Abstract: Nitrogen(N) is the nutrient regulating net plant primary production in most ecosystems, and N nutrient plays a significant role in N cycling. This study selects typical forest types in Xiaoxing'an Mountains of northeastern China in 2016, namely Pinus koraiensis plantation and pristine broadleaved Pinus koraiensis forest, and sampled continuously in 0-5 cm, 5-15 cm and 15-30 cm soil horizons for five months, measuring the contents of free amino acids-, nitrate-, ammonium-N and soluble organic N in the three soil horizons, and moisture content, pH, organic carbon, total N, microbial carbon and N levels in the 0-5 cm soil horizon. By exploring and discussing the dynamics of multi-form N and by incorporating some soil biochemical factors we measured, this study aims to discover the pool size of multi-form N and seasonal dynamics of available N in typical forests of Xiaoxing'an Mountains, with principal component analysis (PCA) analyzing the underlying driver factors of N seasonal dynamics. The final results showed that the composition of amino acids was similar in the two Pinus koraiensis forests of Xiaoxing'an Mountains, with aspartic acid, serine, glycine, histidine, arginine and leucine owning higher contents, but dominant amino acids varied in different months. The three forms of available N pool size followed ammonium>nitrate>free amino acids, and amino acids-N comprised a significant part of soil soluble N pool. The content of multi-form N decreased with soil depth in 0-30 cm soil horizon, with consistent relationship with time.With the temperature and hydrologic circumstances as drivers, free-thaw event, microbial factor, pH and complex biochemical processes changed over time leading to soil available N dynamics; as the variation in potential physical, chemical and biological factors and their combined effects in soils, however, tendency in available N dynamics appeared to be different.