Nitrogen fertilization and biostimulants in sugar beet (Beta vulgaris L.) production: a systematic review, meta-analysis, and nutrient management framework.

Sugar beet (Beta vulgaris L.) is a globally significant industrial crop, yet the interactive effects of nitrogen (N) fertilization and biostimulant application on yield and juice quality remain inadequately synthesized. Excessive N application suppresses sucrose content, elevates molassogenic impurities, and generates environmental externalities; however, no prior synthesis has coupled a systematic review with field-derived path analysis to jointly characterize the N × biostimulant system. The present study quantified the effects of N rate and biostimulant type on root yield, sucrose content, juice purity, and impurity concentrations through a PRISMA-compliant systematic review (68 eligible studies from 1,250 records; 50 with extractable data), random-effects meta-analysis, GRADE certainty grading, and a two-season split-plot field experiment (n = 72; Sakha, Egypt) analyzed by structural equation modelling (SEM). N rate positively predicted root yield (GRADE: MODERATE) but negatively affected sucrose content above 120 kg N ha⁻¹ (GRADE: LOW). All biostimulant outcomes were rated GRADE VERY LOW certainty due to high heterogeneity and publication bias. Path analysis confirmed potassium as the principal quality-limiting impurity and its co-accumulation with α-amino nitrogen (α-AN) under excess N. An optimal range of 90-120 kg N ha⁻¹ is supported by MODERATE-certainty evidence for Nile Delta clay-loam conditions. Biostimulant recommendations remain provisional pending pre-registered multi-location trials.