Effects Of Forest Thinning On Soil Nitrogen Mineralization In Manchurian Ash And Dahurian Larch Mixed Plantations

Soil nitrogen mineralization is an important process of nitrogen supply for plant growth, and this study has important significance for understanding forest ecosystem productivity, nutrient cycling and nitrogen transformation. Thinning is one of the commonly used measures for forest cultivation. To understand the effect of different thinning intensity on soil nitrogen mineralization rate of temperate plantation, the study employed the resin-core technique, investigated the nitrogen mineralization rate of top soil layer (0-10cm) after four thinning density (moderate thinning and keep all logging slash, weak thinning and keep all logging slash, weak thinning and logging slash removed, control) in mixed plantations of Manchurian ash and Dahurian larch. Results showed that:1. Forest thinning increased the soil net nitrogen mineralization rate and net nitrification rate. Seasonal average net nitrogen mineralization rate and nitrification rate from different treatments range as follow:moderate thinning and keep all logging slash, weak thinning and keep all logging slash, weak thinning and logging slash removed, control. No significant difference were detected between the four treatments in the first growing season after thinning (P>0.05). Both net nitrogen mineralization, net nitrification rate showed obvious seasonal dynamic, peaked in July, and ranged from0.13-0.94,0.08-0.78mg·kg-1·d-1, respectively.2. Forest thinning reduced the soil ammonium and nitrate concentrations. In growing season, nitrate nitrogen were the dominate morphology of inorganic nitrogen. Seasonal average ammonium nitrogen concentration peaked in control, declined in weak thinning and logging slash removed and bottomed in weak thinning and keep all logging slash, no significant difference between the four treatment were investigated (P>0.05). There were no significant difference in seasonal average nitrate nitrogen concentration between the four treatments, being highest in control, lower in moderate thinning and keep all logging slash and lowest in weak thinning and keep all logging slash (P>0.05). Both ammonium and nitrate nitrogen had marked seasonal changes, ranged from1.63～13.56mg·kg-1and7.50～22.23mg·kg-1, respectively.3. Forest thinning reduced the soil nutrient concentrations, and the nutrient indexes have less effect on soil nitrogen mineralization rate. Among all treatments, control stands were highest in averaged organic carbon, total carbon, soluble organic carbon and microbiomass carbon content and carbon/nitrogen ration, but the difference between treatments were not significant in the first growing season(P>0.05), and all of the parameters showed clear seasonal dynamic. There were no significant correlation between net nitrogen mineralization with soil organic carbon, total carbon, carbon/nitrogen ratio, soluble organic carbon and microbiomass carbon (P>0.05). 4. Forest thinning increased the soil temperature and water content, and they were significantly effect on soil nitrogen mineralization rate. Control stands were lowest in soil temperature and water content in all the four treatments, but the difference were not significant in the first growing season (P>0.05). Soil temperature and water content were significantly correlated with net nitrogen mineralization and net nitrification rates (P<0.05), indicating they were the main influential factors.For ash and larch mixed plantations, seasonal average net nitrogen mineralization rate and nitrification rate slightly increased in the first growing season after thinning, but the difference were not significant with control. Net nitrogen mineralization was mostly influenced by soil temperature and water content.