Pedosphere 24(2): 251–257, 2014

ISSN 1002-0160/CN 32-1315/P

c© 2014 Soil Science Society of China

Published by Elsevier B.V. and Science Press

Decomposition of Surface-Applied and Soil-Incorporated Bt Maize Leaf

Litter and Cry1Ab Protein During Winter Fallow in South Africa∗1

A. KAMOTA1, P. MUCHAONYERWA2,∗2 and P. N. S. MNKENI3

1Department of Crop Science, Bindura University of Science Education, Private Bag 1020, Bindura (Zimbabwe)2School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209 (South

Africa)3Department of Agronomy, University of Fort Hare, Private Bag X1314, Alice 5700 (South Africa)

(Received April 17, 2013; revised January 21, 2014)

ABSTRACT

Unintended effects of genetic modification on chemical composition of Bt maize leaf litter may have impacts on its decomposition.

In most agricultural systems in South Africa, maize litter is either left on the soil surface or incorporated into the soil during tillage.

A litterbag experiment, using leaf litter of three maize hybrids (DKC80-12B, DKC80-10 and DKC6-125), was carried out at the

University of Fort Hare Research Farm, South Africa, to determine the effects of genetic modification on decomposition of maize leaf

litter when left on the soil surface under field conditions between July and November, the normal fallow period, in 2008. Another

litterbag experiment was conducted at the University of Fort Hare Research Farm and Zanyokwe Irrigation Scheme, South Africa,

using leaf litter of two maize hybrids genetically modified with the cry1Ab gene (MON810), DKC75-15B and PAN6Q-308B, and their

corresponding near isolines, CRN3505 and PAN6Q-121. The degradation of Cry1Ab protein in the litter, both surface-applied and

soil-incorporated, was also investigated. Decomposition of Bt maize litter was similar to that of non-Bt maize litter both when applied

on the surface and when incorporated into soil. Soil-incorporated litter, as well as its Cry1Ab protein, decomposed faster than that

applied on the surface. The leaf litter C:N ratios of PAN6Q-308B and PAN6Q-121 were similar throughout the study, whereas those of

DKC75-15B and CRN3505 declined by similar amounts during a 12-week period. These findings suggested that decomposition of leaf

litter of Bt maize, with the MON810 event, was not affected by maize genetic modification, and that the Cry1Ab protein broke down

together with plant leaf litter during the winter fallow regardless of whether the litter was applied on the soil surface or incorporated

into soil.

Key Words: genetic modification, litterbag experiment, maize hybrid, MON810 event, protein degradation

Citation: Kamota, A., Muchaonyerwa, P. and Mnkeni, P. N. S. 2014. Decomposition of surface-applied and soil-incorporated Bt

maize leaf litter and Cry1Ab protein during winter fallow in South Africa. Pedosphere. 24(2): 251–257.

INTRODUCTION

The cry1Ab gene in Bt crops may alter compo-

sitional quality with possible reduction in litter de-

composition and nutrient release. In most agricultural

practices in South Africa, maize litter is either left on

the surface as mulch in no-till systems or incorporated

into the soil to depths from 10 to 20 cm during mold-

board plough tillage operations. Leaving maize litter

on the surface, including that of Bt maize, is a com-

mon practice where conservation agriculture has been

adopted. It is estimated that dry matter of 6 and 2.5

t ha−1 is left in the field after harvesting maize grain

and plant materials for silage, respectively, and the re-

cycling of nutrients in this litter is governed by the rate

of its decomposition (Zwahlen et al., 2003).

Decomposition of litter is influenced by soil micro-

bial composition, activities of enzymes (Flores et al.,

2005), soil conditions, particularly moisture, tempera-

ture and pH (Donnelly et al., 1990) and lignin and

polyphenol contents and C:N ratio of the litter (Palm

et al., 1999). There are conflicting reports on the effect

of genetic modification on the chemical composition of

Bt maize plant materials.

