The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 226

grasslands, the N cycle was thought to be 'highly conservative', where high
rates of mineralization were balanced by high rates of immobilization, with
minimal net N mineralization or nitrification. However, additions of N in
atmospheric deposition can disrupt the soil N cycle.
Stimulation of the process of nitrification is of key importance, since
NO 3 − is vulnerable to loss by leaching or denitrification. The ratio of gross
nitrification to NH 4 + immobilization has been used to assess the absolute
potential of the soil to exchange N with the wider environment (Fig. 4.1.1;
Tietema and Wessel, 1992). The greater the ratio, the more likely a soil is to
lose N via leaching or denitrification. The lower the ratio, the more likely
the soil is to retain N within the internal cycling of mineralization-
immobilization turnover (MIT). This ratio can be used to assess the 'N
saturation' of forested soils and, along with measurements of NH 4 + supply,
can be used to examine the potential for N loss.
In arable soils, it is generally assumed that nitrification occurs rapidly,
when NH 4 + supply permits. A large pool of NH 4 + is rarely maintained in
arable soils. Nitrifiers respond rapidly to additions of NH 4 + in fertilizer or
manure, where NH 4 + supply is in excess of requirements for maintenance
and growth of the heterotrophic microbial population, or when this is
restricted by carbon availability. In contrast to forests, N uptake by crops
can also be rapid in agricultural systems, representing a further sink for
'surplus N'. It is therefore only during periods when crop uptake is small
that the ratio of gross nitrification to NH 4 + immobilization forms an index
of potential N loss. In humid temperate climates, coincidence of low crop
uptake and high effective rainfall over winter may mean that indices of
potential N loss can be applied in agricultural systems, as well as forests.
This was evaluated preliminarily by Goulding et al . (1998). In this study,
we further evaluated these indices in a range of arable agricultural soils,
Fig. 4.1.1. The soil internal N cycle. The ratio of gross nitrification to NH 4 + immobilization can be
used to indicate the potential for loss of N.