Rongxiao Che

Precipitation shapes communities of arbuscular mycorrhizal fungi in Tibetan alpine steppe

Tibetan Plateau is one of the largest and most unique habitats for organisms including arbuscular mycorrhizal fungi (AMF). However, it remains unclear how AMF communities respond to key environmental changes in this harsh environment. To test if precipitation could be a driving force in shaping AMF community structures at regional scale, we examined AMF communities associated with dominant plant species along a precipitation gradient in Tibetan alpine steppe. Rhizosphere soils were collected from five sites with annual precipitation decreasing from 400 to 50 mm. A total of 31 AMF operational taxonomic units (OTUs) were identified. AMF community composition varied significantly among sites, whereas AMF community composition did not vary among plant species. Path analysis revealed that precipitation directly affected AMF hyphal length density, and indirectly influenced AMF species richness likely through the mediation of plant coverage. Our results suggested that water availability could drive the changes of AMF communities at regional scale. Given the important roles AMF could play in the dynamics of plant communities, exploring the changes of AMF communities along key environmental gradients would help us better predict the ecosystem level responses of the Tibetan vegetation to future climate change.

Increase in ammonia-oxidizing microbe abundance during degradation of alpine meadows may lead to greater soil nitrogen loss

Alpine meadows on the Tibetan Plateau have experienced severe degradation in recent decades. Although the effects of alpine meadow degradation on soil properties have been well documented, there is still a paucity of knowledge regarding the responses of nitrogen-cycling microbes (NCMs) to degradation and their links to the changes in soil properties. Here, we systematically determined the effects of degraded patch formation on soil properties (i.e., total carbon, total nitrogen, ammonium nitrogen, nitrate nitrogen, available phosphorus, dissolved organic carbon, moisture, δ15N, δ13C, and pH) and NCMs (based on nifH, amoA, narG, nirK, and nirS genes and their transcripts) across three Tibetan alpine meadows at different degradation stages. Results showed that compared to the original grassed patches, the contents of most soil nutrients (e.g., carbon, nitrogen, and phosphorus) were significantly decreased in the degraded patches across the study sites. Degraded patches also tended to have higher soil δ15N values and nitrate contents. Among the aforementioned NCMs, soil diazotrophs and denitrifiers only showed weak responses to the patch formation, while ammonia-oxidizing microbes showed the highest consistency and sensitivity in response to the patch formation across the study sites. The abundance of amoA gene and archaeal amoA mRNA significantly increased in the degraded patches, and they were positively correlated with soil δ15N values and nitrate nitrogen contents, but negatively correlated with soil total nitrogen and inorganic nitrogen contents. These results suggest that the increased ammonia-oxidizing microbial abundance may be an important driver of soil nitrogen loss during degraded patch formation in alpine meadows.

Warming decreased and grazing increased plant uptake of amino acids in an alpine meadow

Abstract Organic nitrogen (N) uptake by plants has been recognized as a significant component of terrestrial N cycle. Several studies indicated that plants have the ability to switch their preference between inorganic and organic forms of N in diverse environments; however, research on plant community response in organic nitrogen uptake to warming and grazing is scarce. Here, we demonstrated that organic N uptake by an alpine plant community decreased under warming with 13C–15N‐enriched glycine addition method. After 6 years of treatment, warming decreased plant organic N uptake by 37% as compared to control treatment. Under the condition of grazing, warming reduced plant organic N uptake by 44%. Grazing alone significantly increased organic N absorption by 15%, whereas under warming condition grazing did not affect organic N uptake by the Kobresia humilis community on Tibetan Plateau. Besides, soil NO 3–N content explained more than 70% of the variability observed in glycine uptake, and C:N ratio in soil dissolved organic matter remarkably increased under warming treatment. These results suggested warming promoted soil microbial activity and dissolved organic N mineralization. Grazing stimulated organic N uptake by plants, which counteracted the effect of warming.

Assessing soil microbial respiration capacity using rDNA- or rRNA-based indices: a review

