Nitrate leaching losses mitigated with intercropping of deep-rooted and shallow-rooted plants

Abstract

Purpose: Nitrate (NO₃¯) leaching has been recognized as a worldwide problem, and NO₃¯ is a major pollutant in ground and surface waters. The gradual leaching of residual NO₃¯ into soil layers below the root zone is the main way of nitrogen (N) loss in cropping systems. The interception of NO₃¯ by subsurface roots is crucial for crops or vegetables to utilize N. However, few studies were focused on the mechanisms of NO₃¯ leaching under intercropping of deep-rooted and shallow-rooted plants. Materials and methods: In a greenhouse production system, we selected deep-rooted and shallow-rooted plants for single-season intercropping planting experiments (MP, monoculture of pepper; IPA, intercropping of pepper and alfalfa; IPM, intercropping of pepper and maize). Plants were grown in core drainage lysimeters for 4 months, and leachate was collected by soil solution samplers. The results revealed N transformations between monoculture and intercropping systems by comparing NO₃¯ leaching, N use efficiency, and microbes involved in nitrification. Results and discussion: NO₃¯ accumulation mainly occurred in 25–40-cm soil layer, which was affected by root morphology. Compared with monoculture, intercropping reduced NO₃¯ leaching, in particular, pepper/maize intercropping significantly reduced NO₃¯ leaching losses. In deep soil layers, roots of alfalfa and maize were more developed than those of the other crops, which made this corresponding intercropping system more effective in absorbing N in the subsoil. N uptakes of pepper/alfalfa intercropping and pepper/maize intercropping were 19% and 28% higher than that from the monoculture, respectively. Ammonia oxidizing bacteria (AOB) were significantly stimulated with planting especial in the topsoil, while ammonia oxidizing archaea (AOA) decreased with planting along the soil profile. These results indicated that lush plant roots immobilized N from the deep soil and thus improved N use efficiency and reduced NO₃¯ leaching in the deep-rooted and shallow-rooted intercropping system. Conclusions: Intercropping of shallow-rooted pepper with deep-rooted alfalfa can enhance root nutrient absorption in deep soil layers, increasing N use efficiency and thus reducing NO₃¯ leaching. The nitrification process in the intercropped soil was mainly regulated by AOB.