Assessing impacts of rainfall intensity and slope on dissolved and adsorbed nitrogen loss under bare loessial soil by simulated rainfalls

Nitrogen is of key importance to sustain modern agriculture but how to accurately quantify the loss forms of fertilizer-derived nitrogen has been a challenge for bare loessial soil. We employ 30 simulated rainfalls to evaluate effects of rainfall intensity (45, 60, 75, 90, 105 and 120 mm/h) and slope gradient (5°, 10°, 15°, 20° and 25°) on dissolved and adsorbed nitrogen loss. Here, we show that there was an overall increasing trend of runoff yield with increased rainfall intensity and also an overall increasing trend of sediment yield with increased slope gradient. TN loss at different slopes was more concentrated under rainfall intensities of 45 mm/h and 60 mm/h and more dispersed under rainfall intensities of 75-120 mm/h. t-Test results showed that there were no significant differences for 6 TN samples at each slope (p > 0.05) and 5 TN samples at each rainfall intensity (p > 0.05). All NO3−-N concentration had a slight declining trend at the beginning of runoff yield and then gradually tended to be gentle with the prolongation of rainfall duration. All NH4+-N concentration first showed a sharp decreasing trend and then relatively levelled off with the prolongation of rainfall duration, but it was different and unstable for different rainfall intensities, especially in the first 10 min. Adsorbed TN under bare loess soil occupied 7.0-82.0% of TN loss with an average of 58.6%, while dissolved TN accounted for 18.0-93.0% with an average of 41.4%. NO3−-N and NH4+-N concentrations respectively accounted for 11.8-73.2% and 1.5-13.3% of TN loss under all combinations of rainfall intensity and slope. There were significant differences in variation trends among NO3−-N, NH4+-N and TN loss load under various rainfall intensities and slopes, but they all presented an overall upward trend with increased rainfall intensity. Our results provide the underlying insights needed to guide nitrogen loss control in sloping land of the loess hilly region.