C.R. Rahn et al.
Soil Use and Management (2003) 19, 193±200
193
DOI: 10.1079/SUM2003188
Management of N mineralization from crop residues
of high N content using amendment materials of
varying quality
C.R. Rahn*, G.D. Bending, M.K. Turner & R.D. Lillywhite
Abstract. A potential technique for reducing overwinter leaching from high N containing crop residues is
to immobilize the N released during decomposition by co-incorporating materials of a wider C : N ratio.
This article describes the use of laboratory incubation experiments to investigate the effects of a wide range
of such amendment materials on the mineralization of N from sugar beet and brassica leaf residues in a
sandy loam and a silt loam. These materials were of varying quality, with C : N ratio ranging from 15 : 1 to
520 : 1, and cellulose content from 0 to 34%. Amendments were added at a ®xed rate of 3.5 mg C g±1 of dry
soil, equivalent to around 10 t ha±1 C (to 20 cm depth). The soils were then incubated at 15°C, and net
mineral N derived from the leaves was measured at regular intervals over 168 days. Net mineralization of
residue N was greatest with molasses (C : N ratio of 18 : 1), whereas paper waste (C : N ratio of 520 : 1)
reduced N mineralized by up to 90% compared with a soil-only control. As the concentration of cellulose
and lignin in the amendment materials increased, so the amounts of N mineralized decreased, with 62 and
54% of variance in N mineralized explained by cellulose and lignin content, respectively. Reduced levels of
mineral N were associated with higher levels of biomass-N. The levels of N2O-N lost from sugar beet residues on day 14 were signi®cantly reduced from 66 to 5 g ha±1 where compactor (cardboard) waste had been
mixed into sandy loam, but this effect was not observed in the silt loam. These techniques could lead to
greater ef®ciency of N use in rotations through reduction in N losses, and provide alternative routes for disposal of wastes when the EC Land®ll Directive is implemented.
Keywords: Wastes, crop residues, nitrogen, denitri®cation, mineralization, carbon, recycling, land®ll
R
INTRODUCTION
eturn of crop residues with a high N content to the soil,
particularly in the autumn, can result in considerable
environmental pollution, arising both from NO3± leaching to
water courses, and from the generation of nitrous oxides,
which have been implicated in the greenhouse effect
(Neeteson & Carton 2001). Sugar beet and potatoes typically
produce crop residues with between 100 and 200 kg N ha±1
(Sylvester-Bradley 1993), but crop residues generated by
vegetable brassicas can occasionally exceed 300 kg N ha±1
(Rahn et al. 1992). On this basis it is estimated that sugar
beet, potatoes and vegetable brassica residues that are
produced on 375 000 ha (DEFRA 2001a, b) of land each year
in the UK contain 45 000 t of N, which is equivalent to
chemical fertilizer worth £13 million. Further, fertilizer
residues can be left if errors are made in estimating N
recommendations, which can lead to considerable excess
Horticulture Research International, Wellesbourne, Warwick, CV35 9EF,
UK.
*Corresponding author. Fax: +44 1789 470 552. E-mail: Clive.rahn@
hri.ac.uk
mineral N in soil as well as larger amounts of N in the crop
residues (Rahn et al. 2001). In organic rotations, there is an
even greater need to retain and manage N from crop
residues in the soil crop system (Watson et al. 2002).
Evidence suggests that even when soils are cool,
decomposition of crop residues can still occur rapidly,
providing N that can potentially be leached (Rahn et al.
2002). Cover crops have been shown to reduce leaching in
cereal rotations, but even those planted early may contain
only 30 kg N ha±1 (Garwood et al. 1999), which would be
insuf®cient to control leaching from the above-mentioned
residues. Therefore, alternative methods to reduce N
leaching need to be sought.