Mungai et al. (2005) did not find any consistent

differences in the lignin content between Bt maize and

non-Bt near isolines. However, Saxena and Stotzky

(2001) observed a significantly higher lignin content in

the vascular bundle sheath and the surrounding cells

of Bt maize as opposed to their corresponding non-

Bt near isolines, with the highest lignin content being

found in Bt maize cultivars with event Bt11 followed

∗1Supported by the National Research Foundation of South Africa (NRF) and the Govan Mbeki Research and Development Center

(GMRDC) of the University of Fort Hare (No. GUN62299).∗2Corresponding author. E-mail: muchaonyerwa@ukzn.ac.za.

252 A. KAMOTA et al.

by MON810, with event 176 having the least. Flores

et al. (2005) and Daudu et al. (2009) also reported

significantly higher lignin contents in Bt maize.

In most studies on decomposition of Bt maize litter,

different hybrids with different C:N ratios and lignin

contents among other components are used. These vari-

ations in the background chemical compositions, due

to varietal differences, could be responsible for most of

the differences in decomposition. The effects of genetic

modification on decomposition of maize litter have,

however, not been consistent. Lehman et al. (2008)

and Daudu et al. (2009) reported similarities in de-

composition of litter of transgenic maize lines and their

near isolines incorporated in soil even though, in some

cases, lignin contents and C:N ratios were elevated in

Bt maize. On the other hand, Flores et al. (2005) re-

ported lower decomposition rates of Bt maize residues

than their non-Bt near isolines in a laboratory experi-

ment, which was attributed to differences in the lignin

content but not to C:N ratios or microbial activity.

Environmental conditions could also have an ef-

fect on decomposition of Bt maize litter (Lehman et

al., 2008), and this has generally been ignored. Don-

nelly et al. (1990) reported that microbial biomass and

the decomposition of lignin and cellulose in forest soils

increased with increases in soil moisture and tempe-

rature, with moisture being the dominant factor. In

South Africa, maize litter is either left on the surface

or incorporated into the soil during the fallow period,

which is characterized by cool dry soil conditions (May

to August) followed by a warm dry period. The pre-

sence of higher lignin contents in Bt maize, coupled

with unfavorable conditions during the fallow winter

period, could result in greater persistence of the litter

than reported in previous studies that were carried out

during warmer and wetter months of the year. This

could have significant implications on nutrient release

and availability for the subsequent crop, while provi-

ding an advantage of sufficient soil cover required in

conservation agriculture systems.

Some studies have investigated the decomposition

of Bt litter buried in the soil but little has been

done regarding surface-applied litter under field con-

ditions. With the increase in adoption of transgenic

crops in South Africa (James, 2007), it is imperative

to understand the fate of litter of locally grown Bt

maize hybrids when incorporated in the soil or left on

the soil surface. This study was carried out to deter-

mine the effect of genetic modification on decomposi-

tion of surface-applied and soil-incorporated Bt maize

(MON810) leaf litter and their Cry1Ab protein under

winter fallow field conditions.

MATERIALS AND METHODS

Site description

This study was carried out at the University of Fort

Hare (UFH) Research Farm (32◦47′ S; 26◦50′ E; 508 m

above sea level (asl)) in Alice and at the Zanyokwe Irri-

gation Scheme (ZIS) (32◦43′ and 37◦47′ S; 27◦01′ and

27◦07′ E; 520 m asl) in Eastern Cape Province, South

Africa. The UFH Research Farm experiences a semi-

arid climate, receiving a mean annual rainfall of 575

mm and a mean annual temperature of 18.1 ◦C (Van

Averbeke and Marais, 1991). The soil is classified as

an Oakleaf using the South African soil classification

system (Soil Classification Working Group, 1991) and

a Haplic Cambisol (Eutric) using the World Reference

Base for Soil Resources (Fey, 2010), with 64.2% sand,

16.0% silt, 19.8% clay, 8.4 g kg−1 organic carbon and a

pH of 6.2 (water) (Mandiringana et al., 2005). The ZIS

experiences a temperate to warm sub-humid climate

and receives an average annual rainfall of 590 mm. The

soil is a Valsrivier (the South African soil classification

system) (Soil Classification Working Group, 1991) or

a Haplic Luvisol (Chromic) (the World Reference Base

for Soil Resources) (Fey, 2010), with 66% sand, 22%

silt, 12% clay, 4.4 g kg−1 organic carbon and a pH

of 6.5 (water). Ambient temperatures and rainfall du-

ring the study periods and their long-term averages are

shown in Figs. 1 and 2, respectively.

