Nitrogen uptake and use by competing individuals in a Xanthium canadense stand

We studied differences in nitrogen uptake and use for plant growth among in dividuals competing in a natural dense stand of an annual herb, Xanthium ca nadense. Larger individuals took up more nitrogen than proportionately to t heir size, indicating that the competition 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 increased with plant size. While smaller individuals shared smaller fractions of total plant nit rogen in the stand, they had higher nitrogen concentrations per unit mass. "Turnover" rates of nitrogen influx (r(in)) and outflux (r(out)) were defin ed as the rates of nitrogen uptake and loss per unit aboveground nitrogen, respectively. ri, was higher in larger individuals, whereas r(out) was high er in smaller individuals. Consequently, the relative rate of nitrogen incr ement (r(in)-r(out)) was higher in larger individuals, whereas it was aroun d zero in the smallest individuals. The mean residence time of nitrogen (MR T), defined as the inverse of r(out), was longer in larger individuals. Nit rogen productivity (NP), i.e. the growth rate per unit aboveground nitrogen , was higher in larger individuals. As the product of lifetime MRT and NP g ives the nitrogen use efficiency (NUE), defined as biomass production per u nit flux of nitrogen, higher MRT and NP observed in larger individuals woul d have contributed to their higher lifetime NUE. Shorter MRT in smaller ind ividuals was caused by the abscission of leaves which contained relatively large fractions of total plant nitrogen. Xanthium canadense, as a competiti ve ruderal, tended to produce leaves at higher positions to acquire higher light levels at the expense of older leaves rather than to modify their pro ductive structure to efficiently use low light levels as observed in shade- tolerant species.