Recent ®eld studies have suggested that short-term
rates of N mineralization and subsequent NO3± leaching
can be minimized by the incorporation of paper waste
along with crop residues (Vinten et al. 1998). These
results demonstrated that there is scope to develop novel
strategies for crop residue management based on the
addition of substrates to in¯uence directly the activities
of the decomposer organisms. Such strategies could be
used to either inhibit or stimulate short or long term
194
Management of N mineralization from crop residues using amendments
Table 1. Characteristics of the sandy loam (Whit®eld 1974) and silt loam (Soil survey 1984) soils used in the incubation experiment.
Soil texture
Particle size distributiona
Soil Series
1
Sandy loam
Silt loam2
Dunnington Heath
Wisbech
pH (H2O)
Sand
Fine sand
Silt
Clay
62.2
1.0
13.4
17
9.6
70
14.8
12
CaCO3
Organic C (%)
Water holding capacity (%)
w/w
6.1
8.2
0.1
12.0
0.8
1.6
16.0
25.2
1
a
Chromic Luvisol; 2Calcari-Glegic Fluvisol (FAO 1998).
Size fractions: sand based on 200 mm±2 mm; ®ne sand 60±200 mm; silt 2±60 mm, and clay <2 mm.
mineralization of N and to synchronize N release to the
needs of following crops, depending on the nature and
quantity of material added. Since mineralization of N
from crop residues is affected by both residue quality
(Bending et al. 1998) and soil properties, including
organic matter content (Bending et al. 2002), there is a
need to test such strategies on more than one soil and
reside type. Further, the addition of readily utilized sources
of C to soil has been shown to stimulate denitri®cation
(Weier et al. 1993). Adding amendments might therefore
reduce mineralization of N, and the potential for loss by
leaching, at the expense of increased production of nitrous
oxide. The introduction of the Land®ll Directive (EC 1999)
which aims to reduce the amount of biodegradable waste
going to land®ll, and the increasing costs of land®ll itself,
have forced many producers to seek alternative disposal
routes such as disposal to land. Some wastes, for example
green compost, have fertilizing and soil conditioning
properties, while others, such as paper and cardboard
wastes, might immobilize N and reduce the potential for
leaching.
The objective of this research was to compare the effects
of co-incorporating six amendment materials with a range of
biochemical qualities on mineralization of N in the soil from
high-N crop residues, in order to identify potential
strategies for reducing mineralization and losses of N by
denitri®cation or leaching.
MATERIALS AND METHODS
Soils
A sandy loam soil of the Dunnington Heath series (Chromic
Luvisol, FAO 1998) from HRI Wellesbourne, Warwick,
England (Whit®eld 1974) and silt loam soil of the Wisbech
series (Calcari-Glegic Fluvisol, FAO 1998) from Kirton
Lincolnshire, England (Soil survey 1984) were selected for
the experiment (Table 1).
Soil samples were taken from the top 200 mm of the
pro®le, sieved to produce a crumb size of <3 mm, and airdried. The soils were moistened to 40% of water-holding
capacity 5 days prior to setting up the incubation study.
These soils were then held at 15°C until the residues and
amendment materials were mixed in. More water was added
to bring the soils to 60% water-holding capacity, taking into
account the water contents of the amendment materials. In
total, 100 g of sandy loam and silt loam soil contained 9.6 g
and 15.1 g of water, respectively.
Plant materials
To provide consistent sources of fresh, high-N residues,
sugar beet (Beta vulgaris cv. Saxon) and Brussels sprout
(Brassica oleracea cv. Peer Gynt) plants were grown in a
glasshouse (20°C) for 16 weeks using Levington (F1) peat
compost. Fresh leaves were harvested at the base of petiole.
The lamina were cut into 10-mm squares and the petioles
into 10-mm lengths.
