Agriculture Reference
In-Depth Information
Table 4.2.2.
N mineralization rate in situ(LIXIM calculation) and under controlled conditions
(incubations) expressed as kg N ha
−
1
nday
−
1
(nday = normalised day for residues in treated soils
kept bare. The first period lasts 202 and 100 normalized days for the KL and KZ sites, respectively;
then the second period takes place until the end of the observations (March 1999).
Vp 1st period
Vp 2nd period
LIXIM
Incubation
LIXIM
Incubation
KL PG-0N
1.61
1.41
0.46
0.55
KL PG-200N
2.27
1.92
0.63
KL PG-400N
1.73
1.89
0.62
KL G/WC
2.21
2.0
0.61
KZ PG-250N
1.06
1.85
0.45
0.66
KZ G/WC
1.11
1.86
mineralization of soil OM, while the high rates observed during the first 100
days would correspond to the decomposition of the easily decomposable
OM accumulated during the grass period (8 years). This hypothesis is
supported by the kinetics of C mineralization.
Laboratory incubation and
in situ
estimates of net mineralization gave
a similar range of net mineralization rates (using the normalized days as a
common unit, Table 4.2.2). However, differences between sites did not
appear in controlled conditions, and the period of intense mineralization
lasted longer in field conditions than under controlled conditions (180
versus 100 normalized days). The differences observed might result from
differences in decomposition conditions (Mary
et al
., 1996). Soil-residue
contact was favoured in laboratory incubation by cutting residues and
mixing them with sieved soil, whereas
in situ
decomposition occurred at the
soil surface. Laboratory incubation conditions may have stimulated decom-
position compared with field conditions, as has already been observed for
rapeseed residues (Dejoux
et al
., 2000; Trinsoutrot
et al
., 2000).
Discussion and Conclusion
The study confirms that a high rate of N release follows grassland
destruction, even without soil tillage: ~2 kg N ha
−1
normalized day
−1
in bare
soil during a first period corresponding to the decomposition of fresh
residues. The rate was almost independent of the previous grassland man-
agement. Thus no relationship was observed between N balance during the
grazing period and N mineralization after destruction. Unlike the results of
Hogh-Jensen (1996), no significant difference was observed between
mineralization of pure grass versus grass-clover, but incorporated N and C
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