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Aims: Understanding the accumulation and mineralization of soil organic carbon (SOC) in aggregates under long-term fertilization conditions is of great significance in comprehending the dynamic changes in farmland C pools on the Loess Plateau. Methods: The long-term field experiment has been established for 26 years and includes six fertilization treatments: no fertilization (CK), organic fertilizer (M), organic and nitrogen fertilizers (MN), organic and phosphorus fertilizers (MP), NP and MNP. We determined nutrient content, mineralization characteristics, temperature sensitivity (Q10), and extracellular enzyme activities of soil aggregates (< 0.25 mm, 0.25–1 mm, 1–2 mm, > 2 mm). Results: (1) M, MN, MP, and MNP significantly increased the SOC content of aggregates by 92.58%, 82.84%, 73.55% and 83.23% compared to NP. The SOC in < 0.25 mm aggregates was significantly highest than other particles. (2) Organic fertilizer treatments (M, MN, MP, and MNP) significantly increased the cumulative mineralization of different-sized aggregates by 97.10–178.75% compared to NP. The average mineralization rate and the cumulative mineralization of < 0.25 mm aggregates was the lowest. (3) The mineralization of aggregates is directly and significantly affected by the available soil nutrients and enzyme activities. (4) Fertilization and aggregate size had significant effects on Q10 and the Q10 of MNP was lower than that of M, MP, and MN. Conclusion: < 0.25 mm aggregates had the strongest C sequestration effect. MNP reduced the amount of mineralization by regulating enzyme activities and promoted the accumulation of SOC in farmland on the Loess Plateau by increasing the percentage of < 0.25 mm aggregates. [ABSTRACT FROM AUTHOR]

Aims: Biological nitrogen fixation (BNF), a function performed by diazotrophic microbes, plays an essential role in nitrogen (N) bioavailability in terrestrial ecosystems. However, little is known about the effects of meadow degradation on soil BNF and diazotrophic communities in alpine meadows. Methods: We investigated changes in soil BNF and their potential drivers in alpine meadows along a degradation gradient on the Tibetan Plateau (non-degraded, lightly degraded, moderately degraded, and severely degraded meadows) using real-time quantitative PCR and amplicon sequencing. Results: Soil BNF rates decreased significantly along the meadow degradation gradient with a range of 17.34–79.84 nmol C2H4 g−1 dry soil d−1 across all sites. The highest BNF rate in the non-degraded meadow was 1.5–4.6-fold higher than that in the degraded meadows. The abundance and diversity of diazotrophs measured by nifH abundance and Shannon diversity was also decreased in the degraded meadows, accompanied by decreases in plant biomass, soil moisture, and nutrient content (C, N). Soil BNF rate was correlated with plant biomass, soil nutrient content, and diazotrophic abundance (including Nostoc, Scytonema, Rhodopseudomonas, and unidentified genera within the Rhizobiales and Proteobacteria). The community composition of diazotrophs differed markedly among sites with different levels of degradation, with both autotrophic (Cyanobacteria) and heterotrophic (Proteobacteria) diazotrophs contributing significantly to BNF. The plant functional groups, particularly the sedge family, were the primary drivers of soil BNF rates via mediating soil moisture, nutrient content (dissolved organic C and N), nifH gene abundance, and diazotrophic community composition. Conclusions: Our results reveal the main drivers of decreased BNF during alpine meadow degradation and emphasize the importance of plant functional groups in shaping the diazotrophic community and regulating the BNF rate. This information can be applied to the restoration of degraded meadow ecosystems. [ABSTRACT FROM AUTHOR]

