Root nitrogen acquisition strategy of trees and understory species in a subtropical pine plantation in southern China

Nitrogen (N) is considered as a major factor that limits plant growth and metabolism, and absorptive roots play a critical role in plant resource acquisition. However, little is known about the roles of mycorrhizal colonization, and morphological and architectural traits of absorptive roots in N uptake rates. We examined the uptake rates for ammonium ( NH_4^ + ), nitrate ( NO_3^ - ) and glycine using a 15 N isotope tracer technique and measured mycorrhizal colonization rates and functional traits (morphology, architecture and chemistry) of absorptive roots in a subtropical plantation during the growing season. Results showed that trees, shrubs and herbs all preferred to take up NH_4^ + over NO_3^ - and glycine, likely due to the dominant available N form in native soils. Species coexisting in the subtropical plantation did not show chemical niche differentiation, but there was a temporal niche separation in N acquisition across plant species. Absorptive roots with higher mycorrhizal colonization rates exhibited higher N uptake rates than those with lower colonization. In May, morphological traits (diameter and root tissue density) seemed playing important roles in N acquisition on that the absorptive roots with larger diameter and shorter specific root length (SRL) showed higher uptake rates for NH_4^ + , NO_3^ - , and glycine than those with smaller diameter and longer SRL. While in August, the architectural traits of root branching might be essential to enhance nutrient absorption on that the absorptive roots with intensive branching exhibited higher N uptake rates than those with less branching. Our findings suggested that plant species may evolve effective N acquisition strategies integrated mycorrhizal symbiosis, and root morphological and architectural traits over a temporal scale to acclimate to the changing environments.