Ion exchange resin±soil mixtures as a tool in net nitrogen
mineralisation studies
JuÈrgen K. Friedel
a,*, Anke Herrmann
b, Markus Kleber
c aUniversity of Agricultural Sciences, Institute of Organic Farming, Gregor-Mendel-Street 33, A-1180 Vienna, Austria
b
Department of Soil Sciences, Division of Soil Fertility and Plant Nutrition, Swedish University of Agricultural Sciences (SLU), S-75007 Uppsala, Sweden
c
Martin-Luther University Halle-Wittenberg, Institute of Soil Science and Plant Nutrition, Weidenplan 14, D-06108 Halle, Germany
Accepted 29 February 2000
Abstract
Mixed-bed ion exchange resins (IER) were mixed with intact soil aggregates and incubated at 60% water ®lled pore space in closed polyethylene bags for 12 weeks. To test IER eects on N losses, nitri®cation and net N mineralisation, an arable soil and a grassland soil, diering in organic matter content, were chosen and two crop residues (wheat straw, sugar-beet leaves) with dierent C-to-N ratios were added to the arable soil. It was proposed that IER might exert an in¯uence on N cycling similar to that of plant roots. Nitri®cation was inhibited by adsorption of NH4in the +IER treatments. Net N mineralisation was greater
in the grassland soil than in the arable soil which had less soil organic matter. Without incorporation of additional organic substrates, net N mineralisation was not aected by IER in both soils. Straw addition to the arable soil caused immediate N immobilisation in theÿIER treatment, whereas N mineralisation continued in the +IER treatment. Incorporation of sugar-beet leaves into the arable soil highly increased net N mineralisation and microbial biomass N in theÿIER treatment. In the +IER treatment, the enhancement of both N mineralisation and microbial biomass N was less pronounced. Thus, IER mixed into soil samples can exert either a stimulating (wheat straw) or dampening (sugar-beet leaves) eect on N mineralisation. Soil±IER mixtures can prevent losses and re-immobilisation of mineralised N and mimic nutrient exchange properties of plant roots. It is concluded that in incubation experiments they can better re¯ect conditions in the vicinity of roots than incubations without IER or with incorporation of IER in con®ned resin bags as long as water and aeration conditions are not largely changed. Soil±IER mixtures may also be a useful tool for studying root-induced changes in net N mineralisation.72000 Elsevier Science Ltd. All rights reserved.
Keywords:Ion exchange resins; Soil±resin mixtures; Soil nitrogen cycling; Nitrogen mineralisation; Nitri®cation
1. Introduction
Quanti®cation of the relationships between soil N mineralisation and other ecosystem processes has been restricted by the lack of suitable methods to measure N mineralisation under ®eld conditions. Methods used to measure or estimate patterns of N mineralisation under ®eld conditions include: (i) exposure of
dis-turbed soil in plastic bags buried in the ®eld, (ii) ex-posure of relatively undisturbed soil columns under ®eld conditions, and (iii) measurement of mineral N collected by ion exchange resins placed in the ®eld for extended periods (Raison et al., 1987).
All methods of containment developed to date alter the soil environment through (i) cessation of the car-bon (C) input from decomposing litter and from ®ne root turnover, (ii) increased C inputs from severed roots, (iii) modi®cation of the moisture and tempera-ture regimes relative to bulk soil, and (iv) accumu-lation of inorganic N (Adams et al., 1989). An
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* Corresponding author. Fax: +43-1-47654-3792.
additional source of error is the inability to prevent
mineralised N (Nmin) from being either re-immobilised
into microbial biomass or denitri®ed within the con-tainment period. As assay conditions alter the C and N availability within the containers, none of the methods may be considered to measure mineralisation rates accurately.
The use of intact soil cores (Nordmeyer and Richter, 1985; Raison et al., 1987) may reduce the sources of error due to disturbance of the soil and modi®cations of the moisture and temperature regimes. The central problem however, the absence of living root functions, remains.