Aims: Extensive studies have demonstrated a significant impact of nitrogen (N) addition on microbial community diversity, composition, and functions in various soils. However, microbial co-occurrence patterns and assembly processes under enhanced N levels is less known, especially with respect to the rhizosphere where microbes are closely linked with plant growth. Methods: In this study, we performed a pot experiment using N addition gradient (0, 2.5, 5, 7.5, 10, and 15 g N·m−2·year−1) and amplicon sequencing and metabolomics to investigate the effect of short-term N addition (2 years) on the co-occurrence patterns and assembly process of bacterial communities as well as their potential drivers in the rhizosphere. Results: Short-term N addition significantly altered bacterial community diversity and composition in the rhizosphere soil but not in the bulk soil; i.e., the rhizosphere bacterial diversity increased with N addition but decreased under high N input (>5 g N·m−2·year−1). The rhizosphere co-occurrence networks showed a more complex topology than the bulk soil, and this complexity could be strengthened by the quantity of N added. The N-induced establishment of the bacterial community in the rhizosphere was mainly dominated by deterministic processes with an increase in the variable selection in response to the increase in the N-addition rate, while the bacterial community in the bulk soil exhibited a transition from stochastic to deterministic processes. N addition stimulated the release of amino acids and short-chain organic acids into the rhizosphere, and these changes in root exudates and the soil NH4+-N level were mainly responsible for the changes in the structure of the rhizosphere bacterial community and co-occurrence networks of keystone species. Conclusions: Our results demonstrated the differences in the co-occurrence patterns and assembly processes of bacterial communities between rhizosphere and bulk soils under N addition, which were closely associated with root exudates. These findings improve our understanding of plant–microbial interactions. [ABSTRACT FROM AUTHOR]

Aims: Nitrogen (N) fertilizer is applied to the soil to increase the nutrient level and plant productivity, but the effects of N addition on the soil microbial community diversity and functions are unclear. This study aimed to determine how changes in soil N influence soil microbial community diversity and potential functions. Methods: We evaluated plant characteristics, soil chemical properties, microbial composition, and potential functions (N-fixation and functional fungal guild) after 3 years of urea fertilization at four different levels (0, 25, 50, and 100 kg N ha−1 year−1) in a semiarid grassland in China. Compositions of bacterial and fungal communities were determined by high-throughput sequencing, and their potential functions were predicted by comparing their data with those in the Tax4Fun and FUNGuild databases, respectively. Results: Compared with the grassland without N input (N0), N fertilization significantly reduced soil bacterial diversity, possibly by reducing plant diversity. Fungal diversity increased with N fertilization and peaked in the N50 treatment, while the aboveground biomass showed a parallel increase and peak. A lower abundance of nif genes was found in the N100 than in the N0 treatment because the enhanced NO3− and NH4+ content alleviated the dependence of plants on biological N-fixation and reduced the N-fixation potential. N fertilization increased the relative abundance of saprotrophs (wood, plant, and dung saprotrophs) and pathogens (plant and animal pathogens). However, this promotion could be weakened when the excessive N fertilization was applied due to the lower abundance in the N100 than in the N50 treatment, and these changes could be attributed to the variation in aboveground biomass and soil organic C. Changes in the abundance of arbuscular mycorrhizal fungi were related to their host plants, exhibiting a transient increase in the N25 compared to the N0 treatment and then sharply decreasing. Conclusions: Our results show that N-induced environmental changes have considerable influence on the composition and potential functions of soil bacterial and fungal communities, which are likely to be dependent on interactions between plants and soil. [ABSTRACT FROM AUTHOR]

Background and aims: Afforestation is considered one important means of mitigating climate change. However, it is still controversial whether mixed-species plantations (MP) were more conducive to the soil organic carbon (SOC) stocks than monoculture plantations (PP). Methods: We conducted a meta-analysis based on 21 publications to assess the effects of different afforestation modes as well as controlling factors of SOC stocks on the Loess Plateau of China. Results: Compared with monoculture plantations, mixed-species plantations could significantly increase the response size of SOC stocks by 28%. For different species combinations, tree-shrub mixtures were more conducive to increasing the SOC stocks. In climate-limited regions (MAT <8 °C, MAP <500 mm), the response size of SOC stocks of mixed-species plantations was 34% higher than that of monoculture plantations. Furthermore, significant differences were found in the response size of SOC stocks at young (< 10 yr) and mature stages (> 20 yr). Compared with abandoned land, afforestation on cropland, especially mixed-species plantations, could increase the SOC stocks by 10%. Additionally, compared with monoculture plantations, a stronger coupling relationship was observed between the soil total nitrogen (STN) and SOC stocks in mixed-species plantations. Conclusion: Our results suggest that for the Loess Plateau of China, planting mixed-species plantations containing nitrogen-fixing plants is a more effective approach to enhancing the SOC stocks than monoculture plantations. [ABSTRACT FROM AUTHOR]

