Dry And Wet Cycle On The Loess Plateau With Farmland To Forest Restoration Effects On Soil Microbial Carbon Use Efficiency (CUE)

Terrestrial ecosystems play an important role in climate feedback.Vegetation and soil in terrestrial ecosystems store large amounts of carbon and thus become important global carbon sinks.The pedosphere connects the atmosphere,hydrosphere and lithosphere,and several other spheres conducting material exchange and energy flow.The soil carbon pool is the largest carbon pool in the terrestrial ecosystem.Microbial carbon use efficiency（CUE）is an index of microbial activity,and therefore affecting the ability of soil to sequester carbon.Climate change triggers extreme weather events,leading to frequent downpours and droughts,and impacts on ecosystem carbon（C）,nitrogen（N）,and phosphorus（P）cycles.Heavy rain and drought can lead to changes in soil moisture,and the process of slow drying and re-wetting of soil is called alternating wetting and drying of soil.The alternating wetting and drying cycles of soil will could affect the physical properties,chemical properties and microbial activities of soil.In this study,soil of a farmland ecosystem and two forest ecosystem with different years of restoring farmland to forest（0,16 and 40 years）were collected in the area of the Loess Plateau where the ecological project of returning farmland to forest was implemented.The laboratory experiment was conducted to simulate the alternating dry-wet cycle and different intensity of two alternating dry-wet cycles（high intensity:100%WHC-20%WHC,low intensity:60%WHC-20%WHC）and different number of four alternating wet and dry cycles（1-4 times）were set.Microbial carbon use efficiency was measured using the ¹⁸O isotope labeled water method to explore how the dry and wet alternating cycles changed microbial carbon use efficiency,microbial respiration rate and microbial growth rate.The study reached the following conclusions:（1）The dry-wet cycle is a kind of stress,which led to the reduction of microbial carbon utilization efficiency.From the perspective of respiration rate and growth rate,the dry and wet alternating cycle led to the increase of microbial respiration rate and the decrease of microbial growth rate.This indicated that soil microorganisms used organic matter absorbed by themselves more for respiration metabolism than for growth metabolism when facing with the stress of alternating wetting and drying.（2）The intensity and frequency of dry and wet alternating cycles significantly affected soil microbial carbon use efficiency.The microbial carbon use efficiency under high intensity dry and wet alternating cycles was higher than that under low intensity;With the increase of dry and wet alternating cycles,microbial carbon use efficiency increased gradually.（3）Moisture is an important factor affecting microbial carbon utilization efficiency.With the increase of soil moisture,microbial carbon use efficiency first increased and then decreased,with the maximum value between 20%WHC-40%WHC.（4）The longer the farmland was converted to forest,the greater the microbial carbon use efficiency and the stronger the carbon sequestration capacity.At the same time,the longer the period of returning farmland to forest,the less the soil was disturbed.In conclusion,soil moisture content and moisture changes can change the physiological process of soil microorganisms through affecting microbial biomass,microbial activity,microbial respiration rate and microbial growth rate.The interference stress of alternating dry and wet cycling can significantly reduce microbial carbon utilization efficiency.The intensity of dry-wet alternating cycle affected microbial carbon use efficiency through stress degree and microbial biomass change,and the frequency of dry-wet alternating cycle affected microbial carbon use efficiency through changing microbial community composition and microbial survival strategy.However,soil with different years of returning farmland to forest had different carbon sequestration abilities under alternating dry and wet stress,indicating that vegetation restoration had certain effect on soil carbon.This study elucidated the internal mechanism of alternating dry and wet cycles on microbial carbon use efficiency,and provided a basis for further understanding of soil carbon change in the Loess Plateau after restoring farmland to forest.