Several other plant-growth-promoting bacteria, including Bacillus siamensis, Bacillus megaterium, Bacillus subtilis, and Bacillus cereus, had relatively higher IAA yields at temperatures of 2–135 ◦C and pH 7–8. Three different bacteria isolated from therhizosphere of Stevia rebaudiana also exhibited greater production of IAA at a pH range of 6–9 and a temperature of 35 ◦C to 37 ◦C; these bacteria also increased the root and shoot bio-masses of wheat and mung bean. Various carbon sources are used as an energy source for IAA production and could enhance recycling of cofactors in bacterial cells. Our results revealed that IAA yields of B. seminalis strain 869T2 were slightly better when glucose and fructose were used in media. Several previous publications also indicated that the ability of plant-growth-promoting bacteria to produce IAA was different, depending on the carbon source used in the media. Results from these studies and our study demonstrated that IAA production by different plant-growth promoting bacteria can be influenced by various factors, such as temperature, pH, carbon sources, culture conditions, and bacterial species. In this study, we utilized the colorimetric method to estimate the IAA amounts of B. seminalis strain 869T2 when grown in various in vitro conditions and media. Because the available tryptophan in the rhizosphere and root exudates of plants might be relatively lower than the tryptophan used in the media,black plastic plant pots the IAA production of B. seminalis strain 869T2 when grown in inoculated plants shall be determined with more sensitive and accurate methods, such as high-performance liquid chromatography or ultra-performance liquid chromatography systems.
Apart from the IAA production ability of B. seminalis strain 869T2, this bacterium exhibited siderophore production and phosphate solubilization activities. Iron is an important element for many biological processes in plant growth and development. Most iron in soils is present in the highly insoluble ferric form, which is unavailable for plant absorption. Endophytic bacteria can yield iron-chelating agents such as siderophores, which bind ferric iron and help transport it into plant cells via root-mediated degradation of organic chelate, ligand exchange, or other mechanisms. Phosphorus is another essential macro-nutrient for numerous metabolism processes in plants, such as biosynthesis of macromolecules, signal transduction, photosynthesis, and respiration. Most of the phosphorus in soil is insoluble and not available for root uptake to support plant growth. In order to increase the bio-availability of phosphorus for plants, certain endophytic bacteria turn insoluble phosphate into soluble forms via the processes of chelation, ion exchange, acidification, or production of organic acids. Previous studies have also correlated siderophore production and phosphate solubilization abilities with the plant growth promotion traits of other Burkholderia species, such as the Burkholderia sp. SSG isolated from boxwood and the Burkholderia sp. MSSP isolated from root nodules of Mimosa pudica. Burkholderia cenocepacia strain CR318, which was isolated from maize roots, significantly enhanced maize plant growth by solubilizing inorganic tricalcium phosphate. Other studies have revealed that additional Burkholderia species also have the ability to solubilize inorganic phosphate to increase available phosphorous in agricultural soils and improve agricultural production.
In summary, both previous studies and our results suggest that the IAA synthesis, siderophore production, and phosphate solubilization abilities of B. seminalis strain 869T2 may collectively contribute to the growth enhancement observed in the several plant species tested here. We successfully inoculated and reisolated B. seminalis strain 869T2, which was originally isolated from the monocot plant vetiver grass , in several eudicot plant species of the Brassicaceae, Asteraceae, Amaranthaceae, Solanaceae, and Malvaceae families. Strain 869T2 can significantly improve the growth of both the roots and aerial parts of Arabidopsis and several leafy vegetables, including ching chiang pak choi, pak choi, loose-leaf lettuce, romaine lettuce, red leaf lettuce, and Chinese amaranth. These results suggest that the endophytic bacterium strain 869T2 may have a wide host range. A similar observation was reported for Burkholderia phytofirmans strain PsJN, first isolated from onion roots, which enhanced the growth of Arabidopsis, switch-grass, potato, tomato, maize, wheat, and grapevines. We did not observe significant growth improvement in hot pepper or okra plants after inoculation with strain 869T2; however, we did observe early flowering and better fruit development in these tested plants. These results suggest that the plant growth promotion abilities of strain 869T2might be more apparent in crops with a shorter life cycle or that the latter two tested host plant species might not be fully compatible with this bacterium. The plant colonization process and growth promotion abilities of endophytic bacteria seem to be active processes that are regulated by different characteristics of both the host plants and bacteria. In conclusion, our study revealed the potential of Burkholderia seminalis strain 869T2 for use as a bio-inoculant in agriculture to improve plant growth and production. Broken nutrient and carbon cycles in food systems waste vast quantities of organic resources and contribute to climate change, food insecurity, and soil degradation. Closing these cycles may contribute to global greenhouse gas reduction goals , improve the resiliency of agroecosystems , and advance sustainable development goals . The capture and transformation of human excrement, in particular, represents an enormous and largely untapped resource stream.
