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

Abstract 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. Utilizing 15 N 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 15 N partitioning among plant–soil N pools with seasonal fluctuations of plant functional traits. We found a substantial depletion of 15 N in fine roots (−40% ± 2.8%) and above‐ground tissues (−51% ± 5.1%), and an enhanced 15 N 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 15 N retention in coarse roots and soil organic matter. Particularly, 15 N 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 15 N 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.