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Nitrogen (N) deposition can reduce plant species richness and cause grassland degradation, thus affecting grassland ecosystem stability. Arbuscular mycorrhizal (AM) fungi play an important role in ecosystem stability. However, the influences of AM fungi on grassland ecosystem stability under N deposition remain unclear. We need more information on the impacts of N accumulation on the interactions between AM fungi and the plant community. To test the contribution of AM fungi to grassland stability under N deposition, a 5‐year field experiment was conducted in a temperate meadow with two manipulated factors, namely, N addition and AM fungi suppression. The plant species richness and diversity, biomass stability, litter decomposition, and greenhouse gas emissions were quantified. Under N addition, AM fungi did not affect the plant species diversity and richness but altered the coverages of different functional groups and increased the aboveground productivity and biomass stability. Litter decomposition increased under N addition and increased more in the treatment where AM fungi were not suppressed. The emissions of N2O and CH4 in the AM fungi suppression treatment were much higher than those in the nonsuppression treatment under N addition. Our results suggest that AM fungi can alter the plant community structure, increase plant productivity and community biomass stability, accelerate litter decomposition, and reduce the soil total N concentration and emissions of N2O and CH4 under N addition. Our results highlight that the conservation of AM fungi should be considered to alleviate grassland degradation and maintain grassland ecosystem multifunctionality in the future considering global change. [ABSTRACT FROM AUTHOR]

Studying the responses of phytoliths to warming is a new approach to interpret how grassland ecosystems respond to global change. We used an infrared radiator to simulate warming on Phragmites communis in the Songnen Grassland, China - three warming treatments and a control were used to study the relationships between phytolith size and temperature. Warming changed the size of different types of phytoliths. During June-October, the average size of saddle phytoliths decreased and that of hair cell phytoliths and silicified stomata increased with warming. The responses of phytolith size to warming differed among months. With warming, the size of saddle phytoliths, hair cell phytoliths and silicified stomata increased in July and September, but decreased in October. Overall, the results demonstrate that P. communis phytoliths were sensitive to warming. This study provides experimental evidence for phytoliths as reliable proxy indicators for reconstruction of paleo-environments. [ABSTRACT FROM AUTHOR]

Measuring the response of terrestrial ecosystems to elevated CO2 concentrations is very important for understanding the effects of global change. In this study, OTC (open top chambers) were used to simulate elevated CO2 concentrations in the Songnen Grassland. As well, phytoliths in Leymus chinensis were extracted to study the relationships between phytoliths and CO2 concentration. The results show the rondel is abundant in Leymus chinensis, while the trapeziform polylobate is rare. When phytolith production is increased, the rondel phytoliths grow bigger and the proportions of the different phytolith types changes under high CO2 concentration. These types include elongate hollow, acicular hair cell, square and laminate and are only observed in samples grown under high CO2 concentrations. All this evidence demonstrates that phytoliths in Leymus chinensis are sensitive to CO2 concentration, and indicate that phytolith analysis may have potential use in the study of global change, identifying different ecotypes of Leymus chinensis and for the reconstruction of paleoatmospheric CO2 concentrations. [ABSTRACT FROM AUTHOR]

Global change apart from ecosystem processes also influences the community structure of key organisms, such as arbuscular mycorrhizal fungi (AMF). We conducted a 3-year experiment where we suppressed with benomyl mycorrhiza to understand how AMF alter the plant community structure under warming and nitrogen (N) addition. The elemental content and foliar tissue stoichiometry of the dominant species Leymus chinensis and the subordinate species Puccinellia tenuiflora were studied along with soil nutrient stoichiometries. Overall, N addition enhanced plant N: phosphorus (P) ratios at a greater level than experimental warming did. Under global change conditions, AMF symbionts significantly increased soil available P concentrations, promoted plant P absorption and decreased the plant N:P ratios. AMF alleviate P limitation by reducing plant N:P ratios. Our results highlight that the negative influence of global change on plant productivity might cancel each other out through the additive effects of AMF and that global change will increase the dependency of plants on their mycorrhizal symbionts. Unlabelled Image • N supply increased plant N:P ratios. • Effect of N supply on plant stoichiometry was stronger than that of warming. • AMF increased soil available P concentrations and promoted plant P absorption under global change. • AMF alleviated P limitation by reducing plant N:P ratios under global change conditions. • The negative influence of global change on plant productivity might cancel each other out through the presence of AMF. [ABSTRACT FROM AUTHOR]

Accuracy of paleovegetation reconstruction and understanding of phytolith formation would both be improved by further study of phytolith size in the Phragmites australis under different environmental conditions. Leaves of P. australis were collected from 11 sampling sites in northeast China with differences in temperature, precipitation and habitat. Principal component analysis of environmental factors (climatic and edaphic) indicated that the annual averages of temperature and precipitation were the main factors influencing phytolith size. Moreover, three-way analyses of variance (ANOVAs) further showed that phytolith size differed significantly under conditions of different temperature or precipitation gradients, whereas habitat differences had little effect. The changes in phytolith size with temperature differed in the humid, semi-humid and semi-arid areas of northeast China. In the humid and semi-humid areas, moving from the temperate to the warm temperate zone, increasing temperature reduced phytolith size; whereas in the semi-arid area, phytolith became larger with increasing temperature. In the warm temperate and temperate zones, the changes of phytolith size with precipitation showed the same trend—moving from the semi-arid to semi-humid to humid areas, as precipitation increased, phytolith grew larger. Finally, ANOVA revealed that phytoliths were also sensitive to habitat. These findings demonstrated that the size of P . australis phytoliths was sensitive to environmental factors: for regional research, the annual averages of temperature and precipitation were the major factors influencing size, but in the same climate district, habitat differences seemed to also have a significant impact on phytolith size. Consequently, phytolith analysis has potential utility in the study of global climate change, palaeoenvironment reconstruction, and environmental conservation and restoration. [ABSTRACT FROM AUTHOR]