Aims: Nitrogen (N) addition had differential effects on root respiration. However, the reasons and mechanism have not yet been analyzed. We speculated that the differential effects of N addition are related to the N addition rate, root diameter, and temperature. Meanwhile, tree and understory herb in forest ecosystems maybe another reason that root respiration has different responses. Methods: N addition was performed in a Pinus tabulaeformis forest for six years. P. tabulaeformis and Carex lanceolata fine roots with three size classes (< 0.5 mm; 0.5–1.0 mm; 1.0–2.0 mm) were sampled for respiration and temperature sensitivity measurement at three temperatures and chemical analysis. Results: (1) N addition increased the root respiration of P. tabulaeformis and had the maximum values in 3, 6, and 9 g N m−2 y−1 at 1 °C, 14 °C, and 18 °C, respectively. (2) The root respiration of C. lanceolata significantly decreased under N addition only at 18 °C. (3) The effect of N addition on temperature sensitivity of root respiration was also varied due to N addition rate. (4) N and carbon/nitrogen ratio were the main driving factors of root respiration of P. tabulaeformis, while chemical properties had a slight driving effect on root respiration of C. lanceolata. Conclusions: N addition rate and temperature were the reasons for the differential effect of N addition on root respiration, but not root diameter. Furthermore, root respirations of tree and understory herb have varied changing mechanism, which demonstrated an apparent species specificity. [ABSTRACT FROM AUTHOR]

Aims: While the effects of destocking on soil nutrient and plant productivity are known, the effect on functional groups of fungi has received less attention. The objective of this study was to evaluate the effect of long-term destocking on fungal functional guilds and their association with plants and soil. Methods: We characterized the changes in five fungal functional guilds, including plant pathogens, animal pathogens, wood saprotrophs, dung saprotrophs, and arbuscular mycorrhizal fungi (AMF), along a 35-year chronosequence following destocking at 0–60 cm soil depths on the Chinese Loess Plateau. Fungal community composition was assigned by comparing with the FUNGuild database. Results: After 35 years of destocking, diversities of plant pathogens, wood saprotrophs, and AMF increased, while that of animal pathogens and dung saprotrophs decreased. Destocking had a greater effect in the near-surface layers (0–10 and 10–20 cm) owing to the greater influence of plant biomass and soil nutrients. Among the above- and belowground drivers, plant pathogen diversity was largely associated with plant diversity, while animal pathogens, dung saprotrophs, and wood saprotrophs were associated with aboveground biomass, AMF were responsive to soil conditions (e.g., organic C, NO3−-N, C:N ratio, and moisture). Conclusions: Long-term destocking can be considered to be an important predictor of fungal functional guilds, and changes in these microorganisms were associated with cessation-induced shifts in plants and soil by grazing. Our findings provide insights into the duration of destocking necessary to benefit fungal communities, and how this varies according to soil depth. [ABSTRACT FROM AUTHOR]

