Impact of grassland degradation on soil multifunctionality: Linking to protozoan network complexity and stability.

Soil protozoa, as predators of microbial communities, profoundly influence multifunctionality of soils. Understanding the relationship between soil protozoa and soil multifunctionality (SMF) is crucial to unraveling the driving mechanisms of SMF. However, this relationship remains unclear, particularly in grassland ecosystems that are experiencing degradation. By employing 18S rRNA gene sequencing and network analysis, we examined the diversity, composition, and network patterns of the soil protozoan community along a well-characterized gradient of grassland degradation at four alpine sites, including two alpine meadows (Cuona and Jiuzhi) and two alpine steppes (Shuanghu and Gonghe) on the Qinghai-Tibetan Plateau. Our findings showed that grassland degradation decreased SMF for 1-2 times in all four sites but increased soil protozoan diversity (Shannon index) for 13.82-298.01 % in alpine steppes. Grassland degradation-induced changes in soil protozoan composition, particularly to the Intramacronucleata with a large body size, were consistently observed across all four sites. The enhancing network complexity (average degree), stability (robustness), and cooperative relationships (positive correlation) are the responses of protozoa to grassland degradation. Further analyses revealed that the increased network complexity and stability led to a decrease in SMF by affecting microbial biomass. Overall, protozoa increase their diversity and strengthen their cooperative relationships to resist grassland degradation, and emphasize the critical role of protozoan network complexity and stability in regulating SMF. Therefore, not only protozoan diversity and composition but also their interactions should be considered in evaluating SMF responses to grassland degradation, which has important implications for predicting changes in soil function under future scenarios of anthropogenic change.

Changes of soil nutrients and organic carbon fractions in Caragana korshinskii forests with different restoration years in mountainous areas of southern Ningxia, China.

Vegetation restoration can effectively enhance soil quality and soil organic carbon (SOC) sequestration. In this study, the distribution characteristics of soil nutrients and SOC along soil profile (0-100 cm), and their responses to restoration years (16, 28, 38 years) were studied in Caragana korshinskii plantations in the southern mountainous area of Ningxia, compared with cropland and natural grassland. The results showed that: 1) the contents of SOC, soil total nitrogen (TN), total phosphorus (TP), particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and the proportion of particulate organic carbon to total organic carbon (POC/SOC) all decreased with increasing soil depth. The ratio of mineral-associated organic carbon to total organic carbon (MAOC/SOC) exhibited an opposite trend. 2) The contents of SOC, TN, TP, C:P, N:P, POC and MAOC gra-dually decreased as the restoration years increased. However, the C:N ratio showed no significant change. The POC/SOC ratio initially increased and then decreased, while the MAOC/SOC ratio decreased initially and then increased. 3) In three different types of vegetation, POC, MAOC, and SOC showed a highly significant positive linear correlation, with the increase in SOC mainly depended on the increase in MAOC. The SOC, TN, TP, POC and MAOC contents in natural grassland and C. korshinskii plantations were significantly higher than those in cropland. In conclusion, soil nutrients and POC and MAOC contents of C. korshinskii plantations gradually decreased with the increases in restoration years. However, when compared with cropland, natural grassland and C. korshinskii plantations demonstrated a greater capacity to maintain and enhance soil nutrient and carbon storage.

Soil stoichiometric imbalances constrain microbial-driven C and N dynamics in grassland.

Grassland restoration leads to excessive soils with carbon (C) and nitrogen (N) contents that are inadequate to fulfill the requirements of microorganisms. The differences in the stoichiometric ratios of these elements could limit the activity of microorganisms, which ultimately affects the microbial C, N use efficiencies (CUE, NUE) and the dynamics of soil C and N. The present study was aimed at quantifying the soil microbial nutrient limitation and exploring the mechanisms underlying microbial-induced C and N dynamics in chrono-sequence of restored grasslands. It was revealed that grassland restoration increased microbial C, N content, microbial C, N uptake, and microbial CUE and NUE, while the threshold elemental ratio (the C:N ratio) decreased, which is mainly due to the synergistic effect of the microbial biomass and enzymatic stoichiometry imbalance after grassland restoration. Finally, we present a framework for the nutrient limitation strategies that stoichiometric imbalances constrain microbial-driven C and N dynamics. These results are the direct evidence of causal relations between stoichiometric ratios, microbial responses, and soil C, N cycling.

