Impacts Of Long-term Nitrogen Fertilization On Soil C, N And P In A Chinese Fir Plantation

Soil degradation has been widely occurring due to the successive rotation, short rotation and full tree utilization in Chinese fir plantations. In order to improve the soil fertility, facilitate the nutrient conditions and promote the tree growth, fertilization(especially using nitrogen fertilizer) has become one of the effective strategies for forest management of Chinese fir plantation. The objective of this thesis was to examine the effects of nitrogen addition on soil carbon, nitrogen, phosphorus and their stoichiometric relationships based on the long-term field manipulative experiment in a Chinese fir plantation. The experiment was conducted in January 2004, it has been 10-year-long when we sampled in June, September, December 2013 and in March 2014, which represented the four seasons. The experiment included four treatments: N0(0 kg N·hm-2·a-1), N1(60 kg N·hm-2·a-1), N2(120 kg N·hm-2·a-1) and N3(240 kg N·hm-2·a-1). Soil N mineralization and leaching were analyzed by in situ cultivation method in the field.The results are as followings:(1) Nitrogen fertilization decreased soil p H, the three levels(N1, N2 and N3) reduced by 3.34%, 6.76% and 10.19% compared with control level, respectively.(2) Nitrogen fertilization decreased soil organic matter, the three levels reduced by 11.62%, 12.63% and 17.5% compared with control level, respectively. But nitrogen fertilization increased the total nitrogen of soil. The increments were 17.35%, 21.60%, 25.06% compared with control level, respectively. There was no significant difference among the treatments.(3) The contents of soil ammonium nitrogen and nitrate nitrogen enhanced along the increases of nitrogen application rate and showed obvious seasonal dynamics. The contents of ammonium nitrogen were the highest in summer and lowest in winter, while the nitrate nitrogen had the opposite trend.(4) The effect of long-term fertilization on soil total phosphorus and available phosphorus was not significant. The seasonal dynamics showed the highest in winter and lowest in summer for phosphorus contents.(5) Nitrogen fertilization led a decrease of soil C/N, but there was no significant difference among treatments. N2 treatment decreased C/P. In addition, N1 and N3 led a decrease of N/P.(6) N2 and N3 treatments increased the N mineralization rate and N leaching. The increased degree was positive with the nitrogen treatment level.(7) The average annual increment of diameter at breast height(DBH) did not significantly change among treatments. N2 treatment increased the average annual increment of the low DBH classes. The DBH increment were larger in the high DBH classes than the low DBH classes.