Microbiological Mechanism Of Soil Organic Carbon Affected By Conversion Of Cropland To Forest On The Loess Plateau

Soil organic carbon（SOC）is a huge terrestrial carbon pool and a full understanding of the mechanisms of SOC pool dynamics is important for predicting global climate change.Soil microbes are the key drivers of SOC transformation.Microbial carbon use efficiency（CUE）balances the release of carbon through catabolism and the formation of microbial-derived carbon through anabolism,and thus is emerging as a central parameter affecting SOC.The Grain for Green Program implemented in the Loess Plateau plays an important role in promoting carbon sinks and mitigating climate change.However,the changes in soil microbial physiological metabolism during vegetation restoration are still unclear,which hinders the comprehensive understanding of SOC sequestration mechanisms and the accurate prediction of response to climate change in the Loess Plateau region.Therefore,accurate estimation of microbial CUE is needed to refine the mechanisms of microbially mediated SOC dynamics during vegetation restoration.In this study,soil samples from the cropland,the shrubland and the forestland restored from croplands were collected.The differences in microbial CUE estimates and their response to environmental changes using different measurement methods were elucidated.The effects of vegetation restoration on microbial physiological metabolism and community composition were explored.Microbial communities and CUE-mediated changes in SOC during vegetation restoration were revealed.The main results were as follows:1.The ¹⁸O-labeled H₂O method,¹³C-labeled glucose method,¹³C-labeled litter method and stoichiometric modeling（SM）method were used to explore the differences between microbial CUE measurement methods.The results showed that CUE estimates varied significantly between methods,exhibiting ¹⁸O<SM<¹³C-litter<¹³C-glucose and differing responses to warming.The CUE estimates with the SM method were insensitive to temperature,while the CUE determined by the ¹⁸O and ¹³C methods decreased with increasing temperature,which likely implies an unclear estimate of CUE and its response to climate change.Vegetation restoration led to an increase in microbial CUE,but a greater decrease with warming.Therefore,more attention needs to be paid to the response of soil microbial CUE to climate change under different vegetation types,which may facilitate the prediction of SOC dynamics.2.Microbial CUE was measured using the ¹⁸O-labeled H₂O method to reveal the controlling factors of soil microbial CUE changes under vegetation restoration.The results showed that microbial CUE remained constant at the early stage and increased significantly at the later stage of vegetation restoration in the 0-15 cm soil layer.Soil p H and nitrogen availability were the main predictors of changes in microbial CUE from the cropland restoration to the shrubland and the forestland,respectively.Vegetation restoration reduced microbial CUE in the 15-30 cm soil layer,probably due to soil homogenization caused by cropland tillage,resulting in similar habitats in different soil layers and greater CUE.However,microbial CUE gradually increased with increasing restoration years.The changes in CUE from the cropland restoration to the shrubland were mainly controlled by carbon and nitrogen availability,while the changes in CUE from the cropland restoration to the forestland were mainly controlled by phosphorus availability and phosphorus-acquisition enzyme activity.3.The changes in soil bacterial and fungal community composition and diversity during vegetation restoration were explored by high-throughput sequencing techniques.The results revealed that both soil bacterial and fungal diversity increased more rapidly with increasing restoration years from the cropland to the forestland compared to the shrubland.Soil nutrient content and microbial biomass were closely coupled with changes in bacterial and fungal community composition and diversity during vegetation restoration.Vegetation restoration increased bacterial and fungal network complexity,mainly exhibiting synergistic interspecific interactions,implying that vegetation restoration may be beneficial for increased microbial community stability.4.The mechanism of microbially mediated SOC changes during vegetation restoration was explored by establishing the relationship between soil properties,microbial properties and SOC content.The results indicate that vegetation restoration resulted in a decrease in SOC content,but a significant increase with increasing restoration years.Soil properties,microbial community composition and diversity mediated SOC changes by driving the response of microbial CUE to control living biomass and necromass formation.The study highlighted the central role of microbial CUE in SOC accumulation during vegetation restoration by regulating the"biomass-necromass"relationship.The results supported that the incorporation of microbial-derived carbon pools and CUE into soil carbon cycle models may help to improve predictions of SOC dynamics under global change.The enzyme activity plays an important role in enhancing microbial CUE and SOC accumulation in the 15-30 cm soil layer with more resource limited.The restoration from the cropland to the forestland resulted in more SOC accumulation compared to the shrubland,which was driven by bacterial and fungal communities,respectively,implying that the planting of trees in the central region of the Loess Plateau（～500 mm precipitation）may be more favorable for SOC sequestration.In summary,soil properties,microbial community composition and diversity drive the response of microbial CUE to control the formation of microbial biomass and necromass,thereby mediating the process of SOC change.This study will help to improve the microbial sequestration mechanism of SOC during vegetation restoration in the central Loess Plateau and provide theoretical and data support for the ecosystem model.