Aims: High plant diversity is usually linked with high soil microbial diversity, which is hypothesized to be attributed to a high diversity of components in the soil leachate, but experimental evidence is scarce. The aim of this study was to determine if the variation in soil leachate caused by plant diversity could affect the soil microbial community. Methods: A microcosm experiment was conducted to determine the effect of plant diversity on the soil microbial community by measuring soil leachate in a gradient of plant richness from levels 1 (one species) to 3 (three species). Results: Plant richness significantly affected the diversity of soil leachate and microbial communities. The amount and diversity of soil leachate, microbial biomass carbon (C), basal respiration, β-1,4-glucosidase activity, β-1,4-N-acetylglucosaminidase activity, bacterial biomass, fungal biomass, total microorganism biomass, and microbial diversity (Shannon diversity index and evenness) were highest at richness level 3. Changes in the microbial community were best explained by variation in the amount and diversity of leachate. Linear regression and correlation analyses indicated that leachate diversity had a close association with microbial Shannon diversity and evenness, whereas leachate amount had a close association with microbial biomass C, total microbial biomass, bacterial biomass, enzyme activities, and abundance of microbial groups. An ordinary least squares multiple regression and the structural equation model demonstrated that leachate amount had a greater effect on microbial biomass than leachate diversity, which had a greater impact on microbial Shannon diversity and evenness. Conclusions: Our results indicate that plant diversity drives changes in soil microbial communities by altering the amount and diversity of leachate in the soil. The diversity of soil leachate determined the diversity of the microbial community to some extent. [ABSTRACT FROM AUTHOR]

Aims: Initial substrate chemical characteristics are the most important factor in the regulation of fine root decomposition. However, it remains unclear how nitrogen (N) deposition changes the decomposition process by affecting initial substrate chemical characteristics with different fine root diameter sizes. Methods: We compared the root decomposition processes across three diameter sizes (very fine roots, < 0.5 mm; intermediate fine roots, 0.5–1.0 mm; largest fine roots, 1.0–2.0 mm) of Pinus tabulaeformis treated with N addition (control, low, medium, high N are 0, 3, 6, and 9 g N m−2 y−1 respectively) for two years. Results: (1) The root decomposition rates, which were mainly determined by initial N, phosphorus (P), cellulose and lignin concentrations, and carbon (C)/N and lignin/N ratios, increased with the root diameters. (2) The effect of N addition on fine root decomposition rate was not significant (P > 0.05), but low N addition enhanced the correlation coefficients between initial chemical indexes and decomposition rates. (3) Low N addition increased the release rates of C and cellulose in the very fine roots but not intermediate fine and largest fine roots, while the medium and high N addition decreased the release rates of N, P, cellulose and lignin in the very fine and intermediate fine roots by affecting the initial C, N, P, starch, cellulose and lignin concentrations. (4) Release of compounds from large diameter fine roots is less responsive to N addition than that from the small ones. Conclusions: The initial substrate chemistry plays an important role during the N addition affecting fine root decomposition and release of chemical compounds. Our results suggest that N deposition may change the biogeochemical processes of forest ecosystems by affecting the release of compounds from fine roots. [ABSTRACT FROM AUTHOR]

Background and aims: The changes in the characteristics of Panicum virgatum, an exotic invasive species, after invading various plant communities on the Loess Plateau in China and the main soil nutrient factors in these communities closely associated with invasion remain unclear. Methods: A pot culture experiment was carried out to simulate the changes in photosynthesis, biomass, and biomass allocation in P. virgatum and to identify the main soil nutrient factors in various soils collected from local plant communities. P. virgatum was grown in soils collected from communities of P. virgatum (PS treatment), Setaria viridis (SS treatment), Bothriochloa ischaemum (BS treatment), and Artemisia sacrorum (AS treatment) and in a mixed soil from the communities of S. viridis, B. ischaemum, and A. sacrorum (MS treatment). Results: Photosynthesis in P. virgatum differed significantly among the soil treatments. Net photosynthetic rate, stomatal conductance, and photochemical efficiency (Fv/Fm) were highest in PS, whereas single-photon avalanche diode values were highest in PS and SS. The variation of biomass differed significantly in different tissues of P. virgatum in the treatments. Leaf and stem biomasses were highest in PS and SS, and root biomass was highest in PS and MS. Total biomass differed significantly among the treatments, except between BS and MS. Both the leaf to total and stem to total biomass ratios were highest in AS and SS, but the root to total biomass ratio was lowest in these two treatments. A constrained redundancy analysis and a path analysis suggested that the water-soluble nitrate-nitrogen (W-NN) concentration of the soil could significantly affect photosynthesis, biomass, and biomass allocation in P. virgatum. Conclusions: Photosynthesis, biomass, and biomass allocation in P. virgatum differed significantly when grown in soils from different local plant communities on the Loess Plateau. The soil W-NN concentration in these local plant communities likely has a large impact on the invasive success of P. virgatum. [ABSTRACT FROM AUTHOR]

