Influences of plant traits on the retention and redistribution of bioavailable nitrogen within the plant-soil system

The supply of bioavailable nitrogen (N) from atmospheric deposition, microbial mineralization, and biological N fixation determines the N limitation status of the ecosystems. This N is either retained in the ecosystem through microbial immobilization and plant uptake or lost through leaching and gaseous emission. The retention and turnover of bioavailable N vary greatly among ecosystems with different vegetation compositions. However, it remains unclear how plant traits modulate the fates of bioavailable N within the plant-soil systems. We conducted a dual 15N/13C labeling experiment on 6 grassland species with distinct traits in a climate-controlled chamber. We quantified the retention and redistribution of bioavailable N in plants, short-lived particulate organic matter (POM), and relatively stable mineral-associated organic matter (MAOM), and investigated how they were affected by plant traits. Total 15N tracer retention by the plant-soil system averaged 67 %, of which plants retained 33 % and soils retained 34 %. Plant species with higher root growth rates had a greater capacity to scavenge bioavailable N from soils directly, and less relied on rhizosphere priming effects to acquire N from soil organic matter. Such fast-growing species thus retained more of the 15N tracer and promoted the 15N retention within the plant-soil system. The POM-15N retention was negatively associated with root lifespan, whereas the MAOM-15N retention was positively related to root growth rates. We emphasized that plant traits, especially root growth rates, are of importance in driving the partitioning and recycling of the bioavailable N between plants, POM, and MAOM. Integrating plant traits into models will likely more accurately assess the impact of different vegetation compositions on soil N cycling.