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Background and aims: This study explores trends in soil fungal patterns, potential functions and their responses to carbon and nitrogen applications over alpine grassland succession in the Qilian Mountain area of China. Methods: The soil fungal community was characterized via Illumina sequencing of ITS genes. The FUNGuild database was used to predict functional groups in alpine grassland succession from swamp meadow to alpine meadow and steppe meadow. The levels of soil carbon and nitrogen, vegetation carbon and nitrogen, and soil enzyme activity were also assessed. Results: Soil fungal operational taxonomic units increased from swamp meadow to alpine meadow stage and then reached a relatively stable state in steppe meadow successional stage. This result is consistent with the changing trend of sobs and Chao1 index. Moreover, the soil fungal community differed significantly between different succession stages. During succession, while most phyla of soil fungi followed linear decreasing trends, Ascomycota was the dominant fungal phylum. Its abundance increased significantly, from 60.00% in swamp meadow to 72.26% in steppe meadow. The relative abundances of Pathotroph and Saprotroph fungal functions increased with successional stages, while Symbiotroph did not change significantly. Soil ammonium nitrogen and organic carbon levels dominated the effect on the soil fungal microbial community and functional groups. Conclusion: These findings indicate that the fungal community shifted dramatically in the first stage(swamp to alpine meadow) of succession but then reached a relatively stable state in the second(alpine to steppe meadow) successional stage. The soil fungal function group did not follow the same successional trajectory. Soil ammonium nitrogen and soil organic carbon levels imposed the strongest influence on the fungal community and its function, respectively. [ABSTRACT FROM AUTHOR]

Zhang et al. ([29]) determined that linkages between community assembly processes and the abundance of soil nematode communities under nutrient enrichment may play a major role in the C cycling of meadow steppe ecosystems. China's meadow grasslands possess a total area of some 1.1 million km SP 2 sp , including meadow steppes (60,851 km SP 2 sp ), lowland meadows (252,196 km SP 2 sp ), montane meadows (167,189 km SP 2 sp ), and alpine meadows (637,206 km SP 2 sp ) (Liao and Jia [15]). 1 Outline map of the Chinese meadow steppe vegetation (adapted from Li et al. [11]) Of the 15 papers presented in this Special Issue, 12 concern meadow steppe communities at local and regional scales, based on either long-term stationary manipulative experiments, or off-site incubation tests, and transect surveys. Overgrazing has long been regarded as the most significant factor causing grassland degradation in China, while cultivation of grasslands has been most substantial in the meadow steppe region of Inner Mongolia and meadows in northeast China. [Extracted from the article]

Purpose: In nitrogen (N)-fertilized grasslands, mowing removes large quantity of nutrients from ecosystems due to plant biomass removal at higher levels. This study tested the hypothesis that if mowing could counteract the synergistic interactions of exogenous zinc (Zn) and N supply on plant Zn uptake. Methods: Our hypothesis was tested with three dominant species (Leymus chinensis, Carex duriuscula, and Stipa baicalensis) in a field experiment exposed to Zn addition, N addition and mowing treatments. A pot experiment using a low-Zn soil was further conducted to explore mechanisms of N-driven changes in plant-soil Zn dynamics. Results: Zinc addition increased soil available Zn across all treatments. However, effects of Zn addition on plant Zn uptake were treatment- and species-specific given by higher Zn contents in L. chinensis under N addition, in C. duriuscula under mowing, and in S. baicalensis under mowing and N addition. Nitrogen addition synergistically interacted with Zn addition to increase Zn uptake of L. chinensis. Meanwhile, N addition reduced Zn uptake of C. duriuscula without Zn addition, mainly due to plant biomass suppression in the presence of more dominant L. chinensis. Mowing counteracted the synergistic effect of combined N and Zn addition on Zn content of L. chinensis through biomass removal and lowering soil moisture. Conclusion: Our results indicated that plant Zn uptake was mainly driven by soil Zn availability and species-specific responses of biomass to N addition and mowing. Moreover, counteractive mowing effect on the synergisms between Zn and N addition might necessitate grassland Zn fertilization. [ABSTRACT FROM AUTHOR]

