PS 86-46
Variability in soil nitrogen retention across forest, urban, and agricultural land uses

Friday, August 14, 2015
Exhibit Hall, Baltimore Convention Center
Julie N. Weitzman, Department of Ecosystem Science and Management, Pennsylvania State University, University Park, PA
Jason P. Kaye, Department of Ecosystem Science and Management, Pennsylvania State University, University Park, PA
Background/Question/Methods: Inorganic nitrogen (N) added to soils can be converted to organic form within minutes to weeks, but the extent to which new soil organic N is susceptible to remineralization remains unclear. If the newly converted soil organic N is labile it may become remineralized, potentially impacting water and air quality, as well as plant growth.  In contrast, stable soil N may be a sink that mitigates N pollution on decadal time scales. Forests are known to retain a high percentage of added N (from 30-80%), but not much is known about N retention in urban and agricultural systems, which are both recognized as sources of N pollution. A better understanding of rates and magnitudes of organic N formation and stability should translate into better management of N losses from terrestrial ecosystems and better predictions of N cycling in regions with mixed land-use. This study assessed the rate and magnitude of N retention in the stable organic matter of forest, urban, and agricultural ecosystems. Soil plots in each land use were labeled with inorganic 15N (98% APE) and cored at 15 minutes, 2 days, and 20 days following injection. Subsamples were biologically fractionated to differentiate labile and stable pools. 

Results/Conclusions: Stable 15N formed within 15 minutes in all land uses, with retention greatest (14% of added 15N) in forest systems, followed by urban and agricultural systems. Stable 15N made up a greater proportion of the retained 15N pool as time increased, in contrast to labile 15N, which decreased over time. The form in which 15N was added impacted partitioning between the soil N pools. When injected as 15NH4+ the stable 15N pool formed within 15 minutes and remained fairly constant over time. For 15NO3- enriched sites stable 15N did not form quickly, and small accumulations were seen in forest and urban systems. Results suggest that the magnitude of the stable 15N pool varies among forest, urban, and agricultural ecosystems, but the rate of change over time is similar among the three land-use types.