Biol Fertil Soils (1999) 29 : 319–327
Q Springer-Verlag 1999
ORIGINAL PAPER
Gary D. Bending 7 Mary K. Turner
Interaction of biochemical quality and particle size of crop residues
and its effect on the microbial biomass and nitrogen dynamics
following incorporation into soil
Received: 3 July 1998
Abstract Mineralization of N from organic materials
added to soil depends on the quality of the substrate as
a carbon, energy and nutrient source for the saprophytic microflora. Quality reflects a combination of biochemical and physical attributes. We investigated how
biochemical composition interacts with particle size to
affect the soil microflora and N dynamics following incorporation of crop residues into soil. Four fresh shoot
and root crop residues were cut into coarse and fine
particle sizes, and incorporated into sandy-loam soil
which was incubated under controlled environment
conditions for 6 months. In the case of the highest biochemical quality material, potato shoot (C/N ratio of
10 : 1), particle size had no effect on microbial respiration or net N mineralization. For lower biochemical
quality Brussels sprout shoot (C/N ratio of 15 : 1), reducing particle size caused microbial respiration to
peak earlier and increased net mineralization of N during the early stages of decomposition, but reduced net
N mineralization at later stages. However, for the lowest biochemical quality residues, rye grass roots (C/N
ratio of 38 : 1) and straw (C/N ratio of 91 : 1) reducing
particle size caused microbial respiration to peak later
and increased net immobilization of N. For Brussels
sprout shoot, reducing particle size decreased the C
content and the C/N ratio of residue-derived light fraction organic matter (LFOM) 2 months following incorporation. However C and N content of LFOM derived
from the other materials was not affected by particle
size. For materials of all qualities, particle size had little
effect on biomass N. We conclude that the impact of
particle size on soil microbial activities, and the protection of senescent microbial tissues from microbial at-
G.D. Bending (Y) 7 M.K. Turner
Department of Soil and Environment Sciences,
Horticulture Research International, Wellesbourne, Warwick,
CV35 9EF, UK
e-mail: gary.bending@hri.ac.uk, Tel.: c44-1789-470382,
Fax: c44-1789-470552
tack, is dependant on the biochemical quality of the
substrate.
Key words Crop residues 7 Biochemical quality 7
Particle size 7 Nitrogen cycling 7 Microbial biomass
Introduction
The rate and pattern of decomposition and mineralization of plant material incorporated into soil reflects interaction between its ‘quality’ and the prevailing chemical and physical environment of the soil. Quality,
which can be described as the suitability of the substrate as a carbon, energy and nutrient source to the
organisms that degrade it (Swift et al. 1979), has two
components – its biochemical composition and its physical nature.
Many studies have attempted to relate biochemical
quality to decomposition and mineralization, and a
wide range of components have been found to control
these processes, depending on the type of plant material being tested and the time during decomposition that
mineralization is measured. Generally applicable biochemical quality components that have been correlated
with N mineralization include C/N ratio, and N, lignin
and cellulose contents (Iritani and Arnold 1960; Frankenberger and Abdelmagid 1985; Janzen and Kucey
1988; Vigil and Kissel 1991; Bending et al. 1998), while
polyphenol content has been shown to be important in
controlling N mineralization from leguminous residues
(Palm and Sanchez 1991; Giller and Cadisch 1997). Furthermore, the importance of particular components can
change over the course of decomposition. Bending et
al. (1998) showed that for a range of shoot and root
residues, early net N mineralization was correlated with
phenolic content, while later release was correlated
with soluble N, followed by cellulose, total N and
C/N.
Much less is known of the way in which physical
quality affects mineralization. Features of plant materi-
320
als that represent physical quality include particle size,
toughness, surface properties including cuticle thickness and composition, the presence of defence structures such as spines, and water content (Swift et al.
