EVALUATION OF POST-HARVEST ORGANIC CARBON AMENDMENTS AS A STRATEGY TO MINIMIZE NITROGEN LOSSES IN COLE CROP PRODUCTION - Agricultural Resource Use and Management - page 70

In the CR-control, autumn 0-30 cm SMN concentration ranged from
118 to 368 kg N ha -1 (Figure 1) and SMN was affected by treatment. Af-
ter the early-broccoli system and relative to the CR-control, the 0-30 cm
SMN concentrations were reduced by the OCA treatments of OCA-straw
in 2009 and 2010, OCA-yard in 2010 but not in 2009, and OCA-oil in
2009 and 2010 (Figure 1).
After the late-broccoli system, the only treatment which reduced SMN
compared to the CR-control was the OCA-oil in both 2009 and 2010
(Figure 1). The CC-oat treatment did not have different 0-30 cm SMN
concentrations compared to the CR-control (Figure 1). The CR-removal
reduced the 0-30 SMN levels relative to the control in the autumn after
early-broccoli system, but not after the late-system in both years (Figure
1). No treatment differences were found in the 30-60 cm depth (data not
shown), with SMN concentrations ranging from 17 to 51 kg N ha -1 in the
CR-control.
Due to the lack of effect on SMN by the CC-oat compared to the CR-
control (Figure 1, Table 3), it is suggested that oat cover crops may not
reduce the potential for N losses after broccoli harvest. Conversely, the
establishment of an oat cover crop after green pea production reduced au-
tumn SMN concentrations in southwestern Ontario [41]. It is possible that
the CC-oat had low N uptake compared to the plant available N in the soil
after broccoli harvest, considering that 94 to 210 kg N ha -1 remained as
available N at green pea harvest [41] while 265 to 415 kg N ha -1 remained
at broccoli harvest.
Despite the removal of the N rich crop residue (CR-removal), the 0-30
cm SMN concentrations were only reduced in the autumn after the early-
broccoli and not after the late-broccoli system (Figure 1). The difference
in CR-removal effect between systems was likely a reflection of the cooler
temperatures and slower N mineralization of the crop residue in the late-
system. Regardless, high autumn SMN concentrations (89 to 227 kg N
ha -1 ) remained in the field in the CR-removal treatment (Figure 1). It is
therefore suggested that the soil and/or below-ground crop residue pro-
vided substantial quantities of N during the post-harvest period. Thus, best
management practices that minimize N losses after broccoli production
would be beneficial.