Dissolved nitrogen exports from watersheds with contrasting land uses in sub-humid cool-temperate climates

Knowledge is limited in responses of watershed dissolved nitrogen (N) export to different land uses in Baijiang soil area under sub-humid cool-temperate climates. This study provided comparisons of three adjacent sub-watersheds (SWs) with contrasting land uses (SW 1: mixed land uses of woodland, dry cropland and residen-tial area; SW 2: dry cropland dominated; and SW 3: paddy field dominated) after agricultural land use conversion (from woodland to dry cropland at SW 1 and SW 2 and from dry cropland to paddy field at SW 3) at the Abujiao River watershed, NE China. Exports of total dissolved N (TN), nitrate N (NO3-N) and ammonium N (NH4-N) in base and storm flows during the pre-growing, growing and post-growing seasons were investigated. Water discharge at the sub-watersheds mainly occurred as base flow with mean base flow index ranging within 0.62 - 0.69, while base flow and storm flow N exports were comparable. Across sub-watersheds, TN, NO3-N and NH4-N exported equally through base flow (1.36 - 3.16, 0.56 - 1.69 and 0.24 - 0.63 kg km(-2) d-1, respectively) and storm flow (1.42 - 2.95, 0.54 - 1.66 and 0.42 - 0.63 kg km-2 d-1, respectively). SW 3 presented greater N export than SW 1 and SW 2 through base flow in the pre-growing (3.49 - 3.55, 3.21 - 4.30 and 3.27 - 4.47 times for TN, NO3-N and NH4-N, respectively) and growing seasons (1.95 - 3.32, 2.34 - 2.62 and 1.94 - 2.46 times for TN, NO3-N and NH4-N, respectively) and through storm flow in the pre-growing season (2.73 - 3.97, 2.59 - 4.02 and 3.38 - 4.00 times for TN, NO3-N and NH4-N, respectively). No significant difference in N export were found between SW 1 and SW 2. This study suggested special emphasis on the increased export of dissolved N through base flow and storm flow after dry croplands were converted to paddy fields.