1979). These properties have the potential to affect the
accessibility of substrates to soil organisms, and thus alter rates of colonization and patterns of decomposition
and mineralization. Controlling the physical quality of
plant materials could therefore provide a tool with
which to manipulate patterns of mineralization from
crop residues, to improve synchrony between release of
mineral N and the needs of following crops.
The physical quality parameter that has received
most attention in nutrient cycling studies is particle
size. Several studies have confirmed that particle size
affects these processes, although the nature of the effect seems variable, with some reports that reducing
particle size increases rates of microbial processes
(Sims and Frederick 1970; Nyhan 1975; Amato et al.
1984), while other investigations have shown the reverse effect (Stickler and Frederick 1959; Jensen 1994;
Sørensen et al. 1996).
There is a need to determine whether the contrasting results from the studies could be a result of the varied biochemical qualities of the materials used. Additionally, most previous studies have used plant materials that had been air dried, with the smallest particle
sizes prepared by milling samples. Such preparation
will produce substrates with little relationship to those
which could be generated in the field. In order to determine the practical significance of particle size, studies
need to be conducted using fresh materials.
The aim of this investigation was to determine
whether biochemical quality and particle size interact
to affect microbial activities during decomposition of
crop residues. Fresh residues representing a spectrum
of biochemical qualities were cut into two particle sizes
and incorporated into soil. Over a period of 6 months,
microbial respiration, microbial biomass N and N dynamics were investigated. Additionally, residue remaining as light fraction organic matter (LFOM) was determined 2 months following incorporation.
Materials and methods
Soil
Soil was collected from the top 20 cm of a fallow field at Wellesbourne, UK. The soil is a sandy-loam of the Wick series, with
74% sand, 12% silt and 14% clay, a pH of 5.9, an organic C content of 0.8%, and an organic N content of 0.1% (Whitfield 1974).
Soil was passed through a 2-mm sieve and air dried prior to use.
Plant materials
Crop residues were chosen to represent a spectrum of qualities.
The residues consisted of mature leaves and petioles of Brussels
sprouts (Brassica oleracea L cv. Peer Gynt) and potato (Solanum
tuberosum L. cv. Wilja), and roots of rye grass (Lolium perenne L
cv. Parcour), all of which had been grown under glasshouse con-
ditions in Levington M2 compost for up to 4 months. Prior to use,
roots were washed thoroughly in deionised water to remove adhering soil. Wheat (Triticum aestivum L.) straw was collected
from a recently harvested field at Wellesbourne, air dried and
stored at 4 7C before use. There were two particle size treatments
for each residue, both of which were prepared by manually cutting the material with a pair of scissors. For the shoot treatments,
the coarse and fine particle sizes consisted of 4- and 0.2-cm
squares of leaf and petiole, while the rye roots and wheat straw
were cut into 1- and 0.2-cm lengths.
Residue quality analysis
Quality characteristics of the residues were determined by proximate analysis. Subsamples of the residues were dried at 100 7C
and milled before investigation. A water soluble fraction was removed by extracting the residue with deionized H2O in a boiling
water bath for 2 h. The material was centrifuged, and cellulose
and lignin were determined in the substrate remaining. The material was subjected to hydrolysis with H2SO4 to convert cellulose to
sugars, which were estimated in the acid hydrolysable fraction using a phenol-H2SO4 assay (Dubois et al. 1956). Lignin was determined by weight loss of the acid-insoluble fraction on ashing. Total residue organic C and N were measured by dichromate oxidation and a Kjeldahl procedure, respectively (Anderson and Ingram 1993).
