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Title: Aggregation and Aggregate Carbon in a Forested Southeastern Coastal Plain Spodosol

Author
item SARKHOT, D - UNIV FLORIDA, GAINSVILLE
item COMERFORD, N - UNIV FLORIDA, GAINSVILLE
item JOKELA, E - UNIV FLORIDA, GAINSVILLE
item REEVES III, JAMES
item HARRIS, W - UNIV FLORIDA, GAINSVILLE

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2007
Publication Date: 11/28/2007
Citation: Sarkhot, D.V., Comerford, N.B., Jokela, E.J., Reeves III, J.B., Harris, W. 2007. Aggregation and Aggregate Carbon in a Forested Southeastern Coastal Plain Spodosol. Soil Science Society of America Journal. 71:1779-1787.

Interpretive Summary: Soil aggregates are small particles of soil bound to one another. It has generally been believed that clay content is important for such aggregation. Thus, soil aggregation is influenced by the soil environment and composition and is a factor in soil carbon sequestration (retention of carbon in soil over periods of time). Sandy Coastal Plain soils often do not have the clay to promote aggregation, and thus have not been considered to have high levels of aggregation. This study was conducted to examine the aggregate structure, to quantify aggregate stability and to investigate aggregate protected carbon; as well as to determine the effect of management on aggregation in Sandy Coastal Plain soils. This was accomplished through the use of a 4-year-old loblolly pine plantation in north Florida with two contrasting management regimes. Results showed that aggregates were composed of both organic and mineral fractions and that sonication was useful for estimating the strength of various aggregate fractions (based on size) and their carbon content. Also, aggregate organic matter had a significantly different chemical composition than Particulate organic matter, being higher in polysaccharides (carbohydrates), esters and amides (proteins). In conclusion aggregation in sandy Coastal Plain soils is a significant mechanism for incorporating carbon into surface soils, particularly in the absence of any significant amount of clay, and sonication appears to have great potential in investigation the aggregate hierarchy and strength/stability in these soils and their influence on carbon cycling and sequestration.

Technical Abstract: Soil aggregation is influenced by the soil environment and is a factor in soil carbon sequestration. Sandy Coastal Plain soils often do not have the clay to promote aggregation nor have been considered soils with high levels of aggregation. This study was conducted to examine the aggregate morphology, to quantify aggregate stability and to investigate aggregate protected carbon; as well as to determine the effect of management on aggregation in these important soils. This was accomplished through the use of a 4-year-old loblolly plantation in north Florida with two contrasting management regimes, replicated in 3 blocks. Soils were dry sieved into four particle size classes: 2000 to 250', 250 to 150', 53 to 150' and < 53'. Soil aggregates of varying morphology and stability were observed within each size fraction and elemental dot maps confirmed that they were combinations of both organic and mineral material. Sonication of the size fractions was useful for determining aggregate stability, and for estimating aggregate carbon. Aggregate stability varied with soil particle size fraction and ranged from 2500 J for the least stable macro-aggregates to 16600 J for the most stable micro-aggregates. Aggregate organic matter had a significantly different chemical composition than Particulate organic matter, being higher in polysaccharides, esters and amides. The low intensity management encouraged higher carbon content inside soil aggregates in the 2000-250' fraction, which was most likely due to higher input of understory roots not controlled by herbicides, as was the case in the high intensity treatment. Aggregation in sandy Coastal Plain soils is a significant mechanism for incorporating carbon into surface soils, particularly in the absence of any significant amount of clay. Sonication appears to have great potential in investigation the aggregate hierarchy and strength/stability in these soils and their influence on carbon cycling and sequestration.