In undisturbed soil inhabited by living plants, root N uptake reduces or even inhibits denitri®cation or re-immobilisation of mineralised N and limits microbial N availability. Simultaneously occurring rhizodeposi-tion leads to an increase in the amount of substrate available to soil micro-organisms, enabling them to mineralise ``surplus'' amounts of organic N. Consecu-tive predation by soil fauna may release part of this
extra N as NH4(Clarholm, 1985).
Any attempt to advance the methodology for study-ing ¯uxes of soil mineral-N in situ must therefore seek to ®nd a means which takes root functions into account. Such a procedure should immobilise inor-ganic N throughout the incubation, thus preventing it from being nitri®ed, denitri®ed or re-immobilised into microbial biomass.
If this process occurs with the same magnitude as is performed by living roots, it should eliminate the most prominent source of error.
IER were shown to adsorb NH4and NO3eectively
in soils (Binkley, 1984). Only few attempts (e.g. HuÈb-ner et al., 1991; Binkley et al., 1986; DiStefano and Gholz, 1986) to establish the use of IER as a pro-cedure to measure net N mineralisation were known when growing concern about the reliability of the bur-ied bag method (Eno, 1960) led Zeller et al. (1997) and Bhogal et al. (1999) to investigate further into the sub-ject. In all previous attempts to employ IER as a tool in net nitrogen mineralisation studies, these were applied in ``resin bags'' to the soil. Such design may
prevent Nmin from being leached out of cores as well
as from being washed into them. Due to the low
mobi-lity of NH4in soil, however, nitri®cation and
re-immo-bilisation of NH4 cannot be prevented. Thus, with
resin bags of traditional design, Nmin may continue to
accumulate in soil samples during the incubation. We assumed that by mixing IER thoroughly into soil samples, the accumulation of microbially-available mineralised N in soil samples can be prevented and one of the most important eects of living roots on
soil N transformations (i.e. the removal of Nmin) may
be simulated. Conditions in soil samples containing
IER can therefore be expected to be closer to con- Ta
ditions in the vicinity of roots than conditions in samples without IER. For this reason, N mineralis-ation rates measured in soil samples containing IER should be a more realistic measure for N mineralis-ation rates under ®eld conditions.
To achieve this, we tested (i) if mixing IER into soil
samples can reduce losses of Nmin either by
denitri®ca-tion or immobilisadenitri®ca-tion, (ii) whether nitri®cadenitri®ca-tion can be reduced or even prevented by IER in soil samples, and (iii) if net N mineralisation rates are enhanced in the presence of both IER and microbially-available sub-strate.
2. Materials and methods
2.1. Soil sampling
Soil samples were taken from the A horizons of an arable soil (Stagnic Luvisol; ``Arable soil'') and a humus-rich grassland soil (Cumulic Anthrosol; ``Grass-land soil''). Basic soil properties are given in Table 1.
In both cases, a surface area of 1 m2 was cleared of
plant materials and the soil subsequently sampled down to the horizon boundary. The soil was then freed from root residues and homogenised. This was done with maximum care to preserve crumbs and aggregates. As the material was stone-free, no further sieving was necessary until preparation of individual incubates.
2.2. Ion exchange resin (IER)
When both cations and anions are included as target ions, the various species can be accumulated by the resin if a mixture of cation- and anion-resins are placed into the medium (Skogley and Dobermann,
1996). Consequently, a mixed-bed resin (Amberlite1
MB-3) from CWG GmbH, Mannheim, Germany, was
used. Amberlite1 MB-3 is a mixture of the strong
cat-ion exchange resin Amberlite1
IR-120 and the strong
anion exchange resin Amberlite1
IRA-420 Type 1. Grain size of the moist resin was determined to be in the range 0.3±1.2 mm, with most of the grains within
0.5 and 1.0 mm. The speci®c weight was 0.7 g cmÿ3.
Prior to the experiment, IER was washed once with deionised water. By this treatment, the N content extractable with 2 M KCl (soil : extractant ratio=1 : 20) was reduced to about 50% (data not shown).