Aims: Livestock grazing is one of the most common utilization methods and exerts a significant effect on the carbon allocations between the above- and belowground components of a grassland ecosystem. The major aim of this study were to evaluate the proportions of 13C allocation to various C pools of the plant-soil system of a meadow steppe ecosystem in response to changes of stocking rate. Methods: In situ stable 13C isotope pulse labeling was conducted in a long-term grazing experiment with 4 stocking rate. Plant materials and soil samples were taken at eight occasions (0, 3, 10, 18, 31, 56 and 100 days after labeling) to analyze the decline in 13C over time, and their composition signature of 13C were analyzed by the isotope ratio mass spectrometer technique. Results: We found a significantly greater decline in assimilated 13C of shoot and living root for the heavily grazed swards compared to other stocking rates, with the highest relocation rate of 13C into soil C pool compared to other fractions. In addition, light grazing significantly allocated 13C assimilates in the belowground pool compared to other stocking rates, especially in the live root and topsoil C-pools. Conclusions: In this study, the effects of grazing on the carbon transfers and stocks within the plant-soil system of the meadow steppe were highly grazing pressure dependent. Plant-soil system in light stocking rate presented the highest C utilization efficiency, however, plants allocated more C to soil C pools with heavily stocking rate. [ABSTRACT FROM AUTHOR]

Background and aims: Understanding the effects of livestock grazing on the ecosystem biomass and soil nitrogen processes of grassland ecosystems is critical to improving knowledge on the mechanisms underlying grassland degradation and accurately assessing the influence of grazing management on grassland functions. Methods: We examined the interannual patterns of ecosystem biomass and soil nitrogen mineralization in response to cattle grazing in a Chinese meadow steppe. The soil core incubation method was employed for soil N transfer estimation, whilst the fumigation extraction method, a modified Baermann funnel method and harvest method were used for various measurements of ecosystem biomass parameters. Results: We found that cattle grazing caused consistent significant increases in soil temperature, irrespective of the stocking rate and year, whereas significant effects on soil moisture and edaphic properties were observed only in individual years and/or at specific stocking rates. Consistent positive effects at moderate stocking rates were observed for aboveground net primary production and soil nematode biomass in all study years. The across stocking rate pattern of N mineralization in response to cattle stocking appeared to be year-specific, although negative effects were found in most cases. In contrast, the interannual pattern of N mineralization was determined principally by the interannual patterns in precipitation and soil moisture and was much less affected by cattle grazing. Conclusions: Soil N mineralization in this meadow steppe was affected by cattle grazing via two major mechanisms, i.e., its effects on the aboveground net primary production (ANPP) and thus the quantity of plant litter input into the soil and its effects on soil temperature and moisture. Overall, our study spanned the longest consecutive years with the broadest range of stocking rates thus far of its kind, which revealed for the first time that the soil nitrogen mineralization pattern with respect to stocking rate was year-specific. Our findings have important implications for adaptive management and sustainable utilization of Chinese grasslands. [ABSTRACT FROM AUTHOR]

Aims: Soil nematodes, as key bioindicators, play crucial roles in soil ecological process. Management of grasslands, such as meadow steppes in northeast China, is often done by mowing, which has an impact on soil nematode communities. However, few studies have explored effects of mowing frequency on the community structure and biomass of soil nematodes. Methods: Routine field and laboratory methods concerning plant community, soil properties, and soil nematodes were applied in this study. Soil nematode community structure was analyzed by using nonmetric multidimensional scaling (NMDS) and principal component analysis (PCA). The relationships between nematode genus and biotic and abiotic factors were analyzed by redundancy analysis (RDA). Results: High mowing frequency significantly reduced abundance, biomass, and functional or metabolic regimes of soil nematodes in this ecosystem, whereas moderate mowing frequency enhanced those indices and regimes. Conclusions: Our findings showed that changing patterns in nematode indices across the mowing frequency conformed with the intermediate disturbance theory. Variations in soil nematode community were related to changes in belowground biomass, aboveground litter, soil available nitrogen and acidity, and the effects of edaphic and vegetal traits appeared to be trophic or genus-specific. This study has potential benefits for grassland restoration in northeast China. [ABSTRACT FROM AUTHOR]

