| 摘 要： |
Nitrate export from dry croplands through multiple hydrological pathways driven by rainfall is critical for watershed diffuse pollution control. The movement of nitrate across soil layers is affected by rainfall patterns and agricultural management. However, the mode of nitrate transport and potential sources under different agricultural practices remain elusive. This study was conducted in Baijaing soil with an albic layer at mid-high latitudes of Northeast China, aiming to characterize rainfall-driven nitrate export from dryland during two rainfall events (long-duration and low-intensity, R1; and short-duration and high-intensity, R2) under three agricultural practices (nitrogen fertilization, buffer zone and vegetation type). The constructed runoff plots were used to observe surface and subsurface runoff generation and for soil and water sample collection. Oxygen and nitrogen isotopes of nitrate were employed to identify the sources and transformation processes of nitrate in runoff. The surface runoff volume was comparable to subsurface runoff volume in R1, and shallow subsurface runoff accounted for 78 % of the total subsurface runoff. The surface runoff volume was higher than subsurface runoff volume (63 % vs. 47 %) in R2. Buffer zone did not alter the time and volumes of runoff generation, but suppressed its variability. Nitrate concentrations presented significant differences across rainfall patterns (9.37 vs. 5.30 mg L-1 in R1 and R2), fertilization rates (8.49 vs. 6.17 mg L-1 in fertilized and unfertilized soil), and vegetation types (8.12, 7.69 and 4.28 mg L-1 for maize, soybean and fallow, respectively). The delta O-18-NO3- values in runoff decreased from 14.94 parts per thousand to 2.58 parts per thousand with soil depth increased, while the delta N-15-NO(3)(-)values remained relatively stable (similar to 3.5 parts per thousand). With nitrogen fertilizer application, rainfall and nitrification of soil nitrogen were the predominant sources of nitrate in surface runoff, while nitrification of fertilizer-derived nitrogen predominantly contributed to nitrate in subsurface runoff. Without nitrogen fertilizer application, majority of nitrate in both surface runoff and subsurface runoff came from the nitrification of soil nitrogen. Compared to R1, R2 promoted loss of fertilizer-derived nitrate through subsurface runoff. |
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