Modeling the transformation dynamics of organic and inorganic nitrogen in rivers

Abstract

This study investigated the transformation of organic and inorganic nitrogen in river water mediated by microorganisms through controlled laboratory experiments. Water samples were incubated in darkness for 20 days, and concentrations of particulate organic nitrogen (PON), dissolved organic nitrogen (DON), ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻) were measured. Both labile and recalcitrant fractions of PON and DON exhibited substantial variability among rivers and even across sampling sites within the same river. Decomposition rate coefficients for total organic nitrogen (TON), PON, labile PON (LPON), DON, labile DON (LDON), NH₄⁺, and NO₂⁻ were estimated using three modeling approaches. Data from five Bangladeshi rivers fit well with differential equations based on exponential decay models, yielding an average kTON of 0.025 day⁻¹. In Model 2 (PON → DON → NH₄⁺ → NO₂⁻ → NO₃⁻), mean decomposition rates were 0.028 day⁻¹ for PON, 0.187 day⁻¹ for DON, 0.492 day⁻¹ for NH₄⁺, and 0.830 day⁻¹ for NO₂⁻. In Model 3 (LPON → LDON → NH₄⁺ → NO₂⁻ → NO₃⁻), rates were 0.091 day⁻¹ for LPON, 0.451 day⁻¹ for LDON, 0.648 day⁻¹ for NH₄⁺, and 0.950 day⁻¹ for NO₂⁻. These decomposition coefficients for nitrogen fractions provide valuable parameters for river water quality modeling and nutrient cycling assessments.