The application of plant growth-promoting rhizobacteria enhances the tolerance of tobacco seedling to salt stress

The application of plant growth-promoting rhizobacteria (PGPR) is a novel and an efficient strategy for improving soil degradation and productivity. However, the effect of PGPR on tobacco (Nicotiana tabacum L.) seedling growth under salt stress remains unclear. Here, microcosm experiments were designed to verify the effects of Bacillus cereus TC012 (BC), Bacillus methylotrophicus TC023 (BM), and Bacillus amyloliquefacien TC037 (BA) on tobacco grown in salt-affected soil. The results showed that BC, BM, and BA treatments significantly increased the height of tobacco plants by 38.65%, 91.94%, and 90.66%, respectively. Furthermore, the growth of various components of tobacco plant, such as stem girth, seedling biomass, carotenoid, and chlorophyll were stimulated in salt-affected soils. The changes in the salinity of the tobacco plant mostly relies on the improvement of proline, soluble protein, soluble sugar content, plant protective enzymatic activity, and K+/Na+ ratios. Increases in indole-3-acetic acid, zeatin riboside and gibberellic acid also promoted tobacco growth. Additionally, inoculation with PGPR enhanced the enzymatic activity of laccase, urease, neutral protease, acid phosphatase, and sucrase in soil samples and had positive effects on the physicochemical properties. The soil bacterial communities significantly improved after inoculation with PGPR. In particular, the relative abundance of Pseudomonas and Bacillus significantly increased. Overall, PGPR inoculation has great potential to alleviate salt damage in tobacco plants and may have far reaching benefits to the agricultural community.

Effects of practicing long-term mulched drip irrigation on soil quality in Northwest China

Assessing soil quality variation during the prolonged application of mulched drip irrigation (MDI) is critical to comprehend the sustainability of arid agriculture. To investigate the dynamics of crucial soil-quality indicators caused by the long-term application of MDI, the "space instead of time" methodology was adopted, and six fields were selected to represent the primary successional sequence in Northwest China. A total of 21 vital soil attributes from 18 samples were used as soil quality indicators. Based on the soil quality index calculated from the entire datasets, it was observed that long-term MDI practice enhanced soil quality by 28.21 %-74.36 % due to improvements in soil structure (e.g., soil bulk density, three-phase ratio, and aggregates stability) and nutrients (including total carbon, organic carbon, total nitrogen, and available phosphorus). Compared to natural unirrigated soil, soil salinity in 0-200 cm depth significantly decreased by 51.34 %-92.39 % in cotton fields with increasing years of practicing MDI. In addition, long-term MDI practice restructured soil microbial communities and augmented microbial activity by 259.48 %-502.90 % relative to the natural salt-affected soil. However, soil quality stabilized after 12-14 years of MDI application due to accumulated residual plastic fragments, increased bulk density, and reduced microbial diversity. Overall, practicing long-term MDI promotes soil quality and crop yield by promoting soil microbiome structure and function and soil structure. However, long-term mono-cropping with MDI would result in soil compaction and impair microbial activity.

Dissolved nitrogen in salt-affected soils reclaimed by planting rice: How is it influenced by soil physicochemical properties?

Planting rice is an effective way to reclaim salt-affected soils, but overapplying nitrogen fertilizer has resulted in a large loss in the amounts of soil dissolved nitrogen (SDN) from paddy fields. While the dynamic of SDN and its response to changes in soil physicochemical properties by planting rice are well-studied in non-salt-affected soils, little is known about the relationship between the SDN and soil physicochemical properties in reclaimed salt-affected soils. To fill this knowledge gap, soil samples were collected from bare salt-affected soils and three paddy fields with different reclaimed years (4, 9, 20) in six soil layers. Compared with bare salt-affected soils, soil salinity and sodicity exhibited trends of firstly increasing and then decreasing, whereas organic matter and total nitrogen tended to increase with the extension of the reclamation year. Soil dissolved organic carbon and total dissolved phosphorous showed decreasing trends. The sand content showed an increasing tendency, whereas the silt and clay contents tended to decrease. Ammonium nitrogen concentrations in reclaimed paddy fields were higher than those of bare salt-affected soils, and nitrate nitrogen concentrations in reclaimed paddy fields were smaller than those of bare salt-affected soils. However, the changing trends of dissolved organic nitrogen concentrations were not consistent among paddy fields with different reclamation years. Meanwhile, statistical analysis results revealed significant correlations between SDN and soil physicochemical properties. Moreover, dominant drivers influencing SDN were grouped using principal component analysis, identifying the following factors including soil sodicity, active nutrients, soil texture and water retention. Redundancy analysis also revealed that the soil physicochemical properties explained 69.65% of the variation in SDN and the influenced relationship between soil physicochemical properties and SDN nutrients. This study enhances our understanding of the mechanisms influencing SDN during planting rice and has implications for the management of the nutrient application of reclaimed salt-affected soils.

