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Abstract Grazing intensity and global warming are expected to increase in the forthcoming decades, with uncertain consequences for their interaction on grassland ecosystems and their functions. We investigated the effects of warming, grazing and their interaction in a factorial warming (+1.2–1.7 °C) and grazing (moderate intensity with ca. 50% vegetation consumption) experiment in a Tibetan alpine meadow on microbial communities by studying functional genes involved in soil carbon and nitrogen cycles, using GeoChip technology. Our results showed that microbial functional gene structure and abundances were largely affected by the interactive effect of grazing and warming, rather than the main effect of warming or grazing. Compared to the control, grazing alone significantly increased the functional gene alpha diversity, changed the overall functional community structure, and increased the abundances of C fixation, C degradation, N mineralization and denitrification genes, likely due to the stimulating impact of urine and dung deposition. Warming alone did not change these microbial properties, possibly related to the unchanged soil nutrient status. Despite an increase in soil NO 3 − concentrations and the deposition of urine and dung, the combined treatment did not change functional gene alpha diversity, community structure, or C/N cycling gene abundances, possibly resulting from the limiting effect of water depletion in the combined treatment. Our study revealed antagonistic interactions between warming and grazing on microbial functional gene structure and abundances, which remained stable under the moderate intensity of grazing in future warming scenario in the Tibetan alpine meadow, raising potentially important implications for predicting future soil carbon and nitrogen processes in these systems. Highlights • Grazing alone significantly increased abundances of some key soil C/N cycling genes. • Warming alone had no significant effects on them. • Functional gene structure and abundances remained stable in the combined treatment. • Antagonistic interaction between warming and grazing may result from water depletion. [ABSTRACT FROM AUTHOR]

Abstract Global climate change is projected to intensify soil drying-rewetting (DRW) events with extended drought, especially in arid and semiarid ecosystems. However, the extent to which the soil DRW with intensified drought can alter soil respiration (R s) in forests is still under debate, and subsequent legacy effects on R s are not well understood. Here, we conducted a 180-d soil incubation experiment to investigate how soil DRW with different drought intensities alter the R s in poplar (Populus simonii) and Mongolian pine (Pinus sylvestris var. mongolica) plantations. The incubation experiment included four 30-d cycles of 1) constant moisture treatment (control), 2) DRW with 10-d drying and 20-d rewetting (DRW 10-20) or 3) DRW with 20-d drying and 10-d rewetting (DRW 20-10), and then an extended 60-d incubation under constant moisture. During the four DRW cycles, the direct C release with respiration of Mongolian pine soils (27 g C·m−2 in DRW 10-20 and 140 g C·m−2 in DRW 20-10 , respectively) decreased to a much lower extent than that of poplar soils (228 g C·m−2 in DRW 10-20 and 498 g C·m−2 in DRW 20-10 , respectively). R s did not significantly change during the extended 60-d incubation in the DRW 10-20 treatment compared to control treatment. However, the respired CO 2 were increased by 68 g C·m−2 in the poplar soils and 19 g C·m−2 in the Mongolian pine soils in the DRW 20-10 treatment, which approximately compensated for 14% of the decreases of total respiration during four DRW cycles. This legacy effect induced by the DRW with intensified drought was attributed to the higher amount of remaining substrates and soil microbial biomass. Our study highlights that DRW can cause both direct and legacy effects on R s , but the effects vary with drought intensity and forest type. Highlights • Soil respiration was measured during four drying-rewetting (DRW) cycles and an extended incubation. • DRW with longer drought decreased more soil respiration than with mild drought. • Extended incubation compensated for 14% of the drought-induced soil respiration reduction. • The DRW effects on soil respiration varied with drought intensity and forest type. [ABSTRACT FROM AUTHOR]

