Internal Cycling of Nitrogen in Shortgrass Prairie

Abstract

Study was made of the rates at which and the routes by which nitrogen once taken up by the plant moves into litter and soil compartments and from them to new plant growth. Field mini—plots were given an initial pulse treatment with 15N, following which destructive removals were made over the next 5 growing seasons. In the year oif application, 15N moved primarily to the green herbage; 0.58 of all 15N recovered in plant materials the 1st year was in such herbage. There was late season translocation of N from aboveground to belowground plant parts. The pulse N peaked in the aboveground dead compartment in the winter months following the first and preceding the second growth season. Transfer of 15N from the aboveground litter occurred principally during the second growing season. The 15N content of crowns and live roots did not change significantly during the 5 seasons; that of senescent and detrital roots increased significantly during the same interval. It was concluded that the annual N requirement in ungrazed blue grama is met by a combination of 4 nitrogen—supplying mechanisms. One of these is that of internal translocation whereby N of one season is stored over winter belowground and then moved to new growth in the next growing season. Another is that of mineralization of easily decomposable organic materials; among these are certain herbage components, roots exudates and exfoliates, and short—lived unsuberized roots. A third is mineralization of organic nitrogen synthesized by microorganisms that grow on energy—rich materials such as those named in the immediately preceding sentence. Finally, not all of the plant and microbially synthesized organic N undergoes quick release to the available N pool; a portion of it undergoes polymerization and becomes humic nitrogen, from which there is slow release to the available N pool. It is the first 3 mechanisms that promote quick recycling of N in the blue grama system. Once a given N atom makes its initial entry into the blue grama plant, there is greatly increased probability that the atom will again enter ner herbage growth in each of several following years. Nitrogen that enters the soil humus is very slowly recycled into new plant growth.

INNOVATIVE15N MICROPLOT RESEARCH TECHNIQUES TO STUDY NITROGEN USE EFFICIENCY UNDER DIFFERENT ECOSYSTEMS

“…After 10 yrs, 15 N recovery of the applied N decreased at both topographic positions to 85% in the footslope position and 27% in the backslope position. The earlier results of Clark (1977) and those by Delgado et al (1996b) illustrate not only the strengths of the confined-microplot technique for long-term studies, but also the power of using 15 N labeling to study % 15 N recovery, turnover rates of N in the soil organic matter, and the partitioning of N between the soil and plant material. Much was also elucidated by the use of 15 N about the long-term effect of soil texture, slope position, and grassland systems on the retention of N within this type of ecosystem.…”

“…An early and perhaps classic study of the rates and the routes by which N, once taken up by the plant, moves into litter and soil compartments pools and from them to new plant growth was reported by Clark (1977). Using 15 N tracer, and physical barriers for a study conducted in Colorado in a semiarid soil moisture regime (Soil Survey Staff 1998), 64 individual cylinders (20 cm diameter and 40 cm long) were driven to a depth of 36 cm into otherwise undisturbed shortgrass prairie.…”

“…Selected cylinders were removed over the next five growing seasons and the plant and soil material separated for analy-ses. Among Clark's (1977) conclusions were that, in the year of application, 15 N was removed primarily in the green herbage and that there was late season translocation of N from aboveground to belowground plant parts. The pulse of 15 N peaked in the aboveground dead compartment in the winter months following the first and preceding the second growing season.…”

“…No N was applied in 1977to 1979. In 1974to 1977, irrigation applications of 5, 7.5, or 10 cm of water were made at 2-, 3-, or 4-week intervals, respectively, until a total of 30 cm had been applied. The plots were uniformly irrigated in 1978 to 1979.…”

Productivity, 15N dynamics and water use efficiency in low‐ and high‐input switchgrass systems

“…Warm-season perennial grasses translocate N during senescence from AG to BG biomass (Clark, 1977). For example, Heckathorn & DeLucia (1994) reported that 30% of the N was translocated BG in C 4 perennial grasses in response to drought stress.…”