Methane and nitrous oxide evolution and 15N and 226Ra uptake as affected by application of gypsum and phosphogypsum to Louisiana rice

Abstract

A field study was conducted to determine the effects of gypsum and phosphogypsum on methane and nitrous oxide evolution and on 15N and 226Ra uptake by Oryza sativa L. (rice). Gypsum and phosphogypsum at 2.5, 5.0 and 10.0 tons ha −1 and 15N labelled urea (150 kg N ha −1) were applied pre-flood to experimental plots drill-seeded with `cypress', a semi-dwarf long-grain rice cultivar. Methane and nitrous oxide were measured twice a week over the main cropping season. Plant samples were collected at harvest and 15N uptake and 226Ra activity measured. Three rates of gypsum reduced methane evolution 49, 52 and 66%, respectively, compared with control plots. Phosphogypsum (2.5, 5.0 and 10.0 tons ha −1) decreased emissions of methane 47, 46 and 51%, respectively, over the 84-day study. Nitrous oxide fluxes were low (10–29 g ha −1 day −1) and only detected from control plots on two sampling dates. The assimilation of applied 15N in grain collected from gypsum plots was significantly higher compared with control and phosphogypsum treatments. The measured grain 226Ra activities were low and averaged 0.518 Bq kg −1 for phosphogypsum compared with a control plot activity of 0.222 Bq kg −1.