The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 228

NO 3 − -N concentrations were determined colorimetrically by flow injection
analysis (Skalar SAN PLUS ). The 15 N: 14 N isotopic ratio of the NH 4 + -N or
NO 3 − -N within soil extracts, after preparation by diffusion, was determined
by mass spectrometry (Europa Tracermass). Gross N mineralization and
gross nitrification rates were calculated from the change in pool size, and
the decline in atom % 15 N excess, of the 15 N-labelled NH 4 + or NO 3 − pool,
as soil-derived organic 14 N was ammonified or indigenous 14 NH 4 + was
nitrified (Kirkham and Bartholomew, 1954; Barraclough, 1991). The
rate of microbial consumption of NH 4 + and NO 3 − was also calculated.
Ammonium immobilization was estimated as the difference between total
NH 4 + consumption, estimated from the dilution of 15 NH 4 + , and gross
nitrification, estimated from the dilution of 15 NO 3 − . Although this is not
a direct measurement of immobilization, it does enable the relative
importance of the two major microbial NH 4 + consumptive processes to be
examined.
Calculated gross N transformation rates were used to describe the
internal cycling of N in each soil. The ratio of gross nitrification to NH 4 +
immobilization (N : I) was then used to determine the potential for N loss
from each soil.

Results and Discussion

Gross mineralization rates ranged from 0.56 to 1.66 mg N kg −1 day −1 , and
gross nitrification rates from 0.24 to 0.61 mg N kg −1 day −1 for sites sampled
in spring 1999 (Table 4.1.1). There were no clear patterns with soil type or
with current or previous crop. When ratios of gross nitrification to NH 4 +
immobilization were calculated, these were dominantly < 1 (average 0.7),
except where a cereal followed peas (2.1) or where oilseed rape was grown
(average of 1.0 for three sites). Where the same soil was sampled before and
after cultivation in autumn 1997, the ratio of gross nitrification to NH 4 +
immobilization increased rapidly (within 1 day) (Fig. 4.1.2), and
before NO 3 − leaching was detected, illustrating that the processes which
control soil N dynamics can respond rapidly to management practices
(Goulding et al ., 1998). In all these cases, soil N supply (mineralization)
was stimulated above the immediate heterotrophic microbial demand
(immobilization),

NH 4 +

thus increasing the amount of available for

nitrification.
In these cases, however, increases in soil N supply through cultivation
or incorporation of low C : N residue crop residues or leaf litter are well
known to increase soil N supply, and the use of the complex measurement
of the 'index of potential N loss' was probably not worthwhile. However,
these data show that the index can be applied to arable soils, if careful
interpretation is also used.