Agropyron mongolicum Keng’s Growth in Response to Nitrogen Addition Is Linked to Root Morphological Traits and Nitrogen-Use Efficiency

Abstract

Nitrogen (N) is an essential macronutrient for plant development and growth, and increased N deposition is affecting the diversity and productivity of plants. The objective of this study was to explore the growth response of Agropyron mongolicum Keng to N addition and to determine whether N-induced changes in soil-available nutrients have indirect impacts on the biomass of A. mongolicum via the regulation of root morphological traits and NUE. We conducted a pot experiment subjecting A. mongolicum to five N addition levels (0, 0.8, 1.6, 2.4, and 4.0 g N m−2 yr−1) under greenhouse conditions. N was provided through urea [CO(NH2)2] delivered with two equal applications and added at the seeding and tillering stages. The results showed that the total biomass response of A. mongolicum to increasing N addition appeared unimodal-shaped with an N saturation threshold at 3.20 g N m−2 yr−1. The total biomass was significantly and positively correlated with the root surface area (RSA), volume (RV), length (RL), forks number (RF), N-uptake efficiency (NUpE), and N-utilization efficiency (NUtE) (p < 0.05). N-induced changes in soil-available nutrients had an indirect impact on the total biomass of A. mongolicum via the regulation of root morphological traits and NUE.