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

Abstract

Summary 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. 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. 15N‐labelled pulses were applied to the two Atriplex species in early and mid spring. Sequential time harvests were used to quantify changes in plant 15N content, root length, root length relative growth rate, root N inflow rates, microbial biomass 15N, soil water content and soil inorganic 15N 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. 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 pools were removed from the soil system four times faster than soil pools, allowing sufficient 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. Understanding interactions between root responses, soil processes and pulse timing provides insight into mechanisms underlying competitive interactions and diversity maintenance in pulse‐driven systems.