[Soil microbial carbon pump conceptual framework 2.0]. / 土壤微生物碳泵概念体系2.0.

Microorganisms are essential actors in the biogeochemical cycling of elements within terrestrial ecosystems, with significant influences on soil health, food security, and global climate change. The contribution of microbial anabolism-induced organic compounds is a non-negligible factor in the processes associated with soil carbon (C) storage and organic matter preservation. In recent years, the conceptual framework of soil microbial carbon pump (MCP), with a focus on microbial metabolism and necromass generation process, has gained widespread attention. It primarily describes the processes of soil organic C formation and stabilization driven by the metabolic activities of soil heterotrophic microorganisms, representing an important mechanism and a focal point in current research on terrestrial C sequestration. Here, we reviewed the progress in this field and introduced the soil MCP conceptual framework 2.0, which expands upon the existing MCP model by incorporating autotrophic microbial pathway for C sequestration and integrating the concept of soil mineral C pump. These advancements aimed to enrich and refine our understanding of microbial-mediated terrestrial ecosystem C cycling and sequestration mechanisms. This refined framework would provide theoretical support for achieving China's "dual carbon" goals.

[Advances in the research of transformation and stabilization of soil organic carbon from plant and microbe]. / æ¤ç‰©æºå’Œå¾®ç”Ÿç‰©æºåœŸå£¤æœ‰æœºç¢³è½¬åŒ–ä¸Žç¨³å®šç ”ç©¶è¿›å±•.

Soil organic carbon (SOC) is the core component of terrestrial carbon (C) sink. Exploring the transformation and stabilization mechanism of SOC is key to understand the function of terrestrial C sink which copes with climate change. The traditional perspective is that plant residues are the initial source of SOC. The new concept of "soil microbial C pump" emphasizes that the synthesized products of soil microbial assimilation are important contributors to the stable SOC. This provides a new insight to the sequestration mechanism of SOC. Due to the complex and variable decomposition process of plant residues and the high heterogeneity of microbial residues, the transformation and stabilization mechanism of plant residues and microbial residues into SOC is still unclear. We reviewed research progress in plant and microbial residues, and introduced the characterization methods of quantification and transformation of plant residues and microbial residues, and also summarized the new findings on the transformation of plant and microbial residues into SOC. We further discussed the contribution and driving factors of microbial and plant-derived C to SOC. Finally, we prospected the future development direction and research focus in this field. This review would provide the scientific reference for the research of soil C sequestration in terrestrial ecosystem.

[Distribution characteristics of microbial necromass carbon along soil profiles in different restoration periods of Caragana korshinskii in mountainous areas of Southern Ningxia, China]. / å®å—å±±åŒºä¸åŒæ¢å¤å¹´é™æŸ æ¡æž—åœ°åœŸå£¤å¾®ç”Ÿç‰©æ®‹ä½“ç¢³æ²¿å‰–é¢åˆ†å¸ƒç‰¹å¾.

Microbial necromass, an important and stable source of soil organic carbon (SOC), is an important index to evaluate the contribution of microorganisms to SOC transformation and accumulation. It is not clear about the accumulation of microbial necromass in deep soil layer and its contribution to SOC during the restoration process of Caragana korshinskii forests. Combined with the biomarker method, we investigated the carbon contents of bacte-rial, fungal, and microbial necromass in the soil profiles (0-100 cm) of C. korshinskii forests in 16, 28, and 38 years of restoration, with natural grassland as control. We further examined the contribution of microbial necromass to soil organic carbon. The results showed that: 1) Along the soil profile (0-100 cm), the contents of fungal necromass carbon (FNC), bacterial necromass carbon (BNC), and microbial necromass carbon (MNC) significantly decreased with increasing soil depth in natural grassland and C. korshinskii forests. Except for the significant decrease in FNC/SOC, BNC/SOC, and MNC/SOC in the soil of C. korshinskii forests in 38 years of restoration, FNC/SOC and MNC/SOC generally showed an increasing trend followed by a decreasing trend in other plots, while BNC/SOC gradually decreased. 2) With the increases of restoration years, the contents of FNC, BNC, and MNC significantly decreased in C. korshinskii forests. FNC/SOC and MNC/SOC showed an overall increasing trend followed by a decreasing trend, while BNC/SOC gradually decreased. 3) The average contribution of microbial necromass carbon to SOC was highest in C. korshinskii forests in 28 years of restoration (35.0%), followed by C. korshinskii forests in 16 years of restoration (33.5%), natural grassland (31.0%), and C. korshinskii forests in 38 years of restoration (28.6%). In conclusion, when the restoration years of C. korshinskii forests are 16, the contents of microbial necromass carbon and their contributions to SOC are higher compared to natural grassland, which are beneficial for SOC sequestration.