Incubation study
Soil was moistened to P126 kPa with deionized H2O, and incubated at 15 7C for 5 days prior to use. Replicate pots were set up
by mixing 5, 1.5 and 1 g fresh weight (fw) of the shoot, root and
wheat straw residues respectively into 100 g fw soil. The soil water-filled pore space (WFPS) was 22% at the start of the experiment. The amounts of residue C and N incorporated are given in
Table 1. The shoot and straw additions are within the range 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. Pots were
placed into sealed 15-l plastic tubs, through which moist air was
continually passed to ensure that an aerobic environment was
maintained throughout the experiment. The incubation was conducted at a constant temperature of 15 7C. For each residue and
unamended soil, 3 pots were harvested after 3 days, followed by
weekly intervals for the first month, and monthly intervals for the
following 2 months, with a final harvest 3 months later. At each
harvest, microbial respiration, microbial biomass N, and soil mineral N were measured. The characteristics of residue-derived
LFOM were determined 2 months following incorporation.
Microbial respiration was measured by the method of Bending et al. (1998) using infrared gas analysis of CO2 derived from
20 g fw portions of soil, following incubation in 100-ml containers
at 25 7C for 30 min, using an ADC 225 mark 3 infrared gas analyser (ADC, Hoddesdon, UK). Microbial biomass N was determined
by the fumigation-extraction method of Joergensen and Brookes
(1990). Ninhydrin N released on fumigation was converted to biomass N using a conversion factor of 3.1 (Amato and Ladd 1988).
Soil mineral N was extracted in 0.5 M K2SO4, and NHc
4 measured
by the indophenol blue assay (Scheiner 1976) and NOP
3 by high
performance liquid chromatography (Hunt and Seymour 1985).
LFOM was extracted from 30 g fw soil using a 1.75 g cm P3 solution of NaI (Strickland and Sollins 1987). Total C and N of the
extracted LFOM was determined using a C/N autoanalyser (CN2000, Leco Corporation, Michigan, USA). All analyses were carried out in triplicate.
Data analysis
All treatment effects were calculated by subtracting results obtained in unamended soil.
321
Table 1 Biochemical quality characteristics of residues and amounts of C and N added to soil
Crop residue
Potato shoot
Brussels sprout shoot
Rye grass root
Wheat straw
C/N
10
15
38
91
Residue composition (% dw)
N
Cellulose
Lignin
4.1
2.5
1.0
0.4
25.6
36.5
46.2
62.3
32.8
18.9
37.3
43.9
Results
Residue biochemical quality
The biochemical characteristics of the residues are
shown in Table 1. Using the criteria of Bending et al.
(1998) to define biochemical quality, the residues span
a spectrum, ranging from high quality N-rich potato
shoot, with a low C/N ratio and cellulose content, to
low-quality N-poor wheat straw, with a high C/N ratio
and cellulose content.
Amount C and N added to soil
(mg g P1 dw soil)
C
2180
2978
1039
4037
N
223
199
28
44
root residues, biomass N peaked during the first 28
days following incorporation, after which it declined to
low amounts (Fig. 2a–c). In the case of wheat straw,
biomass N peaked between 21 and 56 days (Fig. 2d).
Particle size had little significant effect on biomass N.
For potato shoot, biomass N was significantly higher in
soil amended with fine than coarse particles after 28
days, but there was no significant difference at any other time interval. Particle size had no significant effect
on biomass N for any of the other materials. However,
for Brussels sprout shoot, rye grass roots and wheat
straw, there was a trend for biomass N to be higher in
soil amended with coarse particles.
Microbial respiration
The effect of crop residue particle size on microbial respiration following residue incorporation is shown in
Fig. 1. Incorporation of Brussels sprout shoot stimulated greatest respiration, followed by potato shoot, rye
grass root and wheat straw. In all treatments, respiration peaked within the first 28 days after incorporation,
declining to low levels after 56 days.
The impact of particle size on respiration depended
on the biochemical quality of the crop residue material.
For potato shoot, particle size had no significant effect
on microbial respiration (Fig. 1a). In the case of Brussels sprout shoot, reducing particle size caused respiration to peak earlier, so that respiration was higher in
soil amended with fine than coarse particles between 7
and 14 days, but the reverse was true after 28 days (Fig.
