PII:
Soil Biol. Biochem. Vol. 30, No. 14, pp. 2055±2065, 1998
# 1998 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
S0038-0717(98)00081-9
0038-0717/98 $19.00 + 0.00
FATE OF NITROGEN FROM CROP RESIDUES AS
AFFECTED BY BIOCHEMICAL QUALITY AND THE
MICROBIAL BIOMASS
GARY D. BENDING*, MARY K. TURNER and IAN G. BURNS
Department of Soil and Environment Sciences, Horticulture Research International, Wellesbourne,
Warwick CV35 9EF, U.K.
(Accepted 1 May 1998)
SummaryÐNet mineralization of N from a range of shoot and root materials was determined over a
period of 6 months following incorporation into a sandy-loam soil under controlled environment conditions. Biochemical ``quality'' components of the materials showed better correlation with net N mineralization than did gross measures of the respiration and N content of the soil microbial community
during decomposition. The quality components controlling net N mineralization changed during decomposition, with water-soluble phenolic content signi®cantly correlated with net N mineralization at
early stages, and water-soluble N, followed by cellulose at later stages. C-to-N and total N were correlated with net N mineralization towards the end of the incubation only. Cumulative microbial respiration during the early stages of decomposition was correlated with net N mineralization measured after
2 months, at which time maximum net N mineralization was recorded for most residues. However,
there was no relationship between microbial-N and net N mineralization. Biochemical quality factors
controlling the C and N content of the residue remaining at the end of the incubation as light fraction
organic matter (LFOM) were also investigated. Both C and N content of LFOM derived from the residues were correlated with residue cellulose content, and the chemical characteristics of LFOM were
highly correlated with those of the original plant material. Incorporation of low cellulose, high watersoluble N-containing shoot residues resulted in more N becoming mineralized than had been added in
the residues, demonstrating that net mineralization of native soil organic matter had occurred. Large
amounts of N were lost from the mineral-N pool during the incubation, which could not be accounted
for by microbial immobilization. # 1998 Elsevier Science Ltd. All rights reserved
INTRODUCTION
The importance of biochemical composition or
``quality'' in determining the rate of decomposition
and mineralization of nutrients from plant materials
has long been recognised (Swift et al., 1979).
Although many studies have been undertaken to
identify quality components that control N mineralization, there has been little agreement between
them, and a wide range of quality factors have been
found to be correlated with N release.
These include the C-to-N ratio (Frankenberger
and Abdelmagid, 1985; Giller and Cadisch, 1997),
N content (Iritani and Arnold, 1960; Janzen and
Kucey, 1988; Vigil and Kissel, 1991), lignin content
(Frankenberger and Abdelmagid, 1985; De Neve et
al., 1994; Giller and Cadisch, 1997), lignin-to-N
(Vigil and Kissel, 1991), polyphenol-to-N (Palm
and Sanchez, 1991), and polyphenol plus lignin-toN (Constantinides and Fownes, 1994) ratios.
The diverse results obtained in such studies could
have several explanations. Much of the discrepancy
arises from the diversity of materials included in
*Author for correspondence.
E-mail: gary.bending@hri.ac.uk
each investigation, with most studies focusing on
residues of similar nature, including those from
legumes (Frankenberger and Abdelmagid, 1985;
Palm and Sanchez, 1991), tree litters (Melilo and
Aber, 1984), and vegetables (De Neve et al., 1994).
However, even those studies that contained diverse
assemblages of materials failed to include residues
from roots. Additionally, many of the studies
measured relationships between quality and N
mineralized at a single point in time, even though
quality factors controlling mineralization are likely
to change over time as the nature of the remaining
substrate changes (Heal et al., 1997).
Quality controls decomposition and mineralization by direct e€ects on the microbes responsible
for these processes, and measurement of the size
and N content of the microbial biomass could serve
as a useful predictor of N mineralization. Although
such measurements of the microbial biomass
include organisms contributing to immobilization of
N and denitri®cation, several studies have established relationships between the size and N content
of the micro¯ora and N mineralization.
Alef et al. (1988) found a close relationship
between N mineralization rate and both microbial
2055
2056
Gary D. Bending et al.
respiration and microbial ATP content in a range
of agricultural and grassland soils, and Dalal and
Meyer (1987) showed that nitrogen mineralization
potential was correlated with microbial biomass in
a variety of soils subjected to long-term cereal cropping. Further, Fisk and Schmidt (1995) found that
seasonal variation of N mineralization in some
alpine tundra soils was related to microbial-N content. There is a need to establish whether these microbial variables have potential for use as
predictors of N mineralization from crop residues.