Response of the C-fixing bacteria community to precipitation changes and its impact on bacterial necromass accumulation in semiarid grassland.

Climate change-induced warming has the potential to intensify drought conditions in certain regions, resulting in uneven precipitation patterns. However, the impact of precipitation-induced changes on soil C-fixing bacterial community composition to changes and their subsequent effect on the accumulation of microbial necromass in the soil remains unclear. To address this knowledge gap, we conducted an in-situ simulated precipitation control experiment in semi-arid grasslands, encompassing five primary precipitation gradients: ambient precipitation as a control (contr), decreased precipitation by 80% and 40% (DP80, DP40), and increased precipitation by 40% and 80% (IP80, IP40). Our findings indicate that while an increase in precipitation promotes greater total bacterial diversity, it reduces the diversity of cbbM-harboring bacteria. The dominance of drought-tolerant Proteobacteria within the cbbM-harboring bacterial community was responsible for the observed increase in their relative abundance, ranging from 8.9% to 15.6%, under conditions of decreased precipitation. In arid environments characterized by limited soil moisture and nutrient availability, certain dominant genera such as Thiobacillus, Sulfuritalea, and Halothiobacillus, which possess cbbM genes, exhibit strong synergistic effects with other bacteria, thereby leading to a high nutrient use efficiency. Linear regression analysis shows that bacterial necromass C was significantly negatively correlated with cbbM-harboring bacterial diversity but positively correlated with cbbM-harboring bacterial community composition. Consequently, in the extreme drought environment of DP80, the contribution of bacterial necromass C to SOC was dramatically reduced by 75% relative to the control. Although bacterial necromass C was preferentially consumed as nutrients and energy for microorganisms, C-fixing microorganisms supplemented the soil C pool by assimilating atmospheric CO2. Bacterial necromass was primarily controlled by accessible C and N rather than by the total bacterial community composition and relative abundance. Our results provide compelling evidence for the critical role of the composition of the bacterial community and its necromass in the accumulation of SOC in semiarid grassland ecosystems.

Mechanism of polyethylene and biodegradable microplastic aging effects on soil organic carbon fractions in different land-use types.

Microplastics (MPs) are widely present in terrestrial ecosystems, but knowledge about the aging characteristics of MPs in different land-use types and their impact on soil organic carbon fractions is still limited. Polyethylene (PE) and biodegradable MPs (Poly propylene carbonate and Polybutylene adipate terephthalate synthetic material (PPC + PBAT, Bio)), at 0 %, 0.03 %, and 0.3 % (w/w) dosages, were added to grassland, farmland, and facility soils for eight-week incubation. The aging degree of MPs was explored by quantifying the carbonyl index (CI). Soil organic C fractions such as SOC, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and microbial-derived C were analyzed. MPs underwent rapid aging after incubation, and the CI value for 0.03 % PE-MPs increased from 0.05 to 0.27 (farmland) and 0.26 (facility) (p < 0.05). The aging degree of 0.03 % and 0.3 % Bio-MPs was most significant in grassland, with CI decreasing by 46.6 % and 69.0 %, respectively. The CI of MPs were negatively correlated with their dosage. The 0.03 % and 0.3 % PE-MPs decreased soil organic carbon (SOC) content by 7.4 % and 8.2 % in grassland, and 3.0 % and 6.0 % in the facility (p < 0.05). POC content of farmland and facility soil was negatively correlated with PE-MPs' CI (p < 0.05). The 0.03 % PE and Bio-MPs decreased fungal necromass C (FNC) by 0.40 and 0.05 g kg-1 in grassland and 0.48 and 0.21 g kg-1 in farmland. Besides, the dosage of MPs regulated FNC content through soil pH, nutrients, and extracellular enzyme activity, either directly or indirectly, ultimately affecting the soil C pool. Therefore, this study demonstrates that MPs strongly affect SOC dynamics by influencing soil microbial enzyme activity and fungal necromass.

