SSSAJ: Volume 71: Number 6 • November–December 2007 1779 Soil Sci. Soc. Am. J. 71:1779–1787 doi:10.2136/sssaj2006.0340 Received 29 Sept. 2006. *Corresponding author (nbc@ufl.edu). © Soil Science Society of America 677 S. Segoe 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. S oil organic C (SOC) management requires an understanding of the processes by which SOC is sequestered. Secondary for- ests in the southeastern United States, covering an area of 1.4 mil- lion km 2 , accumulated C at a rate >70 million Mg yr −1 (Delcourt and Harris, 1980), identifying them as important regional C sinks. Richter et al. (1995) reported that a 34-yr-old loblolly pine (Pinus taeda L.) plantation in South Carolina sequestered C at a rate of 5.16 Mg ha −1 yr −1 . Many of these plantations are underlain by sandy Spodosols (Adegbidi et al., 2002), which represent the dominant soil order in Florida, covering 27% of the state (Stone et al., 1993). Many Spodosols are exceptionally sandy, with <5% silt plus clay and <10 cmol c kg −1 of cation exchange capacity (Carlisle et al., 1981, 1988, 1989; Sodak et al., 1990). Forest fertilization and chemical weed control are two man- agement inputs that have increased the productivity of southern pine stands in these landscapes (Albaugh et al., 1998; Jokela et al., 2004) and altered N and P dynamics and cycling (Polglase et al., 1992a,b,c,d; Grierson et al., 1998, 1999; Gurlevik et al., 2004; Meason et al., 2004; Sanchez, 2004). Fertilization has not promoted an increase in SOC (Harding and Jokela, 1994; Shan et al., 2001), however, while chemical weed control has reduced the SOC content (Shan et al., 2001; Echeverria et al., 2004), presum- ably by reducing detrital inputs of understory plants. The effects of these practices on SOC within aggregates and the development of aggregates have yet to be considered. Soil organic C can be protected from decomposition through four mechanisms: sorption onto clay (chemical protection), incor- poration into aggregates (physical protection), translocation and storage in the subsoil, particularly the Bh horizon, and biochemi- cal transformation into products that are resistant to microbial attack (biochemical protection) (Stone et al., 1993; Six et al., 2002; Blanco-Canqui and Lal, 2004; Jiménez and Lal, 2006). The soil structure of Florida’s Spodosols is described as weak crumb to granular or single grain (Carlisle et al., 1981, 1988, 1989; Sodak et al., 1990), suggesting poor soil aggregation. In these soils, the potential for chemical protection of soil C is limited by the low clay content. The low cation content also limits aggregate forma- tion through clay–polyvalent cation–organic matter complexes FOREST, RANGE & WILDLAND SOILS Deoyani V. Sarkhot N. B. Comerford* Dep. of Soil and Water Science Univ. of Florida P.O. Box 110290 Gainesville, FL 32611-0290 Eric J. Jokela School of Forest Resources and Conservation Univ. of Florida P.O. Box 110410 Gainesville, FL 32611-0410 James B. Reeves III USDA-ARS, Environmental Management and Byproducts Utilization Lab. Bldg. 306, Rm. 101 BARC-East Beltsville, MD 20705 Willie G. Harris Dep. of Soil and Water Science Univ. of Florida P.O. Box 110290 Gainesville, FL 32611-0290 Abbreviations: AOM, aggregate organic matter; DRIFTS, diffuse reflectance infrared Fourier-transform spectroscopy; POM, particulate organic matter; SOC, soil organic carbon. Physical protection of C by aggregates and their response to forest management are important components of soil C management. This study was conducted to examine the morphology and strength of aggregates, to quantify C held by aggregates, and to study the effects of forest management intensity on aggregation. Surface horizon soil (0–5- and 5–10-cm depths) was collected from a 4-yr-old loblolly pine (Pinus taeda L.) plantation in North Florida under two contrasting management regimes (intensive vs. operational fertilization and chemical weed control, called high- and low-intensity treatments, respectively). Samples were dry sieved into four size classes: 2000 to 250, 250 to 150, 150 to 53, and <53 μm. Soil aggregates of varying morphology and strength were observed in the three sand size fractions. Aggregate strength, as measured by sonication, varied with size fraction and ranged from approximately 17 J mL −1 for the least stable macroaggregates in the 2000- to 250-μm fraction to 113 J mL −1 for the most stable microaggregates in the 150- to 53-μm fraction. Aggregate organic matter (AOM) was an important C pool in these soils, accounting for nearly half of the total soil organic matter. The high-intensity management treatment had lower AOM in the 2000- to 250-μm fraction, probably due to lower biomass input of understory roots caused by chemical understory control. Modification of the sonication technique proved useful for studying different aspects of aggregation and gave indications of an aggregate hierarchy even in these extremely sandy soils. Aggregation and Aggregate Carbon in a Forested Southeastern Coastal Plain Spodosol