Decomposition of surface-applied maize leaf litter

This experiment was performed at the UFH Re-

search Farm between July and November 2008. This

period covered the cold winter months to the begin-

Fig. 1 Ambient temperatures and long-term temperature ave-

rages at the two study sites, the University of Fort Hare Research

Farm (UFH) and the Zanyokwe Irrigation Scheme (ZIS), in Eas-

tern Cape Province, South Africa.

LEAF LITTER AND PROTEIN DECOMPOSITION 253

Fig. 2 Annual rainfall and long-term rainfall averages at the

two study sites, the University of Fort Hare Research Farm

(UFH) and the Zanyokwe Irrigation Scheme (ZIS), in Eastern

Cape Province, South Africa.

ning of next summer, which is the normal fallow pe-

riod when maize residues are left in the field after

harvest before the next crop is grown. The field was

previously cropped with three experimental maize cul-

tivars, DKC80-12B (Bt maize), DKC80-10 (non-Bt

near-isogenic line) and DKC6-125 (a locally grown hy-

brid), as treatments, arranged in a randomized com-

plete block design (RCBD) with three replicates, from

February to June 2008. Dried leaf litter of the three

maize hybrids was used in the litterbag-based decom-

position study. Polyethylene litterbags (0.20 m × 0.20

m), with 2-mm mesh size, were filled with 10 g of

dry maize leaf litter (0.15 m in length). The litter was

evenly distributed in the litterbags and sealed. Five lit-

terbags per treatment per block were prepared to allow

for destructive sampling. The litterbags with leaf litter

of DKC80-12B, DKC80-10 and DKC6-125 were placed

in plots previously under the same cultivar. No irriga-

tion was applied and no weeding was done in the plots.

One litterbag was collected from each plot after 0,

2, 4, 8, 12 and 16 weeks of incubation. The last set

of litterbags was retrieved on November 10, 2008. Lit-

terbag samples were brushed free of soil, oven-dried at

40 ◦C to constant weight and ground to < 1 mm. Ini-

tial leaf samples were also oven-dried and ground. All

the samples were analysed for ash-free dry matter and

Cry1Ab protein contents, whereas initial maize leaf lit-

ter was also analysed for C, N and lignin contents.

Decomposition of soil-incorporated maize leaf litter

This experiment was established at both sites, the

UFH Research Farm and ZIS, between August and

October, 2009. The land was fallow in the previous

season. The maize leaf litter of two Bt maize hybrids

(DKC75-15B and PAN6Q-308B) and their near iso-

lines (CRN3505 and PAN6Q-121), used in this experi-

ment, was obtained from North West University, South

Africa. The experiment was arranged in an RCBD

with four treatments (maize hybrids) and five repli-

cates. The blocks represented different positions on the

slope. Litterbags containing 15 g of leaf litter were ran-

domly placed in each block with the Bt maize hybrid

and its near isoline being close to each other to mini-

mize effects of within block variability being detected

as an effect due to Bt maize. The litterbags were buried

to a depth of about 10 to 15 cm, representing the depth

to which crop residues are normally incorporated by

moldboard plough tillage of fields in South Africa. The

positions of the litterbags were marked with pegs. The

litterbags were sampled at 0, 4, 8 and 12 weeks (ter-

mination). The sampled leaf litter was oven-dried to

constant weight at 40 ◦C, ground (< 1 mm mesh) and

analyzed for ash-free dry matter, lignin (only for initial

samples), C and N and Cry1Ab protein contents.

