Agriculture Reference
In-Depth Information
where we have applied
15
N isotopic dilution techniques to examine the
internal cycling of N.
Materials and Methods
Agricultural soils were sampled from the plough layer (0-30 cm) of 12 sites
throughout England with varying textures in arable or ley-arable rotations
(Table 4.1.1). The
15
N isotopic dilution technique requires labelled
15
Nto
be distributed throughout the soil so that the unlabelled soil NH
4
+
pool (or,
in the case of nitrification, the NO
3
−
pool) becomes uniformly enriched
with
15
N. This was achieved by applying 20 ml of (
15
NH
4
)
2
SO
4
at 60.9
atom % or K
15
NO
3
at 61.2 atom % at a rate equivalent to 1.5 mg N kg
−1
dry soil to pre-incubated soil (300 g of soil at field capacity). Solutions were
added dropwise while rotating the incubation vessels (see Willison
et al
.,
1998). A nitrification inhibitor (
acetylene treatments) was added to the
headspace above soil labelled with (
15
NH
4
)
2
SO
4
to minimize problems
associated with preferential nitrification of the applied versus indigenous
NH
4
+
. Soil was removed from the incubation vessels on days 1, 3 and 7 for
15
NH
4
+
-labelled soil and days 1, 7 and 14 for
15
NO
3
−
-labelled soil. At each
sampling, 80 g of soil was removed for mineral N extraction in 160 ml
of 2 M KCl. Values are presented on a dry weight basis. NH
4
+
-N and
±
Table 4.1.1.
Site characteristics, gross mineralization and nitrification rates and N : I ratios for ten
arable soils sampled in spring 1999.
Gross
mineralization rate
(mg N kg
−
1
day
−
1
)
Gross
nitrification rate
(mg N kg
−
1
day
−
1
)
Previous
crop
Current
crop
Ratio
N : I
Soil texture
Sandy loam
W. wheat
W. wheat
0.76
0.44
0.50
Sandy clay loam
W. OSR
W. wheat
0.56
0.24
0.67
Silty loam
Celery
W. wheat
0.57
0.28
0.74
Clay
W. wheat
W. wheat
1.66
0.45
0.27
Clay loam
W. wheat
W. wheat
1.05
0.30
0.44
Clay loam
W. barley
W. OSR
0.96
0.46
0.92
Silty loam
W. wheat
W. OSR
1.28
0.51
1.42
Clay loam
Dried peas
W. wheat
1.05
0.61
2.10
Silty clay loam
W. wheat
W. barley
0.80
0.40
0.75
Clay loam
W. wheat
W. OSR
0.88
0.35
0.66
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