Nitrogen Transformation Kinetics of β-Glucan and Nucleotide Supplementation in a Zero-Water-Exchange Aquaculture System

Nitrogen is a major challenge in zero-water-exchange (ZWE) aquaculture systems. This study examined the effects of dietary β-glucan and nucleotides on nitrogen transformation dynamics, oxidative stress responses, and environmental stability in Nile tilapia (Oreochromis niloticus). A 42-day feeding trial was conducted using diets supplemented with β-glucan (βg1 = 1%, βg2 = 2%, βg3 = 3%) and nucleotides (Nt1 = 1%, Nt2 = 2%, Nt3 = 3%). Final fish weight increased from 21.57 g in the control (Ct) to 28.32 g in Nt3. Total nitrogen (TN) derived from cumulative feed input reached 264.08 mg/L in Ct, whereas nucleotide treatments showed lower accumulation. NH4+ peaked between days 10 and 17, reaching 12.38 mg/L in Nt3, while NO2− peaked at 0.240 mg/L in Nt1 and declined to 0.03–0.07 mg/L by day 42. NO3− accumulated continuously to 31.74–51.47 mg/L, with higher concentrations generally observed in nucleotide treatments. Kinetic modeling yielded higher model-derived apparent nitrogen transfer coefficient (ka) in β-glucan treatments (1.287–1.690/day) and higher model-derived apparent nitrite-to-nitrate conversion coefficients (k2) in nucleotide treatments (4.994–5.748/day). Nucleotide supplementation favored apparent nitrite-to-nitrate conversion, whereas β-glucan was associated with greater apparent transfer of ammonia-derived nitrogen toward the modeled residual nitrogen pool.