Measurements

Ash-free dry matter was determined using the me-

thod of Okalebo et al. (2002). The Cry1Ab protein

content was determined using a double-antibody sand-

wich ELISA technique with a kit developed by AG-

DIA (Elkhart, USA) as described in Daudu et al.

(2009). Carbon and N contents of the leaf litter were

determined using an LECO TruSpec CN autoanalyser

(LECO Corp., USA) as described in LECO Corpora-

tion (2003). Acid detergent lignin content was analyzed

using the filter bag technique (ANKOM200, Ankom

Technology Corp., USA) as described in AOAC (1984).

Statistical analysis

The data for ash-free dry matter content, C:N and

Cry1Ab protein content were subjected to analysis

of variance (ANOVA) using repeated measures proce-

dure in the GenStat statistical package (Lawes Agricul-

tural Trust, 2008) and treatment means were separated

using the least significant difference (LSD) test. For

the soil-incorporated litter experiment, the treatment

effects were also compared across sites and sampling

times.

RESULTS

Initial chemical composition of leaf litter used

Initial ash-free dry matter contents of all the maize

leaf litter applied on the soil surface were similar,

254 A. KAMOTA et al.

whereas the C:N ratio of DKC80-10 leaf litter was lo-

wer than those of DKC80-12B and DKC6-125 leaf lit-

ter, which were similar (Table I). The lignin content of

DKC80-12B leaf litter was lower than that of its near-

isogenic line leaf litter. The initial ash-free dry matter

contents of all the soil-incorporated litter were similar

across the maize varieties (Table I). The leaf litter C:N

ratio of PAN6Q-308B was similar to that of PAN6Q-

121 and lower than that of CRN3505, which was also

lower than DKC75-15B. Only PAN6Q-121 leaf litter

had a higher lignin content than the leaf litter of the

other three varieties. The leaf litter of DKC75-15B had

a higher Cry1Ab protein content than that of PAN6Q-

308B.

Decomposition of Cry1Ab protein and maize leaf litter

applied on the soil surface

The decomposition of the leaf litter, as measured

by decline in ash-free dry matter content, was influ-

enced by incubation time but not by maize variety (Ta-

ble II). The decomposition of litter was similar across

all maize hybrids irrespective of whether they were ge-

netically modified or not (Table II). Relative to initial

levels, a significant decrease in ash-free dry matter con-

tent of 7.5% was observed after 4 weeks, following an

almost linear trend, with 91.4%, 84.5% and 75.4% of

initial ash-free dry matter remaining after 8, 12 and 16

weeks, respectively (Table III). The ash-free dry matter

levels were similar across the maize hybrids irrespective

of sampling time (Table II).

The Cry1Ab protein in the DKC80-12B litter sig-

nificantly degraded over time, firstly at a slow rate,

with 2.2% of the initial content lost during the first 4

weeks, followed by a faster decline to 74.8% (25.2% de-

cline) and 56% (43.9% decline) of the initial contents

after 12 and 16 weeks, respectively (Table III).

Decomposition of Cry1Ab protein and maize leaf litter

incorporated in soil

Decomposition of the maize leaf litter incorporated

into soil was affected by incubation time but not by

maize variety or site (Table IV). The leaf litter ash-

free dry matter contents decreased by 5.1% within 4

weeks and 69.8% of the initial ash-free dry matter re-

mained after 12 weeks (Table V). Ash-free dry matter

levels of leaf litter were all similar across the maize

hybrids irrespective of sampling time (Table IV).

The Cry1Ab protein content of the DKC75-15B

leaf litter (267.0 ng g−1) was greater than that of the

PAN6Q-308B leaf litter (253.0 ng g−1) during incuba-

tion. Degradation of the Cry1Ab protein was only sig-

nificantly influenced by incubation time. The Cry1Ab

protein content decreased by 12.6% and 60.6%, after 4

and 12 weeks of incubation, respectively (Table V).

