Rhus typhina L. triggered greater allelopathic effects than Koelreuteria paniculata Laxm under ammonium fertilization

Most of the allelopathic studies have concentrated on the allelopathic effects of invasive herbaceous plants on the natives, particularly on seed germination and seedling growth (Sge&Sgr), but have ignored the allelopathic effects of invasive trees and the differences in the allelopathic effects of invasive plants (particularly invasive trees) and the natives. Nevertheless, the variations in the ratio of different nitrogen (N) forms in N fertilization can change the differences in the allelopathic effects of the invasive plants and the natives. Thus, it is important to evaluate the differences in the allelopathic effects of invasive plants (particularly invasive trees) and the natives under N fertilization, particularly with different N forms. However, the recent process in this area is limited. This study aimed to estimate the differences in the allelopathic effects (using aqueous leaf extracts) of one of the most destructive invasive trees in China (Rhus typhina L.) and the native tree (Koelreuteria paniculata Laxm) on Sge&Sgr of the horticultural species Lactuca sativa L. under N fertilization with different forms (i.e., N-NH4, N-NO3, urea, and a mixture of the three N forms at a 1:1:1 ratio). The two deciduous Sapindales trees generated significantly allelopathic effects on Sge&Sgr of L. sativa. R. typhina created stronger allelopathic effects on leaf photosynthetic area, plant growth competitiveness, and plant water content of L. sativa than K. paniculate. N solution can intensify the allelopathic effects of the two deciduous Sapindales trees on seedling growth of L. sativa. The allelopathic effects of R. typhina on seedling growth of L. sativa were significantly greater than those of K. paniculata under N-NH4. Thus, the growth competitiveness of R. typhina may be higher than that of K. paniculata under N-NH4 via the stronger allelopathic effects on adjacent species.