Relationships between nitrogen fixation and growth in Robinia pseudoacacia seedlings: A functional growth‐analysis approach using 15N

Abstract

The effects of NO3− fertilization on N2 fixation and growth of black locust (Robinia pseudoacacia L.) seedlings were examined by growth analyses. Seedlings were grown under low (0.25 mM) and high (2.00 mM) NO3− provided with each watering. In one experiment, seedlings were given NO3− with enriched levels of 15N (11.03%) to identify N accumulation from soil and atmospheric sources. Curves describing accretion of total dry weight and N were fit as functions of accumulated photosynthetically active radiation (PAR). Relative growth rates (RGR) and nitrogen productivities (NP) were derived from these curves and examined as functions of seedling size and/or internal N concentration. Early in the experiments seedlings receiving the higher amount of NO3− grew faster, acquired more N, had greater internal nitrogen concentrations (N1) and had higher NP than seedlings receiving the lower amounts. Seedlings receiving higher N also acquired most of their N from NO3−, whereas those receiving lower N acquired most from N2 fixation. Later in the experiments, no differences in growth rae, N1 or NP were observed. Furthermore, midway through the experiments seedlings receiving the higher amounts of NO3− began to acquire the majority of N from N2 fixation. At the end of the two experiments, seedlings given higher NO3− were 16 and 78% larger in dry weight. Differences in size were due to growth differences that occurred only during the first half of the experiments. During this period, the faster growth of seedlings given higher NO3− was attributed to greater access to utilizable N and lower energy costs associated with the acquisition and/or utilization of NO3−, relative to N2. Unlike the situation in non‐N2‐fixing plants, differences in available soil N had no influence on biomass allocation to roots, stems or leaves in black locust. When soil N became limiting, N2 fixation capacity increased, rather than root mass, to acquire more N.