Dynamic transformation and leaching processes of nitrogen in a karst agricultural soil under simulated rainfall conditions

Frequent exchange of surface water and groundwater in karst agricultural areas results in soil nutrient loss during rainfall and consequent deterioration of the aquatic environment. To understand nitrogen (N) transformation and leaching processes from karst soil during rainfall events, two typical N fertilizers were added to karst soil and consequently investigated the nitrogenous species using soil column experiments system. The contents of various N forms in the soil and leachate were analyzed, and the net nitrification and the N leaching rates were calculated. The results revealed that NH₄⁺ and NO₂⁻ accumulation and variation occurred primarily in shallow soils, indicating that shallow soils were the primary sites for ammonification and nitrification processes. The net nitrification rate in the soil was higher with application of urea (12.5 mg N kg⁻¹d⁻¹) than with manure (2.4 mg N kg⁻¹d⁻¹). Peak N content indicated that it took only 7 days for N to migrate from the topsoil to 90 cm depth of soil after applying urea during successive intense rainfall events. NO₃⁻ leaching was predominantly composed of N, followed by DON, with leaching ratios of 31.1 % for the urea column and only 1.7 % for the manure column. These results highlight that the rapid nitrification of urea application coupled with intensive rainfall accelerated nitrate leaching into subterraneous streams in karst hydrological system, supporting a significant contribution from agricultural fertilizer loss during rainfall in karst area.