Agriculture Reference
In-Depth Information
source of this N. Nitrogen reserves for cereals (an N index 0 residue) were
derived mainly from pre-incorporation SMN, whereas for sugarbeet (also
an N index 0 residue) the contributions from pre-incorporation SMN,
mineralized N from below-ground residues, and mineralized N from
above-ground residues were approximately equal. Similarly, for N index 1
residues such as potatoes and cabbages, N reserves were derived mainly
from pre-incorporation SMN, whereas those for oilseed rape were derived
from pre-incorporation SMN and mineralized N from below-ground
residues. The crops considered to be intermediate between N index 0 and
N index 1 (peas and beans) were similar to oilseed rape, except that
pre-incorporation SMN tended to be slightly higher and mineralized N
from below-ground residues was lower.
The significance of these results is twofold. First (in general), the larg-
est proportion of N from arable crop residues is vulnerable to leaching, as it
is already in mineral form in the autumn. Secondly, (for most crops) further
N is derived from mineralization of below-ground residues, implicating root
turnover as the main mechanism for recycling plant N through the SOM;
the effect of above-ground residues is small and generally immobilizes N.
The exception is sugarbeet, for which above-ground residues are also
important as a source of mineral N.
Conclusions
Crop residue returns are an important N source to the following crop. The
amount of N made available to the next crop depends on the amount of N
returned and its 'mineralizability'. This is recognized in current advice.
However, our experiments have shown where there is scope for improving
recommendations and/or N management.
•
For many crops, much of the returned N is already in mineral form
in the autumn. Management options to reduce losses of nitrate are
therefore limited: an early sown crop (or cover crop) to trap some of the
N, or delaying cultivation.
•
Fertilizer advice will be affected by residue incorporation date and
winter rainfall because of effects on mineralization and leaching:
current advice does not do this.
•
Residue incorporation date appeared to affect the
total
N release. This
may have been because for earlier incorporation dates much of the min-
eralized N was leached from the soil system, whereas for later incorpo-
ration, SMN retained in the soil was available for re-immobilization.
The result is in contradiction to accepted wisdom and needs further
investigation.
•
Sugarbeet tops act as a 'nitrogen store' during winter. Consequently,
this organic N is protected from leaching and will be made available to
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