2.3. Incubation experiment
To test the in¯uence of IER on N mineralisation rates under dierent conditions, soils diering in or-ganic matter content and an addition of rich and N-poor substrates were chosen. The treatments were:
``Arable soil ÿ IER'' (low organic matter), ``Arable
soil + IER'', ``Grassland soil ÿ IER'' (high organic
matter), ``Grassland soil + IER'', ``Arable soil +
wheat straw ÿ IER'', ``Arable soil + wheat straw +
IER'', ``Arable soil + sugar-beet leaves ÿ IER'',
``Arable soil + sugar-beet leaves + IER''.
2.4. Preparation of organic additives
Wheat straw and sugar-beet leaves were dried at
508C for 72 h and ground to pass a 1 mm sieve. After
this, wheat straw had a water content of 7.7% and sugar-beet leaves of 16.2%. Wheat straw had a C
con-tent of 409 mg gÿ1 and an N content of 7 mg gÿ1,
yielding a C-to-N ratio of 58. C and N contents in
sugar-beet leaves were 397 and 28.3 mg gÿ1; which
resulted in a C-to-N ratio of 14. Organic additives were added to soil at a 1:30 ratio by weight.
2.5. Preparation of individual incubates
Polyethylene freezer bags manufactured by Fa. Haaf, 85716 Lohhof, Germany, were used as contain-ers for the soil. Bags had an average standard
thick-ness of 50 mm. Polyethylene ®lm is only slightly
permeable to water vapour, while its O2 and CO2
transmission rates are relatively high. Soil structure in the samples was not disturbed, i.e. complete aggregates were incubated. Soil sample size was 300 g. The same amount of resin was added in the treatments with IER addition. As the density of quartz is about double the density of the resin selected, 600 g of quartz sand of a grain size similar to that of the resin were added to treatments without IER in order to maintain identical grain size distribution and soil physical conditions.
For each treatment, 21 replicate samples were
pre-pared and incubated at 158C and at 60% water ®lled
pore space. Three samples per treatment were selected
the next day (t0 = 16 h) at random and analysed to
determine starting values. Every 2 weeks, another three samples per treatment were selected at random from the incubator and analysed in the same manner
to represent a total of 7 (t0...t6) data points spanning
incubation periods from 0 to 12 weeks in 2-week
incre-ments tn1ÿtn2 weeks). Microbial biomass N
(Nmic) was determined att0andt6(in the beginning of
the experiment and after 12 weeks).
2.6. Determination of inorganic N and microbial N
Inorganic N was extracted by 2 M KCl solution (extractant : soil ratio = 40:1) and measured by means
of a Skalar autoanalyser. NH4 was determined
colori-metrically by a modi®ed Berthelot reaction according
to Krom (1980) and NO3 after reduction to NO2 by
-naphthylamine-para-diazoben-zene-parasulphuric acid at 540 nm in an automated analyser (Houba et al., 1987).
Microbial biomass N was measured with the slightly
modi®ed standard fumigation-extraction method
(Brookes et al., 1985). Brie¯y, soil samples (20 g for IER±soil incubates and 30 g for quartz±soil incubates, equivalent to 10 g of wet soil each) were fumigated
with CHCl3for 24 h at 258C. For analysis att6, 250ml
liquid CHCl3 was added to each portion of the soil
samples before fumigation, according to the method recommended by Mueller et al. (1992) for soil samples with a high moisture content. After removal of the
CHCl3, soluble N was extracted from fumigated and
unfumigated samples with 2 M KCl (extractant:soil ratio = 40:1) for 2 h. N in ®ltrated solution was deter-mined by the Griess reaction colorimetrically after an
automatic UV peroxide digestion to NO3 and
re-duction to NO2 by means of a Skalar autoanalyser
(Houba et al., 1987). Nmic was calculated using a kEN
factor of 0.54.
3. Results
3.1. IER eects on nitri®cation
In ÿIER samples, mineralised N was readily
nitri-®ed (Figs. 1 and 2). At the end of the incubation (12
weeks) all Nmin was nitri®ed to NO3 (Table 2). In
+IER samples, NO3 concentrations found at t0
remained constant throughout the incubation (Fig. 2, Table 2). Mineralisation occurring during the
incu-bation merely increased the NH4pool.
