Impacts Of Simulated Nitrogen Deposition On Soil Organic Carbon Pool In Chinese Fir Plantation

Increases in atmospheric nitrogen deposition since the 20th century 70 years have altered carbon cycling of forest ecosystems by affecting productivity and biomass accumulation in terrestrial ecosystems. This thesis studied the effects of nitrogen deposition on soil organic carbon, active SOC content and its dynamic and the activities of the six soil enzymes involving carbon cycle, a field experiment was conducted in a Chinese fir plantation at Shaxian State Forest Farm of Fujian Province, China. Nitrogen loadings were designed at 4 levels as NO (control), N1, N2 and N3 at the doses of 0,60,120 and 240 kg·hm-2·a-1N, respectively, each treatment comprised three replicate. The results obtained showed as following; (1) With increasing doses of nitrogen deposition, the total nitrogen increased, the ratio of C/N decreased at 0-20 cm, the amount of soil organic carbon content and SOC densities had no significant effect. (2) The Fractions of active organic carbon responded differently to nitrogen treatments, nitrogen additions had no significant effect on MBC and POC, but inhibited HWC to some extents. High level of nitrogen loading (N3) showed significant positive impact on DOC. (3) All four soil active organic carbon ratio to TOC was 45.08%, the maximum was POC/TOC (16.23%-40.02%),then MBC/TOC (2.58%-3.39% ) and HWC/TOC (0.45%～1.12%), DOC/TOC (0.36%-0.55%) was minimum. (4) Significant positive correlation between TOC and MBC, POC, HWC, but negative between TOC and DOC. Significant correlation also existed in fractions of soil active organic carbon, positive between MBC and POC, HWC, negative between DOC and MBC, POC, HWC. (5) Soil samples were incubated in the laboratory at 28℃for 28 days, during the incubation period, the daily mineralization of soil organic carbon decreased, but the cumulative mineralization of soil organic carbon increased with increasing time. Generally, the cumulative mineralization of soil organic carbon decreased with increasing doses of nitrogen deposition. The daily mineralization of soil organic carbon decreased in the sequence of N1>N0>N2>N3. (6) The activities of six soil enzymes responded differently to nitrogen treatments. At the soil depth of 0-20 cm, nitrogen additions promoted cellulose and polyphenol oxidase activities, but inhibited amylase and peroxidase to some extents. High level of nitrogen loading (N3) showed significant positive impact on invertase, but negative onβ-glucosidase. Significant correlation was established between some soil enzymes, with cellulose being positively related with invertase and amylase, polyphenol oxidase with amylase and cellulose, peroxidase with invertase, amylase, cellulose and polyphenol oxidase. (7) Soil organic carbon mineralization was also found to be positively correlated withβ-glucosidase and peroxidase, but negatively with invertase and cellulose at the surface soil. Majority fractions of soil carbon (DOC, POC and HWC) and soil enzymes (invertase, cellulose, amylase and peroxidase) were significantly related at 0-20cm, significant positive correlation between MBC and P-glucosidase, polyphenol oxidase.