Merged with duplicate record 10026.1/815 on 15.02.2017 by CS (TIS)The microbiology of nitrification has been extensively studied, but the ecology and
environmental impact of the process has received less attention. The reason for this has
more to do with the difficulties of conducting field experiments to examine the links with
other processes which cause losses of nitrogen, than a failure to appreciate its importance
in agricultural systems. This project was designed to overcome some of the limitations of
existing field techniques to enable simultaneous measurements of nitrification and the
major processes of N supply (mineralization) and N loss (denitrification and leaching) to
be examined.
The study proceeded in three distinct phases: firstly, soils with contrasting N
management histories were examined, using laboratory assays for potential activities.
Clear differences which resulted from higher N inputs were established, with
correspondingly higher nitrifying activities. For example, in a fertilized soil, ammonia-oxidizers
produced 48.4 compared with 1.3 nM N02 gˉ¹ soil hˉ¹ in an unfertilized soil.
Potential nitrite-oxidizing rates were 93.4 and 62.5 nM gˉ¹ hˉ¹ respectively. Assays of
enzyme kinetics, therefore confirmed the higher nitrifying activity in the fertilized soil, but
demonstrated a lower affinity of the enzyme for N02- substrate, with K, values of 436 and
310 µM N02 -N, respectively. Nitrifying rates in soils from grass-clover swards were
intermediate between the fertilized and unfertilized soils.
Secondly, a new field incubation technique was developed and used to obtain actual
rates by concurrent measurements of the major N cycling processes. A strong correlation
was established between nitrification and denitrification (r² = 0. 98). The measurements
were verified by comparison with other independent methods. Net rates of nitrification in
the same soil type ranged from 0.55 - 1.17 kg N haˉ¹ dˉ¹
, with the highest rates in the
fertilized soil. Over 70% of the mineralized N was nitrified, of which 80% was
subsequently lost (i.e. either denitrified or leached).
Thirdly, the practical implications of these findings were examined in greater detail
using 15N labelling techniques which enabled process rates (net and gross) to be established
in a model of the N cycle. When nitrification was inhibited, there were no significant
differences between gross or net mineralization rates in the soils from the three swards,
which indicated that N-immobilization could be directly influenced by the level of
nitrifying activity in these soils.
The influence of nitrification in determining the pathways of N loss from grassland
soils was quantified in this study. From a detailed investigation of the processes involved
in N cycling, it was deduced that nitrification was also one of the major factors in
determining the outcome of competition for inorganic N between plant and microbial
biomasses.The Institute of Grassland and Environmental Researc