There were significant effects of site × incubation

time and maize variety × incubation time on changes

in C:N ratios of the leaf litter (Table IV). Main effects

of site, maize variety and incubation time were also

significant. At the UFH Research Farm, the C:N ra-

tios decreased at a faster rate, from 19.1% to 12.7%,

than at ZIS where a decline from 19.1% to 15.5% was

observed (Fig. 3). The leaf litter C:N ratios of PAN

6Q-308B and its near isoline PAN6Q-121 were simi-

lar, but lower than those of DKC75-15B and CRN3505

throughout the incubation period (Fig. 3). The leaf lit-

ter C:N ratio of DKC75-15B declined (from 23.7) at a

faster rate than CRN3505 (from 21.1) until the ratios

of the two varieties were similar after 4 and 8 weeks

of incubation. A slower decline in the leaf litter C:N

ratio of DKC75-15B, from 17.6 to 16.7, was observed

between 8 and 12 weeks of incubation, whereas that of

CRN3505 declined from 16.4 to 14.1 during the same

period. Overall, the decline in leaf litter C:N ratio was

similar between DKC75-15B (from 23.7 to 16.7) and

CRN3505 (from 21.1 to 14.1) over the 12-week incuba-

tion period.

DISCUSSION

The higher leaf litter lignin contents of DKC80-10

(63 g kg−1) and PAN6Q-121 (68 g kg−1) than their Bt

TABLE I

Initial ash-free dry matter, lignin and Cry1Ab protein contents and C:N ratios of maize leaf litter used in the study

Leaf litter Maize cultivar Ash-free dry matter content C:N ratio Lignin content Cry1Ab protein content

g kg−1 g kg−1 ng g−1

Surface-applied DKC80-12B (Bt) 898 20.6 55 437.0DKC80-10 886 19.5 63 ND

DKC6-125 900 21.0 NDa) ND

Soil-incorporated DKC75-15B (Bt) 889 23.7 58 328.2

PAN6Q-308B (Bt) 886 15.7 57 314.1

CRN3505 888 21.1 55 ND

PAN6Q-121 868 16.0 68 ND

a)Not determined.

LEAF LITTER AND PROTEIN DECOMPOSITION 255

TABLE II

Probability values of F test on ash-free dry matter and Cry1Ab

protein contents of maize litter applied on the soil surface

Source of variation Ash-free dry Cry1Ab protein

matter content contenta)

Incubation time 0.001* 0.036*

Maize variety 0.565nsb)

Incubation time × 0.937ns

maize variety

*Significant at P = 0.05.a)Only one Bt maize variety was analyzed.b)Not significant

TABLE III

Changes in ash-free dry matter and Cry1Ab protein contents

of surface-applied Bt maize (DKC80-12B) litter over a 16-week

period

Incubation Ash-free dry Cry1Ab protein

time matter content content

weeks g kg−1 ng g−1

0 895 437.0

2 883 426.0

4 828 427.0

8 818 388.0

12 756 327.0

16 675 245.0

LSD0.05a) 5.2 120.4

CVb) (%) 6.6 17.1

a)Least significant difference at P = 0.05.b)Coefficient of variation.

TABLE IV

Probability values of F test on ash-free dry matter and Cry1Ab

protein contents and C:N ratio of soil-incorporated maize litter

Source of Ash-free C:N ratio Cry1Ab

variation dry matter protein

content content

Site 0.206nsa) <0.001*** 0.444ns

Incubation time <0.001*** <0.001*** <0.001***

Maize variety 0.157ns <0.001*** 0.036*

Site × incubation 0.282ns 0.002** 0.870ns

time

Site × maize variety 0.330ns 0.323ns 0.840ns

Incubation time 0.123ns 0.031* 0.693ns

× maize variety

Site × incubation time 0.184ns 0.862ns 0.412ns

× maize variety

*, **, ***Significant at P = 0.05, 0.01 and 0.001, respectively.a)Not significant.