Amendment materials
The amendment materials were selected for their differences
in biochemical quality and their potential immobilizing
ability. Materials with a wide C : N ratio were selected to
immobilize N and those of a narrow C : N ratio to stimulate
net mineralization. Materials were also chosen because of
ready availability. Currently, 5 Mt of both paper waste and
green waste are being land®lled annually in the UK (DETR
1999; Wastewatch 2002). Two types of paper waste were
selected for potential N immobilizing capacity. These were
`compactor' waste and `mineral ®bre', which were obtained
from the cardboard and paper recycling industries, respectively. Wheat straw was collected from a recently harvested
®eld at HRI, Wellesbourne. For narrow C : N ratio
materials, composted green waste (from gardens) was
obtained from Worcestershire County Council and liquid
molasses from the sugar beet re®ning industry. Although
now sold as an animal feed, molasses was chosen for its high
carbohydrate content and potential to stimulate rapid
microbial growth and decomposition. Tannin (tannic acid,
Aldrich chemical Co., Dorset, UK) was chosen as an
inhibitor of microbial processes.
Prior to incorporation into soil, coarse particles were
removed by sieving compactor waste through a 3 mm sieve,
and mineral ®bre and green compost through a 6.5 mm
sieve. Tannic acid and molasses were dissolved in deionized
water, and the wheat straw was cut into 10-mm lengths. The
quality characteristics of each material were determined by
measuring carbohydrates, cellulose, and lignin content of the
plant and amendment materials using a proximate analysis
based on H2SO4 hydrolysis, as described by Rahn et al.
(1999). Water-soluble phenolics, total C : N ratio and
mineral N content were determined as described in
Bending et al. (1998). The analyses were carried out on
triplicate samples.
Treatments
For each soil and each amendment material, three
treatments were used:
1. soil and amendment alone
C.R. Rahn et al.
195
Table 2. Chemical characteristics of leaf and amendment materials based on dry weight.
C : N ratio
Leaf material
Brussels sprout
Sugar beet
Amendments
Compactor waste
Green compost
Molasses
Mineral ®bre
Tannic acid
Wheat straw
SED
Crop residues (df = 6)
Amendments (df = 8)
12.7
8.6
%C
3.18
4.37
41
38
15
21
45
14
39
27
54
44
0
0
146
0
0
15
0.11
0.29
0.75
520
15
18
112
[6.2]
[2.7]
[2.9]
[4.7]
82
[4.4]
0.09
0.89
2.18
0.24
0.0
0.54
[0.06]
0.0227
0.028
0.06
NH4-Na
(mg g±1)
%N
NO3-Na
(mg g±1)
27
130
[0.12]
[0.08]
[4.9]
[0.19]
[2.7]
[0.08]
0
164
1668
0
0
2
5.03
Carbohydratea
(mg g±1)
Pha
(mg g±1)
Cell
(mg g±1)
Lignin
(mg g±1)
Ash
(mg g±1)
14
12
161
126
187
153
5
3
[3.9]
2
1
14
2
1000
11
222
40
3
136
0
344
564
156
16
259
0
428
27
561
0.9
274
4
14
[0.23]
0.20
0.234
10.81
18.84
6.44
14.58
2.33
12.37
154
106
[0.04]
[5.08]
[7.40]
[0.23]
[0.98]
[0.22]
11
8
578
8
0
52
5.66
[2.4]
[1.9]
[6.4]
[2.0]
a
Water soluble extracts. Ph, phenolics, Cell, cellulose.
Figures in square brackets were transformed before statistical analysis: X = Loge (X + 1) for nitrate and ammonium; X = Loge (X) for C : N ratio and
carbohydrate.
2. soil and amendment plus Brussels sprout leaves
3. soil and amendment plus sugar beet leaves.
There were also control treatments of soil alone and soil
plus each of the leaf materials. Four replicates were used for
each treatment.
Preparation and sampling
Amendment materials were added to 100 g of previously
moistened soil in 150 ml polypropylene containers (65 mm
tall, 55 mm in diameter) to supply 3.5 mg C g±1 dry soil.
