlation analysis (Pearson) and redundancy analysis (RDA) were used to explore the potential impact of stand factors (tree height, DBH, canopy density), site factors (slope position and slope gradient) and fuel factors (1 h, 10 h, 100 h fuel load, 1 h,10 h,100 h fuel water content) on soil nitrogen.

  Result  (1) Fire intensity, soil layer and years had significant influence on 4 types of soil nitrogen (P < 0.05). Fire intensity and soil layer had an interaction with all kinds of soil nitrogen (P < 0.05) in 2015. Fire intensity and years had an interaction with all kinds of soil nitrogen (P < 0.05). Fire intensity, soil layer and years showed no three-factor interaction with all kinds of soil nitrogen (P > 0.05). (2) Total nitrogen content, ammonium nitrogen content and nitrate nitrogen content increased in 2016 (1 year), especially in heavy intensity areas (H), and the amplification was 464.67%, 397.97%, 185.63%, respectively. But alkali-hydrolysable nitrogen content decreased in each sample plot, especially in M-plot (52.48%). Total nitrogen content, ammonium nitrogen content and alkali-hydrolase nitrogen content increased in 2021 (6 years), especially in moderate intensity areas (M), and the amplification was 368.78%, 209.00%, 427.51%, respectively. But nitrate nitrogen content decreased by 14.31% and 14.34% in low intensity areas (L) and moderate intensity areas (M), respectively. (3) RDA results showed that fuel water content had the most contribution to the change of soil nitrogen content. 1 h fuel water content had an effect on alkali-hydrolase nitrogen content. 100 h fuel water content had an effect on nitrate nitrogen content. Both of them had a positive relationship between fuel factors and soil nitrogen. Stand factors and site factors had little explanatory on soil nitrogen content, which was the indirect effect.

  Conclusion  The change of soil nitrogen content decreases first and then raises up, which is the common action with fire intensity, soil and years. At the early stage after burning (2015, 0 year; 2016, 1 year), fire intensity plays a significant role in affecting soil nitrogen content. However, the influence power of fire intensity decreased in 2021 (6 years). The direct impact of fire intensity on soil nitrogen content is reduced, and the mediation between fire intensity and soil continues to affect the change of soil nitrogen content through soil moisture content, temperature, pH and other indicators. The regeneration and development of vegetation in burnt areas is also affected by mediating effects. The differences in nutrient utilization strategies at different growth stages and the accumulation of vegetation litter are also important reasons for the significant changes of soil nitrogen content in 1 year and 6 years after fire.