Agriculture Reference
In-Depth Information
Nitrogen immobilization is related to the biochemical composition of
the decomposing substrate, and is positively associated with a high C:N
ratio, lignin, and polyphenol content [42]. Incorporation of easily decom-
posable, high C:N ratio materials generally causes a rapid increase in mi-
crobial biomass and consequently SMN depletion as N is assimilated into
microbial cells. Soil amendments such as yard waste, wheat straw, and
used cooking oil/oily food waste have previously demonstrated N immo-
bilization [11,19,23,25,26].
The OCA-yard reduced SMN compared to the CR-control only after
the early-broccoli system in 2010 (Figure 1). The 2009 yard waste was
composed of notably larger particles than that of 2010, thus it is possible
that C decomposition took longer in 2009 and had less influence on mi-
crobial N immobilization. More recalcitrant substrates, such as the lignin-
containing wood pieces of the OCA-yard, can have more limited decom-
position at low temperatures than that of easily decomposed and more
labile material [43]. It is likely that C decomposition of the OCA-yard
was generally low, and consequently little microbial N immobilization oc-
curred. Perhaps if the OCA-yard material was finely chopped, greater de-
composition may occur. As opposed to the present study, previous labora-
tory research showed N immobilization by incorporating yard waste with
broccoli crop residue [11], and field research has found that green waste
compost mixed with cauliflower crop residue immobilized approximately
42 kg N ha
-1
within the first month after incorporation [12]. Yet, in agree-
ment with the present study, green waste composts have not resulted in N
immobilization during autumn after the incorporation with cauliflower or
leek residues in a two-year study [44]. Thus, the composition and substrate
size of OCA-yard greatly influences N immobilization and its applicabil-
ity as a better management practice for minimizing N losses in the autumn
after broccoli harvest.
Although OCA-straw reduced SMN after the early-broccoli system, it
must be noted that some wheat seed germinated and established a cover
crop during both years. Thus, the reduction in autumn SMN may be a
refl ection of cover crop N uptake as well as microbial N assimilation. Con-
versely, it appeared that the OCA-straw treatment after the late-broccoli
system did not suffi ciently lower SMN, compared to the CR-control, to
