Variability in Soil Nitrogen Retention Across Forest, Urban, and Agricultural Land Uses

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• 作者：Julie N. Weitzman ; Jason P. Kaye
• 关键词：land use ; 15nitrogen ; nitrogen retention ; soil organic matter ; tracer experiment ; biological fractionation
• 刊名：Ecosystems
• 出版年：2016
• 出版时间：December 2016
• 年：2016
• 卷：19
• 期：8
• 页码：1345-1361
• 全文大小：
• 刊物类别：Biomedical and Life Sciences
• 刊物主题：Ecology; Plant Sciences; Zoology; Environmental Management; Geoecology/Natural Processes; Hydrology/Water Resources;
• 出版者：Springer US
• ISSN：1435-0629
• 卷排序：19

文摘

In regions of mixed land use, some ecosystems are sinks for N pollution and others are sources. Yet, beyond this gross characterization, we have little understanding of how adjacent land-use types vary in mechanisms of N cycling and retention. This study assessed the rate and magnitude of soil N retention pathways in forest, urban, and agricultural ecosystems. Soil plots in each land use were labeled with inorganic 15N and cored at 15 min, 2 days, and 20 days following injection. Subsamples were biologically fractionated to differentiate labile and stable pools, while gross N transformations were assessed via the 15N isotope dilution method. Stable soil organic 15N formed rapidly (within 15 min) in all land uses when added as 15NH₄+, and became a proportionally larger sink for inorganic 15N over time. Forests had the lowest gross immobilization rates, but the greatest amount of stable N formation. Rapid retention of NH₄+ in forests may be driven by abiotic processes, with root uptake becoming a more important mechanism of retention over time. Urban sites, on the other hand, had the highest gross microbial immobilization rates and highest root N uptake, suggesting that high short-term N retention may be due to rapid biological processing. Agricultural systems, with low root uptake and the lowest stable N formation, had little capacity for retention of added N. These apparently distinct land-use cases can be understood by synthesizing several emerging aspects of N retention theory that (1) distinguish kinetic and capacity N saturation, (2) recognize links between soil C saturation on minerals and N retention, and (3) account for rapid transfers of NH₄+ to stable organic pools.