Deterministic assembly processes of sediment bacterial communities regulate the effect of nitrogen input on water quality

Abstract

The leaching of nitrogen from agricultural fields into rivers substantially impacts the diversity, composition, and function of sediment microbial communities. However, how elevated nitrogen levels affect the assembly processes of these communities and, in turn, influence water quality remains lacking. This study decodes these causal pathways through a microcosm experiment that simulates nitrogen input using urea, focusing on the assembly mechanisms and the subsequent impact of the reassembled community on water quality. The results demonstrated that nitrogen input shifted the bacterial community assembly from stochastic to deterministic dominance (normalized stochasticity ratio <50%), forming a nested structure with a nestedness-resultant dissimilarity index of 0.02 (compared to 0.01 for the control), whereas fungi were less affected. The reassembled dominant bacterial community included anaerobic Bacillota and Bacteroidota. Mantel analysis revealed that Abditibacteriia, Fimbrifmonadia, and Desulfurellia were the core drivers of water quality changes and black-odorous substances. Structural equation modeling indicated that nitrogen input indirectly reduced dissolved oxygen levels (from 7.10 ± 0.01 mg L⁻¹ to 0.65 ± 0.05 mg L⁻¹) and increased chemical oxygen demand (from 4.81 ± 0.00 mg L⁻¹ to 159.45 ± 9.72 mg L⁻¹) and acid-volatile sulfide levels (from 169.22 ± 0.01 mg kg⁻¹ to 363.13 ± 7.30 mg kg⁻¹) by enriching Desulfurellia. Nitrogen input affected ammonium-nitrogen production (from 3.88 ± 0.03 mg L⁻¹ to 98.72 ± 3.93 mg L⁻¹) through direct chemical action and indirect biological action, while nitrate-nitrogen generation (from 1.55 ± 0.05 mg L⁻¹ to 15.35 ± 1.32 mg L⁻¹) was indirectly regulated by enriching Abditibacteriia, enhancing the potential for water self-purification. The findings of the study confirm that the reassembled microbial community driven by nitrogen input further regulated water quality, providing a theoretical basis for aquatic ecosystem restoration.