Purpose

Rice-paddy-dominated watersheds in eastern China are intensively cultivated and lands with two crops receive as much as 550–600 kg ha^–1 year^–1 of nitrogen (N) mainly through the addition of N-based fertilizers. However stream N concentrations have been found to be relatively low. Waterways in the watersheds are assumed to be effective “sinks” for N minimizing its downstream movement. We directly measured net sediment denitrification rates in three types of waterways (ponds streams-rivers and a reservoir) and determined the key factors that control net sediment denitrification. Such information is essential for evaluating the impact of the agricultural N cycle on the quality of surface water.

Materials and methods

The pond–stream–reservoir continuum was sampled every 2 months at nine sites in an agricultural watershed between November 2010 and December 2011. Net sediment N₂ fluxes-net sediment denitrification rates were determined by membrane inlet mass spectrometry and the N₂-Ar technique. A suite of parameters known to influence denitrification were also measured.

Results and discussion

Net denitrification rates ranged between 28.2 ± 18.2 and 674.3 ± 314.5 μmol N₂–N m^–2 h^–1 for the streams 23.7 ± 23.9 and 121.2 ± 38.7 μmol N₂–N m^–2 h^–1 for the ponds and 41.8 ± 17.7 and 239.3 ± 49.8 μmol N₂–N m^–2 h^–1 for the reservoir. The mean net denitrification rate of the stream sites (173.2 ± 248.4 μmol N₂–N m^–2 h^–1) was significantly higher (p < 0.001) than that of the pond sites (48.3 ± 44.5 μmol N₂–N m^–2 h^–1) and the three types of waterways all had significantly higher (p  <&thin