Distribution And Functional Characteristics Of Microbial Communities In Isohumosol Profiles In Northeast China

Northeast China is an important commercial grain production area,and the main soils as grain production are Ustic and Udic Isohumosols,which are crucial in ensuring national food security and regional ecological security.Microorganisms play a pivotal role in the processes of soil organic matter turnover and nutrient cycling.It is essential to understand the mechanisms of microbial communities regulate ecosystem functions by clarifying the change patterns of microbial community structures and functions along soil profiles.However,previous studies on the microbial communities of soil profiles have mostly been based on a uniform depth interval,and its difficult to establish a link with the processes of soil formation.Therefore,this study collected soil samples from eight agricultural fields across Heilongjiang Province of China,which belong to two suborders of Isohumosols（Ustic and Udic）,based on the diagnostic horizons.The physicochemical and biological characteristics of soil samples were comparatively studied by measuring the physicochemical properties and extracellular enzyme activities,while combining quantitative real-time PCR（q PCR）,high-throughput sequencing and metagenomics techniques.The aim is to explore the vertical spatial distribution patterns of microbial communities and functional characteristics of Isohumosols and their relationship with the physicochemical factors of diagnostic horizons,so as to provide a theoretical basis for understanding the soil formation mechanisms and protecting the valuable Isohumosols resources.The main findings are as follows:（1）The change patterns of physicochemical properties in the Isohumosol profiles were revealed,indicating that the soil organic carbon（SOC）,total nitrogen（TN）,total phosphorus（TP）,ammonia nitrogen（NH₄⁺-N）,available phosphorus（AP）and sand contents,as well as the surface mean diameter（SMD）and volume mean diameter（VMD）of soil particles gradually decreased along Isohumosol profiles,while the soil p H,clay and silt contents significently increased with increasing in soil depth.Interestingly,the variations of the physicochemical properties along soil profiles were greater in Udic Isohumosols compared to Ustic Isohumosols,and greater soil heterogeneities were observed in C-horizons（parent material horizons）than in Ah-horizons（humus horizons）.In addition,soil C:N,C:P and N:P decreased significantly with increasing soil depth in both Ustic and Udic Isohumosols,suggesting that the deep soil horizons enhanced soil carbon and nitrogen limitations,and had a greater potential for phosphorus release in the processes of organic matter mineralization.（2）The spatial distribution patterns of microbial communities in Isohumosol profiles were elucidated.The abundances of archaea,bacteria and fungi consistently decreased more than by 90%in C-horizons compared with those in Ah-horizons.For alpha-diversity,it was found that soil archaeal diversity significantly increased,while the fungal diversity significantly decreased with increasing soil depth,and the trend of bacterial diversity varied along soil profiles at different sites.In addition,soil microbial community structures were significantly influenced by soil types and diagnostic horizons.The greater variations in the microbial community structures and diversities were observed in C-horizons than in Ah-horizons.Canonical correspondence analysis（CCA）and random forest（RF）analysis revealed that soil p H was the most important factor regulating the microbial community structures among all tested edaphic variables.Importantly,this study revealed that the microbial data of diagnostic horizons can be served as quantitative indicators for the soil classification based on machine learning methods.Overall,the findings of this study provided strong evidence for the linkage between soil formation processes,physicochemical properties and microbial communities.（3）The coupled changes in microbial functional gene abundances（FGAs）and extracellular enzyme activities（EEAs）in Isohumosol profiles were revealed,indicating that there was a significant positive correlation between the FGAs and EEAs,and both were significantly distinguished by two suborders of Isohumosols and different diagnostic horizons.Additionally,the variations of EEAs and FGAs were greater in Udic Isohumosols compared to Ustic Isohumosols along soil profiles,and greater changes were observed in C-horizons than A-horizons.The both FGAs and EEAs correspondently decreased along the soil profiles.However,when normalized by SOC,the specific EEAs were significantly increased in deep soil horizons,suggesting that microorganisms will input more resources to the production of enzymes to ensure microbial nutrient requirements under resource scarcity.More importantly,this study revealed that soil microbial nutrient demands were limited by carbon and phosphorus,and the limitation significantly increased along soil profiles with a greater carbon limitation observed in Ustic Isohumosols than that in Udic Isohumosols.Overall,the findings of this study provided solid evidences for a potential link between FGAs,EEAs,and microbial nutrient limitations,which would be helpful for a better understanding of the ecosystem processes in soil profiles.（4）The relative contributions of rare and abundant microbial taxa to ecosystem multifunctionality were analyzed in Isohumosol profiles.The rare microbial taxa had significantly higher diversity compared to abundant taxa,and they occupied more core positions in the microbial networks.Additionally,due to the narrower niche breadths of rare taxa,they showed greater community variations among different sites than abundant taxa.The null model analysis indicated that the deterministic processes contributed relatively more to the assembly processes of rare microbial taxa.In contrast,stochastic processes were more important for the assembly processes of abundant microbial taxa.Importantly,both abundant and rare microbial community diversity and assembly processes were significantly associated with ecosystem multifunctionality.Structural equation modelings（SEMs）indicated that community diversities,network properties and assembly processes of rare taxa had a greater impact on ecosystem multifunctionality than those of abundant taxa.This study emphasizes the important role of rare microbial taxa on ecosystem multifunctionality in the profiles of agricultural Isohumosols.（5）The change patterns of microbial functional gene diversities and relative abundances of carbon,nitrogen and phosphorus cycle-related genes in Isohumosol profiles were revealed,indicating that the diversities of microbial functional gene in Isohumosols were significantly influenced by soil types and diagnostic horizons.Additionally,A-horizons significantly increased the relative abundances of genes related to nitrogen mineralization and liable carbon degradation,while C-horizons significantly increased the relative abundances of genes related to recalcitrant carbon degradation,denitrification and phosphorus cycling.Meanwhile,the interactions of microbial functional genes were closer in C-horizons.In addition,compared to Udic Isohumosols,Ustic Isohumosols reduced the relative abundances of carbon degradation genes but increased the relative abundances of genes related to nitrogen mineralization and phosphorus cycling.This study reveals the potential functions of microbial communities in the profiles of Isohumosols,and thus provides new insights for clarifying the biogeochemical cycles of soil profiles.