Aims: Viable seeds in herbivore dung constitute the dung seed bank, and the contribution of livestock dung to this seed bank in grazing pastures is often overlooked. Grazing season (warm and cold), seed characteristics (size and shape), and forage preference are the main factors that affect the size and composition of the dung seed bank and ultimately affect grassland ecology. However, how these three factors interact is unclear. Methods: We collected yak dung as well as seeds of the common plant species from warm- and cold-season alpine meadows in northeastern Qinghai-Tibetan Plateau, and explored how grazing season (warm and cold), seed characteristics (size and shape) and foraging preferences (temporary cages method) affects yak dung seedling density, richness and diversity in an alpine pasture. Results: Forty-three plant species (mainly perennials) germinated from yak dung. Dung seedling density, richness, and diversity did not differ significantly between the two grazing seasons. Small to medium-sized spherical seeds (seed size < 10 mg, shape index < 0.5) had the greatest germination potential. Conclusions: Yaks vary their forage preference depending on the season (phenological period), and endozoochory occurs throughout both grazing seasons. Seed shape and size directly regulate the dung seedling density, richness, and diversity. Dung seedlings increase the heterogeneity of the aboveground vegetation near the microsites of the dung pieces and therefore promote grassland patching. Our study demonstrates that grazing season, seed characteristics, and yak forage preferences affect the dung seed bank in grazing ecosystems. [ABSTRACT FROM AUTHOR]

Background and aims: Seasonal grazing is a traditional grassland management practice in alpine swamp meadows on the Tibetan Plateau, but little information is available on the interactions between plant diversity and soil carbon and nitrogen sequestration in warm- and cold- season grazed alpine swamp meadows. Methods: A multisite survey was conducted to investigate the plant characteristics and soil properties of Tibetan alpine swamp meadows under warm-season grazing (WG) and cold-season grazing (CG). Results: Our study showed that plant biomass, litter mass, soil water content (SW), soil available nitrogen, soil microbial biomass carbon and nitrogen were significantly lower in the WG meadows than in the CG meadows (P < 0.05). However, plant diversity, species richness and the evenness index tended to increase in the WG meadows. Soil C and N storage were significantly lower in the WG meadows than in the CG meadows (P < 0.01). Grazing-induced changes in SW, plant diversity, plant biomass and litter mass were the major factors resulting in the decrease in soil C and N storage. Conclusions: Our results indicate that warm-season grazing is beneficial for species diversity conservation, whereas cold-season grazing is suitable for soil C and N sequestration in alpine swamp meadows. Grazing-induced changes in litter mass greatly contributed to variations in plant diversity and soil C and N storage. In view of the accelerated vegetation and soil degradation in alpine grasslands, periodic warm- and cold-season grazing strategies should be considered to maintain alpine swamp meadow sustainability. [ABSTRACT FROM AUTHOR]

Background and aims: In recent decades, an increasing proportion of alpine shrub meadow has become severely degraded owing to the combined effects of global climate warming and rodent infestation, with significant impacts on soil water retention. The present paper investigates the patterns and controlling factors of soil water retention of alpine shrub meadow under different degrees of degradation, to help inform decisions on the management of degraded alpine shrub meadow. Methods: Four degradation stages were defined: non-degradation (ND); light degradation (LD); moderate degradation (MD) and higher degradation (HD). Pearson correlation and redundancy analysis were used to examine the relationships between soil physical properties and soil hydraulic properties. Results: Sand content increased while clay content decreased with increasing degree of degradation. In HD treatment, the available nitrogen and soil bulk density of surface soil layer was significantly lower than that in the other three stages, whereas the soil organic matter content and soil total porosity of surface soil layer was increased significantly, the soil compaction of 0–10 cm soil depth in HD was reduced significantly. The soil water retention of 0–60 cm soil depth first decreased and then increased with increasing degradation, with the maximum values occurring in HD, and the soil organic matter has an overwhelming effect on soil water retention than soil texture. Conclusions: As the degree of degradation increased, the surface soil structure deteriorated, and available nitrogen reduced while soil organic matter increased sharply in higher degradation, which leads to the highest soil water retention in higher degradation. Our results suggested that the soil water retention in degraded alpine grassland was largely determined by soil organic matter, and the soil organic matter parameters should be incorporated in hydrological models of degraded alpine ecosystem. [ABSTRACT FROM AUTHOR]