maize counterparts with the MON810 event, DKC80-

12B (55 g kg−1) and PAN6Q-308B (57 g kg−1) were

in contrast with the findings of Xue et al. (2011), who

found similarities between Bt maize with the MON863

event (Cry3Bb) and its near isoline. However, the le-

vels of the lignin content in their study (about 50–60 g

TABLE V

Changes in ash-free dry matter and Cry1Ab protein contents of

soil-incorporated maize litter over a 12-week period of incubation

Incubation Ash-free dry Cry1Ab protein

time matter content content

weeks g kg−1 ng g−1

0 883 321.1

4 838 280.8

8 750 243.6

12 616 194.5

LSD0.05a) 19.7 25.94

CVb) (%) 5.4 10.5

a)Least significant difference at P = 0.05.b)Coefficient of variation.

Fig. 3 Changes in soil-incorporated leaf litter C:N ratio of dif-

ferent maize varieties, two Bt maize hybrids (DKC75-15B and

PAN6Q-308B) and their near isolines (CRN3505 and PAN6Q-

121), over a 12-week period of incubation at the two study sites,

the University of Fort Hare farm (UFH) and Zanyokwe Irrigation

Scheme (ZIS), in Eastern Cape Province, South Africa.

kg−1) were similar to those in our study. More-

over, the similarity in lignin content between DKC6-

125B and CRN3505 leaf litter agreed with Xue et

al. (2011). Daudu et al. (2009) found higher concentra-

tions of lignin, cellulose, total polyphenols and lower

C:N ratios in Bt maize (CRN4549B) than the non-Bt

maize (CRN3549) litter. These findings suggest that

differences in chemical composition between Bt maize

varieties and their near-isogenic lines may depend on

256 A. KAMOTA et al.

the background genetic makeup of the maize. In all

these studies, the lignin contents were far lower than

the threshold of 150 g kg−1 to limit decomposition of

the surface-applied residues (Schroth, 2003).

Genetic modification of maize (MON810) did not

affect the decomposition of Bt maize litter under winter

fallow field conditions regardless of whether the litter

were surface-applied or soil-incorporated. Daudu et al.

(2009) found similar results with soil-incorporated lit-

ter in a study conducted during summer. They found

no effects of genetic modification (MON810) on de-

composition of stem and leaf litter of Bt maize incor-

porated in the same soil during the warmer and wetter

summer maize growing period, in the Central Eastern

Cape. The findings were also in agreement with those

of Tarkalson et al. (2008), in a study done over 23

months in Nebraska, and Lehman et al. (2008), in a

study carried out over 658 d in South Dakota (both

in USA), who did not find any differences in the de-

composition of Bt (events MON810 and MON863) and

non-Bt maize litter after burying them in the soil under

field conditions. In field studies conducted at the Cor-

nell University’s Musgrave Farm in Aurora, USA, Xue

et al. (2011) also found similar results with surface-

applied and soil-incorporated leaf, stem and root litter

of Bt maize with the MON863 event and its near iso-

line. Zwahlen et al. (2007) found similar results with

5 000-μm mesh size litterbags but reported faster Bt

maize decomposition when 125- and 20-μm mesh size

litterbags were used. While DKC75-15B had a higher

initial leaf litter C:N ratio than CRN3505 in our study,

the decline in C:N ratio (as a measure for decomposi-

tion of the leaf litter) was the same over a 12-week pe-

riod and the same was observed for PAN6Q-308B and

PAN6Q-121 leaf litter, which had similar C:N ratios.

These findings were in agreement with those of Daudu

et al. (2009), who reported no differences in litter de-

composition even though there were differences in C:N

ratio. A review article by Yanni et al. (2010) showed

that some researchers found differences in chemical

composition, particularly lignin content and C:N ra-

tio, between Bt maize varieties (with various trans-

formation events) and their near isolines, while oth-

ers reported no differences. However, where the diffe-

rences occurred, they did not translate to differences in

decomposition rates. Differences in C:N ratio did not

limit decomposition probably because the ratios were

below the threshold of 30 above which mineralization

of N could be limited (Schroth, 2003). These findings

suggest that genetic modification of maize with the

MON810 event may not affect decomposition and nu-

trient release from the maize litter. The faster decline

in C:N ratio of the maize litter at the UFH Research

Farm could be because of the higher clay content which

may have resulted in greater moisture storage. Howe-

ver, the effect was not significant for ash-free dry mat-

ter content.

