Plant nitrogen capture in pulse-driven systems: interactions between root responses and soil processes

1 Soil resources are heterogeneously distributed in natural systems. In arid systems, for example, soil nitrogen (N) is often supplied in pulses. Mechanisms influencing the ability of a species to exploit N pulses through the season, however, are poorly understood despite the strong potential for temporal variation in N supply to impact growth, survival and competitive interactions in these systems. 2 We examined how plant physiological and soil processes interacted to influence the ability of two dominant perennial Atriplex species to capture N from pulses occurring at different times in the growing season. 3 N-15-labelled pulses were applied to the two Atriplex species in early and mid spring. Sequential time harvests were used to quantify changes in plant N-15 content, root length, root length relative growth rate, root N inflow rates, microbial biomass N-15, soil water content and soil inorganic N-15 pools. Path analysis and structural equation modelling were used to quantify the relative importance of different root parameters for plant N capture from pulses and to evaluate the degree to which these root responses interacted with soil processes to influence plant N capture. 4 Plant N capture was greatest when pulses coincided with high root length relative growth rates. Declining availability of total inorganic N through a pulse had a limited effect on N capture. This was partly because soil NH4+ pools were removed from the soil system four times faster than soil NO3- pools, allowing sufficient NO3- supply to roots, although total inorganic N pools declined. Microbial immobilization rates did not change significantly through a pulse and did not influence plant N capture during a pulse. Instead, plant N capture during a pulse was limited by total root length and uptake capacity per unit root length, which in turn was affected by plant N demand and soil water content. 5 Understanding interactions between root responses, soil processes and pulse timing provides insight into mechanisms underlying competitive interactions and diversity maintenance in pulse-driven systems.