1b). For rye grass root, reducing particle size caused
respiration to peak later, so that maximum respiration
occurred after 7 and 14 days for the coarse and fine
particle sizes respectively (Fig. 1c). In the case of wheat
straw, respiration after 3 days was considerably greater
in soil amended with coarse particles than that to which
fine particles were added. However, subsequent respiration patterns were similar in both treatments
(Fig. 1d).
Microbial biomass N
Fig. 2 shows soil microbial biomass N following residue
incorporation. In the case of the shoot and root residues, the tissues had become brown and senescent after
14 days, indicating that only values obtained after this
time represented biomass N. For the fresh shoot and
Soil mineral N
Net mineralization of N was faster following incorporation of potato shoot than Brussels sprout shoot (Fig.
3a, b), while rye grass root and wheat straw both caused
net immobilization of soil mineral N (Fig. 3c, d). The
effect of residue particle size on soil mineral N depended on the biochemical quality of the residue. In the case
of potato shoot, particle size had no significant effect
on mineralization of N. However, for Brussels sprout
shoot, particle size affected N mineralization, the nature of the effect depending on the stage of decomposition. At early stages of decomposition, net mineralization of N occurred in soil amended with fine particles,
while mineral N was immobilized in soil amended with
coarse particles. However, N was subsequently immobilized in soil to which fine particles were added. In
both treatments net mineralization of N proceeded rapidly between 28 and 84 days, with significantly more N
mineralized in soil amended with coarse than fine particles.
Incorporation of rye grass roots and wheat straw resulted in net immobilization of mineral N, which increased over the course of the experiment and was
markedly greater following incorporation of fine than
coarse particle size material. Net immobilization of N
was greater in soil amended with wheat straw than in
soil to which rye grass roots were added.
Light fraction organic matter
Analysis of LFOM showed that the C/N ratio of all materials decreased during the decomposition process
322
Fig. 1a–d Effect of crop residue particle size on soil microbial
respiration ([ coarse particle size, l fine particle size). Bars represent c/P standard error of the mean. Significance of difference
between particle size treatments at each time interval determined
by paired t-test (* significant P~0.05; ** significant P~0.01)
(Table 2). Fraction size significantly affected the
amount of C remaining in LFOM derived from Brussels sprout shoot. There was significantly less C remaining in LFOM formed from fine particles than from
coarse particles, and the C/N ratio of LFOM derived
from fine particles was significantly lower than that
produced from coarse particles. There were no significant differences in C, N or C/N ratio of LFOM derived
from coarse and fine particles for the other materials.
However, for both rye grass root and wheat straw there
appeared to be a trend for lower C/N ratios in LFOM
derived from fine than coarse particles.
Soil N budget 56 days following addition of crop
residues to soil
The pool of N unaccounted for following subtraction of
N contained in the mineral, microbial biomass and light
323
Fig. 2a–d Effect of crop residue particle size on soil microbial
biomass N ([ coarse particle size, l fine particle size). Bars represent c/P standard error of the mean. Significance of difference
between particle size treatments at each time interval determined
by paired t-test (* significant P~0.05; ** significant P~0.01)
fraction pools after 56 days from the quantity of N added in the residues is shown in Table 3. In the case of
potato shoot, this pool amounted to 40.7 mg N g P1 dry
weight (dw) soil in soil amended with coarse particles,
12 mg N g P1 dw soil higher than that in soil to which
fine particles were added. In soil amended with Brussels sprout shoot, the missing N pool was 141.7 mg N
g P1 dw soil in soil to which fine particles were added,
which was one third greater than the size of this pool in
soil amended with coarse particles. For both of the lowquality materials, more N was present in the measured
soil pools than had been added in the form of coarse
324
Fig. 3a–d Effect of crop residue particle size on soil mineral N ([
coarse particle size, l fine particle size). Bars represent c/P
standard error of the mean. Significance of difference between
particle size treatments at each time interval determined by
paired t-test (* significant P~0.05; ** significant P~0.01)
particles, while over 10 mg N g P1 dw soil remained
unaccounted for in soil amended with fine particles.