Background and aims: Responses of grassland carbon (C) cycling to changes in precipitation and land use remain a major uncertainty in model prediction of future climate. It remains even far from clear on how does clipping regulate the response sensitivity of grassland C sequestration to precipitation change. Methods: We conducted a manipulative field experiment with five precipitation treatments (1/12P, 1/4P, 1/2P, 3/4P, P, and 5/4P, where P is the growing season precipitation) interacting with clipping treatments in an alpine meadow to examine net ecosystem C uptake and its associated processes in response to clipping and precipitation gradients as well as their interaction. Results: (1) clipping significantly increased soil temperature in 2016 and 2017, while had no effect on soil moisture; (2) clipping also stimulated the net ecosystem productivity (NEP) and gross ecosystem productivity (GEP) during the two growing seasons; (3) although there were no significantly interactive effects of clipping with precipitation changes, clipping increased the response sensitivity of C uptakes to soil moisture changes. NEP increased more with soil moisture under clipping (0.21 μmol m−2 s−1 v/v−1) than unclipping treatments (0.11 μmol m−2 s−1 v/v−1). This was due to the compensatory growth and the positive effect of clipping on soil temperature, which stimulated plant growth and productivity. Conclusions: Our findings suggest that clipping will aggravate grassland's sensitivity to precipitation change. These findings highlight the important role of land use change in regulating ecosystem C sequestration under further climate conditions in an alpine meadow. [ABSTRACT FROM AUTHOR]

Background: Alpine ecosystem underlain by permafrost is considered as one of the most vulnerable ecosystems to disturbance, especially the alpine grassland on the Tibetan plateau with an altitude above 4000 m. Plateau pika (Ochotona curzoniae) burrowing can create distinctive bare grounds and cause micro-topographical heterogeneity in alpine grasslands. The burrowing-induced changes in microtopography may directly alter plant and soil interactions as well as ecosystem carbon cycle, which have rarely been studied in Tibetan alpine grasslands. Methods: To test the responses of ecosystem respiration (Re) to pika burrowing-induced changes in microtopography, we investigated plant characteristics, soil properties and Re from the bare grounds and vegetated grounds in the alpine meadow and steppe on the Tibetan Plateau. Results: Our study showed that vegetation cover, species richness, plant biomass, soil moisture (SM), soil organic carbon (SOC), total nitrogen (STN), soil microbial biomass carbon (MBC) and nitrogen (MBN) in the bare grounds were significantly lower than in the vegetated grounds in both alpine meadow and alpine steppe (P < 0.05). However, soil temperature and inorganic nitrogen tended to increase in the bare grounds. The growing season Re was significantly lower in the bare grounds than that in the vegetated grounds (P < 0.01). Pika burrowing had negative effects on Re and its temperature sensitivity in both alpine vegetations (P < 0.05). The relative changes in Re due to burrowing-induced changes in microtopography were positively correlated with the burrowing caused changes of AGB, BGB, SOC and MBC (P < 0.05). Pika burrowing-induced changes in soil temperature, soil moisture, plant biomass and microbial biomass are the major factors for the decrease of Re in the bare grounds. Conclusion: In view of the large number of pika burrows in the alpine grasslands and the loss of soil organic carbon due to pika bioturbation, the impacts of pika burrowing-induced changes in microtopography on Re must be considered in predicting the carbon cycle in alpine grasslands. [ABSTRACT FROM AUTHOR]