Nano-Iron Oxide (Fe3O4) Mitigates the Effects of Microplastics on a Ryegrass Soil-Microbe-Plant System.

To understand microplastic-nanomaterial interactions in agricultural systems, a randomized block 90-day pot experiment was set up to cultivate ryegrass seedings in a typical red sandy soil amended with compost (1:9 ratio). Polyvinyl chloride (PVC) and polyethylene (PE) microplastic (MP) contaminants were added into pot soils at 0.1 and 10%, whereas nano-Fe3O4 (as nanoenabled agrochemicals) was added at 0.1% and 0.5% in comparison with chemical-free controls. The combination of nano-Fe3O4 and MPs significantly increased the soil pH (+3% to + 17%) but decreased the total nitrogen content (-9% to - 30%; P < 0.05). The treatment group with both nano-Fe3O4 and PE had the highest total soil C (29 g kg-1 vs 20 g kg-1 in control) and C/N ratio (13 vs 8 in control). Increased rhizosphere nano-Fe3O4 concentrations promoted ryegrass growth (+42% dry weight) by enhancing the chlorophyll (+20%) and carotenoid (+15%) activities. Plant leaf and root peroxidase enzyme activity was more significantly affected by nano-Fe3O4 with PVC (+15%) than with PE (+6%). Nano-Fe3O4 significantly changed the ryegrass bacterial community structure from belowground (the rhizoplane and root endosphere) to aboveground (the phylloplane). Under MP contamination, the addition of nano-Fe3O4 increased bacterial diversity (+0.35%) and abundance (+30%) in the phylloplane and further intensified the connectivity of ryegrass aboveground bacterial networks (positive association increased 17%). The structural equation model showed that the change in the plant microbiome was associated with the rhizosphere microbiome. Overall, these findings imply the positive influences of nano-Fe3O4 on the soil-microbe-plant system and establish a method to alleviate the harmful effects of MP accumulation in soils.

Divergent contribution of particulate and mineral-associated organic matter to soil carbon in grassland.

Sequestration of soil organic carbon (SOC) is an effective means to draw atmospheric CO2. Grassland restoration is one of the fastest methods to increase soil C stocks, and particulate-associated C and mineral-associated C play critical roles in soil C stocks during restoration. Herein, we developed a conceptual mechanistic frame regarding the contributions made by mineral-associated organic matter to soil C during the restoration of temperate grasslands. Compared to 1-year grassland restoration, 30-year restoration increased mineral-associated organic C (MAOC) by 41% and particulate organic C (POC) by 47%. The SOC changed from microbial MAOC predominance to plant-derived POC predominance, as the POC was more sensitive to grassland restoration. The POC increased with plant biomass (mainly litter and root biomass), while the increase in MAOC was mainly caused by the combined effects of increasing microbial necromass and leaching of the base cations (Ca-bound C). Plant biomass accounted for 75% of the increase in POC, whereas bacterial and fungal necromass contributed to 58% of the variance in MAOC. POC and MAOC contributed to 54% and 46% of the increase in SOC, respectively. Consequently, the accumulation of the fast (POC) and slow (MAOC) pools of organic matter are important for the sequestration of SOC during grassland restoration. Overall, simultaneous tracing of POC and MAOC helps further understand the mechanisms and predict soil C dynamics combined with the input of plant C, microbial properties, and availability of soil nutrients during grassland restoration.

Soil microbial community and their functional genes during grassland restoration.