Discussion
Our results demonstrate that particle size of crop residue materials influences the activities of the soil microbial population following incorporation into soil, and
that the nature of the effects depend on the biochemical quality of the plant material and the stage of decomposition. The soil microbial community that colonizes
325
Table 2 Residue-derived light
fraction organic matter
(LFOM) 56 days following incorporation of residues. Figures in parentheses give standard error of the mean. Significance of difference between
particle size treatments determined by paired t-test
Treatment
LFOM C and N contents
(mg g P1 dw soil)
Particle size
(C-coarse;
F-fine)
C
Potato shoot
Brussels sprout shoot
Rye grass root
Wheat straw
C
F
C
F
C
F
C
F
528
527
533
170
519
661
3820
3884
C/N
N
(71)
(102)
(118)*
(86)
(201)
(55)
(397)
(139)
64.1
72.1
37.4
33.0
29.4
31.1
45.5
51.2
(4.9)
(8.4)
(3.3)
(5.1)
(4.6)
(5.2)
(5.1)
(1.1)
8.2
7.3
14.3
5.2
17.7
21.3
84.0
75.9
(0.8)
(0.2)
(1.2)**
(1.1)
(2.0)
(3.0)
(13.1)
(4.3)
* Significant P~0.05; ** significant P~0.01
Table 3 N unaccounted for following subtraction of the N contained in microbial biomass, mineral and light fraction organic
matter pools after 28 days from N added as crop residues
Treatment
Particle size
(C coarse;
F fine)
N unaccounted for
(mg g P1 dw soil)
Potato shoot
C
F
C
F
C
F
C
F
40.7
28.3
98.3
141.7
P 1.8
10.9
P16.2
10.3
Brussels sprout shoot
Rye grass root
Wheat straw
and degrades organic substrates will be influenced by
particle size in a number of ways. Particle size controls
the surface area available for colonization by the soil
micro-biota, and will influence exchange of water, nutrients and oxygen between the substrate and the soil
matrix (Swift et al. 1979). Additionally, particle size will
influence contact of the material with clay and silt particles, which can act to protect organic materials from
microbial attack (Hassink 1997). The relative importance of these influences will depend on the biochemical and physical composition of the organic material,
together with the physical and chemical environment of
the soil.
In the case of the highest biochemical quality material, potato shoot, rates of colonization by the soil micro-biota appear to have been so rapid that surface
area available for colonization had little effect on microbial activities. Further, it seems that the decomposition of this substrate was so rapid that the greater protection afforded to C and N derived from fine particles
by more intimate association with the soil matrix was
limited, so that rates of net N mineralization over the
course of the incubation were not affected by particle
size. Any effects of particle size on the microflora were
restricted to the very early stages of decomposition,
when there was some evidence for elevated microbial
respiration and biomass N in soil amended with fine relative to coarse particles.
For lower biochemical quality Brussels sprout shoot,
rates of substrate utilization by the soil micro-biota
were slower, so that increasing the surface area available for colonization by reducing particle size induced
earlier utilization of the substrate by soil microbes,
which was shown by the stimulated rate of microbial
respiration. Similarly, Nyhan (1975) showed that reducing particle size of fresh blue gramma grass shoots incorporated into soil stimulated microbial respiration.
In the case of Brussels sprout shoot, the nature of
the effect of particle size on net N mineralization depended on the stage of decomposition. At early stages
more mineral N was released from fine than coarse particles. This could reflect the initial enhanced activity of
the micro-biota in soil amended with fine relative to
coarse particles, arising from an increased surface area
available for colonization. At later stages more mineral
N was produced in soil amended with coarse particles
than was formed in soil into which fine particles had
been added. This could indicate that by this time, increased contact of fine particles with the soil matrix had
resulted in a greater degree of physical protection of
the residual plant materials and N contained in senescent microbial tissues and microbial products from further microbial attack.
