Plant growth promoting rhizobacteria (PGPR) shows an important role in the sustainable agriculture industry. The increasing demand for crop production with a significant reduction of synthetic chemical fertilizers and pesticides use is a big challenge nowadays. The use of PGPR has been proven to be an environmentally sound way of increasing crop yields by facilitating plant growth through either a direct or indirect mechanism. The mechanisms of PGPR include regulating hormonal and nutritional balance, inducing resistance against plant pathogens, and solubilizing nutrients for easy uptake by plants. In addition, PGPR show synergistic and antagonistic interactions with microorganisms within the rhizosphere and beyond in bulk soil, which indirectly boosts plant growth rate. There are many bacteria species that act as PGPR, described in the literature as successful for improving plant growth. However, there is a gap between the mode of action (mechanism) of the PGPR for plant growth and the role of the PGPR as biofertilizer—thus the importance of nano-encapsulation technology in improving the efficacy of PGPR. Hence, this review bridges the gap mentioned and summarizes the mechanism of PGPR as a biofertilizer for agricultural sustainability.

In Malaysia, about 0.5 million ha of acid sulfate soils are found scattered over the east, west, and Sabah and Sarawak regions that can potentially be cultivated with paddy. This type of soil is acidic and not immediately suitable for crop production unless improved by applying some amendments. Thus, the current study was carried out to investigate the effects of various types of soil amendments on the growth, yield, and physiological responses of rice grown in extremely acidic conditions using ground magnesium limestone (GML), basalt, biochar, and compost as soil amendments. The acid sulfate soil with a pH of 3.76 was obtained from a paddy field in Merbok, Kedah. The plant responses were evaluated based on agronomic, physiological, and yield performance. The compost-treated rice showed the best performance in all three criteria. Compost treatment increased the soil pH up to 6.25. Physiological performances such as chlorophyll, photosynthetic rate, and water use efficiency were higher after compost treatment, while transpiration and stomatal conductance showed the highest after GML treatment. It can be concluded that the addition of compost as a soil amendment can increase soil pH and create favorable soil conditions for rice cultivation in acid sulfate soil, leading to improved rice growth performance.

Plant growth promoting rhizobacteria (PGPR) are a group of bacteria that can enhance plant growth. In fact, PGPR are biologically unstable and the bacteria activity degrades over time due to environmental factors, survival rate in soils, the compatibility with the crop and the interaction ability with the indigenous microflora in soil. Therefore, the utilization of PGPR as plant growth promoter agent is a major challenge in the agricultural sectors because of their bioactivity degradation needs to be inhibited to maximize its function as a plant growth promoter. The application of delivery system based on encapsulation technology shows a promising technique to store and deliver PGPR. However, the task to find the appropriate PGPR encapsulation method is the most challenging for agricultural industry. In addition, the lack of knowledge on the action mechanism of encapsulated PGPR, physico-chemical properties and their survival in the environment are the many challenges need to be addressed. In the present review, the encapsulation technology of PGPR and its properties have been reviewed in detail. Moreover, the remaining technical challenges of encapsulation systems including insignificant stabilization of PGPR, instability of the environmental and difficulty of their preparation are also extensively discussed here.

The disposal of industrial paper mill sludge waste is a big issue and has a great importance all over the world. A study was conducted to determine the chemical properties of recycled paper mill sludge (RPMS) and assess its possibilities for land application. RPMS samples were collected from six different paper mills in Malaysia and analyzed for physical and chemical properties, heavy metals, polycyclic aromatic hydrocarbons, ¹³C-NMR spectra and for the presence of dioxins/furans. The RPMS was dewatered, sticky with a strong odour, an average moisture of 65.08%, pH 7.09, cation exchange capacity (CEC) 14.43 cmol ₍₊₎ kg^–1, N 1.45, P 0.18, K 0.12, Ca 0.82, Mg 0.73, Na 0.76 and Al, 1.38%. The polycyclic aromatic hydrocarbons (PAHs) and heavy metals levels were below the standard Class 2 limits. The dioxin and furan were in below the standard concentration of Class 1. The most prominent peak in the ¹³C-NMR spectra of RPMS was centered at 31 ppm, proving the presence of methylene (-CH₂) groups in long aliphatic chains, with lipids and proteins. The signal at 89 ppm and highly shielded shoulder at 83 ppm were due to presence of cellulose carbon C-4, and the peak at 63 and 65 ppm was due to the cellulose carbon spectrum. The RPMS therefore contains significant amount of nutrients with safe levels of heavy metals and PAHs for environment and can be used as a fertilizer and soil amendment for land application.