Soil microbial functional genes are linked with carbon (C) as well as nitrogen (N) cycling processes, and their relative abundances are strongly affected by ecosystem managements. Yet, soil microbial community compositions and their C, N cycling genes' abundance in temperate grasslands remain poorly studied. Here, the Illumina MiSeq sequencing (16 S rRNA gene and internal transcribed spacer [ITS]) and meta-genomic GeoChip sequencing technologies were used to explore the alterations of microbial compositions and functional genes in the topsoil (0-10 cm) following grassland restoration. Grassland restoration increased the relative abundances of the copiotrophs (such as Actinobacteria, Proteobacteria, Bacteroidetes), but reduced the oligotrophs (including Acidobacteria, Chloroflexi, Planctomycetes), suggesting that microorganisms shifted from oligotrophic to copiotrophic groups during grassland restoration. The changes in microbial eco-strategies were also supported by the meta-genomic GeoChip sequencing data. In the early restoration years, the microbial functional genes were dominant with recalcitrant C degradation (pgu, glx, lig, mnp), C fixation (accA, aclB, acsA, rbcL), N fixation (nifH), and nitrification (amoA, hao) related genes. In the later restoration years, the microbial functional genes were dominant with labile C degradation (amyA, amyX, apu, sga, abfA), and denitrification (nosZ, nirS, narG, napA) related genes. The changes in microbial functional genes were mainly related to soil biotic factors (microbial biomass C and N, as well as C- and N-acquiring enzymes). Finally, we made a framework illustrating the changes in microbial eco-strategies and soil C, N cycling processes. This is the first attempt to link microbial functional genes with microbial eco-strategies by incorporating soil microbial meta-genomic information during grassland restoration.

Biogeographic distribution of autotrophic bacteria was more affected by precipitation than by soil properties in an arid area.

Introduction: Autotrophic bacteria play an important role in carbon dioxide fixation and are widespread in terrestrial ecosystems. However, the biogeographic patterns of autotrophic bacteria and the driving factors still remain poorly understood. Methods: Herein, we conducted a 391-km north to south transect (mean annual precipitation <600 mm) survey in the Loess Plateau of China, to investigate the biogeographic distributions of autotrophic bacteria (RubisCO cbbL and cbbM genes) and the environmental drivers across different latitude sites with clear vegetational and climatic gradients. Results and discussion: The soils in northern region with lower precipitation are dominated by grassland/forest, which is typically separated from the soils in southern region with higher precipitation. The community structure of autotrophic bacterial cbbL and cbbM genes generally differed between the soils in the southern and northern Loess Plateau, suggesting that precipitation and its related land use practices/ecosystem types, rather than local soil properties, are more important in shaping the soil autotrophic microorganisms. The cbbL-containing generalist OTUs were almost equally abundant across the northern and southern Loess Plateau, while the cbbM-containing bacterial taxa were more prevalent in the low precipitation northern region. Such differences indicate differentiate distribution patterns of cbbM- and cbbL-containing bacteria across the north to south transect. Our results suggest that the community composition and the differentiate distributions of soil cbbL- and cbbM-containing bacterial communities depend on precipitation and the related ecosystem types in the north to south transect in the Loess Plateau of China.

Deciphering factors driving soil microbial life-history strategies in restored grasslands.

In macroecology, the concept of r- and K-strategy has been widely applied, yet, there have been limited studies on microbial life-history strategies in temperate grasslands using multiple sequencing approaches. Total phospholipid fatty acid (PLFA) analysis, high-throughput meta-genomic sequencing, and GeoChip technologies were used to examine the changes in microbial life-history traits in a chronosequence of restored grasslands (1, 5, 10, 15, 25, and 30 years since restoration). Grassland restoration increased the relative abundances of Actinobacteria, Proteobacteria, and Bacteroidetes but reduced the relative abundances of Acidobacteria, Planctomycetes, and Chloroflexi. PLFA analysis revealed that grassland restoration reduced the fungi:bacteria and Gram-positive:Gram-negative bacteria ratios. Combined with the meta-genomic data, we found that grassland restoration shifted microorganisms from oligotrophic (K-) to copiotrophic (r-) groups, consistent with the increased rRNA operon copy number of the microbial community. Structural equation modeling showed that soil properties positively (p < 0.05) while plant properties negatively (p < 0.05) affected microbial life-history traits. We built a framework to highlight the importance of plant and soil properties in driving microbial life-history traits during grassland restoration. Finally, by incorporating meta-genomic and other microbiological data, this study showed that microbial life-history traits support the idea that rRNA operon copy number is a trait that reflects resource availability to soil microorganisms.

Microplastics trapped in soil aggregates of different land-use types: A case study of Loess Plateau terraces, China.

