Belowground microorganisms are indispensable components for nutrient cycling in desert ecosystems, and understanding how they respond to increased salinity is essential for managing and ameliorating salinization. Our sequence-based data revealed that microbial diversity decreased with increasing salinity, and certain salt-tolerant phylotypes and phenotypes showed a positive relationship with salinity. Using a null modeling approach to estimate microbial community assembly processes along a salinity gradient, we found that salinity imposed a strong selection pressure on the microbial community, which resulted in a dominance of deterministic processes. Studying microbial diversity and community assembly processes along salinity gradients is essential in understanding the fundamental ecological processes in desert ecosystems affected by salinization.

The means through which microbes and plants contribute to soil organic carbon (SOC) accumulation remain elusive due to challenges in disentangling the complex components of SOC. Here we use amino sugars and lignin phenols as tracers for microbial necromass and plant lignin components, respectively, and investigate their distribution in the surface soils across Mongolian grasslands in comparison with published data for other grassland soils of the world. While lignin phenols decrease, amino sugars increase with SOC contents in all examined grassland soils, providing continental-scale evidence for the key role of microbial necromass in SOC accumulation. Moreover, in contrast to clay’s control on amino sugar accumulation in fine-textured soils, aridity plays a central role in amino sugar accrual and lignin decomposition in the coarse-textured Mongolian soils. Hence, aridity shifts may have differential impacts on microbial-mediated SOC accumulation in grassland soils of varied textures.

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a b s t r a c tConcentration-and flux-based O 3 doseeresponse relationships were developed for poplars in China. Stomatal conductance (g s ) of five poplar clones was measured to parameterize a Jarvis-type multiplicative g s model. The maximum g s and other model parameters varied between clones. The strongest relationship between stomatal O 3 flux and total biomass was obtained when phytotoxic ozone dose (POD) was integrated using an uptake rate threshold of 7 nmol m À2 s À1 . The R 2 value was similar between flux-based and concentration-based doseeresponse relationships. Ozone concentrations above 28 e36 nmol mol À1 contributed to reducing the biomass production of poplar. Critical levels of AOT 40 (accumulated O 3 exposure over 40 nmol mol À1 ) and POD 7 in relation to 5% reduction in total biomass for poplar were 12 mmol mol À1 h and 3.8 mmol m À2 , respectively.

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Nitrogen (N) deposition can profoundly alter soil N cycling of grassland ecosystems.Substrates and soil acidification are expected to modify soil N transformations in response to elevated N deposition. Here, we carried out 15 N tracing studies to test the effects of N addition rates (low: 30 kg N ha -1 and high: 90/120 kg N ha -1 ) and soil acidification on gross N transformation rates using two typical Chinese grassland soils, an alpine calcareous soil and a temperate neutral soil. We found that N addition significantly increased the ratio of gross nitrification rate to gross ammonia immobilization rate (N/I) in both soils, but gross N transformation rates changed differently as a function of N addition rates and soil types. In the calcareous soil, N addition increased soil gross N transformations, largely due to mineral N substrates, SOC, TN and fungal dominance. In contrast, low N addition did not affect gross N transformation rates in the neutral soil, but high N addition significantly decreased gross N transformation rates. Although both SOC and TN were increased with N addition in the neutral soil, N-induced soil pH decline decreased gross N transformation rates. Our results indicate that the effects of N addition on grassland soil gross N transformations are highly dependent on mineral N substrates, SOC and TN. Soil acidification played a more important role than SOC and TN in gross N transformation rate changes in response to elevated N deposition. These findings suggest that the different changes of gross N transformation rates in response to N deposition and soil properties (e.g. SOC, TN and soil pH) should be integrated into biogeochemical models to better predict grassland ecosystem N cycling in the future scenarios of N deposition.