This approximates to an application of 10 t C ha±1 incorporated to 200 mm depth. The weight of N added in
amendment materials varied depending on the composition
(Table 2). Brussels sprout and sugar beet residues were
added at a rate of 1.1 mg C g±1 dry soil which delivered 87
and 132 mg N g±1 dry soil respectively. The containers were
sealed with snap-on lids, which had been pierced with two
pinholes to allow for aeration. They were then placed on
slatted trays in a randomized block design (4 replicates) in a
controlled environment room at 15°C. Experiments were
carried out separately for each soil type during the autumn
of 1998.
Suf®cient containers (1008) were prepared to allow
destructive sampling from all four replicates on days 0, 14,
28, 56, 112 and 168. Moisture contents were maintained by
addition of deionized H2O as necessary.
NH4-N and NO3-N were analysed by methods described
by Bending et al. (1998). Microbial biomass N was
determined by the fumigation-extraction method of
Joergensen & Brookes (1990) followed by ninhydrin assay
of the N present in fumigated and non-fumigated samples.
A conversion factor of 3.1 was used to convert ninhydrin-N
to microbial-N (Amato & Ladd 1988).
Measurements of N2O production were made on 50 g
portions of the soil/amendment mixture in new airtight
polypropylene containers of the type described above. These
were sealed and incubated at 15°C for 48 h, after which the
gas in the headspace was sampled using a 20 ml glass syringe
and injected into a pre-evacuated 10-ml Labco Exetainer
glass gas-testing vial. N2O was measured by gas chromatography using an Ai (Analytical Instruments, Cambridge,
UK) gas chromatograph ®tted with a Haynesept Q80/100
column, which was maintained at 60°C. N2 was used as
carrier gas, and N2O detected using electron capture
detection at 300°C.
Data analysis
The amendment materials contained different amounts of
N. By comparing values of mineralized N and soil microbial
biomass-N derived from the amendment materials with and
without sugar beet and Brussels sprout leaves, it was
possible to distinguish between the net-N derived from the
amendment materials and that derived from the leaves.
Statistical analysis was carried out using the analysis of
variance procedures within GENSTAT (Anon 1987). Skewed
data were transformed logarithmically before statistical
analysis. Regression analysis between biomass-N and
mineral-N was carried out using MINITAB (Anon 1993)
with stepwise regressions to determine the modifying effects
of leaf residue, soil type and amendment composition.
RESULTS AND DISCUSSION
Biochemical characteristics of the plant materials
Sugar beet leaves contained smaller amounts of carbohydrates, phenolics, cellulose, lignin and ash than Brussels
sprout leaves (Table 2). Sugar beet leaves also contained less
carbon but more nitrogen than Brussels sprout leaves,
resulting in a lower C : N ratio. Amounts of N in mineral
form were very small in both residues. Characteristics of
these materials were of a similar nature to those determined
in ®eld crops (Rahn 1999), and it was therefore expected that
sugar beet residues would decompose and release N faster
than sprouts (Rahn & Lillywhite 2002).
Characteristics of the waste materials
The amendment materials used covered a wide spectrum of
biochemical quality characteristics (Table 2). Mineral ®bre,
196
Management of N mineralization from crop residues using amendments
Figure 1. Mineral-N derived from leaf residues in laboratory incubation experiments. Sugar beet on (a) sandy loam, (b) silt loam and Brussels sprout on (c)
sandy loam, (d) silt loam. Symbols are: m, green compost; n, wheat straw; ,, tannic acid; j, compactor waste; h, mineral ®bre; d, soil; s, molasses.
Error bars: SED, df = 16.
compactor waste, and wheat straw consist mainly of lignocellulose, but owing to N contents of 0.24, 0.09 and 0.54%,
respectively, they have widely differing C : N ratios. Green
compost has a high lignin content and a large pool of NO3±N. Molasses is composed mostly of carbohydrates, and has a
very high NO3±-N content. Tannic acid consists of phenolics
only.
