Temperature and substrate effects on C & N mineralization and microbial community function of soils from a hybrid poplar chronosequence

Substrate input as well as climatic factors affect C and N cycling and microbial properties in forest soils. We used a microcosm approach to investigate the response of CO2 efflux, net N mineralization, and microbial community-level physiological profile (CLPP) to temperature (5 vs. 15 degrees C) and substrate (with and without sucrose addition) addition in surface mineral soils collected from 4-, 6-, 13-, and 15-year old (ages in 2007) hybrid poplar (Populus deltoides x Populus x petrowskyana var. Walker) stands in northern Alberta. In the early stage of incubation (0-2 h), CO2 efflux was higher at 5 degrees C than at 15 C with little effect from substrate addition, while 24 h after the addition of substrate, CO2 efflux became higher under the 15 degrees C incubation. After 72 h incubation, temperature and substrate addition effects on CO2 efflux subsided and CO2 efflux rates tended to converge among the treatments. Net N mineralization was significantly affected by substrate addition and stand age, while rates of net ammonification were higher at 5 degrees C than at 15 degrees C. Net N mineralization occurred without sucrose addition while net immobilization occurred with sucrose addition. The soil from the youngest stand had the lowest N mineralization rate among the stands for each corresponding substrate-incubation temperature treatment. We used Ecoplates from Biolog (TM) to study sole-carbon-source-utilization profiles of microbial communities at the end of the incubation. Principal component analysis of C utilization data separated microbial communities with respect to substrate addition, incubation temperature and stand age. Our data showed that organic matter mineralization and microbial substrate utilization were affected by incubation temperature, substrate availability and stand age, indicating that the responses of microbial communities in the studied hybrid poplar plantations to temperature changes were strongly mediated by labile C availability and stand development. (C) 2010 Elsevier B.V. All rights reserved.