Abstract:

We analyzed the effects of plant sizes on nitrogen (N) uptake and use in a dense monospecific stand of an annual herb, Helianthus annuus L. (sunflower) and evaluated the consequences of intraspecific competition. Larger individuals obtained more N disproportionately to their sizes, suggesting that the competition for soil N was asymmetric (one-sided) among individual plants in the stand. Nitrogen loss of individuals also increased with plant size. N influx was greater in larger individuals, while N efflux was lower in small individuals. Therefore, the relative rate of N increment was greater in larger individuals, while it was around zero in the smallest individuals. N use efficiency (NUE) was separated into the N productivity (NP) and the mean residence time of N (MRT). Both NP and MRT were positively related to plant size. Larger individuals showed a higher NP and a longer MRT, while smaller ones displayed the reverse pattern. Consequently, NUE (i.e. the product of NP and MRT), was higher for larger individuals. No trade-off between NP and MRT was found among individuals. N resorption efficiency (NRE) was closely related to plant size. The higher NUE at individual-level was partly a result of greater N resorption during senescence. Asymmetric competition among individuals in this stand resulted mainly from lower efficiency in both N uptake and N use by smaller individuals. This study shows that the concept of NUE defined by Berendse and Aerts offers a powerful tool in studying plant strategies within species as well as among species.

Key words: intraspecific competition, mean residence time (MRT), nitrogen use efficiency (NUE), nitrogen productivity (NP), plant strategies, size inequality