Effects of amendment on net mineralization of N from leaves
In both soils, most of the mineral N derived from sugar beet
and Brussels sprout leaves was in the NO3± form. The levels
of NH4+-N reached a maximum of only 10 mg g±1 at 14 days
after the start of incubation, and subsequently fell to very
low levels. Consequently, data for total mineral N (NO3±
plus NH4+) mineralized from the leaves is presented in
Figure 1.
Where sugar beet leaves had been mixed into soils (Figure
1a, b), molasses stimulated net N mineralization at 28 and 56
days, after which net amounts of N mineralized declined.
The other incorporated amendments resulted in a decrease
in net N mineralization compared with the soil-only control.
The largest decreases were for compactor waste and wheat
straw in the sandy loam soil; amounts of N mineralized were
50±90 % below that in the control treatment for most of the
168 days of the experiment. Compactor waste was
considerably more effective than wheat straw at decreasing
net N mineralization during the ®rst 28 days, but there was
little difference between these materials subsequently. The
differences were less marked in the silt loam soil with
reductions of 30±60% in mineral N relative to the control.
For mineral ®bre and tannic acid, the amounts of net N
mineralized were 10±60% and 10±30% less than the control
treatments for the sandy and silt loam soils, respectively,
over the 168 days of the experiment. Green compost was
effective at decreasing net N mineralization for the ®rst 28
days of the experiment on the sandy loam but not on the silt
loam, although there were no consistent differences thereafter. Peak levels of mineral N suggested recoveries of
between 22 and 66% of N from the leaf residues in both
soils.
Production of mineral N derived from Brussels sprout
leaves showed a steep initial rise over the ®rst 28 days
(Figure 1c, d). Molasses stimulated net mineralization
relative to the control, particularly in the sandy loam soil.
C.R. Rahn et al.
Net N mineralization was reduced by 60±80% relative to the
control using wheat straw, and by 30±70% using compactor
waste. However, the effect of wheat straw was delayed
compared with compactor waste. Tannic acid reduced net N
mineralization by up to 40% in the sandy loam soil but there
was a much smaller effect in silt loam. It is estimated that N
from original leaf residues measured as mineral N was
between 21 and 97% in sandy loam and 25 and 51% in silt
loam, depending on treatment.
Other workers have shown that high C : N ratio materials
can immobilize N when mixed into the soil, including two
incubation studies that used of®ce waste with a C : N ratio of
1235 : 1 mixed into a clay loam soil (Motavalli et al. 2000). In
studies carried out by Zibilske (1987) using paper-mill
sludge with an organic C : N ratio of 478 : 1 mixed into a
coarse loam soil, N from soil was immobilized, with net
mineralization inversely proportional to the rate of sludge
applied, and occurring more quickly at 25°C than 12°C.
Further, remineralization was also observed to take place
more rapidly with lower rates (2.9 t C ha±1) of of®ce waste
(Motavalli et al. 2000).
Materials of narrower C : N ratio such as straw were less
effective at immobilizing N from the crop residues. Where
wheat straw had been incorporated to reduce N leaching
from soil, only transitory effects were observed (Catt et al.
1992). However the rates of application were equivalent to
around 2.5 t C ha±1, that is, only one quarter of the rate used
in our experiments. Similarly, Motavalli et al. (2000) found
that a rate of 2.9 t C ha±1 paper waste had limited effects on
N mineralization. Aitken et al. (1998) showed some small
yield bene®ts in the third year after application of 10 t C ha±1
C as paper-mill sludge, which may have been a result of
remineralization of immobilized N. No yield bene®ts were
seen with higher rates of 20 or 30 t C ha±1, suggesting no
remineralization of N. Further research would be needed to
determine how the quantity of C applied in¯uences
immobilization of N.
