Stable Nitrogen and Carbon Pools in Grassland Soils of
Variable Texture and Carbon Content
(Running Head: Stable 15N Retention in Grassland Soils)
Ingrid Burke, Dept of Forest, Range, and Watershed Stewardship, Colorado State University, Fort Collins , CO 80523
(970) 491-1620 indy@cnr.colostate.edu
Abstract: Nitrogen
(N) inputs to many terrestrial ecosystems are increasing and most of these
inputs are sequestered in soil organic matter within 1 to 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 paper, we measured the capacity of grassland soils to retain 15N
in a pool that was not mineralized by microorganisms during one-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 re-mineralization 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
KEYWORDS: Great Plains, Laboratory incubation, 15Nitrogen, Nitrogen
retention, Soil organic matter, Tracer experiment
Related Publication:
Kaye, J.P., J.E. Barrett, and I.C. Burke . 2002 . Stable nitrogen and carbon pools in grassland soils of variable texture and carbon content . Ecosystems . 5 : 461 - 471
03/21/2002