| As an excellent timber species,Chinese fir(Cunninghamia lanceolata)is widely planted in the south of China.Nitrogen(N)is the major elements which are necessary for plant growth,and plants uptake more N than other elements.Therefore,N is the main limiting nutrient for plant growth.Understory plants compete for water and nutrients with trees in plantations,and also play an important function in maintaining ecological balance,such as soil fertility maintenance,water conservation,etc.Study of the growth of Chinese fir plantation should be considered together with understory vegetation.In this study,a 13-year-old Chinese fir plantation was selected to conduct N addition experiment,in order to reveal the changes of soil physicochemical properties and the responses of Chinese fir plantation growth and understory vegetation diversity to N addition.Consequently,a total of seven 20 m x 20 m plots was set up,with 4 plots for N addition treatment and 3 plots for control.The amount of N addition was 100 kg N/hm~2/a,which was used in 3 times in May,July and September.The results obtained are as follows.1.Effect of nitrogen addition on stand growth.The average DBH was higher in the treatment(15.45 cm)than that in control(14.44 cm).The trees with DBH<10 cm contributed 16.4%of the total for the treatment,which was a bit higher than the control(9.5%).The proportion of Chinese fir with DBH between 10-20 cm in fertilizer plot and control plot are 75.7%and 79.3%,the DBH in that diameter class is 16.46 cm compared with the control(15.24 cm).But the proportion of trees with DBH>20 cm in N treatment and control are 11.2%and 7.9%,respectively,indicating that N addition increased DBH growth of Chinese fir for the middle DBH class.However,N addition had no significant effect on the growth of larger DBH class.2.Effect of nitrogen addition on soil properties.Soil DOC decreased with the depth and the DOC concentration was much higher in the treatment than in control.In0-10 cm soil layer,the DOC concentration was higher in the treatment(119.09 mg/kg)than in control(93.89 mg/kg).In 10-30 cm soil layer,the soil DOC content was 108.41mg/kg,and 21.2%higher than that of control(83.58 mg/kg).In 30-50 cm soil layer,the soil DOC content was 67.17 mg/kg,and 17.9%higher than that of control(55.09mg/kg).In 0-10 cm soil layer,the soil DON content was 10.13 mg/kg,and 15.2%higher than that of control(8.59 mg/kg).In 10-30 cm soil layer,it was 9.91 mg/kg,and29.6%higher than that of control(6.97 mg/kg).In 30-50 cm soil layer,it was 7.97mg/kg,and 18.6%higher than that of control(6.48 mg/kg).In 0-10 cm soil layer,the soil MBC content was 124.70 mg/kg,6.8%higher than that of control(116.22 mg/kg).In 10-30 cm soil layer,it was 159.85 mg/kg,27.5%higher than that of control(115.81 mg/kg).In 30-50 cm soil layer,it was 133.95 mg/kg,27.4%higher than that of control(97.19 mg/kg).Soil MBC differed significantly between different layers.The mean MBC content was significantly higher in the treatment than in control.The soil MBNs in different soil layer were not significantly different between treatment and control.With regard to the available soil nutrients,the annual average content of ammonium nitrogen was 4.06 mg/kg for the treatment and 3.40 mg/kg for the control.The annual average content of nitrate nitrogen was 10.62 mg/kg for the treatment and2.75 mg/kg for the control with significant different between treatment and control.Available N content in different soil layers showed a significant difference between the treatment and the control.The annual average net ammonification rate in0-10 cm soil layer was slightly higher in treatment(3.79 mg/kg)than in control(3.53mg/kg).And in 10-30cm soil layer,the treatment(3.32 mg/kg)was significantly greater than the control(2.15 mg/kg).The annual average net nitrification rate in 0-10 cm soil layer was 16.78 mg/kg,was significantly higher than the control(10.46 mg/kg).In10-30 cm soil layer,it was 9.80 mg/kg for the treatment,significantly higher than the control(5.37 mg/kg).3.Effect of nitrogen addition on understorey vegetation.In the treatment,the coverage of undergrowth reached the maximum in August(37.2%),the minimum in November(19.6%),which were slightly higher than in control.Shannon-Weiner index was 0.67-1.19 in the treatment and lower than in control(1.43-0.71).The species richness was the minimum in August,and maximum in November for the treatment and control.Species evenness index changed little for both treatment and control.The N addition did not change understory vegetation biomass,and resulted in reduction of the relative coverage of the ferns. |
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