Chemical characterisation of vegetable andarable crop residue materials: a comparison ofmethodsCR Rahn,* GD Bending, RD Lillywhite and MK TurnerDepartment of Soil and Environment Sciences, Horticulture Research International, Wellesbourne, Warwick, CV35 9EF, UK

Abstract: Although it is widely recognised that chemical composition controls the patterns of

decomposition and N mineralisation of crop residue materials, there has been little agreement as to the

nature of the most important chemical fractions. We investigated whether this could be attributed to

differences in methodologies employed for chemical characterisation of the lignin and cellulose

fractions of plant materials. The cellulose and lignin contents of cauli¯ower, potato, red beet, Brussels

sprouts and wheat crop residues were analysed by a number of contrasting methods. These were forage

®bre and forest products analyses, which utilise KMnO4 and H2SO4 respectively to separate the two

fractions, and a third method, which employs NaClO2. For all the materials, the forage ®bre method

gave substantially lower amounts of both lignin and cellulose than the other methods. There was

correlation between lignin determined by the different methods. Low recovery of lignin by the forage

®bre method was found to arise partly from incomplete deligni®cation by KMnO4. The cellulose

contents given by the different methods were highly correlated. However, it was apparent that the

forage ®bre method underestimated cellulose, since only alpha-cellulose was measured.

The characteristics of crop residues grown on two sites with different N fertiliser treatments were

determined by forest products analysis. The materials were shown to span a range with respect to both

lignin (12±26%) and cellulose (17±71%) contents. The chemical characteristics of the materials were not

signi®cantly affected by the amount of nitrogen used to produce the crop.

# 1999 Society of Chemical Industry

Keywords: vegetables; nitrogen; residue characterisation; lignin; cellulose; methodology

INTRODUCTIONUnderstanding the patterns of decomposition of crop

residue materials and the subsequent availability of

nutrients to plants is essential for predicting the

fertiliser requirements of crops in intensive cropping

rotations. Vegetable crops in particular leave large

quantities of residues at harvest, and these can contain

substantial amounts of N, often in excess of 150kg

Nhaÿ1.1,2 Although this represents a valuable source

of the element, its release is not always synchronous

with the needs of following crops.3 The ability to

harness this N depends on being able to predict N

mineralisation, which requires understanding of the

mechanisms involved in residue decomposition and

mineralisation. Such understanding could be used to

improve existing computer models of N cycling such

as Greenwood et al,4 and for prediction of fertiliser

requirements.5

The importance of biochemical composition or

`quality' in determining patterns of decomposition

and N mineralisation from plant materials is well

recognised.6 Although many studies have attempted to

identify quality factors that control these processes,

there has been little agreement between them.

Signi®cant correlations have been found to exist

between N mineralisation from plant materials and a

number of quality characteristics, including C/N

ratio,7 N content,8±10 lignin,11,12 cellulose10 and poly-

phenols.13 Other studies have shown combinations of

these characteristics, including lignin/N14,15 and lignin

plus polyphenol/N,16 to be effective predictors of N

release.

While differences in results obtained between the

studies may re¯ect the diversity of materials included

and the time over which mineralisation was measured,

there is considerable variation in the methods by which

chemical fractions representing residue quality were

analysed in the different studies, and this could also

contribute to the contrasting results.

Palm and Rowland17 reviewed the minimum dataset

required for characterisation of plant quality charac-

teristics in decomposition and mineralisation studies.

The dataset included lignin, soluble carbon, soluble

phenolic and total N content for both long- and short-

Journal of the Science of Food and Agriculture J Sci Food Agric 79:1715±1721 (1999)

* Correspondence to: CR Rahn, Department of Soil and Environment Sciences, Horticulture Research International, Wellesbourne, WarwickCV35 9EF, UKContract/grant sponsor: Ministry of Agriculture, Fisheries and Food, UK(Received 4 June 1998; revised version received 19 August 1998; accepted 13 May 1999)

# 1999 Society of Chemical Industry. J Sci Food Agric 0022±5142/99/$17.50 1715

term studies, and it was recognised that cellulose

content should also be included in longer-term

studies. While there have been a variety of methods

used to determine each component of the minimum

dataset, the lignin and cellulose content have been

subject to the most divergent array of methods.

Most studies of the lignin and cellulose content of

plant materials have been conducted either by means

of forest products (FP) analysis, in which H2SO4 is

used to separate acid-soluble cellulose from acid-

insoluble lignin,18±20, or forage ®bre (FF) analysis, in

which KMnO4 in used to oxidise lignin to leave

cellulose.21,22

The aim of this study was to determine the chemical

characteristics of low sclerophylous arable and horti-

cultural crop residue materials, and to compare the

applicability of FF and FP methodologies and an

unrelated NaClO2 method to such residues.