In addition to a€ecting patterns of N mineralization from plant materials, quality components are
likely to control the nature of organic material
remaining at all stages of decomposition. Such partially-decomposed organic matter, together with microbial tissues and products, constitute light
fraction organic matter (LFOM), which represents
a labile component of soil organic matter that governs N mineralization patterns in many soils
(Janzen et al., 1992; Sierra, 1996).
Our aims were to (i) investigate the relationships
between residue biochemical quality components
and net N mineralization from a spectrum of crop
residue materials, including roots, over an extensive
period following incorporation into soil. (ii) compare the relationships between quality and N mineralization with the relationships between the size and
N content of the microbial biomass and N mineralization. (iii) determine the residue quality components that control the C and N content of the
plant derived organic materials remaining as light
fraction organic matter.
MATERIALS AND METHODS
Soil
Soil was collected from the top 20 cm of a fallow
®eld at Wellesbourne, Warwickshire, U.K. The soil
is a sandy-loam of the Wick series, with 14% clay,
a pH of 5.9, an organic-C content of 0.8%, and an
organic-N content of 0.1%. Full description is
given in Whit®eld (1974). Soil was sieved (2 mm)
and air dried prior to use. The initial mineral N
content of the soil was 2.9 mg gÿ1 dw soil.
Plant materials
A range of plant materials were chosen to represent a spectrum of crop residues typical of arable,
horticultural and cover crops. The residues consisted of mature leaves and petioles of brussels
sprouts (Brassica oleracea cv. Peer Gynt), ryegrass
(Lolium perenne L cv. Parcour), sugar beet (Beta
vulgaris L cv. Saxon), french bean (Phaseolus vulgaris L. cv. Double White), and potato (Solanum
tuberosum L. cv. Wilja), all of which had been
grown under glasshouse conditions in Levington
M2 compost for up to 4 months. In the case of
brussels sprouts and rye grass, root residues were
also used. The shoot materials were cut into lengths
of approximately 1 cm before use. Roots were
washed thoroughly in deionized water to remove
adhering soil, and the root mat cut into pieces of
approximately 1 cm before use. Wheat (Triticum
aestivum L.) stubble was collected from a recently
harvested ®eld at Wellesbourne, air dried and stored
at 48C prior to use, when it was cut into 1 cm long
cylinders.
Residue quality analysis
Quality characteristics of the residues were determined by proximate analysis. Sub-samples of the
residues were dried at 1008C and milled before investigation. A water-soluble fraction was prepared
by extracting 200 mg of residue with 10 ml of deionised H2O in a boiling water bath for 2 h.
Following centrifugation at 400 g for 20 min, soluble carbohydrate in the water soluble fraction was
determined by reaction with phenol and H2SO4
(Dubois et al., 1956), using glucose as a standard.
Water-soluble phenolic content was determined
using Folin±Ciocalteu reagent (Sigma Chemical
Co., St Louis, USA) as described by Vidhyasekaren
et al. (1992), using tannic acid as a standard.
Cellulose and lignin were determined in the residue
remaining after water extraction by the acid detergent ®bre method (Van Soest and Wine, 1968).
The N and C characteristics of the residues were
determined by the Analytical Services Unit at HRI
Wellesbourne. Total residue and water soluble organic-C were measured by dichromate oxidation
(Nelson and Sommers, 1982), and total residue and
water soluble organic-N by a micro-Kjeldahl procedure (Anderson and Ingram, 1993). NH+
4 and
NOÿ
3 in the water-soluble fraction were determined
by the indolphenol blue assay (Scheiner, 1976) and
high performance liquid chromatography (Hunt
and Seymour, 1985), respectively.
Incubation study
Soil was moistened to ÿ126 kPa with deionized
H2O, and incubated at 208C for 5 days prior to use.
For the shoot materials, replicate pots were set up
by mixing 5 g fw of residue into 100 g fw soil,
which was then poured into rectangular polythene
pots (7 cm width, 8 cm length). The base of each
pot was tapped gently to allow the contents to
settle. Water ®lled pore space (WFPS) was calculated as follows; WFPS = [(gravimetric water content soil bulk density)/total soil porosity], where
soil porosity = [1 ÿ soil bulk density/2.65] and the
particle density of the soil was assumed to be
2.65 mg mÿ3. Bulk density was calculated by the
method given in Anderson and Ingram (1993). The
WFPS was found to be 22% at the start of the
experiment.
