Agronomy Journal Grow Your Career With ASA
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published in Agron J 100:1687-1693 (2008)
DOI: 10.2134/agronj2008.0181
© 2008 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Lehman, R. M.
Right arrow Articles by Rosentrater, K. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Lehman, R. M.
Right arrow Articles by Rosentrater, K. A.
Agricola
Right arrow Articles by Lehman, R. M.
Right arrow Articles by Rosentrater, K. A.
Related Collections
Right arrow Soil Microbiology
Right arrow Microbial Processes
Right arrow Maize
Right arrow Residue management
Right arrow Ecological Risk Assessment
Right arrow Soil Biology

No Evidence That Bacillus thuringiensis Genes and Their Products Influence the Susceptibility of Corn Residue to Decomposition

R. Michael Lehman*, Shannon L. Osborne and Kurt A. Rosentrater

USDA-ARS-North Central Agricultural Research Laboratory, Brookings, SD 57006

* Corresponding author (michael.lehman{at}ars.usda.gov).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The possibility that Bacillus thuringiensis (Bt) corn (Zea mays L.) residues resist decomposition compared to non-Bt residues would present direct (soil carbon turnover times) and indirect (changes in tillage practices) effects on carbon budgets in agricultural systems. We evaluated the relative decomposition of residue from two pairs of Bt and non-Bt corn hybrids from different seed manufacturers buried in the root zone of adjacent Bt and non-Bt corn plots over a period of 384 d. We found no persistent differences in residue decomposition among the different hybrids regardless of the seed manufacturer or the presence of the Bt genes (both cry1Ab and cry3Bb1 genes present in each Bt hybrid) in the residue. No significant differences in residue compositional properties or flexural strength of intact stalk sections were observed among the four hybrids. Both Bt and non-Bt residues buried in the root zone of a Bt corn hybrid decomposed faster than those buried in the root zone of the corresponding, near-isogenic non-Bt hybrid. A subsequent replicated laboratory study showed no difference between the decomposition of cellulose filter paper buried in the root zone of growing and senescing Bt and non-Bt hybrids. We conclude that (i) the presence of Bt genes and their products in chopped residue does not affect its decomposability; (ii) the presence of Bt genes does not affect the mechanical strength of the stalks; and, (iii) Bt products released from growing or senescing Bt corn plants do not adversely affect decomposition activities in the surrounding soil.

Abbreviations: Bt, Bacillus thuringiensis • GM, genetically modified • TDR, time domain reflectometry

Received for publication May 23, 2008.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE ADOPTION OF genetically-modified (GM) crops has occurred rapidly in the United States and other countries primarily due to their immediate and local benefits to agricultural producers in pest and herbicide management that provide increased quantity or quality of crop yields. Although some human health and ecological risk assessments have been performed before release of GM varieties, full consideration of potential unintended, long-term, or distributed effects of this technology has not occurred (NRC, 2000, 2002; Rombke et al., 2003; Wolfenbarger and Phifer, 2000). The rapidly changing availability of GM varieties or hybrids challenge the demands for long-term field studies and contribute to a lack of peer-reviewed studies for many crop-trait combinations.

Insect-resistant, transgenic corn hybrids are being cultivated that contain genes derived from the Bt bacterium that produce proteins (endotoxins) possessing selective insecticidal activity. The potential for adverse ecological effects relating to the differences between engineered organisms and their protein expression as compared with their naturally-recombinant counterparts are not well-understood. The possibility that some Bt corn hybrids have higher lignin content than their non-GM counterparts (Saxena and Stotzky, 2001; Stotzky, 2004) represents an unintended effect with no known mechanism. Increased lignin content in plant residue should translate into slower rates of residue decomposition (Heal et al., 1997), apart from any direct effects due to the presence of the endotoxin in the residue or exuded into the soil. Flores et al. (2005) attributed slow degradation of Bt corn residues in laboratory studies to compositional changes in the residue. There continues to be wide-spread anecdotal information within the agricultural community that plant residue from some GM corn hybrids may be resistant to degradation, and implement manufacturers now market improved or alternative tillage machinery to deal specifically with "tough Bt corn residue." If Bt corn produces highly-resistant residue, then carbon budgets and soil quality would be affected directly by changes in soil carbon residence times and indirectly by changes in tillage practices.

