Nitrogen Management and Methane Emissions in Direct-Seeded Rice Systems

Published in Agron. J. 106:968–980 (2014) doi:10.2134/agronj13.0491 Copyright © 2014 by the American Society of Agronomy, 5585 Guilford Road, Madison, WI 53711. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. ABSTRACT Rice (Oryza sativa L.) establishment systems based on resource-conserving production practices are gaining popularity globally. To investigate the potential for improved N management and mitigation of methane (CH4) emissions, field experiments were conducted in California on three crop establishment systems: water-seeded (WS) conventional, WS stale seedbed, and drill-seeded (DS) stale seedbed. Fertilizer nitrogen recovery efficiency (NRE) and rice yield as affected by N rate, source, and application timing were evaluated for 2 yr in each system. Methane emissions were monitored over a full annual rice production cycle (growing season plus fallow period). Results indicated that neither split N applications nor ammonium sulfate increased yields or NRE compared with a single application of urea, regardless of system. However, the economic optimum N rate increased by approximately 30 kg N ha-1 in WS stale seedbed compared with the conventional system. Since NRE generally remained similar across N treatments that maximized yields, applying the appropriate N rate as a single dose before the permanent flood would satisfy both agronomic and environmental goals of N management within each system. Both WS systems resulted in similar growing season CH4 emissions. However, the DS system reduced CH4 emissions by 47% compared with the conventional WS system, possibly due to a decreased period of anaerobic soil conditions. This study highlights the importance of assessing benefits as well as tradeoffs when evaluating opportunities for increasing the sustainability of direct-seeded establishment systems with respect to N management and CH4 emissions.