In the case of the root materials and straw, 1.5
and 1 g fw respectively, were added to 100 g soil.
Fate of N from crop residues
2057
Table 1. Quantities of residue C and N incorporated into soils
Quantity incorporated into soil (mg gÿ1 dw soil)
Residue
Residue (dw equivalent)
Brussels sprout shoot
Brussels sprout root
French bean shoot
Potato shoot
Ryegrass shoot
Ryegrass root
Sugar beet shoot
Wheat straw
7600
1412
6385
4805
6165
1427
4610
8995
Quantities of shoot and straw additions are typical
of residue inputs to soil following cropping
(Sylvester-Bradley, 1993), while the root additions
represented the equivalent root biomass produced
by the glasshouse grown plants. A control treatment containing unamended soil was also set up.
The quantities of C and N incorporated in each
treatment are shown in Table 1. Pots were placed
into loosely-wrapped black polythene bags which
were perforated to allow aeration, and incubated in
a fan aerated room at 158C. For each residue and
unamended soil, 3 pots were destructively harvested
after 3 days, followed by weekly intervals for the
®rst month, and monthly intervals for the following
3 months, with a ®nal harvest 2 months later. At
each harvest, soil mineral-N, microbial-N, microbial
respiration and soil moisture content were
measured. At each harvest, soil was mixed
thoroughly prior to analysis.
Soil mineral-N was extracted in 0.5 M K2SO4,
ÿ
and NH+
4 and NO3 in the extract determined by
the methods described above. Microbial respiration
was measured by infra-red gas analysis of CO2 derived from 20 g fw portions of soil, after incubation
in 100 ml cylinders at 258C for 30 min, using an
ADC 225 Mark 3 infra-red gas analyzer linked to a
data logger through a gas handling unit (ADC,
Hoddesdon, Surrey, U.K.). Microbial-N was determined by the fumigation±extraction method of
Joergensen and Brookes (1990). Ninhydrin-N
released on fumigation was converted to microbialN using a conversion factor of 3.1 (Amato and
Ladd, 1988). Following calculation of soil moisture
content at each harvest, pots were amended with
deionized H2O as necessary to bring soil water potential to ÿ126 kPa.
To determine the characteristics of the plant derived organic materials remaining after 168 days,
the light fraction organic matter (LFOM) was
extracted from 30 g fw soil using a 1.75 g cmÿ3 solution of NaI (Strickland and Sollins, 1987).
Quantities of LFOM extracted were low, and replicates from each treatment were pooled before
analysis of organic-C and organic-N content, as
described above.
Total C
Total N
2763
500
2310
1672
2150
557
1203
3666
272
40
210
234
212
31
196
35
Data analysis
All measurements were calculated by subtracting
results obtained in unamended control soil. Since
fresh materials were used in the study, treatments
di€ered in both the amount of dry material and N
added (Table 1). In order to allow comparison
between residues, soil mineral-N, microbial-N and
LFOM-N and -C were calculated as % of N or C
added in the residue, and microbial respiration on
the basis of CO2 released per g dw of residue incorporated. The residue water soluble mineral-N contents were subtracted from soil mineral-N
measurements before relationships between net N
mineralization and the biochemical quality and microbial parameters were determined.
RESULTS
Biochemical characteristics of the plant materials
The materials used in the study spanned a spectrum with regard to the quality characteristics
measured (Table 2). Shoot materials had larger N
contents and carbohydrate and water-soluble organic-C contents than the root materials and straw,
and generally had lower C-to-N, cellulose and lignin
contents. The water-soluble N consisted primarily
of organic-N in most residues, although in sugar
beet almost 70% of this fraction was mineral-N.
Soil moisture content
Since fresh materials were used, the moisture content of residue amended soil increased as the materials released H2O during tissue breakdown. In
soil amended with fresh plant materials, soil moisture content increased during the ®rst 28 days of the
incubation, the WFPS peaking at 14 days for all the
shoot materials, when it reached between 42 and
50.6%, compared to control soil in which WFPS
was 26%. WFPS in soil amended with roots peaked
at 31±35% after 14 days. Moisture content of soil
amended with straw was no di€erent to that of the
control soil. After 56 days, there was no di€erence
in WFPS of control and residue amended soils,
2058
Gary D. Bending et al.
