Comparison of Nitrogen Cycling Between Old Growth Forests and Secondary Forests in the U.S. Mid-Atlantic

Abstract:

Throughout the eastern United States, forests are experiencing regrowth, and the sequestration of carbon (C) associated with this regrowth makes these forests a key component of greenhouse gas mitigation strategies (Albani et al., 2006). Through production and decomposition of plant biomass, the C and nitrogen (N) cycles are closely coupled, suggesting that N has a major impact on the cycling of C in N-limited Mid-Atlantic forest systems. The majority of C and N in a temperate forest system is located in the soil organic matter (Templer et al., 2012), so understanding soil N is important for estimating the potential for C sequestration in soils as Mid-Atlantic forests mature (Knicker, 2010). Due to the scarcity of old growth forest stands in the region, previous empirical studies of Mid-Atlantic forests in the old growth stage of succession are limited. I sampled soil C and N in twenty-five remnant old growth forests and matched secondary stands in the Mid-Atlantic to identify differences in soil organic C and N mass and concentrations of nitrate and ammonium. No significant differences were observed between the old growth and secondary growth concentrations of inorganic N species, N fraction, and C:N ratio. Rather, secondary growth values for these variables were found to have significant, positive linear relationships with old growth values, indicating that biotic and abiotic factors varying on a regional scale are driving variability seen in these N characteristics. Further, this suggests that as forest stands reach approximately 75 years in age, these N characteristics are largely established and not likely to change significantly as stands enter the old growth successional stage. Both N fraction and O-horizon depth were shown to have significant negative correlations with old growth stand age. These results indicate that old growth forest stands have a more efficient microbial decomposer community, which could have significant implications for both soil N and C pools. This study highlights the importance of analyses conducted at the regional-scale for understanding ecosystem-level processes, and further analysis of mature and old growth forests conducted at the regional-scale is needed to better understand the dynamics of forest N cycling through succession.