An interplay of soil salinization and groundwater degradation threatening coexistence of oasis-desert ecosystems

Salinization of shallow groundwater in the Jiaokou Irrigation District and associated secondary environmental challenges

Understanding groundwater salinization of irrigation areas and related secondary environmental challenges is important for ensuring sustainable development. However, the mechanism under which groundwater salinization forms under the influence of long-term anthropogenic activities remains unclear. Therefore, this study analyzed the spatiotemporal variation in groundwater salinization and the underlying mechanism, and discussed the secondary environmental challenges in an irrigation area. The Jiaokou Irrigation District, North China, was adopted as a case study. The results showed a slight downward trend in groundwater salinity over the past two decades at a rate of 0.0229 g/L/y. Higher groundwater salinity was observed in areas with shallow groundwater depth. This correlation was mainly attributed to evaporative concentration, with secondary processes including natural weathering, depth of water-table, and fertilizer leaching. Drainage ditches may reduce groundwater salinity. Groundwater was transformed from freshwater to salt water and then to brackish water during the runoff process. The former transformation is mainly related to evaporation and fertilization. The latter transformation could be related to the inverse relationship between the distance to the Wei River and sediment permeability, with sediment permeability positively related to groundwater flow and leading to the discharge of salt into the Wei River. The secondary environmental challenges related to groundwater salinization in irrigation areas, mainly manifested in deterioration of irrigation water quality, soil salinization, and increased fluorine concentration. This study can act as a theoretical and practical reference for the development and utilization of water resources, ecological protection, and soil salinization in typical irrigation districts.

Application of rhizobacteria to improve microbial community structure and maize (Zea mays L.) growth in saline soil

The utilization of plant growth-promoting rhizobacteria (PGPR) has emerged as a prominent focus in contemporary research on soil microbiology, microecology, and plant stress tolerance. However, how PGPR influence the soil bacterial community and related ecological functions remains unclear. The aim of this study was to investigate the effects of three natural PGPR inoculations (YL07, Planococcus soli WZYH02; YL10, Bacillus atrophaeus WZYH01; YL0710, Planococcus soli WZYH02 and Bacillus atrophaeus WZYH01) on maize (Zea mays L.) growth under two salt stress conditions (S1, ECe = 2.1 ~ 2.5 dS/m; S2, ECe = 5.5 ~ 5.9 dS/m). The results revealed that compared to the control (CK), the average plant height of maize seedlings significantly increased by 27%, 23%, and 29% with YL07, YL10, and YL0710 inoculation under S1 conditions, respectively, and increased by 30%, 20%, and 18% under S2 conditions, respectively. Moreover, PGPR inoculation positively influenced the content of superoxide dismutase, catalase, soluble sugar, and proline in maize under salt stress. Subsequent analysis of alpha diversity indices, relative microbial abundance, principal coordinate analysis, cladograms, and linear discriminant analysis effect size histograms indicated significant alterations in the rhizosphere microbial community due to PGPR inoculation. FAPROTAX analysis demonstrated that YL10 inoculation in S2 rhizosphere soil had a notable impact on carbon cycle functions, specifically chemoheterotrophy, fermentation, and phototrophy. Thus, this study provides evidence that PGPR inoculation improves soil microbial communities and plant indices under salt stress. These findings shed light on the potential of PGPR as a viable approach for enhancing plant stress tolerance and fostering sustainable agricultural practices.

Microbial community structural response to variations in physicochemical features of different aquifers

Introduction

The community structure of groundwater microorganisms has a significant impact on groundwater quality. However, the relationships between the microbial communities and environmental variables in groundwater of different recharge and disturbance types are not fully understood.

Methods

In this study, measurements of groundwater physicochemical parameters and 16S rDNA high-throughput sequencing technology were used to assess the interactions between hydrogeochemical conditions and microbial diversity in Longkou coastal aquifer (LK), Cele arid zone aquifer (CL), and Wuhan riverside hyporheic zone aquifer (WH). Redundancy analysis indicated that the primary chemical parameters affecting the microbial community composition were NO3 -, Cl-, and HCO3 -.