Background and aims: In addition to improving vegetation, grazing prohibition changes the soil micro-environment. Soil enzymes and microbes, which are the chief constituents responsible for mediation of most of the processes involved in energy and material fluxes of ecosystems, are very sensitive to changing environments. This study aimed to illustrate the effects of grazing prohibition on vegetation and soil enzyme characteristics, and to elucidate the responses of soil fungal and bacterial function and community composition on grazing prohibition for 10 years in a Qinghai-Tibetan alpine meadow. Methods: About 6 hm2 lightly degraded alpine meadows were selected in 2006, and half of which were fenced to completely prohibit grazing throughout the year, while the other half were subjected to free yak grazing. Four different plots in each site were selected as experimental fields in 2016. Soil microbial characteristics were analyzed by high-throughput gene detection. Results: Grazing prohibition had no significant impact on vegetable Shannon-Wiener diversities or below-ground biomass, but dramatically changed the above-ground biomass and soil phosphatase activity. In the Qinghai-Tibetan alpine meadow, bacteria are dominant members of the soil microbial community. Grazing prohibition significantly increased the fungal species diversity and the abundance of pathotrophs, saprotrophs and symbiotrophs, while it had no effect on soil bacterial diversity and functional structure. A total of 100.0% of the variations in fungi and bacteria could be explained by plant variables. The reciprocal action among plant biomass (i.e. above-ground biomass, below-ground biomass), plant diversities (i.e. Shannon-wiener diversity, species number, coverage) and species important value have a great impact on soil microbial composition variations. Conclusion: Long-term grazing prohibition restrained the phosphatase activity and increased the abundance of pathotrophic fungi but had no effect on the abundance of functional bacteria, which participate in the soil biogeochemistry cycling process. Grazing prohibition for 10 years did not improve the community structure of functional microorganisms related to carbon, nitrogen and sulfur, and appropriate grazing could maintain the sustainable utilization of grassland resources. [ABSTRACT FROM AUTHOR]

Aims: Soil aggregates constitute spatially-separated microbial habitats and architectural units for biogeochemical reactions. However, little is known about how aggregates varying in size can affect soil carbon (C) storage and mediate the direction of plant-soil interactions. Methods: In a meadow steppe, we assessed soil aggregate C storage affected by nitrogen (N) addition and mowing and to what degree plant-soil interactions differed among soil aggregate-size classes. Results: Nitrogen addition increased plant biomass, soil C concentration in macroaggregates and relative abundance of bacteria, but decreased fungal relative abundance and the activity of oxidative enzymes. However, mowing counteracted N effects on plant productivity and enzyme activity. In macroaggregates, close linkages with plant biomass and species richness were found for soil available nutrients and ratio of fungi to bacteria (F:B ratio). In contrast, strong physical protection and restricted fungal dominance and activity contributed to C stabilization in microaggregates, where plant community and soil properties were less linked with each other. Conclusions: Our study clearly demonstrated that stronger plant-soil linkages occurred in macro- vs. microaggregates. This linkage was associated with a rise in plant biomass and soil C accumulation but a drop in F:B ratio in macroaggregates under N addition. [ABSTRACT FROM AUTHOR]