Background and aims: Global nitrogen deposition alters grassland ecosystems. Whether added nitrogen changes root production and turnover by root orders is unclear. Methods: We compared the root dynamics across four root orders of Bothriochloa ischaemum treated with nitrogen addition (0-10 g N m year). Results: The higher order roots exhibited lower production, turnover, number, length, and biomass, indicating a hierarchical system of B. ischaemum. At whole root system level, nitrogen addition increased length production, biomass production and turnover. At root order level, nitrogen addition increased length production, biomass production, and turnover of the first two order roots but not of the third- and fourth-order roots. Nitrogen addition reduced root biomass, and the belowground to aboveground biomass ratio, supporting the functional equilibrium hypothesis. The increased root production, turnover and decreased root number, length and biomass were mainly attributed to the increasing ammonium and nitrate nitrogen. Conclusions: Nitrogen addition increased the length production (7-30%), biomass production (10-34%) and turnover (8-35%) of the first two order roots but not of the higher order roots compared with the control pots. The discrepancy in root characteristics and their responses to nitrogen availability among root orders should be considered in establishing root dynamic models . [ABSTRACT FROM AUTHOR]

A correction to the article "Above-and belowground dynamics of plant community succession following abandonment of farmland on the Loess Plateau, China," published in the online issue of the journal of July 16, 2009 is presented.

