Soil Science Society of America Journal Mention of trade names or commercial products in this publication is solely for the purpose of providing speciic information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Soil Sci. Soc. Am. J. 77:1659–1666 doi:10.2136/sssaj2012.0423 Received 17 Dec. 2012. *Corresponding author (lisamfultz@gmail.com). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA 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. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Aggregate Carbon Pools after 13 Years of Integrated Crop-Livestock Management in Semiarid Soils Soil Biology & Biochemistry I n the semiarid Texas High Plains, depletion of irrigation sources, frequent and intense drought, high winds, and intensive agricultural practices have contrib- uted to soil degradation resulting in poorly structured soils with low organic matter (Bronson et al., 2004). Integrated crop-livestock systems are an alternative to monoculture cropping, which can help alleviate or reverse soil degradation while meeting the food and iber needs of the growing population (Franzluebbers, 2007; Allen et al., 2012). Compared to CTN, the dominant crop system in this region, ICL systems employ reduced or no-till management and high residue-returning forages (i.e., low disturbance, high input systems). By increasing the amount of res- idue inputs at the surface while minimizing soil disturbance with conservation till- age practices, ICLs should build soil organic matter and, thus, enhance aggregate Lisa M. Fultz* Jennifer Moore-Kucera Dep. of Plant and Soil Science Box 42122 Texas Tech Univ. Lubbock, TX 79409-2122 Ted M. Zobeck Veronica Acosta-Martínez USDA-ARS Cropping Systems Research Lab. 3810 4th St. Lubbock, TX 79415 Vivien G. Allen Dep. of Plant and Soil Science Box 42122 Texas Tech Univ. Lubbock, TX 79409-2122 Integrated crop-livestock (ICL) systems that utilize perennial or high-residue no-till annual forages may build soil organic matter and, thus, enhance aggre- gate stability, water retention, nutrient cycling, and C storage. We examined long-term effects of ICL management on soil organic C (SOC) pools com- pared with continuous cotton [CTN; (Gossypium hirsutum L.)] at the system and individual vegetation levels, both using limited irrigation (65 and 77% replacement of evapotranspiration, respectively). Soil samples collected in 1997 (baseline) and 2010 were fractionated into four water stable aggre- gate-size classes: macroaggregate (>250 μm), microaggregate (53–250 μm), and silt + clay (<53 μm), and three intra-aggregate size classes: particulate organic matter (>250 μm), microaggregates (53–250 μm), and silt + clay (<53 μm). Reduced tillage and increased vegetation inputs under WW-B. Dahl Old World bluestem [Bothriochloa bladhii (Retz) S.T. Blake; bluestem], a com- ponent of the ICL, resulted in increased mean weight diameter (1.5 mm in bluestem vs. 0.40 mm in CTN) and higher proportions of macroaggregates (59%) than under CTN. A continued increase in SOC was measured in the ICL following 13 yr with 22% more SOC relative to CTN. The results from the detailed soil aggregate C fractionation revealed that an ICL under limit- ed irrigation enhanced SOC stored in protected, recalcitrant aggregate pools (intra-aggregate microaggregate SOC of 8.2 and 5.4 mg g -1 macroaggregate in the ICL and CTN, respectively). These beneits impart important ecosys- tem services such as potential C sequestration and reduced erosion potential, which are especially important in these semiarid soils. Abbreviations: CTN, continuous cotton; ICL, integrated crop-livestock; POM, particulate organic matter; RCWF, rye-cotton-wheat-fallow; SOC, soil organic C; WFRC, wheat- fallow-rye-cotton.