A plant nitrophily index based on plant leaf area response to soil nitrogen availability

This article reports a new experimental method to measure plant nitrophily. Knowledge of the nitrophily of plant species has many potential applications such as studying the long-term evolution of flora and designing nitrogen management strategies in cropping systems. Plant nitrophily is commonly measured by the Ellenberg N score based on the natural occurrence of species along soil nitrogen gradients. The Ellenberg N score is known for species from restricted geographical areas representing a small proportion of total plant species diversity. In addition, measuring Ellenberg N score is not convenient. We propose a new definition of plant nitrophily referring to plant leaf area response to nitrogen availability. We compared habitat-based and response-based values of nitrophily to design a simple method to estimate a plant nitrophily index (NI). Eleven monocotyledonous and dicotyledonous plant species were grown in a greenhouse experiment at two levels of soil nitrogen. Nine species were weeds covering the range of the Ellenberg N score. Two crop species with unknown nitrophily, wheat and oilseed rape, were studied to illustrate our method. Plant leaf area was measured at one date for each species x nitrogen treatment combination. A NI was calculated as the ratio of leaf area at high nitrogen to leaf area at low nitrogen. Our results show for weeds a high interspecific diversity of the NI, ranging from 1.0 to 4.2. The NI was strongly and positively correlated to Ellenberg N score, with R (2) of 0.73. The more nitrophilic a species according to habitat preferences, the more leaf area responded to increasing nitrogen supply. This is the first time that a quantitative relationship is found between Ellenberg N score and a growth variable measured non-destructively for both monocotyledonous and dicotyledonous species. Therefore, calculating the response of leaf area to nitrogen supply provides a new simple and non-destructive method that can be used for the assessment of a NI of any species. As an illustration of the method, a NI was estimated for new species. It was at 2.1 and 4.5 for wheat and oilseed rape, respectively, indicating that they were respectively moderately and highly nitrophilic. This method will help to assess the relative nitrophily of weeds vs. crops. Such knowledge could be used to design nitrogen management strategies promoting crop growth but not weed growth, thus reducing the use of herbicides.