(1987) Seasonal Variation of Potentially Mineralizable Nitrogen in ...

BONDE & ROSSWALL: NITROGEN IN FOUR CROPPING SYSTEMS 1511
Table 1. Total mineral N produced during a 13-week incubation
of top soil (0-27 cm) collected from four cropping systems
at four sampling occasions.
Sampling
date
21 April
7 June
7 August
22 October
Form of
mineral N
Total mineral N
(NO, + NOJ-N
NH.-N
Total mineral N
(NO, + NOJ-N
NH.-N
Total mineral N
(NO, + NOJ-N
NH.-N
Total mineral N
(NO, + NOJ-N
NH 4 -N
Cropping systemf
BO B120 GL LL
321adi
190
(120)§
259ad
117
142
188ae
68
121
269ad
140
129
Kg iia
371abd 483cd
232 338
(139) (145)
354bcde
210
144
268abe
119
150
SOOabde
177
124
405cde
255
150
330be
185
144
364bde
222
141
420bcd
254
(166)
279abde
104
175
255be
89
167
299abde
142
157
t BO = Barley without addition of N fertilizer; B120 = barley receiving
120 kg N ha-' yr 1 ; GL = grass ley receiving 200 kg N ha' 1 yr 1 ; LL =
lucerne ley receiving no N fertilization.
t Means that do not differ significantly (p = 0.05) are followed by the same
letter; a, b, c rows; d, e columns;
§ An estimate based on the mean of the last three sampling dates.
culated net mineralization rates of 105, 128, 257, and
211 kg ha~' yr- 1 for BO, B120, GL, and LL, respectively
(Rosswall and Paustian, 1984). If N 0 is a definable
quantity and the only major source of mineralizable
N, a decrease in N 0 during the growing season
should equal the amount of N mineralized. On a yearly
basis this will, however, be an underestimation, since
autumn mineralization is important despite an increase
in A^ due to addition of root litter and aboveground
litter plowed under. In addition, our data
probably underestimate net N mineralization during
the growing season, since the 13-week incubations were
too short to allow a statistically justifiable determination
of both N A and N R .
Fertilizer N (120 kg) was applied on 16 May to the
B120 and GL treatments, and the grass ley further
received 80 kg of N on 2 July. Application of fertilizer
N has been demonstrated to add to the mineralizable
soil N and, in particular, to the readily available pool
of TV,, (El-Haris et al, 1983). Fertilization would then
tend to prevent a decrease in the amount of mineralizable
N during the growing season. To estimate net
N mineralization by a substrate disappearance method,
it is thus necessary to take into account possible additions
to the substrate pool during the period investigated.
The amounts of organic N added through the system
by root and aboveground litter input have been
estimated to be 40 and 55 kg ha~' yr" 1 for BO and
B120, respectively (A.C. Hansson, 1986, personal
communication). This amount is not sufficient to explain
the observed increase of the mineralizable N
fraction in BO from August to October (81 kg ha" 1 ).
The larger amount might be caused by a period of
stimulated microbial activity after harvest, due to
added C-rich crop residues, which would result in
greater immobilization of N and thus larger amounts
of N in live and dead microbial cells and microbial
metabolites (Campbell and Biederbeck, 1982). Nitrogen
in these forms are thought to be a large part of
the readily mineralizable soil N (7V 0 ). This apparent
5 6 7
TIME (WEEKS)
9 10 11 12 13
Fig. 2. Accumulated amounts of mineralized N during 13 weeks of
incubation of soils collected from four cropping systems on 7 June.
Error brackets denote accumulated standard deviation.
rise in the active part of soil N took place during a
period with net mineralization and total N loss from
the system (Bergstrom, 1986). The net effect in the N
transformations during this period is thus a decrease
of the slow fraction rather than the active one. The
smaller increase in the mineralizable fraction in B120
(32 kg), in spite of a larger amount of litter added to
the system, is surprising, but may be caused by a greater
net immobilization of mineral N in these plots.
Cumulative N Mineralization Patterns
Patterns of cumulative N mineralization from soil
samples collected on 7 June are shown for all four
treatments in Fig. 2. The shapes of the mineralization
curves for soil samples collected on the other three
dates were very similar to those in Fig. 2 and are not
shown. The initially high rates of N mineralization in
soil samples collected from all four treatments at all
four dates declined to low, fairly constant rates by
week 6 (Fig. 1), resulting in the apparently linear increase
in total cumulative inorganic N after about week
4 (Fig. 2).
Stanford and Smith (1972) calculated an initial N 0
value and the k value on the basis of the curve of
accumulated mineralized N from week 2 to week 30.
The mineralized N during the first 2 weeks was added
to the initially calculated N 0 value, giving the final 7V 0 .
This was motivated by a poor model fit to the data
set if the first 2 weeks were included, which was probably
a result of pretreatment of soil samples. Numerous
reports on the effect of air drying, sieving, grinding,
and rewetting are available (e.g., Birch, 1960;
Agarwal et al., 1970). Soils respond to drying and rewetting
with a flush of C and N mineralization of short
duration, the magnitude depending on soil characteristics.
To overcome the problem of an initially rapid
mineralization rate, a two-component model (a sum
of two first-order equations; Molina et al., 1980; Griffin
and Laine, 1983), a three-component model (Richter
et al., 1982), or a modified one-component model
(Marion et al., 1981) have been used. These models