Effects of contour reverse-slope terrace on nitrogen/phosphorus stoichiometric ratios and loss fluxes of sloping cropland in the upper reaches of Chishui River Basin

Objective To investigate the effect of deploying contour reverse-slope terrace (CRT) measure on the reduction of runoff and nitrogen/phosphorus loss from sloping cropland in the upper reaches of Chishui River Basin of Yunnan Province, as well as the effect on nitrogen/phosphorus stoichiometric characteristics and flux loss.

Method CRT were deployed on sloping cropland with three different crops (kiwifruit, corn, and corn intercropped with cabbage, respectively) in the upper reaches of Chishui River Basin, and the original sloping cropland was used as the control (CK). A field observation study of rainfall and runoff was conducted during the 2024 rainy season to analyze the change characteristics of the loss concentration, loss amount, stoichiometric ratios, and flux losses of each form of nitrogen/phosphorus in runoff under the deployment of CRT measure in the upper reaches of Chishui River Basin, based on the eight natural rainfall-produced flow events.

Result Runoff loss of sloping cropland was mainly affected by rainfall amount, and the deployment of CRT reduced runoff loss by 55.48%. The CRT effectively intercepted and reduced the loss concentration and loss amount of runoff total nitrogen (TN), nitrate nitrogen (\mathrmNO_3^- -N), ammonium nitrogen (\mathrmNH_4^+ -N), total phosphorus (TP) and orthophosphate phosphorus (\mathrmPO_4^3- -P), with the most significant reduction effects observed during periods of moderate rainfall amount. The nitrogen/phosphorus stoichiometric ratio of runoff from sloping cropland in the upper reaches of Chishui River basin was in nitrogen limitation, while the nitrogen/phosphorus stoichiometric ratio of CRT was significantly higher than that of CK, indicating that deployment of CRT could effectively alleviate nitrogen limitation. Runoff was the key factor driving the migration of each form of nitrogen/phosphorus, and CRT significantly reduced the loss flux of nitrogen/phosphorus from each form by reducing the runoff volume. The flux loss of TN was 2.52−5.55 times that of TP, so priority attention should be given to the control of nitrogen loss in the prevention and control of surface pollution in sloping cropland.

Conclusion CRT measure not only significantly intercepts runoff and nitrogen/phosphorus loss of sloping cropland, but also effectively alleviates nitrogen limitation in regional runoff, blocks and controls the output of surface source pollutants, and then can significantly enhance the soil and water conservation capacity of sloping cropland.