Hartz et al. (1996) showed that green composts
immobilized N in spite of narrow C : N ratios. However,
in our experiments, green compost had little effect on the
net mineralization of N from crop residues. Wheat straw was
more effective at immobilizing N than the mineral ®bre in
spite of a narrower C : N ratio. These observations support
the views of Vinten et al. (1998) that C : N ratios of
amendment materials alone are not suf®cient to predict
immobilizing capacity accurately. In our experiments and
those reported above, C was evidently still in an available
form to support the growth of micro-organisms in materials
with very wide C : N ratios, such as compactor waste. Aitken
et al. (1998) also observed that the carbon substrates in
paper-mill sludge waste are not uniformly available, with
cellulose and hemicellulose C representing the most labile
pools. Similarly, Bending et al. (1998) found that cellulose
was the key fraction controlling net N mineralization from a
range of crop and root residue materials. In our experiments,
the amounts of N mineralized on day 168 were related best
to cellulose and lignin contents, which accounted for 62%
and 55%, respectively, of the variance in N mineralized
where 3.5 mg C g±1 of dry soil was applied (10 t±1 C to 20 cm
depth; P <0.001).
197
Y ˆ 72:5 ÿ 0:084A ÿ 14R
r2 ˆ 0:62; df ˆ 21
1†
Y ˆ 71:4 ÿ 0:048B ÿ 14R
r2 ˆ 0:54; df ˆ 21
2†
where Y = N mineralized from leaf residue (mg g±1 dry
soil); R = 1 (sugar beet), or 2 (Brussels sprout); A = cellulose
content (mg g±1); and B = lignin content (mg g±1).
In a stepwise regression, once cellulose content had been
taken into account, no additional variance was accounted for
by including soil type or lignin content. More commonly
used measures of residue quality, including percentage N
and C : N ratio, less successfully explained variation in net N
mineralized with, respectively, 43 and 35% of variance in N
mineralized by day 168 accounted for. It may be possible to
use equations (1) and (2) for generic estimation of the
immobilizing ability of amendment materials in the ®eld.
In our experiments, there was little difference in the net
mineralization of N by day 168 in sugar beet leaves between
the two soils. Similarly, Whitmore & Groot (1994, 1997)
found no difference in direct mineralization of N from sugar
beet leaves in sandy and silty clay loams. However, in their
later work when N was immobilized from Brussel sprouts,
remineralization took place more quickly in the sandy soil.
In our experiments, there was a tendency for mineralization
from Brussels sprout leaves to be delayed relative to that
from sugar beet, perhaps due to the wider C : N ratio of the
Brussels sprout leaves. Similar delays have been observed in
the ®eld (Rahn et al. 2002). Whitmore & Groot (1997) also
observed longer delays in mineralization with wider C : N
content in sugar beet crowns compared with leaves. Tannins
are able to bind to organic N compounds, including amino
acids and proteins, resulting in the inhibition of degradative
enzymes, and reduction in the accessibility of bound organic
N to microbial degradation (Bending & Read 1996). These
processes probably contributed to the reduced mineralization of N from residues in the tannin treatment seen in our
experiments.
Effects of amendment materials on microbial-N derived from
leaves
The amounts of net biomass-N derived from the leaves are
shown in Figure 2. The proportion of N initially present in
the leaves of Brussels sprout that was recovered in biomassN was double that recovered from sugar beet leaves.
Following mixing of sugar beet leaves into the sandy or
silt loam soils, the largest changes in biomass-N occurred
during the ®rst 28 days of incubation (Figure 2a, b).
Similarly Whitmore & Groot (1997) showed that peak
amounts of biomass-N were found within a week following
single additions of leaf or crown material to soil. At day 14
there were signi®cant differences between treatments, with
compactor waste having up to 20 times more biomass-N
than the unamended or molasses amended soils. After 28
days, compactor waste and wheat straw treatments had
greater amounts of biomass-N in the sandy loam than the
silt loam. Biomass-N decreased in all treatments between 28
and 56 days, after which it remained relatively constant.
