Nitrogen Deposition Affects Carbon And Nitrogen Turnover On The Early Stage Of Forest Litter Decomposition

Over the last several decades, atmospheric nitrogen (N) deposition due to fossil fuel combustion and agricultural intensification has more than doubled in many areas throughout of the world Increasing N supply has greatly altered biogeochemical cycling in such regions.Litter decomposition play a crucial role in soil formation and the availability of nutrients to plants and microorganisms in forest ecosystems. The carbon (C) and N originating from litters might take different pathways during decomposition and be either incorporated in stabilized (remaining) soil organic matter, released as CO₂ and N₂O or transported to the mineral soil as dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN). The increased N input may influence C and N turnover during litter decomposition process by changing chemical and microbial environment in forest floor.Samples including two types needle litters and three types leaf litters were collected from four typical forest ecosystems representative of the northern slope of the ChangBai Mountain (China). Different doses of N (equal to 0, 30 and 50 N ha^-1y^-1, as NH₄NO₃) were added to litter during 13-week experiment time. Leachate was analyzed for, dissolved inorganic nitrogen (DIN) and dissolved organic nitrogen (DON). Litter respiration as CO₂ and N₂O was measured once a week. The aim of our study was to investigate effects of nitrogen (N) availability and tree species on the turnover of carbon (C) and N in the early stage of decomposition of forest litter and effects of internal and external nitrogen on the early stage decomposition of Korean pine litter.Our experiment showed that the litter decomposition rate indicated as mass loss and respiration increased significantly along the N availability under the given circumstances. The leaf litter showed higher mass loss and cumulative CO₂ emission than the needle litter. C quality imposed an overriding constraint on litter decomposition in our study. The dissolved organic carbon (DOC) concentrations in litter leachate varied widely with species but were not greatly affected by nitrogen treatments. A positive correlation (0.89, P<0.05) was found between cumulative DOC leaching and decomposition rate in control samples. Neither N treatments nor species showed significant effect on dissolved organic nitrogen (DON) concentrations in litter leachate. No relationship was found between DON leaching and litter mass loss. In spite of part of input N passed directly through the litter, there was still as much as 51-91% of the added N retained by the litter. Percents of N immobilization were positively correlated with the litter mass loss, indicating forest floor with easily decomposition litters may have greater potential of N sinks than that with hardly decomposition litters.Our experiment also showed addition of external N increase the decomposition rate significantly (P<0.05), high external N treated samples showed greater weight loss and respiration rate than low external N treated and controls. No increase in decomposition rate was observed by internal Ngradient, while Initial lignin content correlated best with decomposition rate of litters. The releases of dissolved organic carbon (DOC ) were not significantly affected by internal or external N ,but external N show a positive effect on dissolved organic carbon (DON) leaching. In spite of part of inorganic N leaching, the litter still can immobilize 40-55% external N, the retention of inorganic N generally increased with higher levels of external N.