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.

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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.

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.

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.

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.

Plant and soil traits driving soil fungal community due to tree plantation on the Loess Plateau.

It is widely accepted that soil fungi plays a crucial role in biogchemical cycle in terrestrial ecosystems, and soil fungal community can be shaped by plant and soil traits; however, we still know very little about the combined impacts of plant and soil traits on soil fungal community due to tree plantation, especially on the Loess Plateau. In doing so, we provided a conceptual framework bridging knowledge on plant, soil traits and soil fungal community, which tested the combined impacts of plant and soil traits on soil fungal community due to tree plantation compared with natural restoration (CK) on the Loess Plateau. There was a disproportionate influence of tree plantation on soil fungal community by using nonmetric multidimensional scaling (NMDS) (p < 0.05) and the interaction networks. Additionally, soil organic carbon (SOC), soil pH, C/N, biomass in litter and root were highly related to the dominant soil fungal community (such as Ascomycota and Basidiomycota), which can be considered as the main drivers for soil fungal community. Most importantly, litter traits and root traits were considered as the key predictors in shaping soil fungal community in terms of tree plantation (especially litter and root C/N), while soil traits and root traits were considered as the key predictors in terms of natural restoration. Besides, structural equation modeling (SEM) indicated that soil fungal community was co-mediated by soil and plant traits due to tree plantation, and the total effects of soil traits, plant traits, litter traits and root traits on soil fungal community were higher in tree plantation, suggesting that tree plantation had a large effect on soil fungal community compared to natural restoration. Finally, we build a conceptual framework to clarify the combined impacts of plant and soil traits on soil fungal community, providing a new sight to understand the crucial role of plant and soil traits in shaping soil fungal community due to tree plantation, and the interactions among plant and soil and also soil fungal community need further studies.

Plant functional diversity drives carbon storage following vegetation restoration in Loess Plateau, China.

Ongoing climatic changes induced by human activities increases in atmospheric carbon dioxide (CO2), which have considerable effects on the structure and function of ecosystems, including carbon (C) storage, plant functional traits and therefore on a wide set of ecosystem services. Plant functional diversity is benefit to improve plant photosynthesis and enhance C efficiency and therefore decrease CO2. Here, the focus of this article is on integrating of plant functional diversity and C storage, which aims to contribute to C sequestration for climate change mitigation following vegetation restoration in Loess Plateau, China. Firstly, the CWM (plant community-weighted mean) traits of the most abundant plant species can account for C storage in AGBC (above-ground biomass C), ALC (above-ground litter C), STC (soil total carbon) and TEC (total ecosystem carbon). Secondly, the CWM of plant height and LCC (leaf carbon concentration) had a positive effect C storage in different part (AGBC, ALC, STC and TEC), while the CWM of LNC (leaf nitrogen concentration) and SLA (specific leaf area) had a negative effect on C storage in different part. Further, the CWM of plant height, LCC, SLA and plant functional dispersion (FDis) can be used to predict C storage by multiple linear regression analysis. Finally, the positive association between FDis and C storage was found in SEM, shedding light on the key role of plant functional diversity driving C storage following vegetation restoration. The findings presented here highlight the importance of both plant traits of dominant species and plant functional diversity in regulating C storage, and show that favorable climate conditions, particularly vegetation restoration, tend to increase C storage and plant functional diversity, which have important implications for improving global C cycling and ecosystem services.

Abiotic and biotic factors modulate plant biomass and root/shoot (R/S) ratios in grassland on the Loess Plateau, China.