Table 2. Biochemical composition of plant materials
Residue composition (% dw)
Water soluble fractions
Residue
C-to-N
N
C
Brussels sprout shoot
Brussels sprout root
French bean shoot
Potato shoot
Rye grass shoot
Rye grass root
Sugar beet shoot
Wheat straw
10.2
12.5
11.0
7.2
10.5
18.0
6.1
104.7
3.6
2.8
3.3
4.9
3.4
2.2
4.3
0.4
7.2
2.4
12.3
5.1
6.0
2.5
6.6
3.0
N*
1.1
0.6
0.9
0.9
1.1
1.0
1.8
0.4
(0.12)
(0.07)
(0.16)
(0.11)
(0.32)
(0.16)
(1.19)
(0.03)
Carbohydrate Phenolics
8.0
2.9
16.2
5.2
6.3
4.7
8.7
3.5
1.8
1.6
1.0
1.0
1.0
1.2
0.7
0.9
Cellulose
Lignin
16.4
20.4
10.0
15.9
22.6
25.4
10.5
41.2
7.3
28.0
5.9
8.4
4.6
7.9
4.2
13.1
*Figures in brackets give contribution of inorganic-N.
which remained between 21 and 28% until day 168
(data not shown).
Soil mineral-N pool
The soil mineral-N pools, expressed as net N
mineralized as % of N added in the residue, are
shown in Fig. 1. NH+
4 was the predominant form
of mineral-N in most treatments during the ®rst
7 days, after which NOÿ
3 dominated, with only very
small amounts of NH+
4 detected after 56 days (data
not shown). Mineral-N was released rapidly following incorporation of the shoot materials (Fig. 1a).
In the cases of all shoot residues net N mineralization had exceeded the quantity of residue-N added
within the ®rst 14±84 days, demonstrating that additional N had been mineralized from soil organic
matter. The peak size of the mineral-N pool in
excess of N added ranged from above 90 mg gÿ1 dw
soil for sugar beet and potato, to 50 mg gÿ1 dw soil
for the other shoot materials (Table 3). The time
for maximum net N mineralization to take place
varied considerably, from 21 days in the case of
sugar beet, to 84 days for ryegrass. After peaking,
the soil mineral-N pool declined rapidly, levelling
out at 168 days to amounts equivalent to 40±60%
of the residue-N added. The rate of decline in the
mineral-N pool varied, occurring very rapidly in the
cases of sugar beet and potato, but slowly for
french bean. Substantial quantities of N were lost
from the mineral pool over this time, ranging
between 143 and 222 mg gÿ1 dw soil for the shoot
residues (Table 3).
Patterns of net N mineralization from the root
residues contrasted with those from the shoots
(Fig. 1b). Net N mineralization from both types of
root peaked within the ®rst 28 days, and maximum
net N mineralization represented a lower % of the
N contained in the residues than was the case for
the shoot materials. Following maximum net N
mineralization, the mineral-N pool declined to
amounts equivalent to 25 and 50% of the N added
in the rye grass and brussels sprout roots respectively. The quantities of N that were lost from the
mineral-N pool over this time were 8 and 25 mg gÿ1
dw soil for the ryegrass and brussels sprout roots
respectively (Table 3). Although incorporation of
straw resulted in net release of mineral-N during
the ®rst 3 days, there was a subsequent net immobilization of soil mineral-N over the course of the
experiment.
Signi®cant relationships between residue biochemical quality variables and net N mineralization
are shown in Fig. 2. Water soluble phenolic content
was correlated with net N mineralization after
3 days only (P < 0.05). The water-soluble N pool
of the residues was strongly correlated with net N
mineralization between 7 and 56 days, while cellulose content was correlated with net N mineralization between 21 and 168 days. Relationships also
existed between C-to-N and % N with net N mineralization between 56 and 168 days. The relationships between net N mineralization and water
soluble-C, water soluble-carbohydrate, lignin, lignin
plus water soluble phenolics, and lignin to N were
not signi®cant at any time interval.
