Differential responses of N benefit mediated by root exudate inputs to N addition between two subalpine forests

Abstract

Root C investment to exchange for the nutrient is a critical nutrient acquisition strategy in regulating soil biogeochemical processes and maintaining the stability of forest structure and functions. However, the response patterns of root C cost-nutrient benefit strategies to N deposition in forests with different initial soil N availability are poorly understood. In this study, root exudation rates were measured by in situ collection during the growing season in two coniferous plantations (i.e., Pinus armandii and Picea asperata plantations with relatively lower and higher initial soil N availability, respectively) to assess whether two plantations differed in root C cost under N addition (25 kg N ha−1 a−1). Simultaneously, the differences in enzyme activity and related soil N mineralization rate between the rhizosphere and bulk soils were quantified to assess whether two plantations differed in N acquisition capacities under N addition. The results showed that N addition to some extent mitigated N-limitation by improving soil N availability, reducing root exudation rate per root biomass and rhizosphere effect of N mineralization by 36.43% and 38.13% in the P. armandii plantation, implying that plant nutrient acquisition strategy shifts from “high C cost-N benefit” to “low C cost-N benefit” strategy. In contrast, in the P. asperata plantation with relatively high initial soil N availability, N addition had little effect on root exudate inputs and related N mineralization capacity, i.e., plant root C cost-nutrient acquisition strategy was independent of N addition in the P. asperata plantation. Collectively, the results suggest that the response patterns of root C cost-N acquisition strategies to N addition between different tree species closely depend on soil N availability, which was more sensitive in forests with lower soil N availability. As a result, variable tradeoffs between root C investment and N acquisition provide new insight into the understanding of rhizosphere nutrient cycling for stability and productivity of forest ecosystem under environmental changes.