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
Search WWH ::




Custom Search