| Under the background of global climate change,nitrogen deposition and precipitation changes affect soil respiration through changing the content of soil moisture and effective nitrogen.This study selected mixed Pinus Koraiensis forest on Changbai Mountain as the research object,using field control experiments simulating nitrogen addition and precipitation reduction since 2009.The effects of nitrogen addition(50kg·hm-2·a-1)and precipitation reduction(-30%through rain)on soil respiration,fine-root morphology,mycelium length,and microbial community structure during the growing season in 2016 and 2017 were studied to provide a scientific basis for accurate climate change response in terrestrial ecosystems.The main results are as follows:(1)Nitrogen addition and precipitation reduction significantly increased soil respiration rates during the growing season.Under increasing nitrogen treatment,the mean annual total soil respiration(Rs)rate,heterotrophic respiration(Rh)rate and autotrophic respiration(Ra)rate increased by 47%,94%and 38%respectively.Under the water reduction treatment,the mean annual Rs rate,Rh rate and Ra rate increased by 18%,53%and 24%respectively.Under interaction of nitrogen addition and precipitation reduction treatment,the mean annual Rs rate,Rh rate and Ra rate increased by 5%,4%and 6%respectively in 2016.The interaction of nitrogen addition and precipitation reduction had no significant effect on Rs and Rh,but significantly reduced Ra in 2017.Rs and Rh were positively correlated with fine root biomass and microbial biomass,while Ra was positively correlated with fine root biomass only.The significant increase in Rs,Rh and Ra may be due to increased root biomass and microbial biomass under nitrogen addition,precipitation reduction and their interactions.Rs and Ra also showed a negative correlation with the total phosphorus concentration in soil.Compared with control treatment,increasing nitrogen significantly reduced Rh contribution to Rs,while precipitation reduction significantly reduced Rh contribution to Rs only in 2017.The results show that nitrogen addition plays a more important role than precipitation reduction in changing the contribution of Rh and Ra to Rs.In addition,nitrogen addition and precipitation reduction significantly increased the temperature coefficient values of Rs and Rh,indicating that nitrogen addition and precipitation reduction changed the response of soil carbon emissions to global warming to a certain extent.(2)Increased nitrogen and precipitation reduction enhanced soil respiration by increasing growing season root biomass,and changes in fine root growth indicators could explain the growing season dynamics of autotrophic respiration.Nitrogen addition,precipitation reduction and their interactions directly affect soil heterogenous respiration by changing the biomass and morphological characteristics of fine roots,and fine roots in the 2030cm soil layer increase more than in other soil layers.During the growing season,the total root length and total surface area of the 0-10cm soil layer under the increasing nitrogen treatment decreased by 2%and 1%respectively,while the fine root diameter increased by 6%on average;under the precipitation reduction treatment,it decreased by 12%,13%and 9%on average;the average reduction of 4%,11%and 4%under the interaction of nitrogen addition and precipitation reduction.During the growing season,the total length,total surface area and diameter of the root system of the 10-20cm soil layer under the increasing nitrogen treatment decreased by 19%,22%,and 4%on average each month;the precipitation reduction treatment decreased by 35%,29%,and 1%;Under the interaction of nitrogen addition and precipitation reduction,the average decrease is 15%,11%and 7%.During the growing season,the total length,total surface area and diameter of the root system of the 20-30cm soil layer under the nitrogen addition treatment increased by 77%,78%and 2%on average each month;The precipitation reduction treatment increased by 60%,82%,and 7%;The average increase of 43%,123%and 45%under the interaction of nitrogen addition and precipitation reduction.Nitrogen addition and precipitation reduction and their interaction significantly increase the fine root biomass.In the 0-10cm soil layer,the fine root biomass has the strongest response to the interaction between nitrogen addition and precipitation reduction,and the fine root biomass in the 10-20cm soil layer has the strongest response to nitrogen addition,and the fine root biomass has the strongest response to precipitation reduction in the 20-30cm soil layer.With the change of sampling time,the specific root length exhibits different dynamic changes between different soil depths under the influence of nitrogen addition and precipitation reduction and their interaction.The results show that the significant changes in fine root morphology during the growing season may be due to the change in fine root biomass and specific root length under the interaction of nitrogen addition and precipitation reduction.(3)Nitrogen addition and precipitation reduction and their interactions significantly changed the total mycelial length of mycorrhizal fungi at different soil depths,which directly affects the respiration of mycorrhizal fungi,and indirectly affects root respiration and soil respiration in the non-rhizosphere zone,and the interactions of nitrogen addition and precipitation reduction interaction showed an antagonistic non-superimposed effect on total mycelial length..During the growing season,nitrogen addition treatment increased the total hyphae length of the 0-10cm and 10-20cm soil layers by 93%and 92%each month,while the 20-30cm soil layer decreased by 40%on average;The average increase was 179%,81%,8%under the interaction of nitrogen addition and precipitation reduction.Nitrogen increase reduced mycelial self respiration at deeper soils and had a weaker effect on root respiration.In addition,the reduction in total mycelial length under temperature change could explain the significant increase in soil respiration in July and August due to nitrogen addition.Furthermore,the effect of mycelium on soil respiration at shallow soils decreased with increasing soil depth.However,the interaction of nitrogen increase and precipitation reduction showed an antagonistic,non-superimposed effect on total mycelial length.(4)Soil microbiota is not the main factor influencing soil respiration;microbial diversity and community structure are the main influences on soil respiration.Bacterial and fungal communities shifted towards species that were more resistant to disturbance under both nitrogen increase and precipitation reduction.Precipitation reduction is involved in the nitrogen cycle of the bacterial community under nitrogen enrichment conditions,and the interaction of nitrogen enrichment and precipitation reduction does not have a single synergistic or antagonistic effect on the bacterial community.Nitrogen addition significantly reduced bacterial and fungal diversity by 26%and 27%.Bacterial and fungal diversity increased by 121%and 159%,respectively,when precipitation was reduced.Under the interaction of nitrogen addition and precipitation reduction,the diversity of bacteria and fungi increased by 185%and 75%,respectively.Under the nitrogen addition treatment,the relative abundance of eutrophic microorganisms decreases,and the relative abundance of some oligotrophic microorganisms increases.The relative abundance of eutrophic microorganisms increased under precipitation reduction.The interaction of nitrogen addition and precipitation reduction treatment has a weak influence on the structure of microbial communities.Under the treatment of nitrogen addition,the relative abundances of nitrification functional groups and nitrate_oxidation bacteria functional groups were significantly increased by 3%and 6%,while the relative abundances of denitrification functional groups,nitrate_reduction functional groups,nitrogen_fixation functional groups,nitrogen_respiration functional groups and saprophytic fungi decreased significantly by 14%,11%,10%,20%and 79%.Under water-reducing treatment,the relative abundance of nitrogen_fixation functional groups and exogenous mycorrhiza decreased significantly by 14%and 96%,while the relative abundances of ammonifiers functional groups,nitrification functional groups and nitrate_oxidation bacterial functional groups increased significantly by 265%,7%and 6%.Under the interaction of nitrogen-increasing and water-decreasing,the relative abundance of ammonifiers functional groups,cellulose-decomposing bacteria functional groups and endogenous bacteria increased by 362%,21%and 987%respectively,while the relative abundance of exogenous mycorrhizae decreased by 99%.To summarize,nitrogen addition and precipitation reduction may change soil respiration by affecting soil nutrients,fine root morphology,total mycelial length,microbial community structure,and the soil carbon cycle during the growing season.In addition,soil depth can also indirectly regulate the response of soil respiration to climate factors(nitrogen addition and precipitation reduction). |
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