A set of long term N-15 studies was initiated during the summers of 19
81 and 1982 on the backslope and footslope, respectively, of a catena
in the shortgrass steppe of northeastern Colorado. Microplots labeled
with N-15 urea were sampled for N-15 and total N content in 1981 and 1
982 and again in 1992. In November, 1982, 100% of the added N was reco
vered in the soil-plant system of the finer-textured footslope, compar
ed to 39% in the coarser-textured backslope microplots. Ten years late
r, N-15 recovery of the applied N decreased at both topographic positi
ons to 85% in the footslope and 29% in the backslope. Average losses s
ince the time of application were 3.5 g N m(-2)yr(-1) in the backslope
and 0.8 g N m(-2)yr(-1) in the footslope. In 1992, soil organic matte
r was physically fractionated into particulate (POM) and mineral assoc
iated (MAON) fractions and 21-day mineralization incubations were cond
ucted to assess the relative amounts of N-15 that were in the slow, pa
ssive and active soil organic matter pools, respectively, of the two s
oils. Our findings confirm the assumptions that POM represents a large
portion of the slow organic compartment and that the MAON represents
a large fraction of the passive compartment defined in the Century mod
el. The N located in the MAON had the lowest availability for plant up
take. Isotopic data were consistent with textural effects and with the
Century model compartmentalization of soil organic N based on the res
idence time of the organic N.