Nitrogen (N) inputs to many terrestrial ecosystems are increasing, and most of these inputs are sequestered in soil organic matter within 1-3 years. Rapid (minutes to days) immobilization focused previous N retention research on actively cycling plant, microbial, and inorganic N pools. However, most ecosystem N resides in soil organic matter that is not rapidly cycled. This large, stable soil N pool may be an important sink for elevated N inputs. In this study, we measured the capacity of grassland soils to retain 15N in a pool that was not mineralized by microorganisms during 1-year laboratory incubations (called "the stable pool"). We added two levels (2.5 and 50 g N m-2) of 15NH4+ tracer to 60 field plots on coarse- and fine-textured soils along a soil carbon (C) gradient from Texas to Montana, USA. We hypothesized that stable tracer 15N retention and stable bulk soil (native + tracer) N pools would be positively correlated with soil clay and C content and stable soil C pools (C not respired during the incubation). Two growing seasons after the 15N addition, soils (0- to 20-cm depth) contained 71% and 26% of the tracer added to low- and high-N treatments, respectively. In both N treatments, 50% of the tracer retained in soil was stable. Total soil C (r2 = 0.72), stable soil C (r2 = 0.68), and soil clay content (r2 = 0.27) were correlated with stable bulk soil N pools, but not with stable 15N retention. We conclude that on annual time scales, substantial quantities of N are incorporated into stable organic pools that are not readily susceptible to microbial remineralization or subsequent plant uptake, leaching losses, or gaseous losses. Stable N formation may be an important pathway by which rapid soil N immobilization translates into long-term N retention.
All Science Journal Classification (ASJC) codes
- Ecology, Evolution, Behavior and Systematics
- Environmental Chemistry