Microbial respiration
In the cases of french bean, ryegrass and potato
shoots, respiration peaked 7 days following incorporation, while for sugar beet and brussels sprout
shoots, respiration peaked after 21 days. By 28 days
following incorporation, only small amounts of respiration were detected in soil amended with shoot
residues (Fig. 3a). Incorporation of roots induced
small amounts of respiration relative to the shoots,
and respiration peaked after 7 days, following
which it declined to low values (Fig. 3b). Straw
induced smaller amounts of respiration than the
root materials, which peaked 3 days following incorporation, following which only little respiration
was detected (Fig. 3b).
There was no relationship between microbial respiration and net N mineralization measured at each
time interval (data not shown). The correlation
coecient between net N mineralization that had
occurred after 56 days, when net N mineralization
was complete for most materials, and the combined
measures of respiration at each time interval over
the ®rst 56 days was 0.83, which is signi®cant at
P < 0.05.
Fate of N from crop residues
2059
Fig. 1. Net N mineralized as a % of residue-N added, calculated following subtraction of mineral-N in
unamended soil. Bars indicate average standard error of the mean at each time interval. (a) Shoots of
brussels sprout, french bean, potato, rye grass and sugar beet (b) Roots of brussels sprout and rye
grass, and straw
Microbial-N pool
It was found that the living plant materials themselves released large quantities of ninhydrin-N on
fumigation (data not shown), making it impossible
to di€erentiate between plant and microbial derived
ninhydrin-N in the period before plant cells died.
For all the fresh plant materials except brussels
sprout, the residue had become yellow or brown
and ¯accid within 14 days from incorporation,
suggesting that ninhydrin-N measured from this
time represented microbial-N only. In the case of
brussels sprout shoot, this stage was not reached
until 21 days following incorporation.
For all shoot materials, the microbial-N pool had
peaked by 21 days or earlier following residue incorporation (Table 4). At its peak, the microbial-N
pool of soil amended with shoot materials was
equivalent to between 18 and 27% of the residue-N
incorporated, declining to between 0.9 and 3.0%
after 168 days. Microbial-N in soil amended with
2060
Gary D. Bending et al.
Table 3. Net amounts of N mineralized from SOM in excess of
that added in residues, and the di€erence between peak net N
mineralized and ®nal measurement of the mineral N pool after
168 days
Peak size of mineral- Di€erence between
peak and ®nal
N pool in excess of
mineral-N pool
that added (mg gÿ1 dw
(mg gÿ1 dw soil)
soil)
Residue
Brussels sprout shoot
Brussels sprout root
French bean shoot
Potato shoot
Ryegrass shoot
Ryegrass root
Sugar beet shoot
Wheat straw
50
NA
50
95
51.0
NA
94
NA
222
25
143
210
148
8
214
NA
NA-not applicable.
brussels sprout and ryegrass roots peaked around
56 days, at which time the micro¯ora immobilized
N equivalent to 61 and 36%, respectively, of the N
incorporated. For brussels sprout roots, microbialN subsequently declined to below that in unamended soil. In the case of straw, the microbial-N
pool was equivalent to a larger proportion of the N
incorporated than for the other materials during
most of the experiment.
There was no signi®cant relationship between the
size of the microbial-N pool and net N mineralization at each harvest, or between the microbial-N
pool at time intervals between 14 and 28 days and
net N mineralization measured at 56 days, at which
time net N mineralization was complete for most
materials (data not shown). Additionally, there was
no signi®cant relationship between the changes in
microbial N, and changes in net N mineralization
between each time interval.
Relationships between residue quality variables and
microbial properties
There were no signi®cant relationships between
either microbial-N content or microbial respiration
and any of the residue quality variables, at any of
the harvests (data not shown).
Light fraction organic matter after 168 days
The C content of LFOM represented between 1.5
and 12% of C added in the shoot residues, and
between 23 and 49% of C added in the root residues (Table 5). The C content of LFOM in the
straw treatment represented over 65% of C added.
In the case of N content, LFOM contained an
equivalent of 0.5±13.5% of the N added in the
shoot residues, and between 24 and 26% of the N
added in the root residues. LFOM contained an
equivalent of 43% of the N added in the straw
treatment. There was a strong relationship between
LFOM C and N content as a % of each added in
the residue (r2=0.948, P < 0.01).
Both LFOM C and N content showed signi®cant
correlations with residue cellulose content C-to-N,
total N (all P < 0.05) and water soluble N contents
(P < 0.05 and 0.01 respectively, Table 6). There
were no signi®cant relationships between LFOM C
or N and any of the other residue biochemical quality variables.