PurposeUnderstanding soil heterotrophic respiration in relation to microbial properties is not only fundamental to soil respiration modelling, prediction, and regulation through management, but also essential to interpreting microbial community dynamics from an ecologically meaningful perspective. This paper reviewed the recent advances in knowledge and proposed future directions for exploring the respiration-microbe relationships by means of rDNA- or rRNA-based indices (i.e. rDNA copies, rRNA copies, and rDNA- or rRNA-based community structures).Materials and methodsWe first elucidated the theoretical basis for using rDNA- or rRNA-based indices to probe into soil microbial respiration. Then, the published studies that simultaneously measured soil microbial respiration and the rDNA- or rRNA-based indices were synthesized, extracted, and analysed to further explore the respiration-microbe relationships. At last, the uncertainties and perspectives for establishing the respiration-microbe links were proposed and discussed.Results and discussionThe rDNA- or rRNA-based indices are theoretically promising for pinpointing the relationships between soil heterotrophic respiration and microbial properties. Our systematic review suggested that the correlations between bacterial rDNA copies and microbial respiration are inconsistent across studies, while the fungal and archaeal rDNA (or ITS) copies showed moderately positive and negative correlations with soil microbial respiration, respectively. Bacterial 16S rDNA-based community structures were significantly correlated with soil microbial respiration in some studies, but not in some short-term situations. Although rRNA copies are widely used as the proxies of microbial activity, no significant correlations between rRNA copies and soil microbial respiration have been found in previous studies. Bacterial 16S rRNA-based community structures were correlated well with the short-term responses of soil microbial respiration to rewetting or labile carbon amendments and clearly outperformed other rDNA- or rRNA-based indices. As respiration-microbe relationships can be affected by many factors, such as soil physicochemical properties and even the analysis methods of microbial indices, the 69 previous studies included in this review actually provided limited information on them, and the aforementioned results still need to be further confirmed in future studies.Conclusions and perspectivesOverall, the relationships between soil microbial respiration and rDNA- or rRNA-based indices are still far from being well established. Future research should be directed to systematically understanding the respiration-microbe links, with more attention to the fungus-, archaea- and RNA-related molecular indices. The relationships between microbial specific lineages and total respiration rates should be explored in future studies, and the effects of edaphic properties on the respiration-microbe relationships should also be evaluated.

The effects of short term, long term and reapplication of biochar on soil bacteria

Biochar has been shown to affect soil microbial diversity and abundance. Soil microbes play a key role in soil nutrient cycling, but there is still a dearth of knowledge on the responses of soil microbes to biochar amendments, particularly for longer-term or repeated applications. We sampled soil from a field trial to determine the individual and combined effects of newly applied (1 year ago), re-applied (1 year ago into aged biochar) and aged (9 years ago) biochar amendments on soil bacterial communities, with the aim of identifying the potential underlying mechanisms or consequences of these effects. Soil bacterial diversity and community composition were analysed by sequencing of 16S rRNA using a Miseq platform. This investigation showed that biochar in soil after 1 year significantly increased bacterial diversity and the relative abundance of nitrifiers and bacteria consuming pyrogenic carbon (C). We also found that the reapplication of biochar had no significant effects on soil bacterial communities. Mantel correlation between bacterial diversity and soil chemical properties for four treatments showed that the changes in soil microbial community composition were well explained by soil pH, electrical conductivity (EC), extractable organic C and total extractable nitrogen (N). These results suggested that the effects of biochar amendment on soil bacterial communities were highly time-dependent. Our study highlighted the acclimation of soil bacteria on receiving repeated biochar amendment, leading to similar bacterial diversity and community structure among 9-years old applied biochar, repeated biochar treatments and control.

Autotrophic and symbiotic diazotrophs dominate nitrogen-fixing communities in Tibetan grassland soils

Biological nitrogen fixation, conducted by soil diazotrophs, is the primary nitrogen source for natural grasslands. However, the diazotrophs in grassland soils are still far from fully investigated. Particularly, their regional-scale distribution patterns have never been systematically examined. Here, soils (0-5 cm) were sampled from 54 grasslands on the Tibetan Plateau to examine the diazotroph abundance, diversity, and community composition, as well as their distribution patterns and driving factors. The diazotroph abundance was expressed as nifH gene copies, measured using real-time PCR. The diversity and community composition of diazotrophs were analyzed through MiSeq sequencing of nifH genes. The results showed that Cyanobacteria (47.94%) and Proteobacteria (45.20%) dominated the soil diazotroph communities. Most Cyanobacteria were classified as Nostocales which are main components of biological crusts. Rhizobiales, most of which were identified as potential symbiotic diazotrophs, were also abundant in approximately half of the soil samples. The soil diazotroph abundance, diversity, and community composition followed the distribution patterns in line with mean annual precipitation. Moreover, they also showed significant correlations with prokaryotic abundance, plant biomass, vegetation cover, soil pH values, and soil nutrient contents. Among these environmental factors, the soil moisture, organic carbon, available phosphorus, and inorganic nitrogen contents could be the main drivers of diazotroph distribution due to their strong correlations with diazotroph indices. These findings suggest that autotrophic and symbiotic diazotrophs are the predominant nitrogen fixers in Tibetan grassland soils, and highlight the key roles of water and nutrient availability in determining the soil diazotroph distribution on the Tibetan Plateau.