Land-use types may affect soil aggregates' stability and organic carbon (OC) distribution characteristics, but little is known about the effects on the distribution characteristics of microplastics (MPs) in the aggregates. Hence, the MPs abundance of soil aggregates and analyzed aggregates' stability, composition, and OC content from two soil layers of four land-use types in Gansu Province were investigated in this study. The total MPs abundances in woodland, farmland (wheat, maize, and potato), orchard, and intercropping (potato + apple orchard) of top and deep soils were 1383.3 and 1477.9, 1324.6 and 931.1, 1757.1 and 1930.9, 2127.2 and 1998.0, 1335.9 and 886.7, and 1777.5 and 1683.3 items kg-1, respectively. The largest MPs abundance was detected in the >5 mm fractions of topsoil in potato (3077.3 items kg-1), followed by maize (3044.7 items kg-1) and intercropping (2718.4 items kg-1). In the topsoil, the total MPs abundance increased significantly with decreasing aggregate stability, and also was positively correlated with bulk density, microbial biomass, and total nitrogen contents of bulk soil. Summarizing, the abundance distribution of MPs correlates with the soil aggregate characteristics of the different land-use types.

Nitrogen fertilization weakens the linkage between soil carbon and microbial diversity: A global meta-analysis.

Soil microbes make up a significant portion of the genetic diversity and play a critical role in belowground carbon (C) cycling in terrestrial ecosystems. Soil microbial diversity and organic C are often tightly coupled in C cycling processes; however, this coupling can be weakened or broken by rapid global change. A global meta-analysis was performed with 1148 paired comparisons extracted from 229 articles published between January 1998 and December 2021 to determine how nitrogen (N) fertilization affects the relationship between soil C content and microbial diversity in terrestrial ecosystems. We found that N fertilization decreased soil bacterial (-11%) and fungal diversity (-17%), but increased soil organic C (SOC) (+19%), microbial biomass C (MBC) (+17%), and dissolved organic C (DOC) (+25%) across different ecosystems. Organic N (urea) fertilization had a greater effect on SOC, MBC, DOC, and bacterial and fungal diversity than inorganic N fertilization. Most importantly, soil microbial diversity decreased with increasing SOC, MBC, and DOC, and the absolute values of the correlation coefficients decreased with increasing N fertilization rate and duration, suggesting that N fertilization weakened the linkage between soil C and microbial diversity. The weakened linkage might negatively impact essential ecosystem services under high rates of N fertilization; this understanding is important for mitigating the negative impact of global N enrichment on soil C cycling.

Response of cbbL-harboring microorganisms to precipitation changes in a naturally-restored grassland.

The impact of the long-term uneven precipitation distribution model on the diversity and community composition of soil C-fixing microorganisms in arid and semiarid grasslands remains unclear. In 2015, we randomly set up five experimental plots with precipitation gradients on the natural restoration grassland of the Loess Plateau (natural precipitation, NP; ± 40% natural precipitation: decreased precipitation (DP), DP40; increased precipitation (IP), IP40; ± 80% natural precipitation: DP80; IP80). In the third and fifth years after the experimental layout (spanned two years), we explored the cbbL-genes, which are functional genes in the Calvin cycle, harboring microbial diversity and community composition under different precipitation treatments. The results showed that the increase in mean annual precipitation significantly changed the cbbL-harboring microbial alpha diversity, especially when controlling for 40% natural precipitation. The response of the dominant microbial communities to interannual increased precipitation variation shifted from Gammaproteobacteria (Bradyrhizobium) to Betaproteobacteria (Variovorax). The structural equation model showed that precipitation directly affected the cbbL-harboring microbial diversity and community composition and indirectly by affecting soil NO3- (mg N kg -1), soil organic matter, dissolved organic N content, and above- and underground biomass. In conclusion, studying how cbbL-harboring microbial diversity and community composition respond to uneven precipitation variability provides new insights into the ecological processes of C-fixing microbes in semi-arid naturally-restored grasslands dominated by the Calvin cycle.

Plant and soil elemental C:N:P ratios are linked to soil microbial diversity during grassland restoration on the Loess Plateau, China.

