Similar to SOC and DOC, P exhibited significant seasonal
variability. Significant decreases occurred at 16 compared
to 11 and 23 months for all compost sources,
application rates, and unamended soil. Available P also
significantly decreased for Bryan and DilloDirt treatments
from 23 to 29 months. The declines at 16 and 29 months
were more pronounced at the highest application rates,
but also occurred for unamended soil at 16 months. For
unamended soil, available P significantly increased 31%
from 16 to 29 months. This increase may have resulted
from soil organic matter or decomposition of clippings
because no compost was applied to unamended soil.
Increasing DOC content in soil is indicative of decomposition
of compost, soil organic matter, or plant residues over
time, and decomposition of these materials likely enhanced
P mineralization rates. These results demonstrate the
importance of the contribution of native soil organic matter
and effects of turfgrass on P cycling in compostamended
soils. Concentrations remained fairly constant
over time, perhaps because P assimilation by St. Augustinegrass
was balanced by mineralization of P from compost
and St. Augustinegrass clippings and residues. Available P was significantly correlated to SOC (r = 0.57)
and DOC (r = 0.45).