EVALUATION OF POST-HARVEST ORGANIC CARBON AMENDMENTS AS A STRATEGY TO MINIMIZE NITROGEN LOSSES IN COLE CROP PRODUCTION - Agricultural Resource Use and Management - page 69

Similar yields, 16 to 35 Mg ha -1 , have been reported in the literature [2,6-
9]. The lower yield in the early-broccoli compared to the late-broccoli
system agrees with previous research, which also found lower total and
head plant mass in summer harvested broccoli compared to autumn har-
vest [39], likely due to different environmental conditions during the broc-
coli growth periods. The crop residue (leaves + stems) fresh weight ranged
from 50 to 75 Mg ha -1 , similar to broccoli and cauliflower crop residue
rates in earlier studies [11,12]. The early-broccoli crop residue contained
202 and 247 kg N ha -1 , while the late-system residue had 212 and 207 kg N
ha -1 , in 2009 and 2010, respectively. Similarly, previous research observed
above-ground broccoli N accumulation of 96 to 465 kg N ha -1 , with fertil-
izer N application rates up to 500 kg N ha -1 [1,2,4,6,40].
At broccoli harvest, 0-30 cm SMN ranged from 68 to 168 kg N ha -1 ,
yet greater quantities of N (207 to 247 kg N ha -1 ) were in the broccoli
crop residue. Thus, between 265 to 415 kg N ha -1 remained in the fi eld
after harvest, which was consistent with other research [1,6]. Clearly,
strategies that minimize N losses during the post-harvest season would
be valuable.
3.3.2 SOIL MINERAL NITROGEN IN AUTUMN
Soil mineral N data were investigated, because both NO 3 - -N and NH 4 + -N
are susceptible to losses. The treatment effect on soil N was similar for
NO 3 - -N and SMN concentrations, but the soil NH 4 + -N was largely unaf-
fected by treatment. Thus, the SMN results were presented. The 0-30 and
the 30-60 cm depths were analyzed separately for the autumn dataset to
assess the downward movement and leaching potential of SMN in au-
tumn. The year variation in autumn SMN between 2009 and 2010 could
be a residual N effect attributed to the type of crop grown the year prior
to broccoli production, which was corn and soybean, respectively. Addi-
tionally, the higher autumn SMN variation in the early- compared to the
late-broccoli systems may be due to higher temperatures during the early
autumn (August to November) which may have permitted more decom-
position and resulted in a greater N mineralization, compared to cooler
temperatures after the late-broccoli system (September to November).