LIGHT-FRACTION SOIL ORGANIC-MATTER - ORIGIN AND CONTRIBUTION TO NET NITROGEN MINERALIZATION

Two investigations were conducted to examine the nature of light-fraction (<1.75 Mg m-3) soil organic matter (OM): (1) determination of the relative contributions of above- and belowground litter to the light fraction (LF) pool in two oak stands (Noe Woods and Wingra Woods) that are part of a long-term (> 30 yr) litter manipulation study in southern Wisconsin, U.S.A.; and (2) evaluation of the monthly variation in LF mass and LF N mineralization potential, plus the contribution of LF to total net N mineralization, for a cornfield, pine stand, and maple stand 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 treatment was 64 and 32% lower for the Noe and Wingra sites, respectively, relative 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-soil C gain under 2 x litter. The results suggest that aboveground foliar litter contributed one-third (Wingra) to two-thirds (Noe) of the LF pool. Additionally, the heavy fraction (HF) declined quickly in response to relatively short-term changes in litter input and functioned as the primary long-term C sink. In the second study, LF represented 11% (corn), 13% (pine), and 2% (maple) of the N mineralization potential (anaerobic incubation) for the whole mineral soil; contribution of LF to net N mineralization could not be determined for the cornfield, but was calculated to be 3 kg N ha-1 yr-1 for both the pine and maple stands. N mineralization potential for the pine and maple sites was consistently lower for LF OM than for HF OM. Both LF mass (corn and pine) and LF N mineralization potential (all sites) showed differences by sampling month. LF constituted 5% (maple), 13% (corn), and 14% (pine) of mineral soil OM. Findings suggest that HF is the primary N source in coarse-textured mineral soils, and also that LF is a relatively minor N source in forest stands with mor-type soil.