The fate at several time intervals of ¹⁵N‐labelled ammonium nitrate applied to an established grass sward

Abstract

SUMMARY 15N‐labelled ammonium nitrate solution was applied in late April to circular, enclosed micro‐plots prepared by pressing open‐ended polypropylene cylinders into an established sward of perennial ryegrass. Cylinders were removed from the ground at intervals between 2 and 370 days after the application and assessments made of the distribution of 15N in plant and soil components. Of the added labelled N, 54.7% was recovered in the herbage which was cut four times during the growing season and again at the final sampling date. After two days, 37% of the labelled N was recovered in the soil microbial biomass. Large fluctuations occurred in the amount of 15N recovered in the soil microbial biomass over the next 14 days suggesting that rapid cycling of 15N occurred between this fraction and the mineral N fraction. After the first cut in late May, translocation of15N occurred more slowly from the roots into the stubble than from stubble into new herbage, so that the amount in the stubble declined more rapidly than did that in the roots. During the winter, there was no net transfer of N from the roots to above‐ground components of the sward. By the end of the growing season, half the 15N remaining in the sward was immobilized in the humified fraction of the soil organic matter; some of this was mineralized in the following spring.

Methodological tests of the use of trace elements as tracers to assess root activity

“…This is shown by Bristow et al (1987) NO3, where the proportion of total N uptake in the harvested plant varied from 2 % on the day of application to 16.8% after 27 days.…”

Within-season grass yield and nitrogen uptake, and soil nitrogen as affected by nitrogen applied at various rates and distributions in a high rainfall environment

“…This suggests that N was stored in the soil in a form not measured as soil extractable inorganic N and/or plant material remaining in the plot. Tracer studies have shown that considerable fertilizer N can be incorporated into organic fractions in the soil (Bristow et al 1987;Ledgard et al 1988;Malhi 1995;Stevens et al 1986), and that the sizes of these fractions increase with time. Whitehead et al (1990) estimated N released by mineralization in the first year after cultivation increased from 70 kg N ha -1 in a 1-yr-old stand to 300 kg N ha -1 in a 15-yr-old stand.…”

Complex dynamics in a carbon-nitrogen model of a grass-legume pasture

“…This may explain why clover content may become low, but few accounts proceed to explain how co-existence is achieved; that is, how a range of mixtures of low clover content are nonetheless sustained. Studies of the dynamics and the cyc1ing and fate of N from urine patches (temporally and spatially heterogeneous environments) have been made (see Ryden, 1984;Bristow, Ryden and Whitehead, 1987;Marriott, Smith and Baird, 1987), and these c1early re1ate to the perturbation and alternation of species advantage explored here. However, the model predicts co-existence even with constant N inputs.…”