Item

Abstract

Soil organic carbon (OC) storage across
regions is influenced by climate and parent materials,
which determine soil properties like clay content and
mineralogy. Within homogeneous soil regions, land
use and management practices are further important
controls for soil OC contents and turnover. Here, we
studied the impact of study region, land use (forest,
grassland), forest management (spruce and beech
forest under age-class management, unmanaged
beech forest), and grassland management (meadow,
mown pasture, unmown pasture) on stocks and
turnover (based on D14C values) of soil OC in density
frations of topsoil horizons. Samples were taken from
36 plots in the regions Hainich–Du¨n (HAI) and the
Schwa¨bische Alb (ALB) in Germany. They were
separated into two light fractions (free light fraction
(LF1), occluded light fraction (LF2)) and the mineralassociated
organic matter (MOM) fraction using
sodium polytungstate with a density of 1.6 g cm-3.
Overall most soil OC was stored in the MOM fraction
(73%). Soil OC concentrations and stocks in the
MOM fraction differed between study regions, probably
due to larger amounts of pedogenic Al- and Feoxides
in the ALB than in the HAI region. Within each
region, forest soils stored significantly higher proportions
of total OC in the two LF (33±1.9 %) than
grassland soils (20±2.3 %). Different management of
forests and grasslands affected the C:N ratio of
density fractions, but not OC storage. While modelled
soil OC turnover in the MOM was longest of all
fractions, all fractions had average D14C values above
atmospheric values, suggesting a significant fastcycling
component in all of them. Different from
stocks, turnover of OC in the MOM fraction were not
affected by study region or contents of pedogenic
oxides. Radiocarbon contents in the LF were higher
for forest than for grassland sites, indicating faster
turnover of OC at grassland sites. However, some of
the observed difference could originate from different
average lifetimes of roots in forests and grasslands.
Applying different lag-times for OC input for forests
and grasslands significantly reduced the differences in
modelled turnover times. Lower D14C values of mown
pastures than pasture soils in both regions suggest a
management effect on soil C turnover in grasslands.
We conclude that OC storage in the MOM of
topsoil layers is more affected by regional differences in soil texture and mineralogy than by land use and management, while its turnover could not be
explained with the studied soil properties. Soil OC
storage and turnover in the two LFs is influenced by
land use (forest or grassland) and management, but ecosystem specific lag-times have to be considered for modelling OC turnover in these fractions.