SG (Slot)

SOIL NITROGEN
In 1988, results for soil nitrogen, at corn planting, from the 0 to 15 em depth
increment (Table 2) were similar for all treatments. About 52 mm of rainfall occurred
between the 2 wk interval of the first and second killing of clover. The corresponding
average maximum air temperature was 20°C. These conditions suggest that
decomposition and N mineralization of the early killed clover (T1) should have
occurred. According to Neely (1990), about 10% of theN from crimson clover should
have been released by 14 days after the first kill. Similar research studies of Neely on
15 N in hairy vetch (Vicia vil/osa Roth) indicated that most of the N from vetch
remained in the upper 7.5 em soil depth over the 161 d period. These similarities,
therefore, suggest that N from the early killed clover may have remained in the upper
soil surface layer; however, the trend is for greater N in T1. Results in spring of 1989
and 1990 showed that plots that reseeded (ground cover > 65%) had much higher
soil N than those that did not reseed adequately. Results for similar treatments were
much higher in 1989 or 1990 than in 1988. Averaged over all treatments soil N
increased 47 and 53%, respectively, by spring 1989 and 1990. However, differences
in soil N between 1989 and 1990 were very small, suggesting N uptake by the corn
crop in 1989. The increase in soil N in the bare fallow in 1989 and 1990 over that in
1988 could be associated with N mineralization during the fallow period. This
phenomenon was reported in Nebraska (Lamb et al. 1985). These results indicate
that growing legumes on the same piece of land, in this case by reseeding, and long
term fallowing will build up soil N, findings which are in agreement with Lamb et al.
(1985), Hargrove (1986), Langdale et al. (1987), Smika (1990).
SOIL CARBON
Soil samples were collected in October of each yr, after harvesting of corn. In
1988, treatments with clover had much higher soil C (Table 3) than bare fallow,
although these differences were not statistically significant at Plains. Similar trends
were observed in 1990. The increase in soil C for bare fallow at Plains is likely due
to dead leaves or roots from corn and the absenceof tillage. Results for soil C were
much lower Plains than at Griffin. This difference is perhaps as a result of high
summer temperatures and long growing season at Plains than at Griffin. McVay et
al. (1989) reported similar values for Plains. These findings as well as those reported
by Hargrove (1986), Langdale et al. (1987), MacVay et al. (1989) clearly indicate that
winter legumes can rapidly improve soil carbon in the upper surface layer. Reseeding
of clover made this possible in our study.
PERCENT WATER STABLE AGGREGATES
In 1988, results for percent water stable aggregates (> 0.25 mm) (Table 4) were
similar for all treatments tested within each site. Although soil C was greater in 1988
for treatments with clover residue (Table 3), it was not sufficient to produce an
increase in the percentage of stable aggregates. There was a marked increase in
aggregate stability in plots with clover in 1990, however. This suggests that the
formation of stable aggregates may take time to develop, even after elevated organic
C levels have been achieved. Aggregate stability for bare fallow decreased at Griffin,
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