DISCUSSION
Our results demonstrate that biochemical quality
variables are more e€ective predictors of N mineralization from crop residues than are gross measurements of the microbial biomass. Residue
biochemical quality attributes controlling mineraliz-
Fig. 2. Correlation coecients between biochemical quality parameters and net N mineralized as %
residue N added. Points above the top horizontal lines and below the bottom horizontal lines are signi®cant at the P level indicated
Fate of N from crop residues
2061
Fig. 3. Microbial respiration in soil amended with residues, calculated following subtraction of respiration in unamended soil. Bars indicate average standard error of the mean at each time interval. (a)
Shoots of brussels sprout, french bean, potato, rye grass and sugar beet (b) Roots of brussels sprout
and rye grass, and straw
ation were shown to change over time. Soluble phenolic content played a role in regulating N-mineralization during the very early stages of
decomposition (Fig. 2). In the living plant phenolic
compounds, including phenolic acids and polyphenols such as tannins, participate in defence against
pathogens (Zucker, 1983), and in terms of residue
quality represent ``modifying'' compounds, which
stimulate or inhibit the decomposer community by
their chemical structure, rather than by acting as
energy or nutrient sources (Swift et al., 1979).
The strong correlation between N-mineralization
and water soluble-N, which consisted predominantly of organic-N in most residues, demonstrated that it was the availability of labile forms
of N which determined N release during early
stages of decomposition. Similarly, Iritani and
Arnold (1960) found that water soluble-N content
was strongly correlated with N mineralization
during the early phase of decomposition of leaves,
roots and straw of a range of agricultural residues.
2062
Gary D. Bending et al.
Table 4. Microbial-N pool in residue amended soils, calculated following subtraction of microbial-N in unamended soil. Figures in
brackets give microbial-N as a % of residue-N added
Microbial-N in amended soils (mg gÿ1 dw soil) Time (days)
Residue
Brussels sprout shoot
Brussels sprout root
French bean shoot
Potato shoot
Rye grass shoot
Rye grass root
Sugar beet shoot
Wheat straw
Standard error of the means
14
21
28
56
84
112
168
ÿ
26.7 (67)
52.6 (25)
62.6 (27)
34.4 (16)
2.8 (9)
27.3 (14)
34.8 (101)
2.2
52.5 (25)
2.4 (6)
13.7 (7)
43.9 (19)
39.4 (19)
1.8 (6)
35.9 (18)
31.3 (91)
2.0
31.0 (15)
9.7 (24)
24.2 (12)
21.4 (9)
3.8 (2)
1.0 (3)
7.6 (4)
8.2 (24)
1.4
23.5 (11)
24.2 (61)
24.7 (12)
19.5 (8)
8.5 (4)
11.3 (36)
11.0 (6)
11.4 (33)
0.8
5.3 (3)
ÿ21.7 (ÿ54)
8.9 (4)
ÿ1.1 (ÿ1)
2.9 (1)
0.8 (0.5)
ÿ0.1 (0.0)
0.8 (2)
0.8
11.4 (5)
ÿ9.2 (ÿ23)
13.1 (6)
ÿ0.6 (ÿ0)
7.3 (3)
1.9 (ÿ0.5)
ÿ1.2 (ÿ0.5)
1.9 (5)
0.4
6.4 (3)
ÿ2.7 (ÿ7)
4.5 (2)
2.8 (1)
1.9 (1)
8.7 (3)
3.9 (2)
8.7 (25)
1.1
Cellulose content was most strongly correlated
with net N-mineralized during the later stages of decomposition, becoming very highly correlated with
net N mineralized after 56 days (Fig. 2), at which
stage net mineralization of N from most materials
was complete. The relationship between N mineralization and cellulose presumably re¯ected the rate
of N immobilization by the saprophytic micro¯ora
as it utilized the compound as an energy source.
A switch in quality variables controlling mineralization re¯ects changes in the nature of the material
as it is degraded over time. Giller and Cadisch
(1997) reported that biochemical quality variables
controlling net N mineralization from Mexican tree
forages changed over time, polyphenol control giving way to control by lignin.