Background and aims: Understanding the influences of environmental variation and anthropogenic disturbance on soil respiration (RS) is critical for accurate prediction of ecosystem C uptake and release. However, surprisingly, little is known about how soil respiration and its components respond to grazing in the context of global climate change (i.e., precipitation or nitrogen deposition increase). Methods: We conducted a field manipulative grazing experiment with water and nitrogen addition treatments in a meadow grassland on the Songnen Plain, China, and assessed the combined influences of grazing and global change factors on RS, autotrophic respiration (RA), and heterotrophic respiration (RH). Results: Compared with the control plots, RS, RA and RH all exhibited positive responses to water or nitrogen addition in the wet year, while a similar effect occurred only for RH in the dry year. The responses of RS to precipitation regimes were dominated by both frequency and amount. However, grazing significantly inhibited both soil respiration and its components in all subplots. Further analysis demonstrated that the plant root/shoot ratio, belowground biomass and microbial biomass played dominant roles in shaping these C exchange processes. Conclusion: These findings suggest that changes in precipitation regimes, nitrogen deposition, and land utilization may significantly alter soil respiration and its component processes by affecting local carbon users (roots and soil microorganism) and carbon substrate supply in meadow steppe grasslands. The future soil carbon sequestration in the studied meadow steppe will be benefited more by the moderate grazing disturbance. [ABSTRACT FROM AUTHOR]

Aims: This study investigated the influences of herbivore grazing intensity and grazing season on decomposition and nutrient release of dung and litter, which aimed to improve our understandings of grazing affecting nutrient cycling in alpine meadows on the Qinghai-Tibetan Platean. Methods: A factorial design experiment comprising 3 grazing intensities (non-grazing, moderate grazing, and heavy grazing) and 2 grazing seasons (summer and winter), was applied to quantify the decomposition and chemistry of dung and litter in an alpine pasture using the litterbag technique. Litterbags were retrieved for analysis of mass loss and nutrient release with 180, 360, 540, and 720 days after placement. Results: Grazing activity accelerated the decomposition of dung and litter and increased nutrient release from dung and litter by increasing soil temperature compared with non-grazing pastures, whereas grazing season had no effect on decomposition. The decomposition time was shorter for dung than that for litter. Conclusions: Herbivores grazing benefited dung and litter decomposition and nutrient cycling directly by increasing soil temperature, which is likely to promote soil microbial activity due to low temperatures in alpine meadows, and indirectly through herbage ingestion and dung deposition which increase the organic debris concentration used for microorganisms growth and reproduction. This study provides insights into the mechanisms of grazing regulating nutrient cycling in alpine ecosystems. [ABSTRACT FROM AUTHOR]

Aims: Determining base cation and trace element dynamics in plant-soil systems is important for sustainable utilization of grasslands affected by global ecosystem nitrogen (N) enrichment. Methods: A 4-year field experiment was conducted with urea addition rates of 0, 2.5, 5, 7.5, 10, 15, 20, and 30 g N m−2 yr.−1 to investigate the dynamics of base cations (Ca, Mg, K) and trace elements (Fe, Mn, Cu and Zn) in soil and a dominant species Leymus chinensis in a semi-arid meadow grassland of northeast China. Results: Soil showed a significant decrease in exchangeable Ca and increases in extractable Fe, Mn and Zn concentrations, while plant Ca and Zn remained unchanged and plant Fe decreased with N addition. For both plant and soil compartments, stoichiometric ratios of trace elements were more variable than base cations, mainly driven by plant physiological requirements, plant nutrient uptake intensity, and elemental interactions. Excessive soil Mn mobilization and plant uptake affected concentrations of Fe, Cu and Zn in the plant and even inhibited Fe absorption (antagonistic interactions), resulting in a plant Fe:Mn ratio lower than 1.5 with high N addition rates, indicating Mn phytotoxicity or Fe deficiency. As a result, plant elemental stoichiometry did not remain homeostatic and was even more variable than in soil. Conclusions: Our results provided direct evidence of decoupled plant-soil elemental responses, stricter homeostatic control of macronutrients than trace elements in plants, and altered stoichiometric variability in the plant-soil system, and suggest that N addition enhances metal nutrient imbalances in the meadow steppe grassland. [ABSTRACT FROM AUTHOR]

The following articles are part of the Special Issue "Challenges and opportunities in Chinese meadow grasslands: perspectives in a land-use change context" Guest Edited Tang Huajun, Li Linghao and Xin Xiaoping and were inadvertently published in a different issue: Li, H., Li, T., Sun, W. et al. Effects of grazing intensity on soil nematode community structure and function in different soil layers in a meadow steppe. [Extracted from the article]