| As the biggest Carbon sink,forestry ecosystem effectively influences the CO2 concentration in atmosphere and the carbon cycle in ecosystem.Aiming to assess how C emission,cycling and relevant ecological processes responded to the climate change,we conduced experiments which included Nitrogen(N)deposition experiment[(Control(0,CK),low-N(50 kg N· ha-1·a-1,TL),medium-N(100 kg N-ha-1·a-1,TM)and high-N(150 kg N· ha-1·a-1,TH)],great fall experiment and snow depth experiment[0 cm(non-snowpack),10 cm,20 cm,30 cm and natural depth)].On one hand,we focused on the effects of simulated nitrogen deposition on soil respiration(Rs),effects of snowpck and simulated nitrogen deposition on soil respiration from Spruce-fir-Korean pine forest[Korean pine(Pinus koraiensis),fir(Abies nephrolepis),Spruce(Picea koraiensis)and(P.jezoensis var.microsperma)]in Lesser Xing’an Mountains in the same year(non-growing season of 2013 to growt season of 2014).On the other hand,we futher focused the effects and mechanism of N deposition on C cycling of typical temperate forest in Lesser Xing’an Mountains within 4 years of long-term experiment.The results showed that:(1)The different components to soil respiration(Rr,Rm,Rl)exhibited seasonal patterns in all treatment plots with the highest rates observed in July and August and the lowest rates in May and October.N deposition did not change the seasonal variation patterns of soil respiration.There was no significant difference in different components to soil respiration(Rr,Rm,Rl)among the different N deposition addition treatments.The mean total soil respiration rate was ranked as:low-N(4.05±0.18)>medium-N(3.69±0.14)>control(3.40±0.13)>high-N(3.14±0.13)μmol CO2·m-2·s-1 during the growing season.Significantly exponential relationship was found between soil respiration rate and soil temperature.There was no correlation between soil moisture and total soil respiration and the components to the Rs in plots among different N deposition addition treatments.The Q10(mean temperature coefficient)were 2.14,2.08,1.97 and 2.35 for soil respiration of Control,low-N,medium-N,and high-N treatments respectively.The mean temperature coefficient of Rh in all N deposition is the most familiar component with the mean temperature coefficient of the total Rs.Our results suggest that the effects of nitrogen deposition on soil respiration(Rs)in spruce-fir-Korean pine forest varied among different stimulated N deposition treatments,and affected soil respiration temperature sensitivity in the temperate forest.(2)Soil respiration(Rs)was influenced by soil temperature and soil water content.The strong rainfalls altered the determining factor that control Rs.Soil water content was stimulated greatly by strong rainfalls.During the early period of wet season,Rs was merely disturbed by strong rainfalls,whereas it decreased Rs during the wet season and increased Rs after the wet season.The influences of strong rainfalls on heterotrophic respiration/soil respiration rate(Rh/Rs)varied during different periods of the wet season.Compared to autotrophic respiration(Ra),strong rainfalls greatly inhibited Rh The relationship between Rs and soil temperature at 5 cm depth(T5)as well as soil water content at 5 cm depth(W5)could be better explained by an exponential equation which contains soil moisture parameter c.Strong rainfalls had varying effects on temperature sensitivities of Rs(Q10)during different periods of the wet season.The soil moisture sensitivity of Rs was decreased after rainfall.The moisture sensitivities(c)during different periods of the wet season were characterized as follow:wet season<early wet season<after the wet season.(3)Seasonal snow pack has significant influence on winter soil respiration in temperate forests during different periods in winter,the seasonal snowpack was significantly increased soil respiration in winter.Rs decreasesed with snowpack reduced(except for late winter).Winter temperature sensitivities(Q10)of Rs were significantly higher than the growing season Q10 under all the N addition treatments.Moderate quantities of N addition(low-N and medium-N)significantly increased temperature sensitivities(Q10)of Rs,but excessive(high-N)addition decreased it during winter.Instead,no evidence for this effects of N on Rs could be found during the growing season.On the other hand,reduction of snowpack decreased winter soil respiration due to accompanied relatively lower soil temperature.Gamma empirical models predicted that winter Rs under the four N addition treatments contributed 4.8±0.3%(control),3.6±0.6%(low-N),4.3±0.4%(medium-N)and 6.4±0.5%(high-N)to the whole year Rs.Our results demonstrate that N deposition will alter Q10 of winter Rs.Moreover,Rs do contribute very few to annual Rs budget in this type of cold temperate forest in northeastern China and potentially decreasing seasonal snowpack will decrease winter Rs and further lessen the winter Rs contribution compared to higher Rs during the growing season based on higher air temperature.(4)The effects of N deposition on the NPP of tree layer,shrub layer and herb were different after 4-year ’s long-term experiment.N addition lead to an increase in NPP of tree layer.low-N and medium-N treatment could increase NPP of tree layer,but high-N treatment could decrease NPP of tree layer.low-N and medium-N treatment could increase NPP of shrub layer,but high-N treatment could decrease NPP of shrub layer.N deposition significantly reduced the NPP of herb layer.N deposition significantly increased litterfall biomass,which could promote the plant leaf C fixation.N deposition increased △CWD.low-N and medium-N treatment could increase △CWD,but high-N treatment could decrease ACWD.The effects of N deposition treatments on soil respiration showed same seasonal patterns among different periods during the growing season.Significantly exponential relationship was found between soil respiration and soil temperature.There was no significant effect between soil moisture and soil respiration.The effects of nitrogen deposition on the temperature sensitivity of the soil respiration and the temperature sensitivity of heterotrophic respiration were different:N deposition decreased the temperature sensitivity of Rs,but no significant effect on the temperature sensitivity of Rh.Mean Rs were 999.50±11.67、1125.11± 11.19 和 983.95± 11.61 g C m-2·a-1 respectively at four quantities of nitrogen addition(control,low-N,medium-N and high-N)treatments during 2010 to 2014.Compared to control,the annual soil respiration at low-N,medium-N treatments increased by 2.68%and 15.59%respectively,but was not signifcantly different for high-N.N deposition had no significant effect on soil organic carbon.Soil carbon pool was not sensitive to N addition.N deposition affected distribution patterns of frorest C pool.The response of the plant C pool to N depositon was more significant,while the soil was a stable ecosystem.Low-N and medium-N increased the proportion of the plant C pool in total C pool,and high-N decreased the proportion of the plant C pool in total C pool.The old-growth spruce-fir-Korean pine forest was found to be a weak C sink with the NEP of 0.93ton C·ha-1·a-1.N deposition initiated strong impacts on C cycling processes.Compared to control,low-N increased NEP by 46.46%,while the medium-N and high-N decreased NEP by 12.51%and 40.41%,respectively.We futher explored the response modes of C emission to future climate change in Lesser Xing’an Mountains.As a weak C sink,the old growth spruce-fir-korean pine forest was with uniqueness and criticality.The future climate change scenarios such as N depositon may change the C carbon source/sink characteristics of spruce-fir-korean pine forest. |
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