The nitrogen (N) cycle is a collection of important biogeochemical pathways mediated by microbial communities and is an important constraint in response to elevated CO 2 in many terrestrial ecosystems. Previous studies attempting to relate soil N cycling to microbial genetic data mainly focused on a few gene families by PCR, protein assays and functional gene arrays, leaving the taxonomic and functional composition of soil microorganisms involved in the whole N cycle less understood. In this study, 24 soil samples were collected from the long-term experimental site, BioCON, in 2009. A shotgun metagenome sequencing approach was employed to survey the microbial gene families involved in soil N cycle in the grassland that had been exposed to elevated CO 2 (eCO 2 ) for >12 years. In addition to evaluating the responses of major N cycling gene families to long-term eCO 2 , we also aimed to characterize the taxonomic and functional composition of these gene families involved in soil N transformations. At the taxonomic level, organic N metabolism and nitrate reduction had the most diverse microbial species involved. The distinct taxonomic composition of different N cycling processes suggested that the complex N cycle in natural ecosystems was a result of multiple processes by many different microorganisms. Belowground microbial communities that mediate N cycling responded to eCO 2 in several different ways, including through stimulated abundances of the gene families related with organic decomposition, dissimilatory nitrate reduction, and N 2 fixation, and suppressed abundances of the gene families in glutamine synthesis and anammox. This study provides a genetic basis of the microorganisms involved in key processes in the N cycle in complex ecosystems, and shows that long-term eCO 2 selectively affects N cycling pathways instead of tuning up every process. [ABSTRACT FROM AUTHOR]

The polar regions, comprising the Antarctic, Arctic and Tibetan Plateau, represent the most extreme environments on Earth. Soils across the polar regions harbor diverse microorganisms, which dominate the biogeochemical cycling. However, polar soil microbial diversity is largely underrepresented, and has not been directly compared with the non-polar regions at a global scale, which hinders our understanding of the potential importance of polar microbial diversity. In this study, we investigated the global microbial diversity and taxonomy by comparing 1114 soils, derived from the Antarctic (203), Arctic (432), Tibetan Plateau (104) and non-polar regions (375) across all continents. Soil microbial diversity was found to increase gradually from the Antarctic < Arctic < Tibetan Plateau and < non-polar regions. Soil microbial diversity dominantly corresponded to mean summer temperature in the polar regions and to soil pH in non-polar regions, respectively. Soil microbial community structure significantly differed across the different biogeographical regions, while the Antarctic exhibited the highest habitat-specificity. Over 26,000 phylotypes were observed across global soils, of which 21.8% were unique to the three poles, and 21.2% were apparently ubiquitous globally. Polar soils were dominated by fewer phylotypes, but individual phylotype showed greater dominance than that in non-polar regions. Our study reveals unique patterns of soil microbial diversity and taxonomic compositions in polar regions, and highlights the importance of environmental stresses in controlling soil microbial community. [Display omitted] • Polar soil microbial diversity (SMD) was compared with global non-polar regions. • 21% of the phylotypes were globally ubiquitous, 22% were three-poles specific. • Actinobacteria was the most ubiquitous, Firmicutes was predominately endemic. • SMD corresponded to summer temperature in poles and to soil pH in non-poles. • Polar soils were dominated by fewer microbial phylotypes with greater dominance. [ABSTRACT FROM AUTHOR]

Abstract: The combustion of fossil fuels and fertilizer use has increased the amount of biologically available N over the last 150 years. Future rates of atmospheric N deposition may slow organic matter decay and alter microbial community composition and function. However, our understanding of how anthropogenic N enrichment may alter the physiological mechanisms by which soil microorganisms assimilate and cycle N in soil are largely unknown. Since 1994, we have experimentally increased NO3 deposition to replicate (n = 4) northern hardwood forest stands across a 500-km climatic gradient in the Great Lakes region of North America. Our goal was to examine how functional genes mediating N-cycle processes in soil microbial communities have responded to experimental N deposition using the functional gene microarray, GeoChip 4.0. Experimental N deposition decreased the abundance and richness of key protein-coding genes in Archaea and Bacteria responsible for N fixation, ammonification, denitrification and assimilatory nitrate reduction; the same was true for bacterial genes mediating nitrification and dissimilatory nitrate reduction. However, the extent to which experimental N deposition decreased abundance and richness was site-specific, which was revealed by a significant site by treatment interaction. Experimental N deposition also caused a community composition shift via dispersion (increased β-diversity) in archaeal and bacterial gene assemblages. In combination, our observations suggest future rates of atmospheric N deposition could fundamentally alter the physiological potential of soil microbial communities. [Copyright &y& Elsevier]