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D.V. Murphyet al.
Can Application of N Saturation Concepts Be Applied in Arable Soils?
4.1
Can Concepts of N Saturation
Developed for Forest Systems
be Applied in Arable Soils?
D.V. M
URPHY
1,3
, N. D
ISE
2
, K.W.T. G
OULDING
1
,
J. M
AC
D
ONALD
2
, C. P
EAKE
1
, P. R
EDFERN
1
AND
E.A. S
TOCKDALE
1
1
Soil Science Department,IACR-Rothamsted,Harpenden,
Hertfordshire AL5 2JQ;
2
Department of Earth Science, The Open
University, Milton Keynes MK7 6AA, UK; and
3
Centre for Land
Rehabilitation, The University of Western Australia,Nedlands, WA
6907, Australia
Introduction
Nitrogen (N) fluxes in soils are dependent on the supply of, and demand
for, mineral N. Both are controlled by interacting biological, chemical and
physical factors. Mineral N can be supplied to the soil as fertilizers, or
returned in animal excreta, as well as through atmospheric deposition and
from ammonification (commonly termed N mineralization) of soil organic
matter (SOM) and crop residues. The development of
15
N isotopic dilution
techniques has provided an opportunity to obtain a much clearer under-
standing of soil N cycling. The principal advantage of these techniques
is that the actual rate of ammonification (termed
gross
N mineralization)
can be estimated without the confounding influence of immobilization.
Similarly, gross rates of nitrification (the total production of nitrate without
any influence of immobilization or gaseous losses of nitrate (NO
3
−
) during
the incubation) can be measured without addition of ammonium (NH
4
+
),
which stimulates the process. The rates of specific pathways of the soil
N cycle can therefore be quantified. The gross N mineralization rate
determines the supply of mineral N from organic matter, while both
immobilization and nitrification rates represent microbial demands for N
within the system (Fig. 4.1.1).
Concepts and measurements of potential N loss have been used for
some years to assess the 'N saturation' status of forest systems (Tietema
and Wessel, 1992; Tietema, 1998). In forest ecosystems and undisturbed
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