Aims: Understanding the linkage between community heterogeneity (β diversity) and ecosystem functioning is crucial for uncovering complex ecological processes that impact ecosystem stability and productivity. However, our understanding of how β diversity is associated with ecosystem functioning across environmental gradients remains limited.We conducted an aridity-gradient transect survey of grassland over 3500 km across the Tibetan Plateau (a total of 80 grassland sites) to investigate the changes of plant and soil microbial diversities along a natural aridity gradient, and to evaluate the above- and belowground biomass and plant nutrient levels connected with the observed variation.Our results showed that plant richness, above- and belowground biomass, plant nutrient concentrations, and soil microbial α and β diversities (including taxonomic and functional) significantly decreased along the aridity gradient. Compared to the α diversities of plant and microbial community, their β diversities were more significantly correlated with above- and belowground biomass and plant carbon, nitrogen and phosphorus concentrations. Metagenomic functional profiles showed that β diversity of microbial functions, including genes of the carbohydrate metabolism responsible for carbon degradation, nitrification, energy production and conversion, material transport and coenzyme metabolism, had closer associations with plant biomass and nutrient concentrations than did their α diversities. These positive β biodiversity-functions association was weakened by the increasing aridity, mainly because the increasing aridity-induced lower biomass of plant functional groups (e.g., sedge and forbs), soil moisture and organic carbon content decreased both plant and soil microbial β diversities.These findings provide novel insight into the biodiversity-ecosystem functions relationship and stress the crucial roles of above- and belowground β diversity in supporting alpine grassland biomass and nutrient levels. Biodiversity conservation to prevent large-scale community homogenization should be attached in alpine ecosystem.Methods: Understanding the linkage between community heterogeneity (β diversity) and ecosystem functioning is crucial for uncovering complex ecological processes that impact ecosystem stability and productivity. However, our understanding of how β diversity is associated with ecosystem functioning across environmental gradients remains limited.We conducted an aridity-gradient transect survey of grassland over 3500 km across the Tibetan Plateau (a total of 80 grassland sites) to investigate the changes of plant and soil microbial diversities along a natural aridity gradient, and to evaluate the above- and belowground biomass and plant nutrient levels connected with the observed variation.Our results showed that plant richness, above- and belowground biomass, plant nutrient concentrations, and soil microbial α and β diversities (including taxonomic and functional) significantly decreased along the aridity gradient. Compared to the α diversities of plant and microbial community, their β diversities were more significantly correlated with above- and belowground biomass and plant carbon, nitrogen and phosphorus concentrations. Metagenomic functional profiles showed that β diversity of microbial functions, including genes of the carbohydrate metabolism responsible for carbon degradation, nitrification, energy production and conversion, material transport and coenzyme metabolism, had closer associations with plant biomass and nutrient concentrations than did their α diversities. These positive β biodiversity-functions association was weakened by the increasing aridity, mainly because the increasing aridity-induced lower biomass of plant functional groups (e.g., sedge and forbs), soil moisture and organic carbon content decreased both plant and soil microbial β diversities.These findings provide novel insight into the biodiversity-ecosystem functions relationship and stress the crucial roles of above- and belowground β diversity in supporting alpine grassland biomass and nutrient levels. Biodiversity conservation to prevent large-scale community homogenization should be attached in alpine ecosystem.Results: Understanding the linkage between community heterogeneity (β diversity) and ecosystem functioning is crucial for uncovering complex ecological processes that impact ecosystem stability and productivity. However, our understanding of how β diversity is associated with ecosystem functioning across environmental gradients remains limited.We conducted an aridity-gradient transect survey of grassland over 3500 km across the Tibetan Plateau (a total of 80 grassland sites) to investigate the changes of plant and soil microbial diversities along a natural aridity gradient, and to evaluate the above- and belowground biomass and plant nutrient levels connected with the observed variation.Our results showed that plant richness, above- and belowground biomass, plant nutrient concentrations, and soil microbial α and β diversities (including taxonomic and functional) significantly decreased along the aridity gradient. Compared to the α diversities of plant and microbial community, their β diversities were more significantly correlated with above- and belowground biomass and plant carbon, nitrogen and phosphorus concentrations. Metagenomic functional profiles showed that β diversity of microbial functions, including genes of the carbohydrate metabolism responsible for carbon degradation, nitrification, energy production and conversion, material transport and coenzyme metabolism, had closer associations with plant biomass and nutrient concentrations than did their α diversities. These positive β biodiversity-functions association was weakened by the increasing aridity, mainly because the increasing aridity-induced lower biomass of plant functional groups (e.g., sedge and forbs), soil moisture and organic carbon content decreased both plant and soil microbial β diversities.These findings provide novel insight into the biodiversity-ecosystem functions relationship and stress the crucial roles of above- and belowground β diversity in supporting alpine grassland biomass and nutrient levels. Biodiversity conservation to prevent large-scale community homogenization should be attached in alpine ecosystem.Conclusions: Understanding the linkage between community heterogeneity (β diversity) and ecosystem functioning is crucial for uncovering complex ecological processes that impact ecosystem stability and productivity. However, our understanding of how β diversity is associated with ecosystem functioning across environmental gradients remains limited.We conducted an aridity-gradient transect survey of grassland over 3500 km across the Tibetan Plateau (a total of 80 grassland sites) to investigate the changes of plant and soil microbial diversities along a natural aridity gradient, and to evaluate the above- and belowground biomass and plant nutrient levels connected with the observed variation.Our results showed that plant richness, above- and belowground biomass, plant nutrient concentrations, and soil microbial α and β diversities (including taxonomic and functional) significantly decreased along the aridity gradient. Compared to the α diversities of plant and microbial community, their β diversities were more significantly correlated with above- and belowground biomass and plant carbon, nitrogen and phosphorus concentrations. Metagenomic functional profiles showed that β diversity of microbial functions, including genes of the carbohydrate metabolism responsible for carbon degradation, nitrification, energy production and conversion, material transport and coenzyme metabolism, had closer associations with plant biomass and nutrient concentrations than did their α diversities. These positive β biodiversity-functions association was weakened by the increasing aridity, mainly because the increasing aridity-induced lower biomass of plant functional groups (e.g., sedge and forbs), soil moisture and organic carbon content decreased both plant and soil microbial β diversities.These findings provide novel insight into the biodiversity-ecosystem functions relationship and stress the crucial roles of above- and belowground β diversity in supporting alpine grassland biomass and nutrient levels. Biodiversity conservation to prevent large-scale community homogenization should be attached in alpine ecosystem. [ABSTRACT FROM AUTHOR]