In the midst of the major soil degradation and erosion faced by tropical ecosystems, rehabilitated forests are being established to avoid the further deterioration of forest lands. In this context, cellulolytic, nitrogen-fixing (N-fixing), phosphate-solubilizing bacteria are very important functional groups in regulating the elemental cycle and plant nutrition, hence replenishing the nutrient content in forest soils. As is the case for other potential plant growth-promoting (PGP) rhizobacteria, these functional bacteria could have cross-functional abilities or beneficial traits that are essential for plants and can improve their growth. This study was conducted to isolate, identify, and characterize selected PGP properties of these three functional groups of bacteria from tropical rehabilitated forest soils at Universiti Putra Malaysia Bintulu Sarawak Campus, Malaysia. The bacteria were isolated based on their colonial growth on respective functional media, identified using both molecular and selected biochemical properties, and were assessed for their functional quantitative activities as well as PGP properties based on seed germination tests and indole-3-acetic acid (IAA) production. Out of the 15 identified bacterial isolates that exhibited beneficial phenotypic traits, a third belong to the genus Burkholderia and a fifth to Stenotrophomonas sp., with both genera consisting of members from two different functional groups. The results of the experiments confirm the multiple PGP traits of some selected bacterial isolates based on their respective high functional activities, root and shoot lengths, and seedling vigor improvements when bacterized on mung bean seeds, as well as significant IAA production. The results of this study suggest that these functional bacterial strains could potentially be included in bio-fertilizer formulations for crop growth on acid soils.

Plant growth-promoting rhizobacteria (PGPR), which include isolates from genera Paraburkholderia, Burkholderia and Serratia, have received attention due to their numerous plant growth-promoting mechanisms such as their ability to solubilize insoluble phosphates and nitrogen-fixation. However, there is a dearth of information on the potential plant growth-promoting effects of these three groups of bacteria on non-legumes such as maize. This study determined the influences of the aforementioned strains on soil properties, maize growth, nutrient uptake and nutrient use efficiency. A pot trial using maize as a test crop was done using a randomized complete block design with 7 treatments each replicated 7 times. The treatments used in this study were: Control (no fertilizer), chemical fertilizer (CF), organic-chemical fertilizers combination without inoculum (OCF) and with inocula consisting of single strains [cellulolytic bacteria (TC), organic fertilizer and chemical fertilizer with N-fixing bacteria (TN), organic fertilizer and chemical fertilizer with P-solubilizing bacteria (TP)) and three-strain inocula (TCNP), respectively. The variables measured included plant growth and nutrient content, soil nutrient content and functional rhizospheric bacterial populations. Paraburkholderia nodosa NB1 and Burkholderia cepacia PB3 showed comparable effects on maize biomass and also improved N and P use efficiencies when compared to full chemical fertilization. Nitrogen-fixing rhizobacteria had a positive effect on above-ground biomass of maize. Paraburkholderia nodosa NB1 improved soil total C and organic matter contents, besides being the only bacterial treatment that improved K use efficiency compared to OCF. The results suggest that P. nodosa NB1 and B. cepacia PB3 have potential usage in bio-fertilizers. In contrast, treatments with Serratia nematodiphila C46d and consortium strains showed poorer maize nutrient uptake and use efficiency than the other single strain treatments. Bacterial treatments generally showed comparable or higher overall N and P use efficiencies than full chemical fertilization. These findings suggest that at least half the amounts of N and P fertilizers could be reduced through the use of combined fertilization together with beneficial bacteria.

21In the midst of major soil degradation and erosion faced by tropical ecosystems, 22 rehabilitated forests are established to avoid further deterioration of forest land. In this context, 23 cellulolytic, nitrogen-fixing (N-fixing), and phosphate-solubilizing bacteria are very important 24 functional groups in regulating the elemental cycle and plant nutrition, hence replenishing the 25 nutrient content in forest soil. As other potential plant growth-promoting (PGP) rhizobacteria, 26 these functional bacteria could have cross-functional abilities or beneficial traits that are 27 essential for plants and improve their growths. This study was conducted to isolate, identify, and 28 characterize selected PGP properties of these 3 functional groups of bacteria from tropical 29 rehabilitated forest soils at Universiti Putra Malaysia Bintulu Sarawak Campus, Malaysia. 30 Isolated cellulolytic, N-fixing and phosphate-solubilizing bacteria were characterized for 31 respective functional activities, biochemical properties, molecularly identified, and assessed for 32 PGP assays based on seed germination and indole-3-acetic acid (IAA) production. Out of 15 33 identified bacterial isolates exhibiting beneficial phenotypic traits, a third belong to genus 34 Burkholderia and a fifth to Stenotrophomonas sp. with both genera consisting of members from 35 two different functional groups. Among the tested bacterial strains, isolate Serratia 36 nematodiphila C46d, Burkholderia nodosa NB1, and Burkholderia cepacia PC8 showed 37 outstanding cellulase, N-fixing, and phosphate-solubilizing activities, respectively. The results 38 of the experiments confirmed the multiple PGP traits of selected bacterial isolates based on 39 respective high functional activities, root, shoot lengths, and seedling vigour improvements 40 when bacterized on mung bean seeds, as well as presented some significant IAA productions. 41The results of this study indicated that these functional bacterial strains could potentially be 42 included in future biotechnological screenings to produce beneficial synergistic effects via their 43 versatile properties on improving soil fertility and possible crop growth stimulation. 44 Keywords: rehabilitated tropical forest soil; cellulolytic bacteria; N-fixing bacteria; phosphate-45 solubilizing bacteria; PGP traits 3 46 Introduction 47 Albeit vast in carbon and biodiversity, tropical ecosystems are facing major 48 deforestations, leading to widespread soil erosion which garnered much recent attention 49 [1-3]. As soil erosion aggravates, the soil loss overrides soil establishment, which 50 results in diminishing soil resources and the ecosystem supports they render [4-5], in 51 particularly manifested as low productivity and farming sustainability on the degraded 52 soils [6]. Planted forest or rehabilitation efforts are undertaken to avoid further 53 deterioration of forest land. 54 Bacteria has large biodiversity composition in soils, and thus, is major 55 participant of soil principal processes that regulate whole terrest...