Growth, yield and arsenic accumulation by wheat grown in a pressmud amended salt-affected soil irrigated with arsenic contaminated water

The study assessed the influence of pressmud (PM) application on soil available phosphorus (P) content, growth, yield, and arsenic (As) accumulation in wheat grains on a salt-affected soil receiving irrigation of As-contaminated water. Wheat seeds (cv. Faisalabad-2008) were sown in pots containing saline soil (EC 11.72 dS m-1; pH 8.07; SAR 31.3 mmol1/2 L-1/2) amended with PM (0, 2.5, 10 and 15 g kg-1) and irrigated with As-contaminated water (0, 25 and 100 µg L-1). The pot experiments had two sets, one was harvested after 30-days of germination while the other at crop maturity. Pressmud application at 2.5, 10 and 15 g kg-1 improved biomass of 30-days old wheat seedlings by 44%, 86% and 90%, respectively compared to unamended soil. Irrigation with As-contaminated waters did not affect seedling biomass or grain yield of wheat. Plant height, fertile tillers, straw biomass and grain yield increased from 57-62 cm, 3-5 no. plant-1, 2.93-5.31 g plant-1 and 3.93-7.11 g plant-1, respectively by 15 g PM kg-1 soil. Moreover, PM application resulted in an 8-fold increase in soil available P content, which resulted in higher grain P uptake. Irrigation with water of 25 and 100 µg As L-1 increased soil available P by 7.6% and 11%, respectively, but its influence on the grain P concentration was non-significant. Pressmud application in combination with As-contaminated water increased accumulation of As in grains. By applying water of 25 and 100 µg L-1 As, accumulation of As in wheat grains increased from 3.12-42.4 and 49.58-91.85 µg kg-1, respectively compared with normal water. However, these concentrations of As in wheat grains were still below the permissible limit of 430 µg kg-1 prescribed for agronomic crops. In conclusion, PM is very effective in improving wheat productivity on salt-affected soils but it can aggravate As accumulation in wheat grains if applied in combination with As polluted water.

Soil salinity under climate change: Challenges for sustainable agriculture and food security

Soil salinity is one of the major and widespread challenges in the recent era that hinders global food security and environmental sustainability. Worsening the situation, the harmful impacts of climate change accelerate the development of soil salinity, potentially spreading the problem in the near future to currently unaffected regions. This paper aims to synthesise information from published literature about the extent, development mechanisms, and current mitigation strategies for tackling soil salinity, highlighting the opportunities and challenges under climate change situations. Mitigation approaches such as application of amendments, cultivation of tolerant genotypes, suitable irrigation, drainage and land use strategies, conservation agriculture, phytoremediation, and bioremediation techniques have successfully tackled the soil salinity issue, and offered associated benefits of soil carbon sequestration, and conservation and recycling of natural resources. These management practices further improve the socio-economic conditions of the rural farming community in salt-affected areas. We also discuss emerging reclamation strategies such as saline aquaculture integrated with sub surface drainage, tolerant microorganisms integrated with tolerant plant genotypes, integrated agro-farming systems that warrant future research attention to restore the agricultural sustainability and global food security under climate change scenarios.

Sustained effects of one-time sewage sludge addition on rice yield and heavy metals accumulation in salt-affected mudflat soil

High-yielding and sustainable production of rice in salt-affected mudflat is restricted by high soil salinity. Although sewage sludge can be used for mudflat amendment especially soil salt reduction, the possibility of potential heavy metal contamination in sludge-amended mudflat especially under paddy cultivation remains unclear, which hinders the further utilization of sewage sludge. In this study, a field experiment was conducted in a newly reclaimed mudflat to assess the sustained effects of one-time sludge input with different addition rates (0, 30, 60, 120, and 180 t ha^-1) on soil salinity, rice yield, and potential metal contamination under paddy cultivation. The results indicated that sewage sludge addition (SSA) significantly decreased soil salinity and increased soil fertility. The increasing SSA rates and amending years led to the gradual increase of rice yield in salt-affected mudflat. The maximum increases in rice yield were 125.1%, 124.7%, and 127.9% in 2016, 2017, and 2018, and the average annual increase in rice yield in sludge-treated mudflat was 1.7%. Sludge addition increased metals accumulation in mudflat soil and metals uptake by rice tissues except Cr, Cu, and Pb in rice grain. The maximum increments in metal concentrations in soil and rice plant all occurred at 180 t ha^-1 sludge addition rate. However, the metal concentrations in rice grain were below the safety limits even in the treatment with the highest sludge addition rate. Metal concentrations in sludge-treated soil and rice plant showed downward trend during the 3-year trial, and the decreases in total amount of soil metals were mainly concentrated in the first amending year, accounting for more than 50%. In summary, one-time sludge input achieved sustained mudflat amendment and efficient rice production. In addition, controlling the total amount of sludge input realized safe utilization of sewage sludge in salt-affected mudflat under paddy cultivation.

