Nitrogen redistribution and seasonal trait fluctuation facilitate plant N conservation and ecosystem N retention

1. Low available soil nitrogen (N) limits plant productivity in alpine regions, and alpine plants thus resorb and reallocate N from senescing tissues to conserve this limited N during the non-growing season. However, the destination and extent of N redistribution during plant senescence among above- and below-ground organs, let alone other processes of translocation outside of plants and into the soil components, remain poorly understood.2. Utilizing N-15 stable isotope as a tracer, we quantified N redistribution among above- and below-ground plant organs and different soil components during senescence in an alpine meadow ecosystem, and explored the relationship between N-15 partitioning among plant-soil N pools with seasonal fluctuations of plant functional traits.3.We found a substantial depletion of N-15 in fine roots (-40% +/- 2.8%) and above-ground tissues (-51% +/- 5.1%), and an enhanced N-15 retention primarily in coarse roots (+79% +/- 27%) and soil organic matter (+37% +/- 10%) during plant senescence, indicating a dual role of roots with coarse roots acting as an N sink and fine roots as a source of N recycling during senescence. In parallel, we observed a temporal variation in plant functional traits, representing a shift from more acquisitive to more conservative strategies as the growing season ends, such as higher coarse root N and coarse root to fine root ratio. The seasonal trait variations were highly correlated with the N-15 retention in coarse roots and soil organic matter. Particularly, N-15 retention in particulate and mineral-associated organic matter increased by 30% +/- 12% and 24% +/- 9%, respectively, suggesting a potential pathway through which fine root and microbial mortality contribute to N-15 redistribution into soil N pools during senescence.
Synthesis. N redistribution and seasonal plant trait fluctuation facilitate plant N conservation and ecosystem N retention in the alpine system. This study suggests a coupled above-ground-below-ground N conservation strategy that may optimize the temporal coupling between plant N demand and ecosystem N supply in N-limited alpine ecosystems.