Extreme Drought Effects on Carbon Dynamics of a Semiarid Pasture

Published in Agron. J. 105:1749–1760 (2013) doi:10.2134/agronj2013.0112 Available freely online through the author-supported open access option. Copyright © 2013 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. T Southern High Plains of Texas was once a vast native grassland dominated by short-grass prairie and, in some regions, tall-grass prairie (Gould, 1975; Allen et al., 2007). The discovery of the vast underground Ogallala aquifer in the early 20th century and its subsequent exploitation for irrigation transformed this semiarid region into one of the most intensively cultivated regions in the United States (Allen et al., 2008). The Texas High Plains is the largest producer of cotton (Gossypium hirsutum L.) in the United States (National Agricultural Statistics Service, 2007). Other crops that significantly contribute to the region’s economy are grain sorghum [Sorghum bicolor (L.) Moench], corn (Zea mays L.), and winter wheat (Triticum aestivum L.). More than 90% of the water pumped from the Ogallala aquifer is currently used for irrigated crop production in the region (Nair et al., 2013). However, the groundwater level is declining in many portions of the region due to excess water withdrawal and slow recharge. In areas where irrigation water is limited, producers are adopting dryland agriculture or planting pasture for biomass and grazing (Texas Alliance for Water Conservation, 2011). WW-B. Dahl Old World bluestem is a warm-season perennial grass adapted to limited irrigation while capable of producing high-quality forage (Philipp et al., 2005; Allen et al., 2012). In a study conducted by Sanderson et al. (1999) in central Texas, WW-B. Dahl had the highest biomass yield compared with several other forage grasses. WW-B. Dahl is gaining popularity with producers in the Texas High Plains region because of high nutritional quality and biomass production (Duch-Carvallo, 2005; Philipp et al., 2007). A change in land use from traditional row crops such as cotton, grain sorghum, and corn to pasture-based agroecosystems has several environment-related implications. Grasslands are known to play a major role in global C dynamics (Scurlock and Hall., 1998; Frank and Dugas, 2001; Vleeshouwers and Verhagen, 2002). Temperate grasslands usually act as C sinks. For example, Ciais et al. (2005) estimated that European grasslands are net C sinks and sequester approximately 74 g C m–2 yr–1. Researchers have also reported grasslands acting as net C sources (Flanagan et al., 2002; Jongen et al., 2011). Variations in climate such as drought impact soil moisture conditions, increase temperatures, and reduce the length of the growing season in many regions and can lead to net C losses. Disturbances such as fire can also alter soil and vegetation functions and can affect C source–sink dynamics (Michelsen et al., 2004; Henry et al., 2005). Grazing of grasslands involves additional management-related factors that can affect the cumulative C balance (Conant et al., 2001; Jones and Donnelly, 2004). Several studies have reported grasslands acting as C sources due to overgrazing, leading to excess removal of green vegetation and reduced primary production (Xianglin, 2009; Vetter et al., 2010). AbstrACt