模拟氮沉降增加对寒温带针叶林土壤 CO 2 排放的初期影响

It is generally thought that high latitude forests are a massive potential global carbon(C) sink.Nitrogen(N) limitation is thought to be the primary reason why these forests are a 'missing C sink' as atmospheric N deposition is shown to significantly promote plant growth and increase C sequestration in forests of these regions.Boreal forests are the second largest biome on earth and are limited by N availability.Soil CO2 emission is an important process of C cycling in boreal forests and it has shown promotion,no change and inhibition responses to N addition.Therefore studies on the CO2 flux from boreal forest soils and its control factors under increased atmospheric N deposition are essential.In this study,a manipulative N addition experiment was created in a cold-temperate coniferous forest in the Daxing′anling region to investigate this.NH4NO3 fertilizer was added at four rates,each with three replicates: control(0 kg N hm-2 a-1),low N(10 kg N hm-2 a-1,the current N deposition level),medium N(20 kg N hm-2 a-1) and high N(40 kg N hm-2 a-1).Each plot had an area of 200 m2(10 m × 20 m) and a 10 m isolation band set between them.During the growing season(May to September),soil CO2 fluxes were measured by the static chamber-gas chromatography method and soil temperature,soil moisture,soil inorganic N and dissolved carbon content(DOC) were simultaneously measured to explore the main factors affecting CO2 emission.Results showed that N addition did not significantly change soil moisture content but significantly reduced soil dissolved inorganic carbon(DIC) in the organic layer and increased soil DOC in the organic and mineral soil layers.Short-term N addition did not increase soil NH+4-N content,but significantly increased soil NO-3-N content.In control plots,soil CO2 flux ranged from 195.24 to 544.91 mg CO2 m-2 h-1,with an average of(357.33±18.72) mg CO2 m-2 h-1.Exogenous N input tended to increase CO2 emission from the coniferous forest soil by 8.3% to 18.2%.Soil CO2 flux was primarily driven by soil temperature,followed by soil moisture and DIC content.The decrease in soil DIC caused by soil acidification was an important factor promoting soil CO2 emission.The overall pattern of soil CO2 flux was dominated by soil temperature but soil moisture appeared to control variation during peak growth periods.These results show that when investigating effects of N addition on soil CO2 flux,the roles of available N and C dynamics should be considered as well as soil temperature and moisture.It is therefore hypothesized that chronic atmospheric N deposition will stimulate CO2 emission from cold-temperate coniferous forest soils in the Daxing′anling region in the short term,which will partially offset C sequestration by plants.