3.2. IER eects on net N-mineralisation
In the ``Arable soil'' without substrate addition,
Nmin accumulated gradually at a low rate during the
incubation (Fig. 3). No signi®cant dierences in Nmin
Fig. 2. Amount of NO3±N in the soil samples during the incubation
experiment. Bars indicate one standard deviation.r= Treatments
without IER, R= treatments with soil±IER mixtures. n= 3 for
each treatment.: treatments dier (Mann and WhitneyU-test,P<
0.05).
Table 2
Inorganic N and soil microbial biomass N (mg gÿ1) in the dierent
soil treatments with or without IER (standard deviation in
parenth-eses) after 16 h (t0) and 12 weeks (t6)
NO3±N NH4+±N Inorganic N Microbial N
t0 t6 t0 t6 t0 t6 t0 t6
Arable soil
ÿIER 8.5 38.6 0.4 0 8.9 38.6 67 113
(0.7) (7.1) (0.6) (0) (0.4) (7.1) (13) (18)
+ IER 7.9 9.7a 3.3a 24.3a 11.1 34.0 ±b 70a
(1.2) (2.5) (2.1) (3.1) (1.8) (5.5) (17)
Arable soil + wheat straw
ÿIER 2.8 0 3.7 0 6.5 0 123 73
(0.7) (0) (3.8) (0) (3.1) (0) (2) (24)
+ IER 9.5a 11.4a 14.4a 35.3a 23.9a 46.6a ±b 40a
(0.5) (3.7) (1.6) (4.7) (1.3) (7.0) (5)
Arable soil + sugar-beet leaves
ÿIER 1.8 528.0 28.6 0 30.3 528 280 654
(1.4) (14.9) (11.7) (0) (10.5) (14.9) (46) (69)
+ IER 29.6a 27.9a 42.3 232.0a 71.9a 260a ±b 167a
(5.9) (10.2) (8.3) (44.6) (5.6) (34.8) (90)
Grassland soil
ÿIER 28.4 124.0 15.1 0 43.5 124 166 133
(2.5) (30.1) (2.8) (0) (5.1) (30.1) (12) (15)
+ IER 22.8 23.7a 23.3a 74.1a 46.1 97.8 ±b 141
(1.6) (1.6) (2.6) (0.9) (1.5) (1.9) (9)
a
Diers signi®cantly from the respective treatment without IER
(Mann and WhitneyU-test,P< 0.05).
b
± data missing due to equipment failure.
n= 3 for each treatment.
Fig. 1. Amount of NH4±N in the soil samples during the incubation
experiment. Bars indicate one standard deviation.r= Treatments
without IER;R= treatments with soil±IER mixtures. n = 3 for
each treatment;: treatments dier (Mann and WhitneyU-test,P<
concentrations occurred between the two treatments
(+IER orÿIER).
In samples of the ``Grassland soil'', Nmin
concen-trations and N mineralisation rate were greater com-pared with ``Arable soil''. Signi®cant dierences in
Nmin concentrations occurred between the two
treat-ments (ÿIER or +IER) only at 4 and 8 weeks of
incu-bation.
In ``Arable soil + wheat straw'', the amount of
Nmin also was generally small in both +IER and
ÿIER treatments. Nmin values were signi®cantly higher
in +IER compared with ÿIER. In this treatment,
values decreased to zero during the incubation.
``Arable soil + sugar-beet leaves'' showed high
min-eralisation activity in both +IER and ÿIER
treat-ments. The Nmin values were signi®cantly lower from 4
weeks until the end of the incubation in the +IER
compared with theÿIER treatment.
3.3. IER eects on soil microbial biomass N
Nmic contents were unrealistically low for +IER
samples at t0, although in a preliminary experiment
results of the chloroform fumigation extraction
method were not aected by mixing of IER into soil samples (data not shown). Therefore, at 12 weeks 250
ml CHCl3 was added to each portion of soil sample
before fumigation to ensure complete fumigation. Results of the chloroform fumigation method still showed a high variability in both the soil + IER and the soil + quartz mixtures and must be considered as
estimates and not exact values for Nmic contents.
