Effects Of Long-term Nitrogen Addition On The Carbon Budget Of Larix Gmelinii Forest In Greater Khingan Mountains

In recent decades,global nitrogen deposition has been increasing,which has a profound impact on the carbon cycle of terrestrial forests.Boreal forests play an important role in the global carbon cycle.The response of different processes of forest carbon cycle to different amounts of nitrogen deposition is different,and the increase of nitrogen deposition is likely to change the role of boreal forest as carbon sink or carbon source.In order to evaluate the long-term effects of different nitrogen gradients on carbon budget in boreal forest,the experiment of nitrogen addition(NH4NO3)was conducted in Larix gmelinii forest in Greater Khingan Mountains for 8 years.Based on the local nitrogen deposition level,four nitrogen addition gradients were set up:control(0 kg N ha-1 yr-1),low nitrogen(LN,kg N ha-1 yr-1),medium nitrogen(MN,kg N ha-1 yr-1)and high nitrogen(HN,75 kg N ha-1 yr-1).We studied the effects of nitrogen addition on aboveground and underground carbon pools,soil respiration,litter decomposition,photosynthetic capacity,nutrient resorption efficiency and fine root growth of Larix gmelinii forest.The research shows that:(1)Larix gmelinii forest is an important carbon sink in Nanwenghe area of Greater Khingan Mountains,but its capacity is usually limited by soil nitrogen level.nitrogen addition not only alleviates nitrogen limitation,but also improves carbon dioxide sequestration in the atmosphere.The carbon sequestration stocks of low-N,medium-N and high-N treatments were 104,20 and 5 gC/gN,respectively.Aboveground trees were the largest carbon pool,followed by 0-20cm soil carbon pool,roots carbon pool and forest floor carbon pool.Nitrogen addition increased the investment of underground roots,increased the underground carbon pool and decreased the forest floor carbon pool of Larix gmelinii.(2)Soil respiration has obvious seasonal variation,with the highest in July and the lowest in September.Under HN treatment,RS was driven by heterotrophic respiration(RH)in June and July,while under low nitrogen treatment,RS was driven by root autotrophic respiration(RAR)in August and September,and RH was always higher than RAR.RH was positively correlated with soil microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN),while RAR was positively correlated with fine root biomass.MN and HN treatments inhibited RH and increased soil carbon fixation.MN treatment significantly increased the temperature sensitivity(Q10)of RS and RH,while MN and HN treatments significantly decreased the Q10 of RAR.LN treatment significantly increased the carbon flux of soil respiration,while MN and HN treatments decreased the carbon flux of RS and RH.The effect of nitrogen addition on soil carbon flux mainly depended on the response of soil microorganisms to nitrogen addition.(3)The decomposition process of needle litter and mixed leaves litter has stage characteristics.Under natural conditions,the decomposition rate of mixed leaf litter was faster than that of needles litter.Nitrogen addition significantly increased the decomposition rate of the two kinds of litter.The decomposition rate of mixed leaf litter was slower under HN treatment.HN treatment changed the C and N release of litter,resulting in the increase of C:N of mixed leaf litter.Meanwhile,HN treatment significantly reduced the abundance of soil animals.The decomposition coefficient of low-quality litter increased after mixing the two litters,which may be related to the transfer of limiting nutrients between litters.Nitrogen addition promoted the release of carbon and inhibited the release of phosphorus.The release of nitrogen showed an enrichment release mode.There was a significant positive correlation between C:N and litter mass loss of needle and mixed leaf litter,and the correlation between C:N and litter mass loss of needle and mixed leaf litter was significantly enhanced by low nitrogen treatment.High nitrogen treatment significantly reduced the total litter biomass in Larix gmelinii forest,and the litter biomass showed a decreasing trend with the increase of nitrogen,showing the inhibitory effect of nitrogen addition on the litter biomass.(4)Low nitrogen treatment significantly increased the photosynthetic rate of Larix gmelinii and the carbon input of the ecosystem.HN treatment significantly decreased soil pH and increased soil nitrogen availability.Nitrogen addition significantly increased TN and N:P in needles and fine roots,and decreased TP in needles.Nitrogen resorption efficiency(NRE)of Larix gmelinii was significantly decreased,litter quality was increased,litter decomposition rate and soil C input were accelerated by HN treatment.Nitrogen addition had little effect on P resorption efficiency(PRE)of Larix gmelinii,and the PRE was very low,which indicated that Larix gmelinii was more inclined to absorb P from soil.Nitrogen addition also made Larix gmelinii distribute more C to higher-order roots,which strengthened the transport function of fine roots rather than the absorption function,which was more conducive to soil P mineralization and absorption.At the same time,low nitrogen treatment significantly increased fine root production,probably because low nitrogen treatment increased the photosynthetic carbon allocation of fine roots,which also led to an increase in root autotrophic respiration and increased C output.In general,long-term nitrogen addition promoted the carbon input and carbon output of Larix gmelinii forest,and the overall performance was "nitrogen promoting carbon sink"...