Isotope identification of nitrogen removal in artificial ponds across a hilly agricultural watershed of southwestern China

Artificial ponds can act as crucial buffers against nitrogen (N) pollutants within agricultural landscapes. Despite the recognized dominance of biogeochemical N removal for N retention by artificial ponds, the mechanisms underlying the coupling of nitrification and denitrification remain little studied. Here, we present an isotope study exploring the N retention patterns in both water and sediments from several artificial ponds across a hilly watershed in southwestern China. We found distinct N retention by the study ponds showing low levels of NO3− in water (0.5 ± 0.3 mg N L–1), but high levels of NH4+ (24.9 ± 7.5 mg N kg–1) and organic N (ON, 0.3% ± 0.1%) in sediments. Isotope signatures of NO3− of pond water (δ15NNO3−, –8.5‰–31.9‰; δ18ONO3−, 2.7‰–18.6‰) show considerable N inputs from N fertilizer, manure, and sewage, confirming the pivotal role of artificial ponds as effective mitigators of anthropogenic pollutants within the watershed. Combining N isotopes of ON, NH4+, and NO3− in sediments and isotope fractionation effects, we estimated the proportional progress of nitrification (fnit, 6.9% ± 4.4%) and denitrification (fden, 48.7% ± 4.1%). The significantly positive correlation between fden and fnit indicated tightly coupled nitrification and denitrification during the N removal, which also contributed to the high NH4+/NO3− ratios detected in sediments. Across those artificial ponds, variations in surface area and water depth associated with the hilly topography could affect C/N ratios in the sediment, thereby governing the progression of both nitrification and denitrification. Therefore, the spatiotemporal heterogeneity of N retention by artificial ponds from that region necessitates further attention, especially in establishing the connection between hydrogeochemical N nutrient flow and biogeochemical N turnover in the dispersed water bodies. Our results pinpoint the importance of the N sink function shown by artificial ponds in the hilly watersheds of southwestern China, which serve as functional units in buffering regional N pollutants from agricultural runoff.