Research comparing Bt and non-Bt corn residue composition and evaluating residue decomposition under laboratory conditions have produced mixed results (Flores et al., 2005; Folmer et al., 2002; Hopkins and Gregorich, 2003; Jung and Sheaffer, 2004; Mungai et al., 2005; Poerschmann et al., 2005). In data collected from a field study, Zwahlen et al. (2007) reported that leaves from a Bt corn hybrid decomposed the same or faster than leaves from a nontransgenic, near-isoline corn hybrid over an 8-mo period. Tarkalson et al. (2008) reported no differences in the lignin content or decomposition of leaves, stalks, or cobs between Bt and non-Bt corn hybrids (two seed manufacturers) in a 23-mo field study. All of the laboratory studies and these two field studies used Bt hybrids containing the cry1Ab transgene active against European corn borer (Ostrinia nubilalis Hübner). In our previous study (Lehman et al., 2008), we found no difference in residue (stems and leaves) composition or residue decomposition rates over 22 mo under field conditions among four corn near-isolines from a single seed manufacturer with the following genetics: (i) cry1Ab transgene; (ii) cry3Bb1 transgene active against corn rootworms (Diabrotica spp.); (iii) containing both transgenes (stacked); and, (iv) base genetics with no genetic modification. In that report, litter bags were buried into incorporated wheat (Triticum aestivum L.) stubble that remained fallow for the duration of the study.

A potential direct effect of Bt endotoxin production by plant tissues is that endotoxins in the residue or released into the soil from residues or root exudates may directly inhibit decomposer organisms. A recent literature review did not find evidence of consistent and substantial effects of Bt plants on soil invertebrates and microorganisms and various measures of their activities (Icoz and Stotzky, 2008), although some individual studies do report significant effects. We are not aware of any published data that compares residue from the same hybrids decomposing within the root zone of growing and senescing Bt and non-Bt corn hybrids.

We hypothesized that the presence of Bt proteins exuded from a growing plant or released by a senescing plant may influence the decomposition of residue from previous or concurrent plantings and result in perceived residue resistance. Therefore, we evaluated the relative decomposition of Bt and non-Bt corn residue buried in the root zones of adjacent plots of Bt and non-Bt corn plants. We used residue from paired Bt (each containing stacked cry1Ab and cry3Bb1 transgenes) and non-Bt hybrids from two different manufacturers to make the results more broadly applicable. Experimental procedures were used to approximate the exposure of the residue when it is chopped and buried during combining and tillage operations conducted in the fall. We report on the compositional (i.e., lignin content) and physical properties (flexural strength of intact stalk sections) of the residue. A replicated greenhouse study was also conducted to confirm that growing or senescing Bt corn plants do not inhibit cellulose decomposition within their root zone.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Overall Experimental Design of Field Study.
Residue from four different corn hybrids (two pairs of Bt and non-Bt hybrids from different seed manufacturers) that were grown in replicated field plots under identical conditions (see below) were analyzed for compositional and physical properties. Decomposition was studied by burying 30 litterbags containing residue for each of the four hybrids in a field plot where a Bt maize hybrid was grown and also in adjacent plot where the corresponding non-Bt, near isogenic, maize hybrid was grown.

Collection and Preparation of Corn Residue.
Residue from four corn hybrids were used: (i) Dekalb (DKC) 46–26 (Conventional, base genetics), (ii) DKC46–25 (YieldGuard Plus stacked with genes for both Cry1Ab and Cry3Bb1 proteins), (iii) Croplan (CL) 344 (Conventional, base genetics), and (iv) CL344CRW/Bt (stacked with genes for both Cry1Ab and Cry3Bb1 proteins). These four hybrids were planted in 0.76 m-spaced rows under no-till conditions with fertilizer application based on soil test recommendations in randomly arranged quadruplicate test plots (3.3 by 3.3 m) under rain-fed conditions in 2005 in Brookings County, South Dakota (44°19' N; 96°46' W). Aboveground biomass (fully mature plants minus corn ears) was harvested from all plants in a 1.5-m section of the two center rows of each plot in October 2005. There was no observable insect pressure at this site during the 2005 growing season. The plants were dried at 60°C for 1 wk. Plant residue (stalks and leaves) for each hybrid was pooled from the replicate field plots and chopped with a gas-powered chipper. The resulting chopped plant residue (nominal size <10 cm) was manually mixed and maintained at 60°C during the subsequent period of litter bag construction (about 1 wk).

