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

BONDE & ROSSWALL: NITROGEN IN FOUR CROPPING SYSTEMS 1513
neralizable pool. "Limited" in the former assumption
implies that the biomass is at a maximum in relation
to the amount of substrate, so that no increase in biomass
occurs at the start of the experiment.
To investigate whether mineralization rates are limited
during incubation according to Stanford and Smith
(1972), literature values (Stanford and Smith 1972,
Campbell et al., 1984) of A^ vs. k 0 for long-term (30
weeks) incubations have been plotted, but no significant
correlation was found.
An inverse relationship between A^ and k 0 values
was evident when data from El-Haris et al. (1983) medium-term
incubations (12 weeks), were plotted. Small
N 0 values were always associated with large k 0 values
when sampling took place in spring as compared with
values obtained when sampling took place in autumn.
As the half-lives of the micrpbial biomass and the active
nonbiomass soil organic matter have been estimated
to be 0.5 and 1.5 yr, respectively (Paul, 1984),
a relatively large amount of biomass N compared to
nonbiomass active N would affect the k 0 value positively.
An inverse relation between N 0 and ko values
can thus be accomplished if the microbial biomass
sustains itself through winter and early spring at the
expense of active nonbiomass soil organic matter.
Schnurer et al. (1986) have not observed any marked
seasonal dynamics of the microorganisms at the Kjettslinge
field, and the variation in N 0 would then be
caused mainly by changes in the amount of nonbiomass
substrate.
Carter and Rennie (1982) compared the flush of C
and N after CHC1 3 fumigation concomitant with a determination
of A^o and t\ j2 (the "half-life" of N 0 ) on
tilled and nontilled soils. This provides an experimental
test of the proportionality between the biomass
N/./VO ratio and k 0 . Biomass N can be calculated
based on C and N flushes (Voroney, 1983), and the
rate constant ko from the formula t\ /2 = In2/k 0 . Analysis
of the data of Carter and Rennie (1982) showed
no significant correlation between the biomass N/A'o
ratio and k 0 , but did show a positive one for conventionally
tilled soils. If one data point, representing a
topsoil of a no-till treatment, was excluded, the correlation
became significant (r = 0.530, p = 0.05, « =
15) for the total data set. These data suggest that a
major part of the microbial biomass is easily mineralizable
and provides some explanation for the observed
relation between N 0 and ko values.
A two- or multicomponent model, however, would
be a better description of the mineralization course,
since several fractions of organic matter are present
in the soil. The single k 0 value associated with the firstorder
model is therefore proposed to be a mixture of
h and k values belonging to a two-component model,
as is NO a mixture of N A and N R . The mineralization
course in this study was well fitted by a simplified twocomponent
model, making it fallacious to determine
a traditional N 0 and fc 0 value. However, the same tendency
of having a small ko value whenever a large N 0
value occurs, is recognizable in the N A and h values.
These parameters are comparable to the N 2 , k 2 values
in the two-component model of Richter et al. (1982),
in the data set of which the same pattern occurred
(Fig. 3). Because the simplified two-component model
was the most justifiable for the data presented in this
4-1
3-
2-
1-
0^
•- DATA FROM THIS PAPER
x-DATA FROM RICHTER ET AL.I1982 )
50 100 150
N 2 or N A (kg ha ,-1i
Fig. 3. Plot of the rate constant h vs. N A for the data from this paper,
and * 2 and N, from Richter et al. (1982).
paper, N A and h values should not suffer from errors
caused by fitting of an inappropriate model. Thus, there
may be a real biological mechanism generating the
observed relationship.
The findings of Paris et al. (1981), that first-order
rate constants were proportional to bacterial numbers,
might explain the relationship between h and N A values
presented in Fig. 3, if bacterial density varied between
treatments and sampling dates. Based on their
findings, a high bacterial density would be associated
with a high measured value for h. In addition, a high
bacterial density could develop at the expense of available
soil organic matter, and therefore be associated
with low N A values. However, an inverse relationship
between bacterial density and available soil organic
matter is not inevitable. Alternatively, a biological
limitation on mineralization imposed by the growth
of active organisms in the early stages of incubation
would account for the relationship between h and N A ,
even if a lag phase did not occur.
ACKNOWLEDGMENT
We are greatly indebted to Dr. S. Simkins for many valuable
discussions regarding the statistical treatment. T.A.
Bonde received financial support from the Inst. of Genetics
and Ecology, Univ. of Aarhus, Denmark. The investigation
formed part of the project, Ecology of Arable Land. The
Role of Organisms in Nitrogen Cycling, supported by the
Swedish Council for Planning and Coordination of Res., the
Swedish Council for Forestry and Agric. Res., the Swedish
Natural Science Res. Council, and the Swedish Environment
Protection Board.