Background As a consequence of global change, intensity and frequency of extreme events such as heat waves (HW) have been increasing worldwide. Methods By using a combination of continuous 60-year meteorological and 6-year tower-based carbon dioxide (CO 2 ) flux measurements, we constructed a clear picture of a HWs effect on the dynamics of carbon, water, and vegetation on the Eurasian Songnen meadow steppe. Results The number of HWs in the Songnen meadow steppe began increasing since the 1980s and the rate of occurrence has advanced since the 2010s to higher than ever before. HWs can reduce the grassland carbon flux, while net ecosystem carbon exchange (NEE) will regularly fluctuate for 4–5 days during the HW before decreasing. However, ecosystem respiration (Re) and gross ecosystem production (GEP) decline from the beginning of the HW until the end, where Re and GEP will decrease 30% and 50%, respectively. When HWs last five days, water-use efficiency (WUE) will decrease by 26%, soil water content (SWC) by 30% and soil water potential (SWP) will increase by 38%. In addition, the soil temperature will still remain high after the HW although the air temperature will recover to its previous state. Conclusions HWs, as an extreme weather event, have increased during the last two decades in the Songnen meadow steppe. HWs will reduce the carbon flux of the steppe and will cause a sustained impact. Drought may be the main reason why HWs decrease carbon flux. At the later stages of or after a HW, the ecosystem usually lacks water and the soil becomes so hot and dry that it prevents roots from absorbing enough water to maintain their metabolism. This is the main reason why this grassland carbon exchange decreases during and after HWs. [ABSTRACT FROM AUTHOR]

Abstract: Songnen Meadow Steppe, which is in northeast China, is increasingly affected by global warming and incremental increases in atmospheric nitrogen deposition. However, the responses of nitrogen (N) and phosphorus (P) in steppe soil, and of the coupling mechanism between them, to the dual effects of global warming and N deposition are still unknown. In this study, the effects of simulated atmospheric warming and N deposition on N and P in Songnen steppe soil, as well as on the coupling between N and P, were examined under in situ conditions. Infrared heaters were used to elevate soil temperature by approximately 1.7 °C since 2006. N additions were treated once a year with aqueous ammonium nitrate at a rate of 10 g m−2 a−1. During the four-year study, addition of N increased the amount of total N, and available N, as well as the rate of N mineralization in the soil. Moreover, the amounts of total P and available P in the soil were considerably reduced. Thus, the N:P ratio increased, and the coupling between N and P decreased. Similar values for the N:P ratio were obtained for the addition of N by itself and for the combination of warming and addition of N, which indicates that a small amount of soil warming in Songnen Meadow Steppe would not have a substantial effect on the ratio. With the growth of China's industrialization, N deposition continues to increase. The study area of Songnen Meadow Steppe, and northeast China in general, are characterized by widespread distribution of saline alkali soil. Therefore, the finding of increased P limitation in the soil of Songnen Meadow Steppe has major implications for ecosystems in northeast China. Reasonable regulation and management of meadow soil nutrients will be of great importance in increasing soil productivity and promoting sustainable use of grassland ecosystems. [Copyright &y& Elsevier]

• Arbuscular mycorrhizal fungi reduced soil carbon (C) and nitrogen (N) stocks. • Neither elevated temperature nor N deposition alter the response of soil C and N stocks to AM fungi. • AM fungi can mitigate the negative effect of increasing N accumulation on ecosystem stability. Arbuscular mycorrhizal (AM) fungi are key players in the soil carbon (C) and nitrogen (N) cycles; on the one hand, AM fungi have been influenced by global changes, while on the other hand, they are involved in shaping the future climate. However, few field studies have combined AM fungi with both elevated temperatures and N deposition to examine their potential combined effects on the stocks of soil C and N in temperate meadow ecosystems. A 5-year experiment was conducted to explore the effects of AM fungal reductions on soil C and N stocks under elevated temperatures and N deposition. The stock of soil C and N and the C:N ratio were measured. The results showed that AM fungi significantly reduced soil C stocks by 8.5% across all treatment combinations compared to the AM fungi suppression treatment, and slight soil C accumulation was detected across the five years. The response of the soil C stocks was not influenced by elevated temperatures and N deposition simultaneously. The soil N stocks in the treatments without AM fungal suppression were, on average, 9.7% lower across all treatments than those in the AM fungal suppression treatment. AM fungi, elevated temperatures, and N deposition did not affect the soil C:N ratio. The robust decreases in soil C and N stocks caused by AM fungi observed here, even when combined with other global changes, suggest that AM fungi might accelerate soil C losses in temperate meadow ecosystems; however, there is a strong potential for reduced soil N accumulations to mitigate the negative impact of increasing N deposition on the stability of grassland ecosystems under future global changes. Overall, our results highlight that the effect of AM fungi on soil C and N stocks has a high resistance to global warming and N deposition in temperate meadow ecosystems. [ABSTRACT FROM AUTHOR]