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.
All Science Journal Classification (ASJC) codes
- Ecology, Evolution, Behavior and Systematics
- Environmental Chemistry