Rapid Response of Runoff Carrying Nitrogen Loss to Extreme Rainfall in Gentle Slope Farmland in the Karst Area of SW China

Nitrogen loss is the main reason for land quality degradation and productivity decline and an important factor in groundwater pollution. Extreme rainfall has occurred frequently in Karst areas of southwest China in recent years. It is of great significance to study the response of soil nitrogen loss to extreme rainfall in Karst areas to prevent and treat land quality degradation and non-point source pollution. In this study, field monitoring and indoor artificial rainfall simulation were used to study the loss characteristics of total soil nitrogen (TN), ammonium (NH4+-N) nitrogen, and nitrate-nitrogen (NO3--N) in Karst bare slope farmland (slope angles of 5 degrees and 10 degrees) under extreme rainfall conditions. The results showed that: (1) Extreme rainfall (90 mm/h) increased the surface runoff, middle soil runoff, and underground runoff by 1.68 times, 1.16 times, and 1.43 times, respectively, compared with moderate rainfall (60 mm/h), so that nitrogen loss increased with runoff. (2) The loss of nitrate-nitrogen in surface, soil, and underground under extreme rainfall conditions was 223.99, 147.93, and 174.02% higher than that under moderate rainfall conditions, respectively; the nitrate losses were 203.78, 160.18, and 195.39% higher, respectively. Total nitrogen losses were 187.33, 115.45, and 138.68% higher, respectively. (3) The influencing factors of total soil nitrogen and nitrate-nitrogen loss in Karst slope farmland were slope > rainfall duration > rainfall intensity, while the influencing factors of ammonium nitrogen loss were rainfall duration > slope > rainfall intensity. Therefore, in controlling nitrogen loss in Karst slope farmland, changing slope degree and increasing farmland coverage may be useful measures to slow the nitrogen loss caused by extreme rainfall.