A significant wastewater source in every household is washed rice water (WRW) because it contains leached nutrients (from washing the rice prior to cooking) that could be used as fertilizer. The paper reviewed the current understanding of the potential use of WRW as a plant nutrient source. WRW was shown to increase vegetables growth, such as water spinach, pak choy, lettuce, mustard, tomato, and eggplant. Different researchers have used various amounts of WRW, and their results followed a similar trend: the higher the amount of WRW, the higher the plant growth. WRW has also been used for other purposes, such as a source of carbon for microbial growth. WRW from brown rice and white rice had nutrients ranging from 40-150, 43-16306, 51-200, 8-3574, 36-1425, 27-212, and 32-560 mg L-1 of N, P, K, Ca, Mg, S, and vitamin B1 (thiamine), respectively. Proper utilization of WRW could reduce chemical fertilizer use and prevent both surface and groundwater contamination and environmental pollution. However, only a few of the studies have compared the use of WRW with the use of conventional NPK fertilizer. The major drawback of WRW studies is that they lack depth and scope, such as determining the initial and (or) final soil physico-chemical properties or plant nutrient contents. Considering the rich nutrient content in WRW, it will impact plant growth and soil fertility when used as both irrigation water and plant nutrient source. Therefore, it is recommended that studies on WRW effect on soil microbial population, plant, and soil nutrient contents to be carried out to ascertain the sustainability of WRW use as a plant nutrient source.
The wastewater from washed rice water (WRW) is often recommended as a source of plant nutrients in most Asian countries, even though most current research on WRW lack scientific rigor, particularly on the effects of rice washing intensity, volumetric water-to-rice ratio (W:R), and condition of the WRW before plant application. This research was thus carried out: (1) to determine how various rice washing intensities, fermentation periods (FP), and W:R would affect the nutrient content in WRW, and (2) to isolate, identify, and characterize the bacterial community from fermented WRW. The WRW was prepared at several rice washing intensities (50, 80, and 100 rpm), FP (0, 3, 6, and 9 days), and W:R (1:1, 3:1, and 6:1). The concentrations of all elements (except P, Mg, and Zn) and available N forms increased with increasing FP and W:R. Beneficial N-fixing and P- and K-solubilizing bacteria were additionally detected in WRW, which helped to increase the concentrations of these elements. Monovalent nutrients NH4+-N, NO3−-N, and K are soluble in water. Thus, they were easily leached out of the rice grains and why their concentrations increased with W:R. The bacteria population in WRW increased until 3 days of fermentation, then declined, possibly because there was an insufficient C content in WRW to be a source of energy for bacteria to support their prolonged growth. While C levels in WRW declined over time, total N levels increased then decreased after 3 days, where the latter was most possibly due to the denitrification and ammonification process, which had led to the increase in NH4+-N and NO3−-N. The optimum FP and W:R for high nutrient concentrations and bacterial population were found to be 3 to 9 days and 3:1 to 6:1, respectively. WRW contained nutrients and beneficial bacterial species to support plant growth.
Washed rice water (WRW) is said to be a beneficial plant fertilizer because of its nutrient content. However, rigorous scientific studies to ascertain its efficiency are lacking. The purpose of this study was to determine the effect of fermenting WRW on the bacterial population and identification, and to measure how fermentation affects the nutrient composition of WRW. Rice grains were washed in a volumetric water-to-rice ratio of 3:1 and at a constant speed of 80 rpm for all treatments. The treatments were WRW fermented at 0 (unfermented), 3, 6, and 9 days. Bacterial N fixation and P and K solubilization abilities in the fermented WRW were assessed both qualitatively and quantitatively. The isolated bacterial strains and the WRW samples were also tested for catalase and indole acetic acid (IAA) production ability. Significantly greater N fixation, P and K solubilization, and IAA production were recorded after 3 days of fermentation compared with other fermentation periods, with increases of 46.9–83.3%, 48.2–84.1%, 73.7–83.6%, and 13.3–85.5%, respectively, in addition to the highest (2.12 × 108 CFU mL−1) total bacterial population. Twelve bacteria strains were isolated from the fermented WRW, and the gene identification showed the presence of beneficial bacteria Bacillus velezensis, Enterobacter spp., Pantoea agglomerans, Klebsiella pneumoniae and Stenotrophomonas maltophilia at the different fermentation periods. All the identified microbes (except Enterobacter sp. Strain WRW-7) were positive for catalase production. Similarly, all the microbes could produce IAA, with Enterobacter spp. strain WRW-10 recording the highest IAA of up to 73.7% higher than other strains. Generally, with increasing fermentation periods, the nutrients N, S, P, K, Mg, NH4+, and NO3− increased, while pH, C, and Cu decreased. Therefore, fermentation of WRW can potentially increase plant growth and enhance soil health because of WRW’s nutrients and microbial promotional effect, particularly after 3 days of fermentation.
