Variability in soil nitrogen retention across forest, urban, and agricultural land uses

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.