Effects Of Nitrogen Deposition On Soil Respiration Of Subalpine Grassland In Qilian Mountains

Soil respiration was not only the main way of carbon output in the soil carbon pool,but also an important part of the terrestrial ecosystem carbon cycle.It was closely related to atmospheric CO₂concentration and the greenhouse effect.Human activities,such as the consumption of fossil fuels and fertilizers,lead to the sharp increase of nitrogen deposition in terrestrial ecosystem,which had an impact on global ecosystem carbon cycle.Since subalpine grasslands were mainly distributed in high-altitude areas,the grassland ecosystem was extremely fragile and sensitive to human disturbance and environmental changes.Under the background of global climate change,the study of the response of subalpine grassland soil respiration to nitrogen deposition was useful for studying grassland ecosystems carbon balance was of great significance,and it can also provide a theoretical basis for the accurate assessment of grassland soil carbon emissions.In this study,seven treatment levels were set up in the subalpine grassland of Tianlaochi watershed basin of Qilian Mountains:N0(0 g N m^-2a^-1),N2(2 g N m^-2a^-1),N5(5 g N m^-2a^-1),N10(10 g N m^-2a^-1),N15(15 g N m^-2a^-1),N25(25 g N m^-2a^-1),N50(50 g N m^-2a^-1),to study the effects of different gradients of nitrogen deposition on soil respiration in subalpine grasslands.The main research results were as follows:(1)With the increased of nitrogen concentration,the content of total nitrogen,nitrate nitrogen and ammonium nitrogen in the soil all showed an increasing trend,which increased the content of available nitrogen in the soil and maked plants faster growth.The above-ground and underground productivity of vegetation increased by30.35%and 14.24%,respectively,compared with that when no nitrogen was added(0g N m^-2a^-1),but the number of species showed a decreasing trend as the amount of nitrogen increased.As nitrogen fertilizer enters the soil to release a large amount of H⁺,the soil p H and total soil phosphorus were reduced by 7.14%and 5.79%on average,but its impact on soil organic carbon was not obvious;(2)The results showed that soil total respiration,heterotrophic respiration and autotrophic respiration all showed a single peak curve pattern with the increase of nitrogen addition.When the nitrogen concentration was 5 g N m^-2a^-1,the soil respiration rate reached the peak,and the soil respiration and its components had obvious seasonal changes.There were obvious and seasonal dynamics in soil respiration and its components,and the peak occurs in August when plants were growing vigorously.In addition,the ratio of soil autotrophic/heterotrophic respiration rate showed an increasing trend with the increase of nitrogen concentration;(3)Through the Pearson correlation method,the correlation of different factors on total soil respiration,heterotrophic and autotrophicity was obtained.The factors positively correlated with total soil respiration were mainly soil ammonium nitrogen,species number,p H,ANPP and BNPP,while negatively correlated with soil total nitrogen,total phosphorus,nitrate nitrogen and organic carbon;the factors positively correlated with soil heterotrophic respiration were mainly soil nitrate nitrogen,total phosphorus,ammonium nitrogen,total nitrogen,BNPP,ANPP,while soil p H,organic carbon and species number were negatively correlated with them;the number of species,BNPP,ammonium nitrogen and p H were positively correlated with soil autotrophic respiration,and other factors were negatively correlated with autotrophic respiration.Simulated nitrogen deposition improved the availability of nitrogen in the soil and promoted the accumulation of biomass,but excessive nitrogen deposition can lead to soil acidification and lower phosphorus content.The response of soil respiration rate to nitrogen deposition showed a single peak curve pattern that first increases and then decreases.Among them,autotrophic respiration was the main component that caused changes in total soil respiration.Compared with other factors,BNPP,total soil nitrogen and organic carbon had a greater impact on soil respiration.Among them,BNPP promoted total soil respiration and the respiration rate of its components,while total soil nitrogen inhibited total soil respiration and autotrophic respiration.Soil organic carbon also had a certain inhibitory effect on total soil respiration.The results of this study can provide a theoretical basis for the accurate assessment of soil carbon emissions from subalpine grasslands in my country.