Mechanism Of Soil Respiration Of The Afforested Vegetation Response To Precipitation In The Hilly Region Of The Loess Plateau

Soil respiration is an essential process of the terrestrial carbon cycle.The dynamic of soil respiration has a profound impact on the soil carbon cycle and global climate.Loess plateau is located in the arid and semi-arid region of China.Since Grain for Green Project was performed,the vegetation in Loess Plateau is restored which had a profound effect on the soil carbon pool,leading to significant changes in soil respiration.Precipitation is the main factor that stimulates soil respiration in the arid and semi-arid regions.Problems emerge when predicting soil respiration under precipitation variations under future climate extremes.Thus,research on the response of soil respiration to precipitation regimes can deepen our knowledge of the carbon cycle of the Loess Plateau and its sink-source characters,which leads to precise predictions of terrestrial carbon emission.In this study,two typical vegetations from the hilly regions of the Loess plateau,Artemisia grassland and Caragana shrubland,were selected as the research object.By conducting observations under natural rainfall and implementing water manipulation experiments,the response of soil respiration to rain regimes was studied under different time scales.Based on that,the response mechanism of soil respiration under precipitation variations was discussed.The relationship between Q10 and soil moisture content was also classified.The main results are as follows:（1）The response of soil respiration and its components to precipitation pulses during different stages of the growing season is identified.The monthly and growing season cumulative soil C emission is also quantified and its relations with controlling factors were analyzed.The result showed that soil respiration was more sensitive to pulses that occurred during the early and middle stages of the growing season,compared to the late growing season.Pulse rainfall during the early,middle and late growing season increased soil heterotrophic respiration by 2.12,2.48,1.06μmol m^-2 s^-1 on average,and soil autotrophic respiration by 1.15,1.94,1.32μmol m^-2 s^-1 on average,indicating that heterotrophic respiration contributes more to the response of total respiration to pulses.The increase in total soil respiration,heterotrophic respiration and autotrophic respiration in the Artemisia grassland decreased by 57.0%,35.0%and 26.0%on average compared with that in the Caragana shrubland.The carbon emission by soil respiration in the growing season of Artemisia grassland is 370-582 g C m^-2,and the cumulative carbon emission in each year is lower than that in the Caragana shrubland(378-748 g C m^-2).Among them,the cumulative carbon emissions of heterotrophic respiration and autotrophic respiration decreased by 15.4%-28.1%and 9.7-157.1%,respectively.The soil moisture pulse induced by rainfall is the dominant factor that drives the increase of soil respiration and its components after rain.The monthly rainfall explains more with the variations of soil respiration（45%）compared to that of temperature（26%）.Soil respiration and its components in Caragana shrubland are more sensitive to rainfall variations than those in Artemisia grassland.Under the context of future rainfall increases,soil carbon will be more likely to emit more carbon for Caragana shrubland.（2）The study clarified the response characteristics and influencing factors of soil respiration and its components,under the conditions of shifting rainfall regimes.Under the same precipitation amount,changed rainfall patterns reduced soil respiration in Artemisia grassland and Caragana shrubland by 5.4%and 14.6%,respectively,and the latter vegetation treatment reached a significant level.Heterotrophic respiration in Artemisia grassland showed a staggered increase and decrease trend in each month of the growing season due to changes in rainfall distribution,while the decrease of heterotrophic respiration in Caragana shrubland occurred throughout the growing season.In the first half of the growing season,soil autotrophic respiration decreased in Artemisia grassland and Caragana shrubland affected by changes in rainfall pattern and increased in the second half,but the increased amount was lower than the decrease in the first half.Soil autotrophic respiration in Artemisia grassland is more sensitive to changes in rainfall distribution than soil heterotrophic respiration,and changes in rainfall distribution increase the proportion of soil heterotrophic respiration/total respiration.On the contrary,the heterotrophic respiration was more sensitive to rain regime shifting.Throughout the whole growing season,soil moisture explains more of the variations of soil respiration and its components,compared with soil temperature.The change of rainfall distribution significantly reduced the soil surface moisture by 16.7%-22%but did not significantly change the soil surface temperature,as well as the temperature sensitivity of soil respiration and its components.（3）The response characteristics and influencing factors of soil respiration and its components to the increase and decrease of rainfall were clarified.50%decrease in rainfall on average reduced total soil respiration in Artemisia grassland and Caragana shrubland by 27.5%and 26.2%,respectively,while 50%increase in rainfall increased the average soil respiration in Artemisia grassland and Caragana shrubland by 24.0%and 24.1%,respectively.The response of each component of soil respiration to the increase and decrease of rainfall was similar to that of total respiration.Rainfall reduced soil heterotrophic respiration by 31.0%and 20.9%respectively,and decreased soil autotrophic respiration by 23.8%and 28.9%in Artemisia grassland and Caragana shrubland,respectively.The increased rainfall increased soil heterotrophic respiration by 29.9%and 18.3%,and autotrophic respiration by 32.9%and 15.9%,respectively.The ratio of soil heterotrophic respiration/total respiration was not significantly changed by increasing or decreasing rainfall,indicating that different components of soil respiration responded similarly to changes in rainfall.The increase or decrease of rainfall did not significantly change the soil temperature,but the decrease in rainfall decreased the sensitivity of soil respiration temperature sensitivity on average by 38.2%and 35.6%,similar to its components.The increase or decrease of rainfall mainly affected the soil moisture in the middle growing season.During the two-year experimental period,the average rainfall decreased the soil moisture of the Artemisia grassland and Caragana shrubland by 19.1%and 24.4%,respectively.The average soil moisture in the shrubland increased by 23.0%and 20.0%,respectively,indicating that changes in rainfall mainly affect soil respiration and its component carbon emissions through soil moisture changes and the interaction effect of soil moisture and temperature.（4）The traditional method of calculating soil temperature sensitivity of soil respiration(Q₁₀)of soil respiration was optimized by creating a sliding sampling method.Using high-frequency in situ observational data,a sliding sampling method was established to quantify Q₁₀ of soil respiration and its components under different soil moisture conditions.The results show that the sample moisture variation used in the calculation of Q₁₀by the sliding sampling method was limited,which can minimize the influence of soil water-temperature covariance on the calculation of soil Q₁₀.The correlation analysis between Q₁₀and the mean value of water showed that the Q₁₀ of soil respiration was positively correlated with soil water,and water could explain 83%and 53%of the variation of Q10 in Caragana shrubland and Artemisia grassland.With the increase of soil moisture,the R² of Q₁₀ fitting was also increased,indicating that the increase of soil moisture promoted the temperature sensitivity of soil respiration.Similar to total respiration,soil heterotrophic respiration Q₁₀ is positively correlated with soil moisture,and soil moisture variation can explain 89%and 77%of the variation in soil heterotrophic respiration Q₁₀ in Artemisia grassland and Caragana shrubland,respectively;Respiration Q₁₀ is significantly positively correlated with soil moisture,and soil moisture can only explain 34%of the variation of Q₁₀.These results indicate that soil heterotrophic respiration Q₁₀ is more sensitive to soil moisture variation.Due to the proportion of soil heterotrophic respiration being higher than that of autotrophic respiration,soil moisture is the main factor affecting total respiration Q₁₀.This study clarified the response dynamics of soil respiration to natural rainfall,quantified the response characteristics of soil respiration to changes in rain amount and frequency,and clarified the response mechanism of temperature sensitivity of soil respiration to rain variations,and clarified the response mechanism of soil respiration to rain regime changes,which provided theoretical support for the accurate estimation of soil respiration carbon emissions in the Loess Plateau under the conditions of future climate change.