MATERIALS AND METHODSMethods of lignin and cellulose analysisTwo variations of the FP method were tested, in which

H2SO4 is used to remove cellulose, leaving acid-

insoluble lignin. In the ®rst variation, plant materials

were subjected to a water extraction before treatment

with acid, and in the other variation there was no pre-

treatment. The FF method was also tested, in which

an acid-detergent solution was used to remove all

components except lignin and cellulose, and potas-

sium permanganate used to delignify the resulting

acid-detergent ®bre. An additional method was also

investigated which has previously been used in studies

assessing straw degradability. This utilises acetic acid

and sodium chlorite as lignin-oxidising agents. A brief

description of each method is given below.

Forest products (FP) method23

Without pre-treatment. One gram of dried and ground

plant material was treated with 20ml 13.8M H2SO4 in

an iced water bath for 2h, following which the acid was

diluted to 0.6M strength using reverse osmosis (RO)

H2O, and the mixture boiled for 2h on a hot plate.

Following cooling, the solution was ®ltered through a

pre-weighed Whatman GF/A ®lter. Material remain-

ing on the ®lter was dried at 100°C for 24h and the

®lter re-weighed to determine acid-insoluble material.

The ®lter was then placed in a furnace at 400°C for

16h to determine ash content. Lignin was determined

by subtracting the weight of ash from the weight of

acid-insoluble material. Cellulose was determined by

measuring sugars in the ®ltrate, using the phenol-

H2SO4 assay of Dubois et al.24

Water pre-treatment. Plant materials (1g) were incu-

bated with 100ml boiling RO H2O for 2h to remove

water-soluble components, following which the resi-

due was collected by ®ltration through a Whatman No

1 ®lter, and dried in a 100°C oven for 24h. The

remaining residue was subjected to treatment with

H2SO4 as described above.

Forage Fibre (FF) method25

Plant materials (0.2g) were re¯uxed with 20ml of a

solution of 60mM acetyltrimethyl ammonium bro-

mide in 0.5M H2SO4 for 1h. The residue remaining

was washed with acetone before being dried at 100°Cand weighed. The acid-detergent ®bre remaining was

deligni®ed by incubation with a 0.3M KMnO4 solu-

tion for 1.5h. The residue was washed with a solution

containing 0.4M oxalic acid, 0.6M HCl and 12M

ethanol, followed by washing with ethanol and

acetone. After drying at 100°C for 24h, the residue

was re-weighed. Lignin was calculated by determining

weight loss following KMnO4 extraction. The residue

remaining was placed in a furnace at 400°C for 16h to

determine ash content. Cellulose content was calcu-

lated by subtracting the weight of ash from the weight

of KMnO4-stable material.

NaClO2 method26

Plant materials (0.2g) were extracted in 20ml boiling

H2O for 1h. The residue remaining was ®ltered

through a Whatman No 1 ®lter and dried at 100°Cfor 24h. An additional step to the original procedure

was added to remove pectin.27 The material was

weighed, following which it was suspended in 150ml

of 0.5% ammonium oxalate for 1h at 70±80°C.

Following centrifugation, further washing and re-

extraction with 0.5% ammonium oxalate the residue

was dried and deligni®ed by incubating in a solution of

0.1M acetic acid and 0.2M NaClO2 at 75°C for 3h.

Following washes with distilled H2O, acetone and

ether, the residue was dried at 100°C for 24h, and re-

weighed. Lignin was calculated by determining weight

loss on treatment with acetic acid/sodium chlorite.

The residue was incubated with 4.3M KOH for 2h,

followed by washing with distilled H2O, acetone and

ether. The remaining residue was dried at 100°C for

24h, and re-weighed. Hemi-cellulose content was

determined by calculating weight lost by KOH

treatment. The residue remaining was placed in a

furnace at 400°C for 16h to determine ash content. a-

Cellulose content was calculated by subtracting the

weight of ash from the weight of KOH-stable material.

Source of crop residue materials, samplepreparation and analysisPotato (Solanum tuberosum L, red beet (Beta vulgarisL), wheat (Triticum aestivum L), and Brussels sprouts

(Brassica oleracea var gemmifera L) crops were grown

on a sandy loam soil at Stockbridge House, North

Yorkshire, UK. Cauli¯ower (Brassica oleracea var

botrytis L) at two planting dates, potato and wheat

crops were grown on a silty loam soil at Kirton,

Lincolnshire UK. The crops were grown with high

and low nitrogen fertiliser rates. There were three

replicated plots for each treatment. Details of crops

grown are shown in Table 1. Conventional applica-

1716 J Sci Food Agric 79:1715±1721 (1999)

CR Rahn et al

tions of pesticides and herbicides were made. Im-

mediately following crop harvest in 1994, residue

material from each experimental block and N treat-

ment was gathered and dried at 60°C before being

ground in a rotary mill to pass through an 500 mm

sieve.