The objective of this study was to investigate the current soil properties in lowland paddy fields in Peninsular Malaysia and to assess the long-term changes in the soil fertility status during 50 years after the Green Revolution. Forty paddy fields were selected close to the study sites surveyed in 1965 and grouped based on six physiographic environments: the brackish swamp (including acid sulfate soils), the brackish alluvium, the freshwater swampand the riverine alluvium in the west coast, and the riverine alluvium and the beach ridges interspersed with swales in the east coast. Soil samples from the depth of 0-15 cm were analyzed for the physicochemical properties. Despite similar fertilizer application rates over Peninsular Malaysia, several differences were found in the current soil properties between different physiographic environments, such as 1) higher levels of clay fraction, 1.4 nm minerals, CEC, exchangeable bases and available Si as well as Mg-and Na-rich status in the brackish environments, 2) higher levels of available N in the riverine alluvium environment in the east coast , and 3) the excessive P accumulation in the acid sulfate soils. The long-term changes well appeared in the dramatic increase of P availability and the alleviation of soil acidity. In addition, the composition of exchangeable bases changed toward Ca-rich and Mg-low status. Large reduction in soil organic matter was found in the swamp environments while those in the riverine alluvium environment in the east coast were increased. It could be concluded that despite the successful increase in rice yield after the Green Revolution, the long-term changes in the paddy soil fertility showed positive and negative aspects depending on physiographic environments. Appropriate fertilizer application schemes taking into account different soil characteristics in different physiographic environments should be required to achieve both efficient, sustainable rice production and environmental conservation.
Over 50 years of the Green Revolution since the 1960s, the global population has increased by 2.5 times, cereal production by 3.3 times and the use of N, P and K fertilisers by 9.4, 4.2 and 4.3 times, respectively. Information is still limited, however, on the influence of these impacts on the fertility status of agricultural soils. Here we investigated the influence of the Green Revolution on 142 paddy soils in three tropical Asian countries, that is, Thailand, the Philippines and Malaysia, during the given period by repeated soil sampling in the 1960s and 2010s at or near the same locations. We revealed that the phytoavailablility indices of three macronutrients – N, P and K, that is, available P, exchangeable K and total N, showed 743% (p < 0.01), 12% and 1% increase on average, respectively, while total C showed 9% decline. Comprehensive investigation of overall fertility status by factor analysis using 11 soil parameters suggested that only the factor scores associated with ‘available P status’ increased drastically in all the three countries (p < 0.01) whereas those associated with ‘organic matter and N contents’ and ‘inherent potentiality’ did not exhibit any consistent changes among the countries. In conclusion, intensive soil/fertiliser management systems under the Green Revolution have successfully improved the nutrient status, especially P status, of paddy soils with slight decrease of soil organic matter over the last 50 years, while a large amount of nutrients applied, especially N and K, has been released from soil to the outer environments. Conversion to a high‐efficiency system of external nutrient inputs with organic matter‐conserving strategies is, therefore, urgently required to secure sustainable food production while restoring the environment during the coming decades. Highlights Paddy soils from 142 locations in three tropical Asian countries were repeatedly surveyed in the 1960s and 2010s. Available P increased by 8.4 times, that of exchangeable K and total N showed 12% and 1% increase, while organic matter level showed 9% decline. Factor analysis indicated significant increase of the factor scores for those associated with ‘available P status’. A high‐efficiency system with organic matter‐conserving strategies is to be established for both sustainable food production and restoration of the environment.
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