Utilizing crops to control invasive weeds are regarded as an environmentally friendly approach to the management of invasive weeds in agro-ecosystems [6, 25]. At the same time, due to the high demand for food, it is important to develop weed control methods to sustain crop yield. N is essential for plant growth and development and is widely used to improve crop vigor and productivity in agro-ecosystems . Our previous studies found that rapeseed may be used to control littleseed canarygrass in agro-ecosystems . It was generally hypothesized that increasing N rates could improve rapeseed yield and enhance the economic benefits . Furthermore, we theorized that different types of N application, such as basal or top-dressing applications might influence responses of the crop and the weed as tested here. Crop and weed responses to N differ and are affected by competition, influencing the outcome of interactions between rapeseed and littleseed canarygrass . We found that increased topdressing N rates decreased rapeseed yield loss in mixed culture, but the yield losses in rapeseed were even more influenced by basal N levels. For basal N application rates of BN3 (150 kg·ha− 1), the yield loss of rape was more than 50%. However, the yield loss of rapeseed was less than 30% when for basal fertilizer rate BN2 (90 kg·ha− 1). Our results suggested that weed control efficiency of rapeseed was greatly affected by N application levels, with basal N levels playing a leading role. Although at higher basal rates rapeseed yield was seriously reduced, intermediate N application rates may reduce the yield loss of rapeseed and could be successfully manipulated in favor of rapeseed,
Competitiveness of crops plays an important role in determining the likelihood of success in the control of invasive weeds [4, 5, 21]. Although increased N levels can improve the competitiveness and plant growth of both crops and invasives [8, 9], the relative effects vary depending on the particular species and other conditions, and adaptation for N uptake and utilization are related to the evolutionary history of invasive species and the artificial selection of crops. .Our evaluation of competitiveness via seed production of littleseed canarygrass provided another view of the effectiveness of various treatments. Just as seen in the biomass measurements, rapeseed showed the highest control efficacy at intermediate basal N rates, while at the highest N rates littleseed canarygrass produced substantially higher seed numbers, resulting in a control efficacies in the 40% range, as compared to 67–80% at intermediate basal N rates, depending on accompanying top-dressing rates, with increased topdressing N rates improving the competitive ability of rapeseed. Thus the N level and type of fertilization must be regulated carefully to favor rapeseed over littleseed canarygrass.
As an essential nutrient element for plant growth, N is integral for plant photosynthesis . Higher rates of photosynthesis can lead to increased growth rates, biomass accumulation and overall production . Our current study showed that net photosynthesis rate (Pn) and chlorophyll content (Chl) of littleseed canarygrass were significantly higher than those of rapeseed (P < 0.05) in December. At that point in the season, the Pn and Chl of littleseed canarygrass increased with increased N whereas by comparison Pn and Chl of rapeseed did not respond as well to the higher fertilizer levels. However, in February, the Pn and Chl of rapeseed were significantly higher than those of littleseed canarygrass (P < 0.05). At this point, increasing basal N continued to produce higher littleseed canarygrass Pn rates, but only increased rapeseed rates over the transition from BN1 to BN2; however, topdressed N consistently increased Pn and Chl in both species. Thus, in the seedling stage, N demand of littleseed canarygrass was stronger than that of rapeseed, with the result that higher N levels did not increase rapeseed productivity as much as the invasive plant littleseed canarygrass. However, by the reproductive stage, increasing topdressing N improved did improve the productivity of rapeseed.
Differences in nutrient requirements of plants at different growth stages is a common phenomenon . If carefully matched to N demands at various plant growth stages, fertilizer availability could also be manipulated to provide ecological management of invasive alien plants. In our study we found that the net photosynthesis rate (Pn) of rape at seedling stage is weaker than that of littleseed canarygrass, while it is stronger at propagation stage. Our finding of a relatively low net photosynthesis rate (Pn) of rapeseed at the seedling stage, the efficacy of rapeseed in controlling littleseed canarygrass could be improved by timing the fertilizer application appropriately. By using fertilization strategically, e.g., by utilizing topdressing methods, N could be applied to deliberately to improve the competitive ability of rapeseed over invasive plants like littleseed canarygrass. Control indices are useful to developing clear measures for evaluating invasive plant control efficacy . Our work was the first study to examine rapeseed yield and littleseed canarygrass weed seed number to evaluate the effect of N regime on their competitive relationship. Our measure of invasive plant control efficacy involved pitting yield of the crop (rapeseed) against the seed production of the weed (littleseed canarygrass), where yield is the measure of crop success and weed seed is essential for the establishment and spread of weed populations. Also key to our approach was showing how the two plants competed at different densities using a De Wit replacement series experiments  using biomass to develop an index to evaluate plant interspecific competition  as well as yield measures. The data from the De Wit replacement series enabled us to calculate competitive balance, which revealed that interspecific competition had a large effect on both species, larger than the effects of intraspecific competition in both cases. It was also clear that top-dressing N application favors rapeseed over littleseed canaraygrass, whereas basal N applications only favor rapeseed to an intermediate level, but at higher levels the competitive balance is tipped in favor of littleseed canarygrass.
With invasive weeds continuing to threaten food safety and agro-ecosystem sustainability, ecological control using crops or native species may provide safe, economical, and environmentally sustainable solutions for invasive weed management [24, 31]. Therefore, choosing crop species with strong competitiveness, high economic value and suitable for large-scale planting have formed the core issues of this study . However, the interspecific competition between plants is dynamic. Environmental factors such as N availability may affect the interspecific relationship between alternative crops and invasive weeds as demonstrated in our study.