Response of Fine Root Carbohydrate Content to Soil Nitrogen Addition and Its Relationship with Soil Factors in a Schrenk (Picea schrenkiana) Forest

Fine roots are vital for water and nutrient uptake in plants. Understanding the responses of metabolic traits to changing environmental conditions is critical, but little is known. In this study, the carbohydrate content of fine roots was characterized for six soil layers and three diameter classes in a Schrenk spruce (Picea schrenkiana) forest on the Tianshan Mountains of China. Soil nitrogen addition can influence carbohydrate content, but the degree is related to soil layer and fine root diameter. Specifically, as nitrogen (N) levels increased, the soluble sugar, cellulose, lignin and structural carbohydrate content (SC) all increased and then decreased; the starch and non-structural carbohydrate content (NSC) decreased gradually. In addition, N addition had significant effects on starch and cellulose content, as well as total carbohydrates. The interaction between root diameter and N addition also influenced soluble sugars, cellulose and NSC, while the three-way interaction among N addition, soil layer and diameter only affected cellulose content and NSC/SC. In the control treatment (no nitrogen addition), the following soil factors influenced fine root metabolism in order of decreasing importance: C:N, C:P, N:P, C, N, water content, bulk density, pH, conductivity, and P. In the N addition treatments, the soil conductivity, pH, and N content were strongly correlated with root metabolic characteristics.