Since the soils had a low WFPS, denitrification
losses during decomposition would have been low
(Weier et al. 1993), and the residue N which could not
be accounted for by addition of microbial biomass, mineral and LFOM pools after 56 days will therefore predominantly consist of N which had been converted to
senescent microbial tissues or soluble components of
the residues that had become stabilized in heavy fraction organic matter associated with mineral fractions.
The fact that this pool was larger in soil amended with
fine than coarse Brussels sprout shoot confirms that
there was greater protection of microbial tissues and
products derived from fine particles from continued decomposition at this time, than was the case for the same
pools formed during the decomposition of coarse particles. However, since particle size had no effect on mineral N produced at the end of the incubation, such
protection was short lived.
326
The extent and duration of physical protection of organic materials by the soil is likely to depend on many
factors, including degree of aggregation, clay content,
compaction and soil moisture content. Additionally,
the size of the particles themselves will be important,
and the strong and long-lived protective effect given to
fine particle size materials that has been seen in several
other studies (Jensen 1994; Sørensen et al. 1996) could
have resulted from the artificial nature of plant materials used, which had been milled to a very fine particle
size prior to incorporation into soil.
In our study, increasing surface area of low biochemical quality residues inhibited microbial respiration in the very early stages of decomposition. This
could have arisen from more rapid release of water soluble components, including inhibitory substances such
as phenolics, causing inhibition of microbial growth
(Bending et al. 1998). This observation contrasts with
other studies, which have found that reducing particle
size stimulates early decomposition of low biochemical
quality residues such as wheat straw (Sims and Frederick 1970; Bremer et al. 1991).
Throughout the incubation, reducing particle size of
low biochemical quality residues resulted in increased
immobilization of soil mineral N. Similarly, Sims and
Frederick (1970) found that reducing particle size of
low biochemical quality corn straw increased net immobilization of soil mineral N. In our study, biomass N
was not consistently different in soil amended with the
different particle sizes, suggesting that enhanced immobilization of N by the soil biomass could not by itself
account for the effect. Additionally, there were only
small differences in N contained in LFOM derived
from the different particle sizes. This suggests that increased protection of residue-derived N, N-containing
senescent microbial tissues and products of microbial
metabolism could have played a role in causing enhanced immobilization of soil mineral N by fine particles.
Both wheat straw and rye grass roots contained high
lignin contents, degradation of which results in the formation of lower molecular weight polyphenols related
to humic and fulvic acids (Stevenson 1994). These compounds have well-known abilities to form insoluble recalcitrant complexes with a variety of organic N-containing compounds including proteins and chitin
(Bending and Read 1996). Enhanced rates of lignin degradation resulting from reduction of particle size of
the wheat straw and rye grass roots could have increased generation of such polyphenols, resulting in increased stabilization of N contained in senescent microbial tissues. This could have contributed to the enhanced immobilization of soil mineral N in soil
amended with fine relative to coarse particle size
pieces. It could also partly explain why the low-quality
materials continued to immobilize soil mineral N over
the time course of the experiment, even though microbial biomass generally declined. This contrasted with
the high-quality fresh shoot materials, which mineral-
ized N as the microbial biomass declined. Soluble polyphenolic compounds generated during decomposition
may therefore play a crucial role in the stabilization of
senescent microbial tissues.
This interpretation is confirmed by evidence from
the soil N budget 56 days following incorporation of residues. This demonstrates that a portion of the N added
in fine particles remained unaccounted for at this time,
presumably in senescent microbial tissues. Further, in
soil amended with coarse particles, more N was present
in the measured pools than had been added in the residues, demonstrating that addition of these materials
had stimulated mineralization of N from background
soil organic matter present in the soil. This suggests
that the amount of N contained in the senescent biomass pool in soil amended with fine particle size rue
grass root and wheat straw residues could actually have
been significantly higher than the N budget indicated.
Acknowledgements We thank the Ministry of Agriculture, Fisheries and Food for financial support.
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