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Nutrient losses from sloping farmland in karst areas lead to the decline in land productivity and nonpoint source pollution. A specially tailored steel channel with an adjustable slope and underground hole fissures was used to simulate the microenvironment of the "dual structure" of the surface and underground of sloping farmland in a karst area. The artificial rainfall simulation method was used to explore the surface and underground runoff characteristics and nutrient losses from sloping farmland under different rainfall intensities. The effect of rainfall intensity on the nutrient loss of farmland on karst sloping land was clarified. The results showed that the surface was the main route of runoff and nutrient loss during the rainy season on sloping farmland in karst areas. The influence of rainfall intensity on the nutrients in surface runoff was more substantial than that on underground runoff nutrients. Nutrient loss was more likely to occur underground than on the surface. The losses of total nitrogen, total phosphorus, and total potassium in surface and underground runoff initially increased and then gradually stabilized with the extension of rainfall duration and increased with increasing rainfall intensity and the amount of nutrient runoff. The output of nutrients through surface runoff accounted for a high proportion of the total, and underground runoff was responsible for a low proportion. Although the amount of nutrients output by underground runoff was small, it could directly cause groundwater pollution. The research results provide a theoretical reference for controlling land source pollution from sloping farming in karst areas.

Nitrogen loss in karst sloping farmland will lead to declining land productivity and environmental pollution, in which the nitrogen loss through underground pore fissures will directly lead to groundwater pollution. The characteristics of total nitrogen (TN) production were studied by simulating the “dual structure” microenvironment of sloping farmland in a karst region using an artificial rainfall simulation method. The results show that rainfall was the main driving factor of TN loss in karst sloping farmland. TN was mainly lost through underground pore fissures when the rainfall intensity was ≤ 30 mm · h−1. TN was lost at the surface and underground when the rainfall intensity was ≥ 50 mm · h−1, TN loss on the surface accounted for a large proportion, and the surface flow was the main carrier of TN loss. The TN loss underground is easily ignored because it is hidden underground. Therefore, TN loss belowground in karst sloping farmland should receive increased attention. It would be interesting to explore the influences of connectivity and type of underground pore fissure system on TN loss in karst sloping farmland. The prevention and control of TN loss in karst sloping farmland should be considered both at the surface and underground. Reducing the formation of slope flows and slowing rainwater filtration by increasing slope vegetation coverage can be considered to reduce TN loss. The results of this study provide a theoretical reference for agricultural non-point source pollution control in a karst region.