Abstract We studied differences in nitrogen uptake and use for plant growth among individuals competing in a natural dense stand of an annual herb, Xanthium cana- dense. Larger individuals took up more nitrogen than proportionately to their size, indicating that the competi- tion for soil nitrogen was asymmetric among individuals, although it was more symmetric than the competition for light. The rate of nitrogen loss of individuals also in- creased with plant size. While smaller individuals shared smaller fractions of total plant nitrogen in the stand, they had higher nitrogen concentrations per unit mass. “Turn- over” rates of nitrogen influx (r in ) and outflux (r out ) were defined as the rates of nitrogen uptake and loss per unit aboveground nitrogen, respectively. r in was higher in larger individuals, whereas r out was higher in smaller in- dividuals. Consequently, the relative rate of nitrogen in- crement (r in –r out ) was higher in larger individuals, whereas it was around zero in the smallest individuals. The mean residence time of nitrogen (MRT), defined as the inverse of r out , was longer in larger individuals. Nitrogen productivity (NP), i.e. the growth rate per unit aboveground nitrogen, was higher in larger individuals. As the product of lifetime MRT and NP gives the nitro- gen use efficiency (NUE), defined as biomass production per unit flux of nitrogen, higher MRT and NP observed in larger individuals would have contributed to their higher lifetime NUE. Shorter MRT in smaller individuals was caused by the abscission of leaves which contained relatively large fractions of total plant nitrogen. Xanth- ium canadense, as a competitive ruderal, tended to pro- duce leaves at higher positions to acquire higher light levels at the expense of older leaves rather than to modi- fy their productive structure to efficiently use low light levels as observed in shade-tolerant species. Keywords Intraspecific competition · Mean residence time · Nitrogen turnover · Nitrogen use efficiency · Nitrogen productivity Introduction In many natural ecosystems, nitrogen is the primary fac- tor that limits plant growth due to its limited availability (Lee et al. 1983; Kachi and Hirose 1983; Vermeer and Berendse 1983; Postor et al. 1984; Tilman 1984), and to the large requirement of nitrogen for photosynthetic pro- teins (Field and Mooney 1986; Evans 1989). Nitrogen is a costly element for plants to assimilate and plants use it efficiently through recycling. When leaves senesce due to shading or aging, a large fraction of nitrogen in leaves is recycled to new organs (Thomas and Stoddart 1980; Mae and Ohira 1981; Hikosaka et al. 1994). Nitrogen that is not recycled from senescing leaves is eventually lost from the plant body. Efficient use of nitrogen may be important to increase the fitness of plants, particularly in an environment where the availability of nitrogen is low (Small 1972). Nitrogen use efficiency (NUE) has been simply defined as dry mass productivity per unit amount of nitrogen flux (Hirose 1971, 1975; Chapin 1980; Vitousek 1982). Berendse and Aerts (1987) have defined the NUE as the product of the nitrogen productivity (NP; growth rate per unit plant, Ingestad 1979) and the mean residence time of nitrogen in a plant (MRT; the inverse of the ni- trogen turnover rate when the plant growth is at a steady state) to make the concept more biologically relevant. Plants may improve NUE either through increasing NP or MRT or both. However, workers suggested that a trade-off exists between NP and MRT: species that grow in nitrogen-rich habitats tended to have higher NP while having shorter MRT, and the inverse was the case in spe- cies that are adapted to nitrogen-poor habitats (Aerts 1989; also Eckstein and Karlsson 1997; but see Garnier and Aronson 1998). NUE, NP and MRT have been applied to plant growth on an annual basis, because in a seasonal environment K. Hikosaka ( ✉ ) · T. Hirose Biological Institute, Graduate School of Science, Tohoku University, Aoba, Sendai 980–8578, Japan e-mail: hikosaka@mail.cc.tohoku.ac.jp Fax: +81-22-2176699 Oecologia (2001) 126:174–181 DOI 10.1007/s004420000517 Kouki Hikosaka · Tadaki Hirose Nitrogen uptake and use by competing individuals in a Xanthium canadense stand Received: 1 February 2000 / Accepted: 2 August 2000 / Published online: 19 October 2000 © Springer-Verlag 2000