Non-point source pollution has been one severe environmental problem over world and was concerned and studied actively. Developed agriculture brought on the loss of fertilizer and pesticide which include much nitrogen and phosphorus. These contaminations led to the eutrophication of water body. However, there are several main factors affecting nitrogen loss such as rainfall intensity, slope, coverage ratio and so on. The factors effected nitrogen loss mainly by influencing runoff, in other words, the polluting ways of nitrogen loss to water would be reflected by nitrogen loss trend in runoff under different conditions.
In order to simulate the nitrogen loss characteristics during crops natural growing processes, according to given conditions such as rainfall intensity, slope and coverage, the experiments of adopting artificial simulated rainfall were designed. The experiments were conducted from August in 2005 to January in 2006 and carried in WSBRZ type automatic control glasshouse in Zhejiang University. There were two wooden runoff troughs with slopes 14 and 21, and they were filled with soil which was taken from Qingshan lake valley in Lin’an city. Cabbages were planted to provide different coverages based on the experiment design. In each rainfall process, 15 runoff samples were collected separately per minute or every two minutes in order. Following on, the runoff samples of 15 were analyzed in the lab, and the concentrations of TN, NH₄⁺-N and NO₃^--N were respectively measured in 24 hours.
Artificial simulated rainfalls of 15 were conducted during the whole experiment. According to the experiment purpose, 6 rainfalls were selected from 15 simulated rainfalls and separated into three groups, while each group has two rainfalls with same slope and coverage and different rainfall intensities. Then, experiment data were analyzed in two ways. Firstly, by charting the curve of nitrogen loss in each group with increasing time, the gradients and numerical values of curves on the same chart were compared with each other respectively, to find out the differences between effects of different rainfall intensities on the nitrogen loss. Secondly, the changing trends of TN, NH₄⁺-N and NO₃^--N concentration with increasing time were compared with each other, in order to get the relations among the concentration, time, and rainfall intensity. It could be inferred that NO₃^--N with bigger percentage was the main component in dissolved nitrogen distinctly.
The study was based on analyzing the experiment data from artificial simulated rainfalls on three indices such as TN, NH₄⁺-N and NO₃^--N. Results in the study were summarized as following: ① the changes of TN concentration in runoff were inapparent with increasing time. However, the total loss content of TN increased evidently because of increasing runoff. Some specifically rules of NH₄⁺-N and NO₃^--N concentration were discovered in loss with increasing time. The rules were that in early stage the concentration was high in runoff, in middle stage the concentration reduced or kept stabilization, and in the end the concentration reascended. ② there were two forming causes of runoff, correspondingly, there were two types of nitrogen loss. One type of nitrogen loss contained more dissolved nitrogen and the other type contained more undissolved nitrogen. Dissolved nitrogen was the main form in two types of nitrogen loss, while NO₃^--N was the main component in dissolved nitrogen. The loss percentage of undissolved nitrogen increased with increasing time under slope <15 degrees. The loss percentages of undissolved nitrogen increasing suggested that re-pollution of nitrogen loss was very important and should be paid more attention to. Re-pollution of nitrogen loss was mainly caused by stronger rainfall intensities. The runoff patterns, which influenced the percentage of undissolved nitrogen loss, changed with different rainfall intensities. However, the loss of undissolved nitrogen was the major reason to non-point source re-pollution.