Kappaphycus alvarezii-derived biostimulant-coated urea is associated with enhanced plant growth, nitrogen-related processes, and shift in rhizospheric soil microbial diversity

Abstract

Excessive use of inorganic fertilizers is associated with declining soil health, reduced microbial diversity, and impaired water quality. This study evaluated LBS27, a novel Kappaphycus alvarezii -derived biostimulant, as a fertilizer-coating solution. In a replicated greenhouse pot experiment, we assessed the effects of LBS27-coated urea (LBS27-CU) on maize growth, plant nitrogen (N) content, and rhizosphere microbial communities during vegetative growth. Compared with uncoated urea, LBS27-CU significantly increased plant height, stem girth, leaf area, and biomass, as well as total flavonoid and phenolic contents. LBS27-CU also increased the relative abundance of plant-growth-promoting rhizosphere taxa, particularly Actinobacteria ( Rhodococcus ) and a nitrogen-fixing Rhizobium-clade genus. FAPROTAX-predicted functional profiles further indicated greater representation of bacteria associated with nitrogen fixation, nitrification, and ureolysis. These findings were consistent with soil microcosm assays, in which LBS27-CU degraded significantly faster than uncoated urea and showed greater early ammonium release. Despite this enhanced ureolytic potential, LBS27-CU reduced measured rhizosphere urease activity by 61% in the planted system, a paradox that our data suggest reflects accelerated plant nitrogen uptake and depletion of the ammonium substrate pool rather than direct enzymatic inhibition. LBS27-CU also increased rhizosphere β-glucosaminidase, arylsulfatase, and phosphatase activities, and upregulated root expression of the nitrogen transporter genes NPF6.6, NPF6.8, AMT1.1a, and AMT1.3. These changes were accompanied by higher glutamate dehydrogenase activity, total amino acid content and total tissue nitrogen content, indicating enhanced nitrogen assimilation. Collectively, these findings support LBS27-CU as a fertilizer-coating strategy that acts through a plant-dependent mechanism linking soil nitrogen cycling with accelerated nitrogen uptake, and plant growth.