Agriculture Reference
In-Depth Information
long-term experiments typically remove at
least 20 kg N ha
−
1
annually (i.e. equivalent
to about 1% of the total stock of N in soil
organic matter), and annual plant uptake in
semi-natural ecosystems could also be of
this order. Although part of this N uptake
will be derived from N deposited in rain or
through dry deposition from the atmos-
phere, in most cases the majority is from the
mineralization of soil organic matter. In nor-
mal agricultural situations in the temperate
region, it is common for N derived from this
source to account for at least 30% of total N
uptake by crops (Macdonald
et al
., 1997).
In tropical and subtropical environments,
the contribution of soil organic matter is
even greater, with an average of 79% of
total N uptake by crops being derived from
soil organic N in one coordinated set of ex-
periments with
13
sites in nine countries
(Dourado-Neto
et al
., 2010).
Such data are derived from experiments
using
15
N-labelled fertilizers; uptake of
un-
labelled
N is taken as indicating that un-
labelled soil organic matter is the source.
However, this can be a slight overestimate,
due to an artefact of the methodology termed
'pool substitution'. Even so, the results still
show that soil organic matter is a major source
of N for agricultural crops. Indeed, a major
thrust of practically oriented research in soil
science and agronomy is to identify ways of
predicting more accurately the quantity of N
that will become available for crop uptake dur-
ing the growing season on a field-specific basis.
The more N that can be derived from soil
sources, the less is required from synthetic fer-
tilizer, which has a large greenhouse gas foot-
print from the manufacturing process. So, any
management practices that can maintain or in-
crease the quantity of organic matter in a soil
have multiple benefits for crop nutrition. These
include: (i) contributing to food security; (ii) in-
creasing the economic efficiency of agriculture
through saving on the purchase of N synthetic
fertilizer by the farmer; and (iii) decreasing the
greenhouse gas emissions associated with N fer-
tilizer. Although total soil organic C or N con-
tent gives some indication of potential N
mineralization under field conditions, the
precision or prediction is poor, and hence
the need for ongoing studies on this.
Figure 7.1
shows an example of agricul-
tural management influencing SOM content,
and this in turn influencing crop yields
through the supply of N. Two cereal crops,
either winter wheat or spring barley, were
grown following three previous cropping re-
gimes. These were: (i) continuous arable
cropping such that organic inputs to soil
were small; (ii)
3
years of a grass pasture that
received N fertilizer - organic inputs from
(a)
(b)
10
10
8
8
6
6
4
4
2
2
0
0
0
50
100
150
200
250
0
5
0
100
150
200
N applied (kg ha
-1
)
Fig. 7.1.
Yields of (a) winter wheat and (b) spring barley in the Woburn Ley-Arable Experiment on a sandy
soil in south-east UK. Graphs show yields of arable test crops (t ha
-1
) following three different previous
cropping sequences:
3
years arable followed by
2
years arable test crops, ♦;
3
years grass ley + N
followed by
2
years arable test crops,
;
3
years grass/clover ley followed by
2
years test crops, ▲.
(From Johnston
et al
., 2009.)
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