About 84.4% of initial ash-free dry matter rema-

ined 12 weeks after surface application, whereas 69.8%

remained after the same period of soil incorporation ir-

respective of site, and this was in agreement with Xue

et al. (2011), who reported that maize litter incorpo-

rated into the soil decomposed at a faster rate. The

faster decomposition rate of incorporated litter could

be a result of greater proximity of the litter to soil

particles and moisture and hence greater exposure to

soil microbial activity. The initial lower decomposi-

tion of surface-applied litter was probably a result of

low temperature and soil moisture, as a result of low

rainfall in July, whereas a faster decomposition was

observed when temperatures and rainfall increased as

summer approached. Microbial activity is more pro-

nounced when temperatures are between 24 and 27◦C (Zwahlen et al., 2003). Daudu et al. (2009) ob-

served that 50% of the dry matter was decomposed

within a two-week period when the maize litter was

incorporated during moist summer months. The dif-

ferences in the findings could be explained by moisture

and temperature conditions. During the experimental

period of Daudu et al. (2009), 236.2 mm rainfall was

received and the temperature ranged from 16.4 to 23.2◦C, whereas in our study rainfall ranged from 98 to 128

mm and temperatures ranged from 15 to 18 ◦C. The

residues could have decomposed at a faster rate if the

study was extended into the summer.

The decrease in the Cry1Ab protein content fol-

lowed the same trend as that in the ash-free dry

matter content, with greater decomposition in soil-

incorporated litter than in litter applied on the soil sur-

face. Like in the decline in ash-free dry matter content,

these differences could be a result of better soil contact

which enhanced decomposition. The longer cool pe-

riod the surface-applied litter experienced (initiated in

July) than the incorporated litter (initiated in August)

may also have caused the difference. These results con-

tradicted those of Zwahlen et al. (2003), who observed

faster degradation of Cry1Ab protein in Bt maize lit-

ter left on the soil surface than that incorporated into

the soil. The difference in the findings could be be-

cause Zwahlen et al. (2003) used maize leaf litter at

the pollen-shed stage, earthworms were included and

the litter was applied on the soil surface, whereas we

used senescent maize leaf litter obtained after harvest

and no effort was made to include or exclude earth-

LEAF LITTER AND PROTEIN DECOMPOSITION 257

worms in any of the experiments. Moreover, activity

of earthworms in our study was minimal considering

the lower moisture and temperature during the study

periods.

The contents of Cry1Ab protein remained > 190

ng kg−1 (> 60% of initial values) after 12 weeks in this

study, whereas Daudu et al. (2009) observed that the

Cry1Ab protein approached undetectable levels within

14 d when Bt maize leaf litter was incorporated into the

soil. This difference could be explained by the higher

rainfall and temperature during the summer months of

February, March and April when the study by Daudu

et al. (2009) was conducted compared to the drier

cooler period for this study. If our study was allowed to

progress into summer, the Cry1Ab protein could have

further degraded to undetectable levels, thus reducing

exposure of the protein to none-target soil organisms.

CONCLUSIONS

The chemical composition of leaf litter of Bt maize

varieties producing Cry1Ab protein (MON810) tested

in this study might be slightly different from their near-

isogenic lines. Such differences did not affect their leaf

litter decomposition when the litter was applied on the

surface or incorporated into the soil during the fal-

low winter months in Eastern Cape Province, South

Africa. Bt maize litter did not limit decomposition and

nutrient cycling and, therefore, may not have an added

advantage of providing longer lasting soil cover in con-

servation agriculture systems or improving C seques-

tration in the soil.

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