Chemical and Microbial Characterization of Washed Rice Water Waste to Assess Its Potential as Plant Fertilizer and for Increasing Soil Health

Abstract

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.

Fermentation of Washed Rice Water Increases Beneficial Plant Bacterial Population and Nutrient Concentrations

“…Conversely, the solubilized K (for F0, F3, F6, and F9) in this study were greater than the 4.29 mg L solubilized by Bacillus mucilaginous MCRCp1 as reported by Sugumaran and Janarthanum [39] after 4 days of incubation. Similarly, our results agree with many studies [13,28,40] that reported an increase in P and K solubilization with a consequent decrease in pH of the culture as the incubation days increased. The increased release of potassium from muscovite is associated with the production of acids, alkalis, or chelates by the bacterial isolates [13].…”

“…More elements were recorded in the fermented WRW relative to the F0 (unfermented) because the bacterial population increased with fermentation, indicating the presence of more N-fixing and Pand K-solubilizing bacteria. The results agree with Nabayi et al [28], who reported an increase in S, P, K, Mg, NH + 4 , and NO − 3 elements with a consequent decrease in C content as the fermentation period progressed. The increase in P, K, Ca, Mg, Zn, NH + 4 , and NO − 3 with fermentation of WRW agrees with Nkhata et al [32], who reported increased P, Ca, Mg, Zn, and Cu as cereals were mineralized by bacteria.…”

“…The partial 16S rRNA gene sequences of the identified strains in this study were deposited in GenBank database (http://www.ncbi.nlm.nih.gov/GenBank/index.html) accessed on 15 December 2020, as reported by Nabayi et al [28]. The sequence was further used to construct a phylogenetic tree using the Maximum Likelihood method.…”

“…Therefore, the lower bacterial population in F0 explained the lowest K solubilization recorded. Bacillus and Enterobacter species are classified as beneficial plant bacteria that could solubilize P and K [13,28,37,38]; therefore, the K solubilization recorded is because of the presence of these PGPB in the fermented WRW. Potassium solubilization rates for the fermented WRW are within the range of isolates used by Tan et al [13], which solubilized 10.7 to 14.15 mg L −1 after 5 days of incubation.…”

Fermentation of White and Brown Rice Water Increases Plant Nutrients and Beneficial Microbes

“…However, significant proportions of water-soluble nutrients are removed because of rice washing. Several studies have indicated that rice washing leads to the leaching of different proportions of nutrients from the rice grains into the WRW (Juliano, 1985;Nabayi, Sung, Zuan, Paing, & Akhir, 2021). For example, Nabayi, Sung, Zuan, Paing, and Akhir (2021) reviewed that WRW contains between 40 to 150 mg L −1 N, 43 to 16306 mg L −1 P, 51 to 200 mg L −1 K, 8 to 3574 mg L −1 Ca, 36 to 1425 mg L −1 Mg, and 27 to 212 mg L −1 S. It indicates that these leached nutrients have enriched the WRW, which potentially could be utilized as a liquid plant fertilizer, rather than WRW being discarded (Nabayi, Sung, Paing, & Zuan, 2021b).…”