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FOREST, RANGE & WILDLAND SOILS

Aggregation and Aggregate Carbon in a Forested Southeastern Coastal Plain Spodosol

^a Dep. of Soil and Water Science Univ. of Florida P.O. Box 110290 Gainesville, FL 32611-0290
^b School of Forest Resources and Conservation Univ. of Florida P.O. Box 110410 Gainesville, FL 32611-0410
^c USDA-ARS, Environmental Management and Byproducts Utilization Lab. Bldg. 306, Rm. 101 BARC-East Beltsville, MD 20705
^d Dep. of Soil and Water Science Univ. of Florida P.O. Box 110290 Gainesville, FL 32611-0290

^* Corresponding author (nbc{at}ufl.edu).

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.

Abbreviations: AOM, aggregate organic matter • DRIFTS, diffuse reflectance infrared Fourier-transform spectroscopy • POM, particulate organic matter • SOC, soil organic carbon