Two investigations were conducted to examine the nature of light-fract
ion (<1.75 Mg m-3) soil organic matter (OM): (1) determination of the
relative contributions of above- and belowground litter to the light f
raction (LF) pool in two oak stands (Noe Woods and Wingra Woods) that
are part of a long-term (> 30 yr) litter manipulation study in souther
n Wisconsin, U.S.A.; and (2) evaluation of the monthly variation in LF
mass and LF N mineralization potential, plus the contribution of LF t
o total net N mineralization, for a cornfield, pine stand, and maple s
tand in western Massachusetts, U.S.A. The long-term treatments at the
Wisconsin plots include no-litter, 2 x litter, and one-time removal of
the A horizon. In the first study, LF mass under the no-litter treatm
ent was 64 and 32% lower for the Noe and Wingra sites, respectively, r
elative to the controls (unaltered litter inputs); LF mass under the 2
x litter treatment was 85% higher at Noe, but 15% lower at the Wingra
site. The LF represented only 6.7 and 4.3% of the total mineral-soil
C losses under no-litter and 7.0% (Noe alone) of the total mineral-soi
l C gain under 2 x litter. The results suggest that aboveground foliar
litter contributed one-third (Wingra) to two-thirds (Noe) of the LF p
ool. Additionally, the heavy fraction (HF) declined quickly in respons
e to relatively short-term changes in litter input and functioned as t
he primary long-term C sink. In the second study, LF represented 11% (
corn), 13% (pine), and 2% (maple) of the N mineralization potential (a
naerobic incubation) for the whole mineral soil; contribution of LF to
net N mineralization could not be determined for the cornfield, but w
as calculated to be 3 kg N ha-1 yr-1 for both the pine and maple stand
s. N mineralization potential for the pine and maple sites was consist
ently lower for LF OM than for HF OM. Both LF mass (corn and pine) and
LF N mineralization potential (all sites) showed differences by sampl
ing month. LF constituted 5% (maple), 13% (corn), and 14% (pine) of mi
neral soil OM. Findings suggest that HF is the primary N source in coa
rse-textured mineral soils, and also that LF is a relatively minor N s
ource in forest stands with mor-type soil.