Nmic contents at the beginning of the incubation (16
h) were smaller in ``Arable soil ÿ IER'' than in
``Grassland soil ÿ IER'' (Table 2). At the end of the
incubation (12 weeks), Nmiccontents were increased in
``Arable soil ÿIER'' and slightly decreased in
``Grass-land soil ÿIER''. Addition of wheat straw and
sugar-beet leaves immediately (16 h) increased Nmic contents
in ÿIER samples by a factor of approximately 2 and
4, respectively. Whereas in ``Arable soil + wheat straw
ÿ IER'', Nmic contents at 12 weeks had decreased to
the original concentrations found in ``Arable soil ÿ
IER'' at 16 h, in ``Arable soil + sugar-beet leaves
ÿIER'' values had further increased.
At the end of the incubation, in ``Arable soil +
IER'' and ``Grassland soil + IER'', Nmic values were
similar to the initial values of the soils (ÿIER) at 16 h.
In both, ``Arable soil + wheat straw + IER'' and
``Arable soil + sugar-beet leaves + IER'', Nmic
con-tents were signi®cantly lower than in the respective ÿ
IER treatments.
4. Discussion
4.1. IER eect on losses of mineralised N and nitri®cation
A prerequisite for preventing losses of mineralised N either due to denitri®cation or immobilisation during the incubation of con®ned soil samples is to prevent
NH4 and NO3 from being transformed by soil
micro-organisms. No indication of removal or
transform-ation of NH4was found in pot incubation experiments
by Binkley (1984), who investigated NH4 adsorbed to
IER in resin bags at dierent rates of cellulose
ad-dition. NO3 adsorbed to IER contained in resin bags
was not transformed during 4 weeks exposure in the ®eld (HuÈbner et al., 1991). Over long deployment
times in the ®eld (up to 44 weeks), NO3but not NH4,
was desorbed from preloaded IER (Giblin et al., 1994).
In our experimental approach, IER and soil were not separated before analysis. Thus, it cannot be
deter-mined directly, if NH4 or NO3once adsorbed to IER
was removed or transformed during the further incu-bation period. However, the lack of nitri®cation in all +IER treatments (Fig. 2) clearly indicates, that in
these treatments, in contrast to the ÿIER variants,
NH4 was not accessible to nitri®ers. The amounts of
NH4 in the +IER treatments either increased or
remained constant throughout the incubation in
con-trast to decreases in NH4 concentrations occurring in
the ÿIER variants (Fig. 1). This also indicates that no
transformation of NH4occurred once it was adsorbed
to the IER. NO3concentrations in the +IER samples
present at the beginning of the incubation were also conserved until the end of the incubation (Table 2). Therefore, we conclude that the mixture of strongly acidic cation exchange resins and strongly basic anion
Fig. 3. Amount of Nminin the soil samples during the incubation
ex-periment. Bars indicate one standard deviation. r = Treatments
without IER;R= treatments with soil±IER mixtures. n = 3 for
each treatment;: treatments dier (Mann and WhitneyU-test,P<
exchange resins mixed into the soil samples in our ex-periment can eectively adsorb mineralised N and pre-vent it from being further transformed by either immobilisation, nitri®cation or denitri®cation. By sep-arating IER from soil samples, e.g. by density
fraction-ation (Thien and Myers, 1991), amounts of Nmin
adsorbed to the IER may be determined separately. This separation procedure might also reduce the
varia-bility in determination of Nmic contents by the
chloro-form fumigation extraction method.
4.2. IER eects on net N mineralisation rates and microbial biomass N
To test the eect of IER on net N mineralisation rates, two soils diering in organic matter contents and two organic substrates diering in C-to-N ratio were chosen. Higher values of mineralised N in the ``Grassland soil'' than in the ``Arable soil'' (Fig. 3) can be explained by higher organic matter contents of the grassland topsoil (Table 1). Without substrate ad-dition, IER had no eect on N mineralisation in both
soils (Fig. 3). Nmic contents slightly decreased in the
``Grassland soil'' (both ÿIER and +IER) during the
incubation (Table 2), indicating no eect of IER on N
in soil micro-organisms. The increase in Nmic contents
from 16 h to 12 weeks in the ``Arable soil ÿ IER''
treatment cannot be explained since no substrate con-taining easily available organic matter was added here. We assume that this value may be overestimated due
to the heterogeneity of the samples. Nmic contents
remained unchanged in the ``Arable soil +IER''
treat-ment at 12 weeks compared with ``Arable soilÿIER''
at 16 h, also indicating a non-existing IER eect on N in soil microorganisms.
