Stable nitrogen and carbon pools in grassland soils of variable texture and carbon content

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Abstract:

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 (15)N 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 (15)NH(4)(+) 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 (15)N 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 (r(2) = 0.72), stable soil C (r(2) = 0.68), and soil clay content (r(2) = 0.27) were correlated with stable bulk soil N pools, but not with stable (15)N 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.