Isotopic studies on the uptake of nitrogen by pasture grasses. I. Recovery of fertilizer nitrogen from the soil : plant system using rhodes grass in pots

Abstract

Results are reported of the first of a series of studies on the fate of nitrogen applied to soil under pasture. Two series of pretreatments (nil and 200 lb/ac N) were imposed on Rhodes grass, Chloris gayana Kunth, grown on a light-textured soil in pots in a glasshouse. Subsequently the grass was cut 3.3 cm above the soil surface and labelled ammonium nitrate (15NH4NO3) was added in amounts up to the equivalent of 800 lb/ac N. The fate of this added nitrogen was determined at the end of the experiment by analyzing the total contents of each pot for nitrogen and for 15N. The quantity of total nitrogen found in the pots (which included both labelled and unlabelled forms), and that of labelled nitrogen, were each linearly related to the amount of ammonium nitrate added. Regression analysis showed that 93.6% of added total nitrogen, and 94.0% of added labelled nitrogen, was recovered from the soil : plant system. These recoveries did not differ significantly from one another, indicating that the loss fell equally on labelled ammonium nitrogen and some unlabelled form of nitrogen; it is possible that this loss took place from the added ammonium nitrate fertilizer. There was no evidence to indicate the stage at which this loss occurred. Nitrogen pretreatment had a marked effect on the weight of stubble and roots in the pots when the ammonium nitrate was applied, but had no effect on nitrogen loss. Errors encountered during chemical and mass spectrometric analysis of the soil and plant material are discussed.