Impacts Of Nitrogen Addition On Net Primary Productivity In A Typical Broadleaved-Korean Pine Mixed Forest

As industry developed rapidly, more and more artificial nitrogen (N) compounds were put into atmosphere and resulted in serious N deposition. It has impacted our environment badly. The ascending concentration of carbon dioxide in the atmosphere leads to the global warming problem. Forest ecosystem is the main part of terrestrial ecosystem, which could sequestrate more than two thirds carbon. So it is of great meaning to research on the effects of N deposition on forest carbon sequestration. A field work was conducted in typical broadleaf-Korean pine mixed forest in Xiaoxing’an Mountains, and four levels of N addition were control (N0,0 kg N ha-1·a-1), low N (N1,30 kg N ha-1·a-1), medium N (N2,60 kg N ha-1·a-1) and high N (N3,120 kg N ha-1·a-1). Inventories of the study site were made in 2008 and 2014, then we calculated the carbon density and net primary productivity of the forest ecosystem and its components with data collected in the field. Carbon density of the whole forest ecosystem ranged from 355.3 to 436.7t C·hm-2.Carbon density of vegetation ranged from 97.8 to 152.7t C·hm-2, accounting nearly 30 per cent of the whole ecosystem. Carbon density of detritus ranged from 12.3 to 17.9 t C·hm-2, accounting for 3 to 4 per cent. And carbon density of soil ranged from 236.3 to 288.5 t C·hm-2, accounting for almost 60 to 70 percent. There were no significant difference on carbon density among components of forest. In the overstory carbon pool, carbon density of stem ranged from 71.38 to 103.96 t C·hm-2, accounting for 69.43 to 82.14 per cent. Carbon density of branches ranged from 5.98 to 11.321 C·hm-2, accounting for 6.3 to 14.6 per cent. Carbon density of coarse root ranged from 8.87 to 20.5 t C·hm-2, accounting for 9.4 to 15.0 per cent. Carbon density of foliage ranged from 2.06 to 4.64t C·hm-2, accounting for 2.2 to 4.5 per cent. There were no significant difference among different N treatments in the same soil layer, but the soil carbon density decreased significantly (at 0.05 level) with depth of soil layer. Net primary productivity of the whole forest ecosystem ranged from 5.63 to 7.64 t·hm-2·a-1, while the aboveground net primary productivity contributed 3.85 to 5.18 t·hm-2·a-1 and the belowground net primary productivity contributed 1.43 to 2.46t·hm-2·a-1. Distribution of net primary productivity of different components showed in an ascending order is understory (0.70-2.84%), coarse roots (6.54-8.66%), fine roots (18.89-26.98%), foliage (25.85-32.44%) and stem and branch (37.88-41.28%). There were no significant difference (at 0.05 level) among forest ecosystem and its components under different N treatment. Net primary productivity of needle ranged from 0.33 to 0.51 t·hm-2·a-1 and the broad leaf one ranged from 0.70 to 0.95 t·hm-2·a-1. N treatment suppressed the net primary productivity of needle,but N2 and N3 treatment promoted the broad leaf one. Finally, we came to a conclusion that N addition did not affect the net primary productivity of broadleaf-Korean pine mixed forest, but have an effect on the foliage net primary productivity, as N treatment suppressed the net primary productivity of needle, but promoted the one of broad leaf.