Samples from both Stockbridge House and Kirton

(Table 1) were analysed by the FP method. Samples

marked with an asterisk (Table 1) were additionally

analysed by the FF and NaClO2 methods. Some

samples from Stockbridge House included a compari-

son of pre-treatments prior to acid hydrolysis using the

FP method.

Statistical design and analysisCorrelation coef®cients and analysis of variance were

performed using Genstat 5.2. The analysis took

account of the arrangement of the growing crops in

the ®eld at both sites. At Stockbridge House for the

comparison of analytical methods shown in Tables 3

and 5, the crop data were analysed as a 3-row 4-

column incomplete Latin square with methods treated

as subplots. Where the effects of nitrogen treatments

(Table 4) were analysed they were included as

subplots. At Kirton the data in Table 4 were analysed

as a split plot design with crop as main plot and

nitrogen as subplot. There were three replicates at

each site for all treatments.

RESULTS AND DISCUSSIONChemical characterisation of non-sclerophyllouscrop residuesThe results of the chemical characterisation by the FP

method are shown in Table 2. Almost half the dry

matter in brassica crops was soluble in water and

consisted mostly of sugars, while one-third or one-

quarter of dry matter, in potatoes and red beet

respectively was water-soluble. In contrast wheat straw

contained very little water-soluble material. The

proportion of the residue soluble in acid exceeded

one-third of the dry matter with little difference

between crops except for straw which contained

around two-thirds. Wheat straw had the largest

content of cellulose with double or more the amount

present in other materials. Acid-insoluble lignin levels

were highest in wheat straw and lowest in the brassica

residues. Brassicas and cereals had ash contents of

around 10%, the ash content of potatoes was higher

and that of red beet crops was extremely high.

Chemical characteristics of the materials were not

signi®cantly affected by the amount of nitrogen

applied. In a further statistical analysis comparing

characteristics of potatoes and wheat between sites,

the composition of wheat was not signi®cantly

different. In potatoes the acid-soluble fraction, cellu-

lose and ash contents were signi®cantly different

(p =0.05). De Neve et al12 reported chemical compo-

sition of several non-sclerophyllous materials by a

modi®ed Stevenson fractionation28 which identi®ed

six different fractions of crop residue materials. All

values were in similar ranges to those determined by

the FP method but the water-soluble and lignin

fractions were smaller. The difference in water-soluble

fraction may be attributed to the prior removal of fats,

waxes, oils and resins in the Stevenson method. The

lignin and cellulose contents may have been different

as De Neve et al12 separated residue into leaf blade and

stem which can have widely different composition.

Comparison of methods of chemicalcharacterisation of residue materialsA comparison of the results of the lignin determina-

tions (Stockbridge House) for each method of analysis

Table 1. Details of crops grown, N fertiliser applied, planting and harvesting dates

Site Crop Planting N rate (kg haÿ1) Date & N application Harvest date

Stockbridge Brussels sprouts 18 May 94 250 14 May 100, 14 Jun 150 22 Aug 94

125* 14 May 100, 14 Jun 25

Stockbridge Wheat 17 Mar 94 100 27 Apr 100 14 Aug 94

50* 27 Apr 50

Stockbridge Potato 30 Mar 94 275 29 Mar 150, 27 May 125 22 Aug 94

135* 29 Mar 110, 27 May 25

Stockbridge Red beet 09 May 94 175 06 May 100, 14 Jun 75 01 Sep 94

88* 06 May 88

Kirton Potatoes 30 Mar 94 270 30 Mar 220, 23 Jun 50 24 Aug 94

175* 30 Mar 175

Kirton Wheat 08 Mar 94 175 19 Apr 125, 23 Jun 50 16 Aug 94

125* 19 Apr 125

Kirton Early summer 21 Mar 94 300 30 Mar 125, 29 Apr 125 & 12 May 50 24 Jun 94, 01 Jul 94

cauli¯ower 250* 30 Mar 125, 29 Apr 125

Kirton Summer cauli¯ower 25 May 94 276* 19 Apr 138, 05 Jul 138 19 Sep 94

136 19 Apr 69, 05 Jul 67

* Samples selected for lignin and cellulose analysis by FF, FP, and NaClO2 methods.

