Gross Soil Nitrogen Mineralization across a Managed Northern Temperate Forest Chronosequece.

Despite being one of the pivotal biogeochemical processes in temperate-boreal forest ecosystems, there are limited measurements of soil gross nitrogen (N) mineralization and consumption rates. Quantification of these processes is critical for understanding biogeochemical cycling processes in these soils that are N-dependent. In particular, there is limited information about how these processes change in relation to forest harvesting activities and through depth in soil profiles. Here we measured gross N mineralization and consumption rates using a ¹⁵N dilution technique in orthic hummo-ferric podzol forest soils of Nova Scotia, Canada. The study was conducted at two different depth range (0-10 cm and 20-40 cm) in soil profiles of a native red spruce chronosequece that included 125 ±yr, 80 yr, 15 yr old forests along with 3 yr and 3-month old clearcut harvested sites characterized by similar soil type, topography, and climate. During the study period, both mineralization and consumption rates were highest in the surface layer (0-10 cm) of the 15 yr forest site (2.18 ± 0.21 and 2.13 ± 0.06 mgN kg^-1 d^-1 respectively) with minimums observed in the deeper layer (20-40 cm) of the 3-month old harvested site (0.62 ± 0.17 and 0.68 ± 0.33 mgN kg^-1 d^-1 respectively). The difference in mineralization rates among sites was significant for both surface (p = 0.012) and deeper (p = 0.0) layers but was insignificant (p > 0.09) for consumption rates. With the exception of the 3 yr old harvested site, both mineralization and consumption rates were higher in the surface layer, indicating higher microbial activity in the top of soil profile. Residence time of N in the ammonium pool of soil at each site suggested the highest residence time at the 3-month old harvest site (6.5 days), with a minimum at the 80 yr site (0.60 days). This demonstrates a more rapid turnover of ammonium at older forested sites than recently harvested ones.