Agriculture Reference
In-Depth Information
Table 4.1.
Gross rates of N transformations and pool sizes in soils from Rothamsted and
Lakenheath.
Pools
(mg N kg 1 )
Processes
(mg N kg 1 day 1 )
Land
use
Organic
N
Gross
mineralization
NH 4 +
consumption
Gross
nitrification Immobilization a
Site
NH 4 +
NO 3
Broadbalk
Arable 30, 920
0.2
6.0
0.7
0.8
0.2
0.6
Park Grass
Grass
4,740 16.1
2.0
18.3
21.9
0.2
21.7
Knott Wood Wood
3,590
8.9
35.5
2.5
3.1
1.2
1.9
Lakenheath Wood
30,950
6.4
53.1
3.5
4.7
4.9
0.2 b
Lakenheath Arable 29,390
2.0
19.1
3.6
4.2
1.9
2.3
a By difference; b i.e. zero.
yields of only 1 t ha −1 year −1 grain in continuous wheat and 2 t ha −1 year −1
in rotation, and unlikely to lose much N. This is confirmed by measure-
ments which show that, on average, only 12 kg N ha −1 year −1 are leached
from this plot. Park Grass has a rapid rate of gross mineralization but a
relatively slow rate of nitrification. This soil is unlikely to lose N because
it is cycling so rapidly between mineralization and immobilization. We
thus have two N-limited soils, which have very different rates of gross
mineralization-immobilization turnover (MIT).
To begin to explore the links between land use and MIT, gross N trans-
formations were also measured on soil from Knott Wood (300-year-old
coppiced deciduous woodland at Rothamsted) and from drained fenland
peat at Lakenheath in Suffolk, which had been under either intensive arable
cropping for the last 80 years or a poplar wood for the last 30 years
(Willison et al ., 1998) The pool sizes and process rates for N transforma-
tions in all the soils are shown in Table 4.1.
The dominance of immobilization over nitrification on the Park Grass
plot could be because its acidity (pH currently 5.2 in water) suppresses
nitrifiers. However, Knott Wood soil is also acid, but N cycling there is
not dominated by immobilization: the soil is N-saturated and losing N.
Generally, for immobilization to dominate over nitrification and for N to
be conserved, sufficient C is needed to drive the heterotrophic organisms
that immobilize ammonium. Thus C and N availability determine the rate
and balance of MIT and the supply of nitrate available for plant uptake or
loss to the environment. Exudates from the dense root mass on Park Grass
could be the source of the C needed to cause immobilization to dominate.
This would explain why such rapid rates of MIT were measured there but
not on the Broadbalk 'Nil' plot with its similar atmospheric input of N.
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