模拟氮沉降增加对寒温带针叶林土壤 CO2排放的初期影响

研究大气氮沉降增加情景下北方森林土壤CO2排放通量及其相关控制因子至关重要。在大兴安岭寒温带针叶林区建立了大气氮沉降模拟控制试验,利用静态箱-气相色谱法测定土壤CO2排放通量,同时测定土壤温度、水分、无机氮和可溶性碳含量等相关变量,分析寒温带针叶林土壤CO2排放特征及其主要驱动因子。结果表明:氮素输入没有显著改变森林土壤含水量,但降低了有机层土壤溶解性无机碳(DIC)含量,并增加有机层和矿质层土壤溶解性有机碳(DOC)含量。增氮短期内不影响土壤NH+4-N含量,但促进了土壤NO-3-N的累积。增氮倾向于增加北方森林土壤CO2排放。土壤CO2通量主要受土壤温度驱动,其次为土壤水分和DIC含量。虽然土壤温度整体上控制着土壤CO2通量的季节变化格局,但在生长旺季土壤含水量对其影响更为明显。在分析增氮对土壤CO2通量的净效应时,除了土壤温度和水分外,还要考虑土壤有效碳、氮动态的影响。

Early nitrogen deposition effects on CO2 efflux from a cold-temperate coniferous forest soil

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.