Differential Urea Application Alters Soil Properties and Nitrogen Dynamics in Acidic Soil

Nitrogen fertilization with urea profoundly alters soil acidity, mineral nitrogen dynamics and microbial activity, particularly in acidic soils where nitrification-mediated proton release influences fertility and biological. The selected urea concentrations were used under controlled laboratory conditions to evaluate short-term soil and microbial responses rather than field-equivalent application rates. The present study evaluated effects of three urea levels (0, 0.1, and 0.2%) on physicochemical properties, mineral nitrogen pools, microbial populations, enzyme activities and early plant growth responses in acidic soil. Urea increased soil pH from 5.51 (control) to 5.59 and 5.71, electrical conductivity from 0.22 to 0.36 and 0.44 dS m⁻¹, and cation exchange capacity from 6.29 to 7.12 and 7.99 cmol kg⁻¹. Mineral N pools rose progressively, with ammonium increasing from 12.70 to 18.72 and 23.47 mg kg⁻¹ and nitrate from 21.92 to 28.75 and 33.47 mg kg⁻¹. Microbial indicators showed directional responses: bacterial counts increased from 40.25 to 44.50 while saprophytic fungi declined from 30.00 to 27.75. Urease, dehydrogenase, and FDA hydrolysis increased with urea dose, indicating enhanced microbial activity. Shoot and root lengths improved significantly, suggesting enhanced early plant establishment. These findings demonstrate that moderate urea addition stimulates biological activity and N availability but the nitrate increase signals potential long-term acidification risk.