Aggregation and Aggregate Carbon in a Forested Southeastern Coastal Plain Spodosol

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. Soil organic C (SOC) management requires an understanding of the processes by which SOC is sequestered. Secondary forests in the southeastern United States, covering an area of 1.4 million km2, 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 cmolc kg −1 of cation exchange capacity (Carlisle et al., 1981, 1988, 1989; Sodak et al., 1990). Forest fertilization and chemical weed control are two management 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), presumably 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), incorporation into aggregates (physical protection), translocation and storage in the subsoil, particularly the Bh horizon, and biochemical 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 formation through clay–polyvalent cation–organic matter complexes FO R EST, R A N G E & W LD LA N D SO LS Deoyani V. Sarkhot N. B. Comerford* Dep. of Soil and Water Science Univ. of Florida P.O. Box 110290 Gainesville, FL 32611-0290