Rhizosphere microbial communities are of great importance to mediate global biogeochemical cycles, plant growth, and fitness. Yet, the processes that drive their assembly remain unclear. The perennial shrubs Caragana spp., which is well known for their role in soil and water conservation, provides an ideal system to study the biogeography of rhizosphere microorganism communities within natural ecosystems. In order to detect how bacterial rhizosphere communities vary in terms of community diversity and composition, the rhizosphere bacterial community of three Caragana species, Caragana microphylla Lam., C. liouana Zhao, and C. korshinskii Kom., which distributed in arid and semi-arid region of northern China were investigated. Across species, Proteobacteria (61.1%), Actinobacteria (16.0%), Firmicutes (8.6%), Bacteroidetes (3.0%), Acidobacteria (3.5%), Gemmatimonadetes (1.4%), and Cyanobacteria (1.0%) were the most dominant phyla in the rhizosphere of the three Caragana species. The relative abundance of Cyanobacteria was significantly higher in rhizosphere of C. korshinskii Kom. compared with C. microphylla Lam. and C. liouana Zhao, while the opposite was found for Gemmatimonadetes in rhizosphere of C. microphylla Lam. relative to C. liouana Zhao. Stepwise multiple linear regression analysis showed that both diversity and richness of the bacterial rhizosphere communities significantly and positively correlated with soil pH (p < 0.01). Distance-based redundancy analysis indicated that soil properties and non-soil parameters detected there accounted for 47.5% of bacterial phylogenetic structure variation (p < 0.01) all together. Meanwhile, soil total phosphorus accounted for the greatest proportion of community structure variance (9.7%, p < 0.01), followed by electrical conduction (6.5%), altitude (5.8%), soil pH (5.4%), mean annual precipitation (3.6%) and total nitrogen (3.6%, p < 0.05 in all cases). Furthermore, partial Mantel test suggested that bacterial rhizosphere community structure significantly correlated with geographical distance, indicating that the less geographical distant sample sites tend to harbor more similar bacterial rhizosphere community. Our results shed new light on the mechanisms of coevolution and interaction between long-lived plants and their rhizosphere bacterial communities across environmental gradients.

The effective and safe use of FGD gypsum in agricultural land is still debated in some countries even though its effectiveness in soil management has been reported in many studies. Thus, the changes in the levels of soil salinity, alkalinity, crop yield, and other physicochemical properties in different soil types and crops after reclamation and planting with FGD gypsum over four years are evaluated in this paper. The main aim of this paper is to review the effects of six treatment technologies in addressing soil salinity and sodicity and crop production in soils, with a focus on the basic theory, key technologies, and industrialized applications. This paper also shows that soil conditions can be improved and crop yields can be increased by using FGD alone or in combination with humic acid or fertilizer. FGD gypsum plus K–Zn–Mn fertilizer increased the yield of rice by 135%. In alkaline, salinized, and secondary salinized soils, FGD gypsum combined with organic fertilizer or organic plus chemical fertilizer increased the yield of rice by 21.2% and 60.4%, the yield of sunflower by 2.4% and 23.6%, and the yield of medlar by 18.81% and 20.78%, respectively. The application of FGD gypsum also increased the salt tolerance of salt-tolerant plants. Combined with drainage, laser field levelling and tillage decreased soil salinity by more than 63.76% and increased the yield of oil sunflower by up to 96.96%. This study provides convincing evidence of the benefits of the application of the six treatments to reclaim saline–alkali soils. It is suggested that comprehensive measures should be taken to improve saline–alkaline soil.

Leaf stable carbon isotope (δ¹³C) composition provides comprehensive insight into plant carbon cycles and water use efficiency and has also been widely used to evaluate the response of plants to environmental change. In the present study, leaf δ¹³C was analyzed in samples of Caragana microphylla Lam., C. liouana Zhao, and C. korshinskii Kom. from 38 populations. These species provide great environmental benefits and economic value and are distributed east to west continuously across northern China. We studied the relationship of δ¹³C to altitude, mean annual precipitation (MAP), mean annual temperature (MAT), mean annual relative humidity (RH), leaf nitrogen (N), and phosphorus (P) concentrations to examine the patterns and controls of leaf δ¹³C variation in each species. Results indicated that, across the three species, leaf δ¹³C significantly decreased with MAP, RH, and leaf N and P concentrations, while it increased with altitude and MAT. However, patterns and environmental controls of leaf δ¹³C varied proportionally with species. C. korshinskii was mainly controlled by MAP and leaf N concentration, C. liouana was controlled by both MAT and MAP, and C. microphylla was mainly controlled by MAT. Further analysis indicated significant differences in leaf δ¹³C between species, which tended to increase from C. microphylla to C. korshinskii. Overall, these results suggest that the three Caragana species may respond differently to future climate change due to different controlling factors on leaf δ¹³C variation, as well as differentiation in water use efficiency, which likely contributes to the geographical distribution of these species.

An increase in foliar nitrogen isotope composition (δ ¹⁵ N) with decreasing precipitation has been shown to occur widely in non-N ₂ -fixing plant species. However, similar patterns have not been identified in N ₂ -fixing species. Here, we tested the relationships of foliar δ ¹⁵ N with local environmental factors and leaf properties in two leguminous shrub species ( Caragana korshinskii and Caragana liouana ) sampled from 30 populations. Results indicated that the mean annual precipitation (MAP) primarily accounted for the variation of foliar δ ¹⁵ N in the two species. Further analysis revealed strong negative correlations between foliar δ ¹⁵ N and MAP within and across species. These results suggest that the foliar δ ¹⁵ N of leguminous shrub species also shift along precipitation gradients, which augments our understanding of the relationships between foliar δ ¹⁵ N and climatic factors.