Strong variability in nitrogen (N) removal rates in typical agricultural pond from hilly catchment: Evidence from diel and monthly dissolved N2 measurement

Ponds, depressional submerged landscapes that can store and process nitrogen (N)-enriched runoff from surrounding uplands, are recognized as biogeochemical hotspots for N removal. Despite their strong potential for N removal, information is limited concerning the specifics of their changing nature. Here, we investigated the dynamics of N removal rate in a typical agricultural pond from a hilly catchment, by unraveling the monthly and diel patterns of N₂ concentrations and fluxes. Our observations showed that the N pollution in the pond was severe. Its averaged total N level reached 3.6 mg L⁻¹, of which ∼72% consisted of NO₃–N. Meanwhile, the water samples were supersaturated with N₂, demonstrating N removal occurring in the pond. Further estimates of net N₂ fluxes indicated that N removal rates exhibited obvious day-and-night and monthly differences. On the diel scale, N removal rates exhibited a distinct diurnal cycle, with nocturnal rates around 20% higher than during the day. Such a diel pattern can be mainly explained by the fluctuation in DO levels, showing that at nighttime when photosynthesis is absent, low DO environments are conducive to N removal. On a monthly scale, the monthly rates ranged from 0.02 to 0.49 mmol N₂ m⁻² h⁻¹ (mean: 0.23 mmol N₂ m⁻² h⁻¹), with generally higher removal rates in the warmer and concurrently rainy months (June–September). N levels in the pond were the corresponding primary explanatory variables. Assembled data from both monthly and hourly scales provided a more complete picture of the changing nature of N removal in ponds. Future work should carefully consider the effects of altered environmental conditions triggered by hydrological events to better reveal the control mechanisms behind the time-immediate N removal from lowland ponds.