Cellulose has not previously been considered to
be an important variable controlling N mineralization, despite it being the major component of all
plant materials. In most studies of quality factors
a€ecting mineralization from plant inputs, the cellulose fraction has been divided into its hemicellulose
and a-cellulose components, which have each been
used separately to determine their suitability as Nmineralization predictors (Janzen and Kucey, 1988;
De Neve et al., 1994; Quemada and Cabrera, 1995).
Since there is no relationship between hemicellulose
and a-cellulose content in plant tissues, this
approach may have led to cellulose being overlooked as an e€ective controller of mineralization.
Several other investigators have found lignin to
be the most important plant polymer controlling N
mineralization (Frankenberger and Abdelmagid,
1985; De Neve et al., 1994). The fact that it was
not important in our study could suggest that the
micro¯ora utilized lignin only slowly over the time
course of the experiment. This could have led to the
more readily-degraded cellulose becoming the dominant residue derived C substrate used by the microbial community during the period of N
mineralization from the residues. This would have
resulted in cellulose controlling re-immobilization of
mineralized N by the micro¯ora. The metabolic
capabilities of the microbial community of a given
soil, which will control the relative breakdown rates
of the di€erent residue components, will ultimately
determine which residue component is best related
to N mineralization.
Our data demonstrate that microbial measurements would provide less e€ective prediction of N
mineralization from freshly-incorporated plant materials than biochemical quality variables. However,
since a range of biochemical quality variables have
been found to control N mineralization from di€erent types of plant material, with little consensus
between studies, changes in microbial respiration
patterns could have potential for use as a generally
applicable predictor of N mineralization rates.
Net rate of N ¯ow through the micro¯ora, which
determines N mineralized, could not be estimated
by analysis of the microbial-N content at any single
Table 5. Residue-derived light fraction organic matter (LFOM)
after 168 days. C and N contents were calculated following subtraction of C and N in LFOM from unamended soil. Figures in
brackets give LFOM C and N as % of C and N added in the
residue
Table 6. Correlation coecients between residue-derived light fraction organic matter (LFOM). C and N content as a % of residue
C and N added and residue biochemical quality variables. See
Table 2 for key to residue biochemical quality variables. N and C
contents were calculated following subtraction of C and N in
LFOM from unamended soil
LFOM C and N contents
(mg gÿ1 dw soil)
Residue
Brussels sprout shoot
Brussels sprout root
French bean shoot
Potato shoot
Rye grass shoot
Rye grass root
Sugar beet shoot
Wheat straw
C
N
42.2 (2)
242.1 (48)
274.1 (12)
186.6 (11)
232.0 (6)
128.7 (23)
55.1 (5)
2386.0 (65)
3.3 (1)
10.3 (26)
28.4 (14)
13.8 (6)
13.8 (7)
7.7 (25)
0.9 (0.5)
14.9 (43)
C-to-N
12.8
23.5
9.7
13.5
16.8
16.7
61.2
160.1
LFOM-C and N as % of residue-C and N
added
Residue quality
variable
C±N
N
WS±N
Cellulose
*Signi®cant P < 0.05.
**Signi®cant P < 0.01.
C
N
0.791*
ÿ0.820*
ÿ0.777*
0.786*
0.809*
ÿ0.915**
ÿ0.788*
0.831*
Fate of N from crop residues
time period, or changes in microbial-N over time.
The communities degrading the di€erent substrates
will possess varying chemical compositions and
degradative abilities, which together with the
characteristics of the residue remaining will a€ect
patterns of N turnover and recycling of N into the
micro¯ora as new organic-C is utilized.
Although the sum of microbial respiration
recorded during the early stages of decomposition
was correlated with peak net N mineralized, the relationship was not as strong as that between the
biochemical quality variables and N mineralization.
Again this will re¯ect the distinct saprophytic communities that the di€erent quality substrates supported, which are unlikely to show the same
respiration characteristics even when utilizing the
same substrate at the same rate. In addition to
measuring activities of organisms involved in mineralization of N, microbial respiration measures activities of organisms involved in immobilization of
N and denitri®cation. Further, the rates of these
processes will change during the decomposition process.
Patterns of mineralization and decomposition of
root and shoot materials were considerably di€erent
(Fig. 1). In agricultural systems, di€erences in biochemical quality between shoot and root materials
will be augmented by other factors that will a€ect
quality. Roots are protected from attack by soil
organisms via several mechanisms which do not
apply to shoot residues. These include physical and
microbial defences presented by the rhizosphere.
