Soil microbial biomass and nitrogen transformations among five tree species of the Catskill Mountains, New York, USA

Abstract

We measured soil microbial biomass nitrogen (MBN), microbial uptake of 15N, potential net mineralization and net nitrification in the laboratory to determine the influence of tree species on nitrogen (N) transformations in soils of the Catskills Mountains, New York, USA. Organic horizon soils were taken from single species plots of beech ( Fagus grandifolia), hemlock ( Tsuga canadensis), red oak (Quercus rubra), sugar maple ( Acer saccharum) and yellow birch ( Betula alleghaniensis). 15NH 4Cl was added to the soils and N pools were sampled at 1, 3, 10 and 28 days to examine microbial uptake of 15N over time. Soil MBN was about 60% lower in red oak and sugar maple soils than in the other three species. Soil pools of NO 3 − and rates of net nitrification were significantly greater in soils associated with sugar maple than hemlock, red oak and yellow birch. With the exception of sugar maple soils, microbial recovery of 15N was significantly greater after 10 and 28 days compared to 60 min and 1 day following 15N tracer addition. Microbial 15N recovery declined significantly within sugar maple stands within the first 3 days of incubation. Soil carbon to nitrogen ratio (C:N) was lowest in sugar maple soils and highest in red oak soils. However, correlations between soil C:N and MBN or rates of net mineralization and nitrification were not significant. Soil moisture could account for 22% of the variation in MBN and 36% of the variation in net mineralization. Soil microbial transformations of N vary among tree species stands and may have consequences for forest N retention and loss.