IER eects on N mineralisation or Nmic contents
reported in most of the other studies (e.g. Binkley et al., 1986; Zeller et al., 1997; Bhogal et al., 1999) are not comparable with our experiments, because in such investigations IER were used in resin bags and not mixed into the soil. Only Saeed et al. (1994) used
mix-tures of soil with K+ saturated IER in anaerobic
incu-bation experiments with lowland rice soils. Nitrogen mineralisation was higher in IER±soil mixtures than in control soil samples and the eect was greater in
strongly NH4-®xing soils. The authors concluded that
N mineralisation is underestimated by standard an-aerobic incubations due to ®xation of mineralised
NH4. The contrasting eects of IER (without any
sub-strate addition) on N mineralisation in our study is probably due to the dierent experimental conditions
(NH4-®xing capacity of the soils, anaerobic versus
aerobic incubations).
Addition of wheat straw with a wide C-to-N ratio of
58 at a high rate (33 mg gÿ1) led to immobilisation of
inorganic N at the beginning of the incubation in the
``Arable soil + wheat straw ÿIER'' treatment (Fig. 3,
Table 2). Throughout the incubation period, no
remi-neralisation of N occurred. Nmic values at t016 h
were almost doubled compared with ``Arable soil ÿ
IER'' without addition of organic substrate (Table 2), also indicating immediate N immobilisation. Addition of straw to soil samples also resulted in N immobilis-ation for several weeks in incubimmobilis-ation experiments of Aoyama and Nozawa (1993) and Jensen (1997).
In the ``Arable Soil + wheat straw + IER'' treat-ment, however, IER prevented microbial transform-ation of mineralised N due to immobilistransform-ation (Fig. 3). Throughout the incubation period, in ``Arable soil'' N mineralisation after wheat straw addition was slightly greater than in the treatment without substrate ad-dition (Fig. 3, Table 2). Thus, in this soil sample, N mineralisation from the soil organic matter was obviously little aected by straw addition when N im-mobilisation was excluded by IER. This ®nding is in accordance with the concept of dierent microbial groups, living independently on dierent substrates in soil (Cochran et al., 1988). Results also indicate that,
in the ``Arable soil + wheat straw ÿ IER'' treatment,
gross N mineralisation continued, but was overcom-pensated by a greater N immobilisation, leading to net N immobilisation. In accordance with this assumption, N mineralisation and N immobilisation have been shown to occur simultaneously in soil by gross N min-eralisation measurements (Sparling et al., 1995).
In ``Arable soil + sugar-beet leaves ÿ IER'',
ad-dition of sugar-beet leaves with a narrow C-to-N ratio
of 14 at a high rate (33 mg gÿ1) strongly enhanced N
mineralisation rates during the ®rst 6 weeks (Fig. 3)
and increased Nmic contents at 16 h and 12 weeks
(Table 2). In other studies, increases in N mineralis-ation (Aoyama and Nozawa, 1993; Aulakh et al.,
1995) and Nmic contents (Aoyana and Nozawa, 1993),
mostly after a short lag period, have been described after addition of crop residues with a narrow C-to-N ratio.
In the ``Arable soil + sugar-beet leaves + IER'' treatment, N mineralisation rates were dampened in the presence of IER from week 4 until the end of the
incubation (Fig. 3). Signi®cantly reduced Nmiccontents
at the end of the incubation (Table 2) indicate that IER mixed into the soil prevented or strongly reduced microbial N immobilisation after incorporation of sugar-beet leaves, as in the variants with straw
ad-dition. The eect of IER on Nmin concentrations
re-used by the micro-¯ora in the ``Arable soil +
sugar-beet leaves ÿ IER'' treatment. In the +IER
treatment, this reduced N availability obviously limited
microbial growth (Nmic contents) and activity (N
min-eralisation rates). This eect of IER on N release, to our knowledge, has not been described before.
