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

Abstract

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 residential 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.