Responses of Root System Architecture, Biomass Allocation, and Soil characteristics to Water-Nitrogen Coupling in Piptanthus nepalensis Seedlings

Abstract

Water scarcity and nitrogen (N) limitation restrict vegetation restoration on the Qinghai-Tibet Plateau. We investigated the root foraging strategies and soil feedback of the pioneer shrub Piptanthus nepalensis under water-N coupling. One-year-old seedlings were subjected to five water gradients and five N rates. We analyzed rhizosphere soil properties, root system archite cture, and biomass allocation. Water availability predominantly drove phenotypic plasticity, w hile N addition modulated growth under suitable water conditions. Root morphology and bio mass accumulation followed a unimodal pattern, with moderate water (40%–60% field capacit y) and medium N (2 g·plant⁻¹) maximizing root extension (>350cm·plant⁻¹) and total dry bio mass (approx. 25.05 g·plant⁻¹).Severe drought induced a conservative strategy, inhibiting root extension while maintaining stable construction costs. Furthermore, soil electrical conductivity (EC) was strongly associated with plant phenotypic traits, suggesting the osmotic environment plays a key role in regulating adaptation. P. nepalensis adapts to heterogeneous habitats pri marily by adjusting biomass allocation. We recommend moderate water deficit combined with medium N supply for seedling establishment to optimize resource acquisition and stress toler ance.