LONG-TERM N-15 STUDIES IN A CATENA OF THE SHORTGRASS STEPPE

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.