Interactive effects of warming and nitrogen addition on fine root dynamics of a young subtropical plantation

Abstract

Forest aboveground production and soil carbon (C) pools are closely linked to fine root dynamics. Uncertainty about the effects of warming and nitrogen (N) deposition on fine root dynamics limits our ability to predict how C will transfer between biological and atmospheric pools in tropical and subtropical forests. In order to examine the effects of warming and N deposition on fine roots in subtropical plantations, we used a randomized complete block design with factorial soil warming (ambient, ambient + 5 °C) and N deposition (ambient, ambient + 80 kg N ha−1 yr−1) manipulation. Minirhizotrons were used to monitor fine root production, mortality and turnover rate of Chinese fir (Cunninghamia lanceolata) seedlings for two years, and seedling growth was measured. We found warming had positive effects on annual fine root production, mortality and turnover rate both under ambient and increased N addition. N addition had positive effects on annual fine root production, mortality and turnover rate in the warmed plots, but had no influence on annual fine root production, mortality and turnover rate in the unwarmed plots. Warming and N addition had an additive (not interactive) effect on fine root production, mortality and turnover rate. There was an interaction between warming and N addition on living fine root biomass in the second year. These changes can be largely attributed to belowground/aboveground C allocation. There was no evidence of root respiration acclimation to warming. In addition, increased fine root turnover rate after warming implies accelerated root C inputs to soils, which may affect soil C and nutrient dynamics. Nitrogen addition may exacerbate this. There was no acclimation of root respiration to warming, which may alter C balance and cause more CO2 release to the atmosphere through autotrophic respiration.