Abstract
ABSTRACT Enrichment with beneficial microorganisms may increase the benefits of organic compost. The aim of this study was to evaluate the enrichment of a mature compost with plant growth-promoting bacteria and arbuscular mycorrhizal fungus (Rhizophagus clarus), using brachiaria (Urochloa brizantha) as host plant, totaling seven treatments: control (compost with no bacteria, brachiaria or mycorrhizal fungus); compost + brachiaria; compost + brachiaria + mycorrhizal fungus; compost + brachiaria + mycorrhizal fungus + Azorhizobium sp.; compost + brachiaria + mycorrhizal fungus + Azoarcus sp.; compost + brachiaria + mycorrhizal fungus + Bacillus subtilis; and compost + brachiaria + mycorrhizal fungus + Azotobacter sp., in a completely randomized design with three replicates. Brachiaria shoot biomass, N and P concentrations, mycorrhizal colonization, and chemical characteristics of the compost were assessed five times over 183 days. B. subtilis and Azotobacter increased ammonium-N concentration in the compost in two and three sampling dates, respectively. In contrast, Azotobacter and Azoarcus decreased the concentrations of nitrate-N in at least one sampling. Despite high P availability in the compost (951-2927 mg kg-1), mycorrhizal colonization reached up to 53%. In a field trial with the produced composts, in a randomized block design with six repetitions, the composts with brachiaria doubled the mycorrhizal colonization of corn (Zea mays) and wheat (Triticum aestivum), independent of the growth-promoting bacteria and, depending on the associated bacteria, increased grain yields.
Highlights
Composting allows the biological stabilization of organic residues of different origins, which can be used as source of nutrients for crops (Andrade et al, 2018)
The aim of this study was to evaluate the enrichment of a mature organic compost with free-living N-fixing plant growth-promoting bacteria and arbuscular mycorrhizal fungi (AMF) using brachiaria (Urochloa brizantha) as host plant, follow the dynamics of nutrients in the compost during the enrichment process, and assess its effect on corn (Zea mays) and wheat (Triticum aestivum) mycorrhizal colonization, P and N nutritional status, and yield in the field
The colonization rate may have been slow in this study because of the high P availability in the compost at the beginning of Different letters in the columns indicate statistical differences among treatments by the Tukey’s test at p ≤ 0.05; AMF - Arbuscular mycorrhizal fungus; CV - Coefficient of variation; “-” - Not assessed the experiment (951 mg kg-1), which increased three times by the end of the experiment (2927 mg kg-1) owing to the mineralization of organic P (Andrade et al, 2018; Sousa et al, 2018)
Summary
Composting allows the biological stabilization of organic residues of different origins, which can be used as source of nutrients for crops (Andrade et al, 2018). Plant growth-promoting bacteria (PGPB) and arbuscular mycorrhizal fungi (AMF) may interact synergistically (Pereira et al, 2013) and promote plant growth by several mechanisms including P-solubilization, P-mineralization, biological nitrogen fixation (BNF), nutrient cycling, synthesis of siderophores, and phytohormones (Rodrigues et al, 2012). Some PGPB may perform free-living nitrogen fixation and enrich the compost with N (Sousa et al, 2018), whereas AMF may reach microsites with their external hyphae that are inaccessible to root hairs, increasing the uptake of low-mobility nutrients such as P (Brito et al, 2017). The tripartite interaction among the plant, AMF, and PGPB may result in a synergistic effect on plant performance (Pereira et al, 2013). Enriching organic compost with beneficial microorganisms may yield even better results than the commonly used composting process
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