Fire Effects on Amounts of Soil Organic Matter (Carbon) and Nitrogen

Prescribed fire

Prescribed fire in pine forests of the southern coastal plain of the United States has the clear effect of decreasing organic matter (carbon, or C) and nitrogen (N) in the forest floor, while having either no effect or a weak positive effect on organic matter and N in the mineral soil (Table:Effects of Prescribed Burning on Soil Carbon and Nitrogen in Pine Forests of the Southern Coastal Plain). The effect of prescribed fire on total (forest floor + mineral soil) C and N appears to be negative.  However, N fixation likely plays a role in preventing excessive N loss from coastal plain ecosystems (Stone 1971; Fisher and Binkley 2000).

There are pressing research needs involving organic matter in prescribed burn ecosystems in the Southern Coastal Plain as well as in other prescribed-burn southern ecosystems. Research in these areas would further understanding of fire’s effects on soil fertility and would also aid in the management of forests for maximum soil C storage.

• The effects of prescribed burning on soil organic matter and N in southern ecosystems outside of the Coastal Plain (e.g. mountains, piedmont) should be assessed as this topic has received little or no attention.  These areas differ in moisture, temperature, and topography from the Coastal Plain, and would likely exhibit different trends in organic matter and N.
• Carbon at depth (more than 20-50 cm below the mineral soil surface), as related to prescribed fire, has received little attention even though some fire-impacted southern soils are known to contain appreciable amounts of organic matter at depth.  For example, Stone et al. (1993) estimated that half of soil organic matter in Florida Spodosols was contained in the spodic horizons.  It is likely that fire was a factor in the genesis of Florida Spodosols and certain other soils of the South, as nearly all forest types in the southern costal plain were subject to frequent fires (Stanturf et al. 2002).
• The decreased probability of wildfire should be reflected in the soil C “accounting” of ecosystems managed by prescribed burning.  The “losses” of soil C due to prescribed burning are probably offset somewhat by decreased wildfire risk.

Wildfire

There is little information on wildfire effects on soil carbon and nitrogen in the South.  Results from one study are described below:

One year after a lightning-caused wildfire in Shenandoah National Park, Virginia, Groeschl et al. (1991) measured C and N amounts in low- and high-intensity areas of the wildfire area as well as in an adjacent unburned area.  Most of the soil C and N loss occurred in the forest floor,  with total losses (forest floor plus mineral soil to depth of 10 cm) of  26 and 68% (C, low- and high-intensity areas) and 10 and 65% (N, low- and high-intensity areas)  The losses in high-intensity areas are conservative estimates as erosion was observed in these areas.  In the low-intensity area of the wildfire, the relatively low N loss (10%) compared to C (26%) may reflect nitrogen fixation during the recovery period.