Results

The species and quantity of microorganisms in the river-groundwater interaction area were considerably higher than those in areas with high salinity [Shannon: WH (6.28) > LK (4.11) > CL (3.96); Chao1: WH (4,868) > CL (1510) > LK (1,222)]. Molecular ecological network analysis demonstrated that the change in microbial interactions caused by evaporation was less than that caused by seawater invasion under high-salinity conditions [(nodes, links): LK (71,192) > CL (51,198)], whereas the scale and nodes of the microbial network were greatly expanded under low-salinity conditions [(nodes, links): WH (279,694)]. Microbial community analysis revealed that distinct differences existed in the classification levels of the different dominant microorganism species in the three aquifers.

Discussion

Environmental physical and chemical conditions selected the dominant species according to microbial functions. Gallionellaceae, which is associated with iron oxidation, dominated in the arid zones, while Rhodocyclaceae, which is related to denitrification, led in the coastal zones, and Desulfurivibrio, which is related to sulfur conversion, prevailed in the hyporheic zones. Therefore, dominant local bacterial communities can be used as indicators of local environmental conditions.

The crucial interactions between climate and soil

Since the birth of soil science, climate has been recognized as a soil-forming factor, along with parent rock, time, topography, and organisms (from which humans were later kept distinct), often prevalent on the other factors on the very long term. But the climate is in turns affected by soils and their management. This paper describes the interrelationships between climate - and its current change - and soil, focusing on each single factor of its formation. Parent material governs, primarily through the particle size distribution, the capacity of soil to retain water and organic matter, which are two main soil-related drivers of the climate. Time is the only unmanageable soil-forming factor; however, extreme climatic phenomena can upset the soil or even dismantle it, so as to slow down the pathway of pedogenesis or even make it start from scratch. Topography, which drives the pedogenesis mostly controlling rainfall distribution - with repercussions also on the climate - is not anymore a given factor because humans have often become a shaper of it. Indeed humans now play a key role in affecting in a plethora of ways those soil properties that most deal with climate. The abundance and diversity of the other organisms are generally positive to soil quality and as a buffer for climate, but there are troubling evidences that climate change is decreasing soil biodiversity. The corpus of researches on mutual feedback between climate and soil has essentially demonstrated that the best soil management in terms of climate change mitigation must aim at promoting vegetation growth and maximizing soil organic matter content and water retention. Some ongoing virtuous initiatives (e.g., the Great Green Wall of Africa) and farming systems (e.g., the conservation agriculture) should be extended as much as possible worldwide to enable the soil to make the greatest contribution to climate change mitigation.

The Role of Soil Salinization in Shaping the Spatio-Temporal Patterns of Soil Organic Carbon Stock

Soil salinization is closely related to land degradation, and it is supposed to exert a significant negative effect on soil organic carbon (SOC) stock dynamics. This effect and its mechanism have been examined at site and transect scales in previous studies while over a large spatial extent, the salinity-induced changes in SOC stock over space and time have been less quantified, especially by machine learning and remote sensing techniques. The main focus of this study is to answer the following question: to what extent can soil salinity exert an additional effect on SOC stock over time at a larger spatial scale? Thus, we employed the extreme gradient boosting models (XGBoost) combined with field site-level measurements from 433 sites and 41 static and time-varying environmental covariates to construct methods capable of quantifying the salinity-induced SOC changes in a typical inland river basin of China between the 1990s and 2020s. Results showed that the XGBoost models performed well in predicting the soil electrical conductivity (EC) and SOC stock at 0–20 cm, with the R2 value reaching 0.85 and 0.81, respectively. SOC stock was found to vary significantly with increasing soil salinity following an exponential decay function (R2 = 0.27), and salinity sensitivity analysis showed that soils in oasis were expected to experience the largest carbon loss (−137.78 g m−2), which was about 4.84, 14.37, and 25.95 times higher than that in the saline, bare, and sandy land, respectively, if the soil salinity increased by 100%. In addition, the decrease in the soil salinity (−0.32 dS m−1) from the 1990s to the 2020s was estimated to enhance the SOC stock by 0.015 kg m−2, which contributed an additional 10% increase to the total SOC stock enhancement. Overall, the proposed methods can be applied for quantification of the direction and size of the salinity effect on SOC stock changes in other salt-affected regions. Our results also suggest that the role of soil salinization should not be neglected in SOC changes projection, and soil salinization control measures should be further taken into practice to enhance soil carbon sequestration in arid inland river basins.