Effect Of Exogenous Nitrogen Input On Soil Fine Roots Of Poplar Plantation In A Coastal Area, China

Nitrogen deposition is one of hot topic issues of global change. The understanding offorest ecosystems in response to this continuous increase and rapid nitrogen cycle, and feedbackmechanisms of forest ecosystems to nitrogen deposition are vitally for the maintenance of forestecosystem health and productivity. As fine root is sensitive to changes of the environment, theincrease of exogenous nitrogen may alter the distribution structure of fine roots, such as verticaldistribution and diameter size distribution formed in the long-term evolution processes, also mayalter the carbohydrates compositions in forest ecosystem, so as to affect the metabolism activityof fine roots and the growth of the plants. We chose three different ages (4years,8years and15years old) of poplar plantations in a coastal area in Dongtai city, Jiangsu. The experiment usedrandomized block design with five nitrogen deposition concentration treatments, i.e. N0(0gN·m-2·a-1), N1(5g N·m-2·a-1), N2(10g N·m-2·a-1), N3(15g N·m-2·a-1), N4(30g N·m-2·a-1) in thefield, to explore the response of fine root biomass and carbohydrates compositions to nitrogendeposition. Important findings:(1) The surface enrichment characteristic of fine root biomass was obvious, with70%~80%of fine root biomass allocated to0-20cm soil layer. Fine root biomass was mainly distributedin the diameter class of0~0.5mm and0.5~1.0mm, and about50%of the fine root biomass (<2.0mm) allocated in the level of0~0.5mm diameter. Exogenous nitrogen increased the proportionof very fine root biomass (0-0.5mm), especially in the young plantation.(2) Four factors (plantation age, soil layer, diameter class, and nitrogen concentration) in thecomprehensive can explain66.3%variation of fine root biomass. Plantation age, soil layer,diameter class, respectively to explain17.6%,16.1%,10.4%variation with significantly effects(P<0.01), and the increase of nitrogen only explain0.24%variation of fine root biomass, withnot significant effects (P>0.05).(3) The maximum content of soluble sugar and starch in the fine roots appeared inDecember, and soluble sugar in one year had a largest change range, the highest and lowest differabout seven times. While the seasonal variation of structural carbohydrates (cellulose and lignin)was not significant, and the cellulose and lignin content was relatively stable.(4) With the increase of exogenous nitrogen input, soluble sugar, starch and NSC content inthe fine roots increased and then decreased, and the largest content at N2, while N4(the mostnitrogen input) appeared below N0(control treatment). SC had no significant difference (P>0.05)among of levels of nitrogen increase treatments.(5) There were significantly relationship between fine root biomass and NSC, and nosignificant relationship between soil C, N and carbohydrates compositions, as well astemperature and humidity.