Effect Of Simulated Nitrogen Deposition On Soil Carbon Budget Progress In A Larix Olgensis Plantation Forest

To explore the effects of different levels of nitrogen (N) deposition on soil carbon input (litter decomposition and fine root turnover) and output (soil respiration) progress, and reveal the effect mechanism of soil carbon pool changes, a 3-year-simulated N deposition experiment using NH4NO3 was conducted with four different N supply levels, including control (CK,0gN·m-2·a-1), low (N1,5 gN·m-2·a-1), intermediate (N2,10 gN·m-2·a-1) and high level (N3,15 g N·m-2·a-1), in a 26-year-old Larix olgensis plantation forest in Mao’ershan Experimental Forest Farm of Northeast Forestry University. We found:(1) Carbon releasing rate of leaf litter decomposition was significantly increased under N1 and N2 treatment (P<0.05), while N3 had no difference compared with control in a Larix olgensis plantation forest (P>0.05). Leaf litter N content increased with N deposition increasing, and carbon-nitrogen ratio decreased.(2) Annual fine root production and turnover rate were decreased under simulated N deposition, both significantly decreased under N3 treatment (P<0.05) and had no difference compared with Nl and N2 treatment (P>0.05), in a Larix olgensis plantation forest. Annual fine root production calculated by minimum-maximum calculation method was lowest (31.44～62.63 g·m-2·a-1), by balancing transfer method was highest (97.25～193.86 g·m-2·a-1), while integral method was intermediate (33.76～86.32 g·m-2·a-1).The trend of carbon releasing rate of fine root decomposition was consistent with leaf litter, both increased under N1 and N2 treatment (P<0.05), had no difference under N3 treatment (P>0.05), fine root N content increased with N deposition increasing,and carbon-nitrogen ratio decreased.(3) With simulated N deposition time increasing, the trend of soil respiration rate changed in a Larix olgensis plantation forest. At first, soil respiration rate significantly increased under different N deposition treatment (N1, N2, N3) (P<0.05), and then, N1 treatment still increased, however N2 and N3 treatments turned to decreased (P<0.05). Soil microbial respiration rate was improved under N1 treatment (P>0.05), and decreased under N2 and N3 treatment (P>0.05). Fine root respiration rate had no changed under N deposition treatment (P>0.05)(4) Simulated N deposition significantly affected soil organic carbon content in a Larix olgensis plantation forest. Soil total organic carbon (TOC) and dissolved organic carbon (DOC) content was significantly improved under N1 and N2 treatments (P<0.05), while N3 treatment had no difference compared with control (P>0.05); N1 treatment significantly increased soil microbial organic carbon (MBC) and labile organic carbon (LOC) content (P<0.05), and N2 and N3 treatments had no difference on MBC content compared with control (P>0.05), while significantly decreased LOC content (P<0.05). Soil pH value was significantly decreased with N deposition increasing (P<0.05)In summary, soil carbon releasing progress (soil respiration) was promoted under N1 treatment, however was inhibited from promoted under N2 and N3 treatments in a Larix olgensis plantation forest; soil carbon input (litter and fine root decomposition) progress was improved under N1 and N2 treatments, and had no changed under N3 treatment compared with control; annual fine root production and turnover rate were decreased under N deposition treatment. All in all, low level N deposition (N1) may improve soil carbon cycle rate, and medium and high level N deposition (N2 and N3) may increase soil carbon accumulation in a Larix olgensis plantation forest at Mao’ershan farm in a short term of simulated N deposition.