Dissolved organic C and SOC were significantly correlated
(r = 0.70). Both SOC and DOC exhibited cyclical patterns
after compost application, as evidenced by lower
concentrations during the winter sampling time at 16
months. Soil organic C was significantly lower for all compost
treatments and unamended soil at 16 months than at
11 and 23 months, suggesting that the transient decline in SOC was not compost related, but likely influenced by the
reduced growth of St. Augustinegrass during the onset of
dormancy in winter, in addition to precipitation. Warmseason
turfgrass growth is more vigorous from late spring
to summer, with a dormant period from late autumn to
winter (Trenholm et al., 2000). Turfgrasses often produce
high levels of belowground biomass (Trenholm et al.,
2000), which ultimately increase soil C levels. Moreover,
the decomposition by-products of belowground biomass
and aboveground clippings likely contributed to SOC
and eventually DOC. Since St. Augustinegrass production
levels are lower in cooler months than in warmer months, a
lower contribution of turfgrass to soil organic matter might
explain the lower DOC at 16 months (Nov. 2002). St.
Augustinegrass was irrigated during the growing season,
which may have promoted leaching of DOC and nutrients
from the soil surface at the 16 and 29 month sampling
times. A similar decrease in SOC at 29 months compared
to 23 months, although to a lesser extent, was also
observed from compost-amended but not unamended soil.
Studies have indicated that dissolved organic matter added
with compost to a sandy soil was leached from the root
zone within a year (Kaschl et al., 2002). Thus, changes in
SOC and DOC in our study over time may have been
due not only to decomposition of compost materials, but
also to decomposition of native soil organic matter and
production of DOC from St. Augustinegrass residues. This
supports the hypothesis of seasonal patterns of SOC and
DOC dynamics related to St. Augustinegrass growth.