Nitrogen reallocation during alpine plant senescence contributed to plant nutrient conservation and ecosystem nitrogen retention

To conserve limiting nitrogen (N) in alpine ecosystems, herbaceous plants resorb and reallocate N from senescing tissues. However, the extent of N resorption and reallocation in aboveground tissues, coarse roots, fine roots and their relative contributions to whole-plant N conservation and ecosystem N retention remain poorly understood. Utilizing N stable isotope (¹⁵N) as a tracer, we quantified N partitions and N retranslocation efficiencies (NRE, % of N changes for each N pool) during senescence among different plant organs in a Tibetan alpine system. We found that compared to the N pools at the peak biomass stage, substantial ¹⁵N infine roots (FR, 39.93%) and aboveground tissues (shoot, 50.94%) was retranslocated primarily to coarse roots (CR, an increase of 79.02% in ¹⁵N) and non-extractable soil organic matter (an increase of 37.39% in ¹⁵N), corresponding to a temporal shift of plant trait syndrome from poor conservation to strong conservation during senescence. ¹⁵N in particulate organic matter and mineral-associated organic matter fractions during the senescence stage increased by 29.80% and 24.30%, respectively, but microbial biomass ¹⁵N significantly decreased. Our results illustrate the key role of N retranslocation to coarse roots and organic matter in N retention and the dual role of plant roots and organic matter as N sink and source in the plant-microbe-soil system. These findings suggest that plant N retranslocation and seasonal trait alternation facilitate the spatial and temporal coupling between plant N demand and bioavailable N supply in N-limiting alpine systems.