Peak recovery of biomass-N was reached within the ®rst 28
days and was estimated to range from 5 to 23% and 10 to
198
Management of N mineralization from crop residues using amendments
Figure 2. Net biomass-N derived from leaf residues in laboratory incubation experiments. Sugar beet on (a) sandy loam, (b) silt loam and Brussels sprout on
(c) sandy loam, (d) silt loam. Symbols are: m, green compost; n, wheat straw; ,, tannic acid; j, compactor waste; h, mineral ®bre; d, soil; s, molasses.
Error bars: SED, df = 16.
20% of the N in the original leaf residues on the sandy loam
and silt loam, respectively.
The greatest stimulation of biomass-N by Brussels sprout
leaves occurred within the ®rst 28 days. There were
signi®cant differences between the treatments at 14 days
with compactor waste and mineral ®bre treatments containing larger amounts of biomass-N than the two unamended
soil controls (Figure 2c, d). After day 28, the amounts of
biomass-N fell and there were subsequently only small
differences between treatments. Peak recovery of biomass-N
represented from 22 to 70% and 22 to 43% of N in the
original leaf residues in the sandy loam and silt loam,
respectively.
Where compactor waste had been applied, very large
amounts of biomass-N were measured on day 14, which
declined as net formation of mineral N increased. This time
scale was unexpectedly similar to previous studies in which
easily degradable sugars had been added to soil (Wu et al.
1993, Keeling et al. 1996). Additionally, Fauci & Dick
(1994) found that organic inputs with easily available N had
a large effect on soil biological response, which was
controlled by residue cellulose or lignin content.
Regression analysis showed that as net mineral N levels
derived from the leaves increased, amounts of microbial
biomass-N decreased, with 38% of the variance accounted
for (P<0.001, df = 138). Using stepwise regression (Table 3),
and including the cellulose or lignin (data not shown) in the
amendment material, the variance accounted for increased to
44% in both cases. This relationship was further improved
by taking into account residue type and date of mineral N
measurement, but not soil type. There was no advantage
from additionally including values of C : N ratio.
The recovery of N derived from the original leaf residue
in the biomass-N and mineral-N pools was only 37% and
46% for sugar beet and Brussels sprout, respectively.
Although additional N may have been found in the
residue-derived light fraction of the organic matter pool,
this source is likely to be small after 168 days, probably
amounting to less than 1% of the N in the added leaves
(Bending et al. 1998). Under-recovery of residue N could
not be explained by N2O production, but N2 produced by
denitri®cation may have contributed to the poor recovery.
C.R. Rahn et al.
199
Table 3. Stepwise regression of mineral-N derived from leaves (by difference) and biomass-N content (n = 120).
Constant
Biomass-N
Cellulose content of amendment
Crop ± Brussels sprout or sugarbeet
Date of Sampling
Soil type ± sandy loam or silt loam
% variance accounted for
Step 1
Step 2
Step 3
Step 4
Step 5
43.9
±0.99 (±8.52)
46.8
±0.88 (±7.58)
±0.0350 (±3.51)
59.0
±0.77 (±6.74)
±0.038 (±3.97)
±8.8 (±3.72)
51.6
±0.47 (±3.70)
±0.047 (±4.95)
±10.4 (±4.63)
0.09 (4.17)
53.4
±0.46 (±3.49)
±0.045 (±5.0)
±10.5 (±4.64)
0.09 (4.19)
±1.3 (±0.59)
38.1
44.0
50
57
57
T ratio of coef®cient shown in parenthesis. If: T >62, P=0.05; T >62.58, P=0.01; T >63.29, P=0.001.
Kelly & Stevensen (1995) found that up to one-third of
fertilizer N added to soil can be immobilized into humic
substances after passage through micro¯ora. It is therefore
possible that residue N could have been immobilized into
heavy fraction organic matter associated with clay particles.