J Sci Food Agric 79:1715±1721 (1999) 1717

Characterisation of crop residue materials

is shown in Table 3. The NaClO2 and FP methods

gave similar results, which were higher than those of

the FF method. Excluding the water pre-extraction in

the FP method increased the acid-insoluble lignin

fraction. This could be a result of acid precipitation of

water-soluble phenolic materials, resulting in their

inclusion in the lignin fraction. Figure 1 and Table 4

show that, though results differ in magnitude between

methods, there was a correlation between them, with rvalues of 0.78, 0.77 and 0.81 for FP vs FF, FP vs

NaClO2 and NaClO2 and FF respectively (df=23)

(Table 4).

The cellulose contents (Stockbridge House) from

the FF analysis were markedly lower than those from

either the NaClO2 or FP methods. (Table 5) The

effects of omitting the water pre-treatment in the FP

method led to a much larger recovery of cellulose after

acid hydrolysis in Brussels sprouts. The cellulose

content of other materials with a lower water-soluble

content was not signi®cantly affected by omitting the

water pre-treatment. Water pre-treatments are there-

fore essential for accurate determinations of cellulose

where large amounts of residue are water-soluble.

Figure 2 shows that, although results from the

different methods of characterisation differ in magni-

tude, there was a correlation between them, with rvalues of 0.87, 0.90 and 0.95 for FP vs FF, FP vs

NaClO2 and NaClO2 vs FF respectively (df=23)

(Table 4). These correlations do allow the possibility

of extrapolating cellulose data obtained by different

methods of analysis for vegetable and arable crops in

decomposition models as suggested by Ryan et al.29

However, it has been reported by Rowland and

Roberts30 that up to 65% of the hemi-cellulose was

Table 2. Chemical characterisation of crop residue materials with water pre-treatment by forest products method

Crop & site N rate (kg haÿ1)

Water-soluble

fraction (%)

Water-soluble

sugar (%)

Acid-soluble

fraction (%) Cellulose (%) Lignin (%) Ash %

Stockbridge House

Brussels Sprouts 125 41.9 25.9 43.4 34.5 14.1 7.2

250 42.2 26.2 41.8 36.9 15.4 7.4

Wheat 50 11.1 3.4 63.6 68.1 25.9 3.7

100 12.4 2.0 62.4 71.4 25.7 3.8

Potato 135 31.7 8.5 43.3 36.2 18.2 20.3

275 34.3 8.8 43.8 38.5 18.7 19.9

Red beet 88 25.5 9.6 27.7 26.0 14.5 56.2

175 33.3 11.5 21.6 16.6 12.3 43.1

SED (16df) 4.0 3.3 5.0 4.7 1.7 6.3

Kirton

Potato 175 36.7 10.8 39.6 27.9 17.7 33.3

270 32.2 4.3 42.0 27.4 22.0 24.8

Wheat 125 10.7 4.4 62.9 71.1 25.5 4.9

175 11.9 2.8 61.9 68.2 24.3 6.1

Early summer cauli¯ower 250 48.7 21.3 36.1 28.0 15.1 13.5

300 46.0 20.7 37.7 25.4 16.1 13.1

Summer cauli¯ower 138 46.4 20.9 36.4 22.7 16.9 11.6

276 49.4 18.4 35.4 26.8 15.0 13.5

SED (16df) 3.1 1.9 2.0 2.1 2.1 1.2

Table 3. Comparison of analysis methods,percentage lignin determined1

Forest products

Crop NaClO2 Forage ®bre Water No water Mean

Brussels sprouts 14.2 6.6 14.2 18.9 13.5

Wheat 20.6 11.9 25.9 28.6 21.7

Potatoes 16.7 8.8 18.8 22.1 16.6

Red beet 12.0 3.4 14.3 15.3 11.2

Mean 15.9 7.7 18.3 21.2

1 Samples taken from one N rate at Stockbridge House, see Table 1 for details.

ErrorsBetween crops df=3, SED=0.90

Between methods df=24, SED=0.58

Between methods same crops df=24, SED=1.15.

1718 J Sci Food Agric 79:1715±1721 (1999)

CR Rahn et al

dissolved and removed by the acid-detergent extrac-

tant in the FF method suggesting that the cellulose

determined by this method was likely to be largely

alpha-cellulose. Figure 3 con®rms that the cellulose

content measured by the FF method is closely

correlated to the alpha-cellulose determined by the

NaClO2 method (r =0.94, n =24). However, the

values derived from the FP analysis were higher due

to the extraction of a variable proportion of the hemi-

cellulose. In addition, as there is no relationship

between hemi-cellulose and alpha-cellulose content

of plant materials,31 use of the FF method for chemical

characterisation of plant materials may lead to under-

estimation of total cellulose content. The discrepan-

cies between studies of the importance of the cellulose

fraction in controlling decomposition and mineralisa-

tion for plant materials therefore could be due to the

different methods of analysis used.