Additionally, in above-ground crops, roots will not
be disturbed by harvesting, and could remain alive
until they are ploughed-under, further protecting
the C and N they contain from mineralization.
The C and N contents of LFOM generated
during the incubation were correlated with residue
cellulose content (Table 6), con®rming that cellulose
content controlled decomposition at the later stages
of the experiment. The correlations between residue
C-to-N and total N with LFOM C and N content,
and the fact that C and N remaining were correlated, demonstrates that the characteristics of the
LFOM produced during decomposition are largely
controlled by the nature of the residue.
LFOM represents a labile component of SOM,
and the size of the soil LFOM pool controls the
rate of N mineralization in many soils (Janzen et
al., 1992; Sierra, 1996). Golchin et al. (1994) used
CP MAS 13 C NMR spectroscopy to determine the
chemical nature of LFOM. They found that in ®ve
Australian soils, C contained in LFOM was largely
O-alkyl in nature, corresponding to carbohydrates,
including cellulose, with smaller amounts of alkyl-C
and aromatic-C, derived from lipids and lignin respectively, re¯ecting the composition of the plant
inputs from which it would have been derived.
2063
Incorporation of N-rich shoot materials caused
enhanced mineralization of N from soil organic
matter (SOM). It is well known that fertilizer and
crop residue addition to soil can stimulate apparent
mineralization of N from SOM. Jenkinson et al.
(1985) suggested that in most cases, stimulation of
N release from SOM following addition of fertilizer-N to soil, termed an ``added N interaction''
(ANI), results from pool substitution, in which 15 N
labelled fertilizer-N becomes diluted as mineralization and immobilization reactions of SOM proceed.
Studies of ANI resulting from incorporation of
plant materials are limited, with values ranging
from 8±29 mg g dw soil (Azam, 1990; Fox et al.,
1990; Azam et al., 1993). However, ANI resulting
from fertilizer application have been well studied,
and values up to 86.5 mg g dw soil have been
recorded (Hart et al., 1986). The high quality shoot
residues used in our study possessed large water-soluble C and N contents, which constitute the most
labile components of plant residues. Vanlauwe et al.
(1994) found that the water-soluble fraction of
maize straw enhanced decomposition of the less
labile cell wall fraction of this material. The large
ANIs detected in our study could have resulted
from stimulation of degradation of the biologicallyactive pools of SOM in which pool substitution
occurs, resulting from stimulated microbial activity
arising from the presence of the labile water soluble
components from the residues. This would decrease
the potential for immobilization of N released from
residues, and re-immobilization of N released from
SOM, the basis for the pool substitution e€ect.
In our study the size of the soil mineral-N pool
declined substantially following maximum net N
mineralization (Fig. 1). Other studies have revealed
considerable apparent losses of applied N following
incorporation of residues or fertilizers. Xu et al.
(1993) found that incorporation of leguminous residues resulted in loss of 25±41% of the N applied,
and attributed the loss to denitri®cation, while
others have ascribed similar sized losses to immobilization in the micro¯ora or soil organic matter
(Muirhead et al., 1985; Rochester et al., 1993).
Our data indicates that the decline of mineral-N
was not due to immobilization by the micro¯ora
(Table 4). Although loss by denitri®cation could
account for the decline of mineral-N (Maag and
Vinther, 1996), denitri®cation would be expected to
be limited at WFPS within the range that existed in
our study (Weier et al., 1993; Flessa and Beese,
1995). Additionally, any denitri®cation would have
peaked at the early stages of the experiment when
labile C was available (Weier et al., 1993).
However, anaerobic zones could have developed
around residue pieces during the period of intense
microbial respiration in the initial stages of decomposition.
2064
Gary D. Bending et al.
Immobilization into SOM could have contributed
to the apparent mineral-N loss. Such immobilization accounts for up to one-third of fertilizer-N
added to soil, and is believed to result from incorporation of N into humic substances following passage through the micro¯ora (Kelley and Stevenson,
1995). The LFOM did not contain sucient N to
account for the bulk of the decline in mineral-N,
suggesting that such immobilization would have
been into heavy fraction organic matter associated
with mineral surfaces or contained within microaggregates.
AcknowledgementsÐWe
thank
the
Ministry
of
Agriculture, Fisheries and Food for ®nancial support,
Simon Elliot and Su Lincoln for technical assistance, and
Julie Jones and Kath Phelps for statistical advice.
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