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Aims: Global environmental changes are known to affect terrestrial ecosystems functions (i.e., plant growth and carbon storage). However, how plants and soil microorganisms respond to warming and nitrogen deposition in dry ecosystems with strong seasonal precipitation remains largely unexplored. Methods: Based on a 13-year manipulative field experiment, we investigated the effects of warming and nitrogen addition on soil microbial communities and plant net photosynthetic rates during dry and wet months in a desert steppe in Inner Mongolia. Results: We found that in the wet month, warming and nitrogen addition significantly increased soil microbial biomass. Yet, warming and nitrogen addition significantly increased soil inorganic nitrogen and leaf nitrogen concentration, thus promoting an increase in the net photosynthetic rate of Stipa breviflora. Moreover, warming and nitrogen addition significantly shifted soil microbial composition with an increase in soil bacterial phospholipid fatty acids (PLFAs) but a reduction in fungal PLFAs. The increased soil inorganic nitrogen indirectly enhanced leaf nitrogen and plant photosynthesis by changing soil microbial community structure. These changes were not significant in the dry month. Conclusions: Our study indicates that warming and nitrogen addition can promote plant photosynthesis by increasing soil nitrogen availability and changing soil microbial community structure. These changes only occurred when there was sufficient precipitation. These results highlight the crucial role of the soil microbial community and precipitation availability in influencing plant growth responses to global change drivers such as warming and nitrogen deposition. These findings suggest the importance of manipulating multiple factors rather than single factors in global change experiments and the role of plant and soil microbial interactions in ecosystem functions (i.e., plant growth and carbon storage) under global change. [ABSTRACT FROM AUTHOR]

Background and aims: The intensity of extreme rainfall events is increasing in many regions of the world, with important impacts on community dynamics and ecosystem functioning especially in water-limited ecosystems. The impact of the intensification of extreme precipitation on mixed communities of CAM plants and dryland C4 grasses remains poorly understood. Methods: Through a set of greenhouse experiments we investigated the effect of increasing intraseasonal rainfall variability on two separate pairs of CAM and C4 grass species (C. imbricata and B. eriopoda; O. phaeacantha and B. curtipendula) that coexist in arid grasslands across the southwest USA. Results: The increased rainfall variability did not significantly change the biomass of C. imbricata while increasing the biomass of B. eriopoda when these two species coexisted. More extreme rainfall regimes caused a 24.8% decrease in B. curtipendula (grass) biomass, compared with a 71.3% decline in the O. phaeacantha CAM plant. Significantly lower nocturnal carbon assimilation rates and higher stem volume-to-surface area ratios were found in CAM plants in mixture than in monoculture. Conclusions: Our study suggests that C4 grasses may outcompete CAM plants through greater access to deep soil water under increased rainfall variability and highlights the important role of hydrologic conditions as determinants of the competitive relations between CAM plants and grasses, their community composition, and ecological resilience in dryland ecosystems. [ABSTRACT FROM AUTHOR]

Aims: Global climate change may greatly alter the structure and stability of drylands, creating an urgent need to recover their functions and services. Biological soil crust (biocrust), an interface between the soil and atmosphere, plays a crucial role in ecohydrological processes, and thus in influencing the restoration dynamics of dryland ecosystems. Previous studies have generally investigated the influences of biocrust on ecohydrological processes as an exogenous factor. However, it remains unclear how biocrusts, as an integral part of many ecosystems (i.e., as a system state variable), will change under global climate change.Methods: This study developed a new ecohydrological model with biocrust cover as a system state variable, and explored the response of dryland ecosystems to altered rainfall regimes.Results: Biocrust cover responded with an inverted U-shaped curve relationship to increasing annual rainfall and linearly to increasing rainfall frequency. Vascular plant (grass and shrub) cover showed an increasing trend with increasing annual rainfall and a decreasing trend to increasing rainfall frequency. Therefore, biocrust usually dominated over vascular plants (i.e., high biocrust cover and low vascular plant cover) under low annual rainfall. Furthermore, an increasing rainfall frequency would amplify the range of environmental (rainfall) conditions dominated by biocrust from an annual rainfall 0-100 mm under a rainfall frequency of 0.025 day−1 to 0-500 mm under a rainfall frequency of 1 day−1.Conclusions: This study developed a model framework to predict dryland dynamics for surfaces covered by biocrust under global climate change. We suggest that restoration efforts could target at biocrust-dominated state in deserts, especially in a (future) drier climate. [ABSTRACT FROM AUTHOR]

Background and aims: We investigated the response of a model facultative CAM plant (Mesembryanthemum crystallinum) and its competition with a C3 grass (Bromus mollis) to changes in rainfall regime.Methods: Seedlings of M. crystallinum and B. mollis in both monoculture and mixtures growing in shallow and deep pots were subjected to three levels of intra-seasonal rainfall variability and rainfall seasonality in both high water and low water conditions. Response of plants were evaluated by nocturnal carboxylation and biomass.Results: A high rate of water drainage beneath root zones in coarse soil led to a negative response of M. crystallinum and B. mollis in monoculture under increased intra-seasonal rainfall variability. Seasonal rainfall shifts to later dates during the growing season generally favored the growth of M. crystallinum and B. mollis in monoculture, with the exception of high water stress conditions whereby drought-intolerant species B. mollis was disfavored. Rainfall seasonality but not intra-seasonal rainfall variability affected nocturnal carboxylation by M. crystallinum in monoculture.Conclusions: We suggest that soil texture, root depth, and rainfall gradient are important mediators of plant growth under increased intra-seasonal rainfall variability. Drought severity and the ability of a plant to tolerate drought and can greatly affect its response to the seasonal timing of rainfall. Nocturnal carboxylation by M. crystallinum in response to rainfall variability depends on the timescale. [ABSTRACT FROM AUTHOR]

Background and aims: Determining which strategies confer a competitive advantage in variable environments with strong short-term resource fluctuations (i.e., seasonal drought and flooding) is critical for understanding ecosystem structure and function, and predicting how ecosystems will respond to global change. Here, we ask how functional traits of plant species vary in response to drought and flooding and whether trait plasticity is a better predictor of plants' cover than average functional trait values. Methods: We transplanted vegetative recruits (similar size and age) of eight common species from a moderately grazed alpine wetland in the Eastern Tibetan Plateau to a common garden where we evaluated the trait responses of these species to drought and flooding. We also measured the coverage, biomass, and height of these plant species in the field communities. Results: The leaf morphological and physiological traits of these eight species generally showed different responses to drought and flood conditions. Variation (plasticity) of leaf morphological or physiological traits rather than average values of these traits were better predictors of cover and biomass in the field community. Species with lower plasticity in leaf morphological traits had higher plasticity in leaf physiology and the higher coverage and biomass in the field community. Conclusions: Our results suggest that the ability to quickly adjust leaf physiology to changing environmental conditions provides an advantage in this ecosystem, where intra-annual rainfall variability is high and thus resource fluctuations are pulsed in time. In contrast, greater functional trait plasticity in leaf morphology was a disadvantage, possibly because the benefits of these adjustments did not outweigh their initial costs. [ABSTRACT FROM AUTHOR]