Wednesday, November 7, 2007
314-2
The Role of Long-Term Forest Management in Soil Aggregation and Soil Organic Matter Storage in a Sandy Soil of the Coastal Plain.
Chemical weed control is a common and effective forest management practice in southeastern sandy soils; yet it may decrease soil organic matter (SOM) in the surface soil of managed forests. Preliminary work on soil aggregation has shown that approximately half of the SOM is held in microaggregates that appear to be influenced by rooting. The objective of this study was to determine to what degree total weed removal and subsequent forest productivity influenced soil aggregation and SOM storage in an 18-year old Pinus taeda plantation growing on a Spodosol near Gainesville , FL. The studies treatments were: control and sustained weed removal. Soil samples, taken from three replicates of each treatment, were dry-sieved into 4 size fractions. Soil aggregation and aggregate stability of the largest three size fractions were measured by sonication. Aggregate organic matter was measured by loss on ignition as aggregates were destroyed by sonication. When aggregate organic matter was expressed as percent of total soil organic matter in a soil fraction (AOM%), weed removal increased the AOM% only within aggregates with high stabilization energies. There was also a difference among size fractions in the portion of total OM that was incorporated into microaggregates. The 250 to 150 micron fraction had the highest portion of aggregate OM , followed by the 150 to 53 micron fractions, with the 2000 to 250 micron fraction having the lowest amount. When aggregate organic matter was expressed as g OM per g of soil fraction (AOMg), it was clear that more aggregate organic matter was found in the weed removal treatment. That treatment resulted in more OM incorporated into high stability aggregates. More AOMg was found in the weed removal treatment within the 2000 to 250 and 150 to 53 micron size fractions. Therefore, the weed removal treatment, operating over 18 years clearly increased both the AOMg and the AOPM% held within the most stable aggregates. Weed removal at least doubled the above-ground growth of loblolly pine. Weed removal increased above ground litter input, but the effect on root amount and turnover over that period is currently unknown. What is causing the higher stability of aggregates in which more AOMg is incorporated under weed removal is also unknown. These data potentially conflict with two published reports from the southeast where weed removal as a forest management practice decreased soil organic carbon. They argue for investigating the controls on aggregate stability and how pine root inputs differ from understory plant inputs in the development of stable aggregates in these uniquely sandy soils.
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