Plant and soil elemental ratios of carbon (C), nitrogen (N) and phosphorus (P) play a central role in shaping the composition and structure of microbial communities. However, the relationships between plant and soil elemental C:N:P ratios and microbial diversity are still poorly understood. Here, we evaluated the effects of C:N:P ratios in plant-soil systems on microbial diversity in a chronosequence of restored grasslands (1, 5, 10, 15, 25, and 30 years since restoration) on the Loess Plateau. We found that C and N concentrations, C:N and C:P ratios in leaf, root, soil and microbial biomass, bacterial and fungal diversity (Shannon-Wiener index) gradually increased with year since grassland restoration. Microbial C:N:P ratios ranged from 17.8:4.5:1 to 24.3:6.6:1, and C:P ratio increased from 17.8:1 at the 1-year site to 24.3:1 at the 30-year site, indicating the increasing P limitation for soil microorganisms during grassland development. Soil microbial diversity increased with root, soil, and microbial C and N concentrations, and decreased with P concentration (p < 0.05). Structural equation modeling indicated that soil and microbial C:N and N:P ratios had the greatest influences on soil bacterial and fungal diversity, and elemental C:N:P ratios had a greater effect on soil fungal than bacterial diversity. Our findings emphasize the importance of elemental C:N:P ratios on soil microbial diversity, which is critical for formulating policies for sustainable biodiversity conservation in terrestrial ecosystems.

Soil Rehabilitation Promotes Resilient Microbiome with Enriched Keystone Taxa than Agricultural Infestation in Barren Soils on the Loess Plateau.

Drylands provide crucial ecosystem and economic services across the globe. In barren drylands, keystone taxa drive microbial structure and functioning in soil environments. In the current study, the Chinese Loess plateau's agricultural (AL) and twenty-year-old rehabilitated lands (RL) provided a unique opportunity to investigate land-use-mediated effects on barren soil keystone bacterial and fungal taxa. Therefore, soils from eighteen sites were collected for metagenomic sequencing of bacteria specific 16S rRNA and fungi specific ITS2 regions, respectively, and to conduct molecular ecological networks and construct microbial OTU-based correlation matrices. In RL soils we found a more complex bacterial network represented by a higher number of nodes and links, with a link percentage of 77%, and a lower number of nodes and links for OTU-based fungal networks compared to the AL soils. A higher number of keystone taxa was observed in the RL (66) than in the AL (49) soils, and microbial network connectivity was positively influenced by soil total nitrogen and microbial biomass carbon contents. Our results indicate that plant restoration and the reduced human interventions in RL soils could guide the development of a better-connected microbial network and ensure sufficient nutrient circulation in barren soils on the Loess plateau.

Identifying the Biogeographic Patterns of Rare and Abundant Bacterial Communities Using Different Primer Sets on the Loess Plateau.

High-throughput sequencing is commonly used to study soil microbial communities. However, different primers targeting different 16S rRNA hypervariable regions often generate different microbial communities and result in different values of diversity and community structure. This study determined the consequences of using two bacterial primers (338f/806r, targeting the V3-V4 region, and 520f/802r, targeting the V4 region) to assess bacterial communities in the soils of different land uses along a latitudinal gradient. The results showed that the variations in the soil bacterial diversity in different land uses were more evident based on the former pair. The statistical results showed that land use had no significant impact on soil bacterial diversity when primer pair 520f/802r was used. In contrast, when primer pair 338f/806r was used, the cropland and orchard soils had significantly higher operational taxonomic units (OTUs) and Shannon diversity index values than those of the shrubland and grassland soils. Similarly, the soil bacterial diversity generated by primer pair 338f/806r was significantly impacted by mean annual precipitation, soil total phosphorus (TP), soil total nitrogen (TN), and soil available phosphorus (AVP), while the soil bacterial diversity generated by primer pair 520f/802r showed no significant correlations with most of these environmental factors. Multiple regression models indicated that soil pH and soil organic carbon (SOC) shaped the soil bacterial community structure on the Loess Plateau regardless of what primer pair was used. Climatic conditions mainly affected the diversity of rare bacteria. Abundant bacteria are more sensitive than rare bacteria to environmental changes. Very little of the variation in the rare bacterial community was explained by environmental factors or geographic distance, suggesting that the communities of rare bacteria are unpredictable. The distributions of the abundant taxa were mainly determined by variations in environmental factors.