Plant biomass and the root/shoot ratio (R/S) are key parameters for estimating terrestrial ecosystem carbon (C) stocks. However, how environmental driving factors (abiotic and biotic factors) modulate plant biomass and R/S has not been well investigated on the Loess Plateau. Here, we tested the impacts of abiotic and biotic driving factors on plant biomass and R/S and whether they are in accordance with optimal partitioning theory in natural grassland in this region. The results showed that above-ground biomass (AGB) and below-ground biomass (BGB) were 63.96 g·m-2 and 311.18 g·m-2, respectively, and that R/S ranged from 0.13 to 0.46, with high spatial heterogeneity. There was a strong positive linear relationship between AGB and BGB (p < 0.05) in accordance with optimal partitioning theory. A principal component analysis (PCA) indicated that the topographic properties (Slope position, Slope gradient and Altitude) were negatively correlated with the soil physical properties (Ec,Electric conductivity; BD, Bulk density; ST, Soil temperature; and SM, Soil moisture) and positively correlated with the soil chemical properties (SOC, Soil organic carbon; TN, Total nitrogen; SMBC, Soil microbial biomass carbon and SMBN, Soil microbial biomass nitrogen), while soil total phosphorus (TP) was not correlated with the soil physical properties (p > 0.05). Structural equation modeling (SEM) suggested that R/S is indirectly driven by plant properties (Height, Density, Coverage), which are determined by soil and topographic properties. However, only 5% of R/S was explained by the soil physical properties and topographic properties, suggesting that these factors had no significant effect on R/S. The data do, however, provide information for quantifying C stocks in natural grassland on the Loess Plateau. Further, ecologists should focus on mechanistic and fresh approaches to understanding the abiotic and biotic factors influencing plant biomass and R/S.

Testing association between soil bacterial diversity and soil carbon storage on the Loess Plateau.

Bacteria are widely distributed and play an important role in soil carbon (C) cycling. The impact of soil bacterial diversity on soil C storage has been well established, yet little is known about the underlying mechanisms and the interactions among them. Here, we examined the association between soil bacterial diversity and soil C storage in relation to vegetation restoration on the Loess Plateau. The dominant phyla among land use types (artificial forest, Af; natural shrubland, Ns; artificial grassland, Ag; natural grassland, Ng; slope cropland, Sc) were Acidobacteria, Actinobacteria, Alphaproteobacteria, and Betaproteobacteria, which transited from Acidobacteria-dominant to Actinobacteria-dominant community due to vegetation restoration. Soil C storage and the Shannon diversity index of soil bacterial community (HBacteria) showed the order Ns > Ng > Af > Ag > Sc, whereas no significant difference was found in Good's coverage (p > .05). Further, a strong relationship was observed between the relative abundance of dominant bacterial groups and soil C storage (p < .05). Additionally, soil bacterial diversity was closely related to soil C storage based on the structural equation model (SEM) and generalized additive models (GAMs). Specifically, soil C storage had the largest deterministic effects, explaining >70% of the variation and suggesting a strong association between soil C storage and soil bacterial diversity. Overall, we propose that further studies are necessary with a focus on the soil bacterial groups with specific functions in relation to soil C storage on the Loess Plateau.

Links between Soil Fungal Diversity and Plant and Soil Properties on the Loess Plateau.

Previous studies have revealed inconsistent correlations between fungal diversity and plant/soil properties from local to global scales. Here, we investigated the internal relationships between soil fungal diversity and plant/soil properties on the Loess Plateau following vegetation restoration, using Illumina sequencing of the internal transcribed spacer 2 (ITS2) region for fungal identification. We found significant effects of land use types (Af, Artificial forest; Ns, Natural shrub; Ag, Artificial grassland; Ng, Natural grassland; Sc, slope cropland) on soil fungal communities composition, and the dominant phyla were Ascomycota, Basidiomycota, and Zygomycota, which transitioned from Basidiomycota-dominant to Ascomycota-dominant community due to vegetation restoration. The Chao1 richness, Shannon's diversity and ACE indices were significantly influenced by land use types with the order of Ns > Af > Ng > Ag > Sc, and the total number of OTUs varied widely. In contrast, Good's coverage and Simpson's diversity indicated no significant difference among land use types (p > 0.05). Correlation analysis showed that plant and soil properties were closely related to fungal diversity regardless of land use types. In addition, soil organic carbon (SOC) and Hplant (plant richness, Shannon-Wiener index) were strong driving factors that explained fungal diversity. As revealed by the structural equation model (SEM) and generalized additive models (GAMs), fungal diversity was directly and indirectly affected by soil and plant properties, respectively, providing evidence for strong links between soil fungal diversity and plant and soil properties on the Loess Plateau.