Seasonality of soil respiration under gypsum and straw amendments in an arid saline-alkali soil

Soil respiration (or CO₂ production) is often determined by measuring CO₂ efflux; however, there are differences between them in saline-alkali soils of arid land. The purpose of this study is to test a hypothesis that CO₂ production exceeds efflux in arid saline-alkali soils under organic and gypsum amendments. We conducted a modeling study that was based on a two-year field experiment with four treatments: control, gypsum addition, wheat straw incorporation, and gypsum-straw combination. A diffusion model was forced by soil CO₂, temperature and moisture that were continuously recorded at 0, 8 and 15 cm, and calibrated by measured CO₂ efflux. We then applied the model to calculate CO₂ production and efflux over 2014-2015, and found a strong and similar seasonality in both CO₂ production and efflux under all treatments (i.e., highest in summer with one peak in 2014 and two peaks in 2015). Our results showed enhanced CO₂ production and efflux over short period following rainfall. There were significantly exponential relationships between CO₂ production/efflux and temperature. While straw incorporation significantly increased CO₂ production and efflux, straw incorporation combined with gypsum amendment caused a decrease in CO₂ production and efflux. CO₂ production exceeded CO₂ efflux mainly in the first half year, and annual difference was 33-130 g C m^-2, with larger differences under gypsum amendment. Our study implies that a portion of respired CO₂ is transformed into other forms and stored in saline-alkaline soils in arid land.

Current states and challenges of salt-affected soil remediation by cyanobacteria

Natural and human activities lead to soil degradation and soil salinization. The decrease of farmlands threatens food security. There are approximately 1 billion ha salt-affected soils all over of world, which can be made available resources after chemical, physical and biological remediation. Nostoc, Anabaena and other cyanobacterial species have outstanding capabilities, such as the ability to fix nitrogen from the air, produce an extracellular matrix and produce compatible solutes. The remediation of salt-affected soil is a complex and difficult task. During the past years, much new research has been conducted that shows that cyanobacteria are effective for salt-affected soil remediation in laboratory studies and field trials. The related mechanisms for both salt tolerance and salt-affected soil remediation were also evaluated from the perspective of biochemistry, molecular biology and systems biology. The effect of cyanobacteria on salt-affected soil is related to nitrogen fixation and other mechanisms. There are complicated interactions among cyanobacteria, bacteria, fungi and the soil. The interaction between cyanobacteria and salt-tolerant plants should be considered if the cyanobacterium is utilized to improve the soil fertility in addition to performing soil remediation. It is critical to re-establish the micro-ecology in salt-affected soils and improve the salt affected soil remediation efficiency. The first challenge is the selection of suitable cyanobacterial strain. The co-culture of cyanobacteria and bacteria is also potential approach. The cultivation of cyanobacteria on a large scale should be optimized to improve productivity and decrease cost. The development of bio-remediating agents for salt-affected soil remediation also relies on other technical problems, such as harvesting and contamination control. The application of cyanobacteria in salt-affected soil remediation will reconstruct green agriculture and promote the sustainable development of human society.

Microbial Inoculants Assisted Growth of Chrysopogon zizanioides Promotes Phytoremediation of Salt Affected Soil

Restoration of salt-affected soil through cultivation Chrysopogon zizanioides is a promising approach. The two way benefit of such an approach is that reclamation of salt-affected soil concomitant to improve plant growth and increased yield of essential oil produced in the plants roots. Earlier studies showed physiological changes and reduced growth of C. zizanioides under salinity. In the present study, plant growth promoting microorganisms viz. Pseudomonas monteilii, Bacillus megaterium, Azotobacter chroococcum and Rhizophagus intraradices were used as bio-inoculants for cultivation of C. zizanioides under salt-affected soil. Bio-inoculants in combination with vermicompost significantly increased the growth and productivity of C. zizanioides under salt-affected soil, and simultaneously improved soil health. When compared to control, the soil physico-chemical and biological properties of bio-inoculants treated plants was significantly improved. The reclamation of salt-affected soil was evident by the significant decrease in the level of soil pH (11.0%), electrical conductivity (23.5%), sodium adsorption ratio (15.3%), and exchangeable sodium percent (12.4%) of bio-inoculants treated plants. The improvement of soil cation exchange capacity indicated the decrease in soil salinity. Whereas increase in the microbial count (four-fold), AMF spores (447 spores), dehydrogenase (six-fold), acid (two-fold) and alkaline phosphatase (five-fold) activities in rhizosphere soil of bio-inoculant treated plants indicated the improved biological properties. A positive correlation of plant biomass production to soil organic carbon, total Kjeldahl nitrogen, available phosphorus and cation exchange capacity depicted improved nutrients content in rhizosphere soil of bio-inoculant treated plants. The findings of this study suggest that P. monteilii and R. intraradices with vermicompost can be effectively used as bio-inoculants for encouragement of phytoremediation in salt-affected soil.