Due to this, we have to modify our initial hypothesis that net N mineralisation rates would be enhanced in the presence of both IER and microbially-available substrate. This increasing eect of IER in the presence of available substrate was only found for substrate with a wide C-to-N ratio (wheat straw, N-limited con-ditions), in accordance with the positive eect of
grow-ing roots (through NH4 uptake and deposition of
C-rich exudates) on net N mineralisation described by Clarholm (1985). IER in the presence of substrate with a low C-to-N ratio (sugar-beet leaves, conditions of high N availability) diminished net N mineralisation
rates due to their limiting eect on Nmin availability
and consecutive microbial growth and activity.
4.3. Use of IER to approximate conditions in the rhizosphere
Mixing IER into soil may alter soil physical con-ditions with respect to micro-sites of diering water and aeration potential and change net N mineralis-ation rates in this way. First, conditions and N miner-alisation rates in the disturbed samples may dier from undisturbed conditions and N mineralisation rates in situ. We tried to reduce this disturbing eect by preserving natural soil aggregation. Secondly, we assume that we could reduce artefacts due to mixing IER into the soil to a minimum by mixing sand of a grain size equivalent to that of the moistened resin into the control soil samples (see Section 2.5).
IER mixed into the soil accumulate nutrients that diuse through water ®lled pore spaces. Hence, they may be used to assess plant-available nutrient quantity as a function of diusion (Yang et al., 1991). IER also mimic exchange properties of plant roots (Yavitt and Wright, 1996). Consequently, we conclude that
adsorp-tion of Nmin to IER with a suciently high sorption
capacity can be regarded as a functional analogy to the N uptake by roots.
IER used in our experiment had a speci®c surface
area in the range of 30±60 cm2cmÿ3(calculated from a
mean diameter of 0.5±1.0 mm, assuming a spherical shape, see Section 2.2). The speci®c surface area of IER in the IER±soil mixture (1:1 by weight) therefore
is about 18±36 cm2 cmÿ3 (assuming a bulk density of
0.7 g cmÿ3for IER, see Section 2.2 and 1.0 g cmÿ3for
the soil). This is many times higher than the active
root area of herbaceous crop plants in soil (11 cm2
cmÿ3; Larcher, 1994, 21). Regarding the speci®c
sur-face area and the sorption capacity of IER, they were
added in surplus compared with the conditions in the rhizosphere. We regard this useful because in soil±IER mixtures there is no water ¯ow due to transpiration di-rected to the sink as in the rhizosphere. No depletion of the sorption capacity of the IER was visible in our experiment.
Our results indicate, depending on microbial N availability, either positive (straw addition) or negative (sugar-beet leaves addition) IER eects on net N min-eralisation. Assuming a functional analogy of IER and plant roots, this suggests that, by the combined eects of N uptake and rhizodeposition, plant roots may either enhance (under N-limited conditions) or dimin-ish (under conditions of high N availability) net N mineralisation.
4.4. Conclusions
In IER±soil mixtures, NH4mineralised from organic
compounds is readily adsorbed by the IER due to its intimate contact with the soil and short diusion ways. Nitri®cation, accumulation and subsequent losses of
Nmin are prevented. Therefore, we suggest net N
min-eralisation rates may be quanti®ed more precisely by IER±soil mixtures than by traditional methods (buried bags, soil cores, resin cores).
IER may mimic sink functions of plant roots, and nutrient sorption can be regarded as a functional ana-logy to nutrient uptake by roots. From this, we assume that net N mineralisation rates of soil±IER mixtures approximate conditions in the rhizosphere better than results of traditional approaches, as long as soil water and aeration conditions are not largely changed.
Due to this assumed functional analogy, soil±IER mixtures may also be a useful model system to study root-induced N mineralisation eects.
Acknowledgements
We would like to thank Corinna Kuûmaul, Martin-Luther-University Halle-Wittenberg, and Irma Schu-macher, University of Hohenheim, for their technical assistance. We are indebted to Martin Kaupenjohann, University of Hohenheim, for his contributions to the experimental set-up and the interpretation of the results, and Karl Stahr, University of Hohenheim, for his readiness to ®nancially support the experiment.
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