An estimated 4.1 billion people do not have access to a sanitation system that includes waste treatment , including 892 million people still practicing open defecation . The release of organic matter and nutrients embedded in untreated feces and urine into the environment impairs water quality and emits GHGs, resulting in public health concerns. Ecological Sanitation is a strategy for improving access to sanitation and full cycle treatment of human waste. EcoSan is implemented throughout the world , and is particularly important in areas where water access, financial resources, and infrastructure are limited . There are several models of EcoSan, with technologies ranging from constructed wetlands to composting toilets and scales ranging from simple household installations to complex decentralized systems . Regardless of implementation, all EcoSan systems share the common objective of the closed-loop management of human waste and a systemic philosophy that connects sanitation and agriculture . Here, we consider EcoSan implementations that combine container-based toilets for capture and transport of waste, with aerobic, thermophilic composting for the treatment process to sanitize human feces and produce an organic soil amendment . EcoSan offers potential solutions to critical sustainability challenges in the nexus of sanitation, water, health, and agriculture . In particular, EcoSan may solve three important sustainability challenges by providing safely managed sanitation for presently underserved communities and reducing the spread of intestinal-born pathogens, returning nutrients and organic matter to degraded agricultural soils,drainage pot and mitigating climate change by reducing GHG emissions compared to alternative waste disposal methods, bio-energy production, and/or promoting soil carbon sequestration in agricultural ecosystems. EcoSan currently makes up a minor fraction of global sanitation services. Current social challenges and technical challenges exist to its widespread implementation. However, as population expands in rapidly urbanizing cities and as water availability declines, cost-effective EcoSan operations, such as container-based implementations we consider here , may grow to serve a larger proportion of the global population . Quantitative data are required to critically evaluate the potential of EcoSan to address these sustainability goals. Here, we focus on the goal of reducing GHG emissions because emissions from EcoSan composting technologies are an important, yet unknown, variable in their overall sustainability. Greenhouse gas emissions vary widely among waste treatment technologies, depending on the bio-geochemical conditions, operating conditions, and associated collection and discharge systems . Wastewater treatment plants are globally significant contributors to GHG emissions . Several sources provide estimates of CH4, N2O, and CO2 emissions from wastewater treatment plants, including national inventories, primary literature, and life cycle assessments . Fluxes of CH4 and N2O are biologically-produced, and therefore fluxes can be reduced by technologies that remove or stabilize carbon and nitrogen during the waste treatment process . Systems that use anaerobic digestion of sludge tend to be a major source of CH4 emissions though some anaerobic digestion systems aim to contain and burn emitted CH4 for energy production . In contrast, GHG emissions data from EcoSan and other non-sewered sanitation systems are sparsely available in primary literature and monitoring inventories. Greenhouse gas emissions from non-sewered sanitation systems, including pit latrines, septic systems, and container-based toilets, are poorly constrained due to their decentralized locations and high level of operational variability.
Direct measurements of CH4 and N2O from septic systems are few and have differed from modeled emissions factors . A recent analysis of pit latrines concluded that globally, pit latrines accounted for 1% of anthropogenic CH4 emissions . The relatively large contribution of pit latrines to global CH4 sources can be attributed to the global extent of pit latrine use e approximately one-quarter of the global population e as well as the wet and unventilated conditions that drive anaerobic CH4 production. EcoSan relies on aerobic conditions to treat waste and has the potential to considerably reduce the GHG footprint of waste management . In aerobic thermophilic composting, CH4 emissions are typically low because of the presence of oxygen. However, anaerobic microsites created by uneven distribution of water in pores and hot spots of labile carbon can create conditions leading to CH4 emissions . The use of bulking agents and pile turning can be used to reduce the occurrence of anaerobic CH4-producing conditions and, when effective, carbon emissions from composting are in the form of CO2, which is considered to be climate-neutral because of its biogenic origin . Composting can, however, produce biogeochemical conditions prime for N2O emissions through nitrification or denitrification, including large sources of reactive nitrogen, dynamic and spatially varying levels of oxygen, and labile carbon sources. Quantifying the magnitude and balance of CH4 and N2O emissions in a given sanitation system is critical as the two gases have 100-year global warming potential values of 34 and 298, respectively. EcoSan systems utilizing aerobic, thermophilic composting are promising because they may mitigate GHG emissions from the waste and agricultural sectors, however these emissions reductions have not yet been quantified. Further, measurements of GHG emissions from management of solid organic wastes are especially limited from tropical climates , where implementation of EcoSan solutions are likely to be greatest. To our knowledge, no direct measurements of GHG emissions exist from EcoSan systems that deploy container-based toilets and thermophilic composting of human excrement. Our primary objective was to characterize and quantify the GHG emissions resulting from the aerobic composting of human waste in EcoSan settings. We considered two operations that employed similar compost practices, but differed in the physical infrastructure that could alter bio-geochemical conditions mediating GHG dynamics. We also compared the GHG footprint of EcoSan with alternative waste management pathways present in the region, including waste stabilization ponds and unmanaged disposal on grass fields. Finally, we undertook an investigation of the effects of compost management options that help reduce EcoSan GHG emissions.Greenhouse gas fluxes were measured from three sanitation pathways in Haiti: two waste stabilization ponds, two EcoSan operations, and a grass field where the illegal disposal of sewage was observed . The waste stabilization ponds were located in Croix ed Bouquets near Port-au-Prince, Haiti and operated by the Haitian government agency, Direction Nationale de l’Eau Potable et de l’Assannissement . Ponds consisted of uncovered concrete basins with effluent pipes connected to secondary overflow ponds. Two ponds were included in the sampling: a pond that received mostly septic tank waste , and another that received mostly pit latrine waste . Solid sludge was scraped out occasionally and stockpiled on-site. Solid and liquid waste from septic tanks and pit latrines were transported to the site and emptied into the waste stabilization ponds. The waste stabilization ponds represent the primary pathway of centralized waste treatment as advanced municipal wastewater treatment technologies are not present in the country.Eighteen and 54 L container-based toilets were collected from households and communities, respectively, in each of the regions.Coarsely ground sugarcane bagasse was added throughout use to prevent flies and reduce odor. Urine was disposed of on-site, and only solid material was transported to the compost facilities. Both facilities used a similar aerobic composting process consisting of a static thermophilic stage, followed by pile turning and maturation in windrows. At the Cap-Haiti€ en operation, hereafter referred to as “Compost CH,” the ground was lined with cement to prevent leaching and an aluminum roof covered the area. Roofs and cement-lined floors were absent at the Port-au-Prince EcoSan operation, hereafter referred to as “Compost PAP”.