| Large combustion of fossil fuel caused a great deal of harmful gas emissions.Human activity resulted in great changes of nitrogen in terrestrial ecosystems.At the same time,the excess nitrogen input to forest ecosystems altered carbon and nitrogen cycling and transformation process,which have significant impacts on the functioning of terrestrial ecosystems.On the background of globalization and nitrogen deposition,China as a zone of high nitrogen deposition,studies on the effects of high nitrogen input on soil properties and soil respiration in forest ecosystems have an important practical significance.Our study site is located in the subtropical zone,Chinese fir?Cunninghamia lanceolata?as typical evergreen coniferous tree species.A simulated N deposition experiment was designed,with one N treatments?100 kg N·hm-2·a-1?and a control.The objective of the study is to reveal the impact of N addition on soil nitrogen transformation and soil respiration in a mid-aged Cunninghamia lanceolata stand.The results are as follows:In 0-10 cm soil layer,annual average content of soil total nitrogen?TN?was1.80g·kg-1in the N treated stand and 1.38 g·kg-1in CK.The average inorganic N?IN?were13.92 mg·kg-1 and 4.72 mg·kg-1 for treatment and CK.In 10-30 cm soil layer,the TN were 1.70 g·kg-1 and 1.24 g·kg-1 in the treatment and CK.The IN were,respectively,10.39 mg·kg-1and 3.22 mg·kg-1in the treatment and CK.Nitrogen addition appeared a significant effect on soil N transformation in Cunninghamia lanceolata stand.In the 0-10 cm soil layer,that the average annual ammonification rates were 69.89 ?g·kg-1·d-1and 62.05 ?g·kg-1·d-1in treatment and CK,and the annual nitrification rates were 103.37 ?g·kg-1·d-1and 90.59 ?g·kg-1·d-1.The annual mineralization rates were 167.76 ?g·kg-1·d-1and 149.54 ?g·kg-1·d-1 in treatment and CK.In the 10-30 cm soil layer,the average annual rates of ammonification,nitrification,and mineralization were all higher in the treatment(49.58 ?g·kg-1·d-1,66.29 ?g·kg-1·d-1,and106.38 ?g·kg-1·d-1)than in the control(42.85 ?g·kg-1·d-1,46.48 ?g·kg-1·d-1,82.87?g·kg-1·d-1).After the N treatment,the nutrient concentrations in fresh branches,leaves and fine root of Cunninghamia lanceolata were high with comparison to the control.The nutrientconcentrations in foliage and twigs ranked as N>Ca>K>Mg>P,while in fine-root was as N>K>Ca>Mg>P.Nitrogen addition showed an inhibitory effects on soil microbial biomass carbon?SMBC?,and an improving effect on soil microbial biomass nitrogen?SMBN?.In the0-10 cm layer soil,the average content of SMBC was higher in CK(127.28 mg·kg-1)than the treatment(124.59 mg·kg-1),with the maximum in summer and the lowest in winter.The annual average SMBN was higher in treatment(7.32 mg·kg-1)than in CK(6.84mg·kg-1),with the highest in summer and lowest in spring for CK and in autumn for treatment.Soil respiration has obvious seasonal variation in both treatment and control.The annual rates of soil respiration were higher in CK?0.90 ?mol·m-2·s-1?than in treatment?0.86 ?mol·m-2·s-1?.The soil respiration rates were significantly correlated with soil microbial biomass carbon,fine-root biomass?p<0.05?.The Q10 values of soil respiration were 1.97 and 2.02 for the N treatment and CK,respectively.This indicates N addition decreased the soil respiration sensitive to temperature to a certain extent. |
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