Equally, it is also possible that some of the N may be present
in senescent (dead) microbial residues (Bending & Turner
1999). The implications of the increases in soil biomass that
might occur with repeated applications of amendment
materials needs further investigation, since repeated incorporation of cover crops over several years has the potential to
increase overwinter leaching (Garwood et al. 1999).
Gaseous losses of N
Emissions of N2O were largest during the ®rst 28 days of
incubation for both soils, reaching an equivalent of 295 g
N ha±1 day±1 as N2O in the molasses treatment (Figure 3a,
b). On day 14, where sugar beet had been added to the sandy
soil (Figure 3a) compactor waste signi®cantly (P=0.004)
reduced emissions of N2O-N from an estimated 68 to 5 g
ha±1compared with soil containing sugar beet residues only.
Emissions from soil treated with just Brussels sprout
residues were at a similar level. Beyond day 14, there were
no signi®cant differences between treatments, but there was
a tendency for soils amended with wheat straw or green
compost to show higher emissions of N2O. In the silt loam,
N2O emissions were generally lower than in the sandy loam
(Figure 3b). The largest emissions were measured on day 14,
where there were signi®cant differences between treatments,
with molasses and tannin amended treatments showing the
highest emissions. Compactor waste did not reduce emissions compared with soil with sugar beet residues only. By
day 28, differences were still signi®cant, though the order
had changed considerably and the molasses treatment
showed relatively low emissions. In the silt loam beyond
day 28, emissions fell and treatment effects were small.
The amounts of N2O emitted from sugar beet residues on
day 14 from silt and sandy loam were equivalent to 27 and
66 g N2O-N ha±1 day±1, respectively. These compare with
emissions of up to 67 g N2O-N ha±1 day±1 from ®eld soils
after lettuce residues had been incorporated (Baggs et al.
2000). Vinten et al. (1998) measured mean losses of 25 and
13 g N2O-N ha±1 day±1 following incorporation of lettuce
residues or cultivation, respectively, over a 60-day period. In
Vinten's experiments, the application of paper-mill waste
did not signi®cantly reduce emissions. In fact, where
44 t DM ha±1 paper-mill waste had been applied, emissions
Figure 3. Emission of N2O from sugar beet leaf residues in presence of
amendment materials on (a) sandy loam and (b) silt loam. Symbols are: m,
green compost; n, wheat straw; ,, tannic acid; j, compactor waste; h,
mineral ®bre; s, molasses; r, soil; e, soil with Brussels sprout residues; d,
soil, no residues. Error bars: SED, df = 16.
rose immediately after incorporation to 61 g N2ON ha±1 day±1. Mineral ®bres derived from paper industry
had little effect on emissions compared with leaf and soil
treatments in our experiments. Beyond day 14 in sandy
loam, the bene®ts of compacter waste were not signi®cant.
Although green compost increased emissions on day 56, the
amendment materials had little other effect. In the silt loam,
emissions declined sharply beyond day 14, where molasses
or tannic acid had been applied. By day 56 there were no
signi®cant differences between treatments.
200
Management of N mineralization from crop residues using amendments
Practical use of amendment materials
There are several bene®ts that could result from the use of
amendment materials in agriculture. As we have demonstated in this article, those rich in cellulose, such as
compactor waste, could be used to modify the dynamics of
N mineralization, thereby providing a tool with which to
manage leaching losses of N. Amendment materials may also
have other agronomic bene®ts, particularly the improvement
of soil organic matter. Additionally, the use of soil as a
disposal route for biodegradable waste materials has
economic advantages, which will grow as the cost of disposal
to land®ll sites increases.
ACKNOWLEDGEMENTS
Thanks are due to Julie Jones for assistance with the
statistical analysis, Colin Webster (Rothamsted) for assistance with N2O analysis and DEFRA for providing funding.
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Received January 2002, accepted after revision February 2003.
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British Society of Soil Science 2003