Searching for hot spots and hot moments of soil denitrification in northern hardwood forests

Abstract:

Denitrification is a key biogeochemical process that affects nitrogen (N) availability, N losses to aquatic systems, and atmospheric chemistry. In upland forests, denitrification has not been thought to be a major N pathway because it is an anaerobic microbial process that requires nitrate, labile carbon (C), and low oxygen (O₂) conditions, which do not occur broadly or consistently throughout forest soils. However, there may be enough spatial and temporal heterogeneity at fine scales to support denitrification rates that are relevant at the landscape scale. To quantify the importance of spatial and temporal variability in soil denitrification in northern hardwood forests at the Hubbard Brook Experimental Forest (HBEF; New Hampshire, USA), we developed two related projects: 1) we sought to identify hot spots of biogeochemical activity, including soil denitrification potential, based on hydropedologic settings and flowpaths in a catchment during the growing season; and 2) we investigated the influence of simulated rainfall events on soil O₂ and nitrous oxide concentrations, denitrification rates, and soil respiration during different seasons at HBEF.

In the first study, we expected to find that sites dominated by soils with thick Bh horizons (zones of C accumulation) would have the highest denitrification rates. However, despite the variation among soil profiles found in different hydropedologic settings, we did not find significant differences in denitrification potential. Rather, when areal coverage and horizon thickness for the contrasting hydropedologic settings were accounted for, catchment-scale estimates of denitrification potential were about 1/3 higher than conventionally calculated estimates. In the second study, soil O₂ in surface horizons only decreased following additions of labile C. Responses of soil respiration and denitrification to simulated rainfall were also influenced by season. While these studies highlight the complex heterogeneity in forest soils, they also contribute to our understanding of the spatial and temporal variability of denitrification, help us identify the meaningful temporal and spatial scales for observing variability in denitrification and its controls, and improve our ability to extrapolate measurements of denitrification to broader scales.