The FF analysis method has integral problems

which limit its consistency, and therefore its applic-

ability as a standard method. These dif®culties arise

largely from the delignifying KMnO4 extraction step.

Figure 1. Relationships between crop residue lignin content determined bythe forest product, NaClO2 and forage fibre methods. Forest product ligninvs NaClO2 lignin (*) and forage fibre lignin (&). Filled symbols representKirton samples, and open symbols Stockbridge House.

Table 4. Regression equations (Y=mX�c) forcomparison of analytical methods1

Material X Y m c r

Lignin FP NaClO2 0.7 (�0.29) 5.2 (�5.39) 0.77

Lignin FP FF 0.6 (�0.26) ÿ2.7 (�4.76) 0.78

Lignin NaClO2 FF 0.9 (�0.38) ÿ7.3 (�6.57) 0.81

Cellulose FP NaClO2 0.9 (�0.36) 0.2 (�15.32) 0.90

Cellulose FP FF 0.5 (�0.20) 5.5 (�8.58) 0.87

Cellulose NaClO2 FF 0.6 (�0.21) 5.4 (�8.20) 0.95

1 Samples taken from Kirton and Stockbridge House as indicated in Table 1.

S E are in parenthesis. Calculated by functional regression analysis taking into account variance in X

and Y, Regression lines for FP vs FF and FP vs NaClO2 are shown in Figs 1 and 2.

Table 5. Comparison of analysis methods,percentage cellulose determined1

Forest products

Crop NaClO2 Forage ®bre Water No water Mean

Brussels sprouts 29.8 21.4 35.3 58.3 36.2

Wheat 66.6 39.2 67.4 66.0 59.8

Potatoes 38.7 23.0 38.2 29.7 32.4

Red beet 17.9 12.6 26.4 22.7 19.9

Mean 38.2 24.0 41.8 44.2

1 Samples taken from one N rate at Stockbridge House, see Table 1 for details.

ErrorsBetween crops df=3, SED=2.29

Between methods df=24, SED=2.24

Between methods same crops df=24, SED=4.47.

Figure 2. Relationships between crop residue cellulose contentdetermined by the forest product, NaClO2 and forage fibre methods. Forestproduct cellulose vs NaClO2 cellulose (*) and forage fibre cellulose (&).Filled symbols represent Kirton samples, and open symbols StockbridgeHouse samples.

J Sci Food Agric 79:1715±1721 (1999) 1719

Characterisation of crop residue materials

Low sclerophyllous samples are very rapidly deligni-

®ed, which can result in loss of cellulose during this

step.21 Additionally, KMnO4 penetrates larger parti-

cles poorly, resulting in slow deligni®cation. Rowland

and Roberts30 reported that some treatments actually

increased in weight following this step, and that when

further oxidation was applied to delignify these

materials, quantities of cellulose were removed. These

problems limit the reproducibility of the technique

between laboratories, and formal Tropical Soil Biology

and Fertility (TSBF) collaborators were encouraged to

use common external laboratories for lignin analysis.25

In our study, permanganate failed to penetrate

larger fragments, even though our samples were

<0.5mm. We used a range of different substances,

ranging from very low sclerophyllous brassica tissues

to relatively high sclerophyllous straw. Because the

nature of the component particles of the materials

varied, with leaf materials possessing a larger propor-

tion of ®ne powder than the straw samples, it was felt

that subjecting the samples to further deligni®cation

would have resulted in variable loss of cellulose.

CONCLUSIONSMethods are available which identify the differences in

lignin, cellulose, and water-soluble composition of

non-sclerophyllous arable and horticultural crop

residues. Different methods, notably the FF method,

gave lower quantities of lignin and cellulose compared

to FP and NaClO2 methods. Even within similar

methods, the exact steps taken, such as stages to

remove water-soluble compounds, can infuence sub-

sequent removal of lignin and cellulose. However,

there are correlations between the results obtained

from the different methodologies tested which may

offer opportunities for comparing results of degrada-

tion studies based on them.

ACKNOWLEDGEMENTSThanks to C Paterson and J Hembry from Kirton and

Stockbridge House respectively for providing the

samples, A Mead Wellesbourne for statistical advice

and the UK Ministry of Agriculture Fisheries and

Food for funding the project.

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Characterisation of crop residue materials