Effects Of Nitrogen Addition On Soil Respiration In Larix Olgensis Plantation

Soil respiration is the primary pathway for C loss in global terrestrial ecosystems,both nitrogen addition and atmospheric nitrogen deposition have significant effects on soil respiration,but the effect of nitrogen application on soil respiration varies with different species and site conditions,so its influence rule still needs to be further clarified.In order to reveal the effects of nitrogen addition on soil respiration rate,soil autotrophic respiration rate and heterotrophic respiration rate of young larch plantations in northeast China,to better assess the impact of forest management pracatices on soil carbon cycling in plantation forest ecosystems,11-year-old Larix olgensis plantations were selected and two nitrogen addition treatments(N1:5g·m-2·yr-1:N2:12.5g·m-2·yr-1)and the control check(N0:0g·m-2·yr-1)were set up to measure soil respiration rate using LI-6400 infrared gas exchange analyzer within three years' fertilization experiment in Northeast Forestry University Maoer Mountain experiment forest farm,Shangzhi,Heilongjiang Province,and soil temperature,water content,total nutrients,available nutrients and soil microbial biomass carbon and nitrogen were also measured,the results were as follows:(1)Soil respiration rate varied from 0.69 to 5.72?molCO2·m-2·s-1 in Larix olgensis plantations with different nitrogen addition treatments.Nitrogen addition caused the mean soil respiration rate decrease among different growing seasons,but it didn't have significant effects on soil respiration.Nitrogen addition also didn't have significant effects on mean soil heterotrophic respiration rate and autotrophic respiration rate among different growing seasons,but the autotrophic respiration rate of high nitrogen treatment(N2)in some months decreased significantly in the second and third years after nitrogen addition.(2)There were significant exponential regression correlations between soil respiration rate and temperature at the depth of 5cm and 10cm,soil temperature was the main drive factors that caused soil respiration generate seasonal variations.Nitrogen addition didn't have significant effect on soil temperature,but nitrogen addition made Q10 have a decrease trend,which reduced the sensitivity of soil respiration rate to temperature increase.There were insignificant regression correlations between soil respiration rate and water content at the depth of 5cm and 10cm,therefore,Soil water content was not the main factor affecting soil respiration rate in Larix olgensis plantations.(3)Nitrogen addition didn't have significant effects on soil total carbon and nitrogen,but it had significant effects on soil ammonium nitrogen and nitrate nitrogen.Nitrate nitrogen accounted for more than 95%of soil available nitrogen(ammonium nitrogen+nitrate nitrogen),and the soil nitrate nitrogen content increased with the increase of nitrogen addition.Soil nitrate nitrogen concentration was negatively correlated with soil respiration rate.Therefore,the increase of soil nitrate nitrogen concentration after nitrogen addition has the potential risk of reducing soil respiration rate.(4)Nitrogen addition didn't have significant effects on soil microbial biomass carbon and nitrogen,although soil nitrate nitrogen concentrate increased significantly after nitrogen addition,it did not significantly inhibit soil microbial biomass.In conclusion,the two nitrogen addition treatments in this study didn't cause significant effects on soil respiration rate and heterotrophic respiration rate in larix olgensis plantations,but nitrogen addition inhibited soil autotrophic respiration rate to some extent.This indicates that the increase of nitrate nitrogen content may lead to the decrease of fine root biomass.