Residue Compositional Analyses.
Subsamples of residue from each hybrid were reduced in size using a Wiley mill (<2 mm) for analysis of acid detergent fiber, acid detergent lignin, and neutral detergent fiber using an Ankom Fiber Analyzer and AOAC Method 973.18 (AOAC, 2003). Ground residue was milled further (Udy mill, <1 mm) for total C and N analysis by dry combustion (LECO CN 2000 analyzer, Leco Corp., St Joseph, MI) (Nelson and Sommers, 1996).

Strength Testing of Intact Residue.
To determine if potential differences among hybrids in composition (e.g., lignin content; Saxena and Stotzky, 2001) or structural properties (e.g., lignin molecular structure; Poerschmann et al., 2005) may be manifested in physical resistance of the plant tissue, mechanical strength parameters of intact stalk sections were measured with an Instron compression tester (Model 5564, Instron Corporation, Canton, MA), using a 1 kN loadcell with a three-point bending attachment. Twelve stalks from each hybrid were analyzed (three replicate stalks per test plot, four test plots per hybrid). From each stalk, stem samples between aboveground nodes 2 and 3, 4, and 5, 6, and 7 (herein referred to as internodes 3, 5, and 7, respectively) were cut, dried for 24 h at 60°C, and then were compression tested. The loadcell was lowered at a constant rate of 0.1 mm/min. During testing, the force applied to the plunger was measured, as was progressive travel distance; thus stress (MPa) and strain (%) were determined over time via the compression tester's computer control software, and flexural strength (i.e., the point of mechanical failure) and Modulus of Elasticity (i.e., material stiffness) were determined. During the testing of each internode section, at least 4000 individual stress-strain data points were collected.

Residue Decomposition.
The relative decomposition of residue prepared from the aboveground biomass of the four corn hybrids was evaluated using the litter-bag technique according to the recommendations of Harmon et al. (1999). Nylon mesh (3.2-mm, Memphis Net and Twine) and poly thread were used to construct the litter bags (20 by 20 cm). Each bag was filled with 10 g of dried plant residue and closed with rust-resistant staples. Sixty (60) bags were prepared for each of the four hybrids for a total of 240 litter bags. Completed bags were kept at 60°C (<1 wk) until burial.

Litter bags for each hybrid were buried in the root zones of both Bt and non-Bt corn following emergence. Adjacent four-row plots of Croplan 344 (Conventional) and Croplan 344CRW/Bt (both Bt transgenes) were planted on 15 May 2006 in 0.76 m-spaced 20-m rows. Application of fertilizer (same for both hybrids) was based on soil test recommendations. The crop was rainfed and grown under no-tillage conditions at the Eastern South Dakota Soil and Water Research Farm, Brookings, SD with soybean as the previous crop. The litter bags were buried (8 and 9 June 2006) in the interrows in vertical slits so that the top of each bag was five cm below the soil surface. The Barnes clay loam soils (fine-loamy, mixed, superactive, frigid Calcic Hapludoll) at this site are reported to have clay content of about 280 g kg–1 (Pikul et al., 2007) with smectite the predominant clay mineral followed by mica, kaolinite, and quartz (Soil Survey Staff, 2007). Sets of 40 litterbags composed of 10 randomly-selected bags of each residue type were buried in each of three interrows for both the CL344 and CL344CRW/Bt corn plants. The growing corn plants were allowed to mature, the grain was harvested manually in October 2006, and the plants left standing for the duration of the experiment (through 27 June 2007). Weeds were controlled by hand.

At three intervals following burial (73, 160, and 384 d), sets of 40 bags were randomly selected and retrieved from the root zones of CL344 and CL344CRW/Bt plants and the amount of residue remaining was measured. Soil loosely adhering to the exterior of the bags was brushed off and each litter bag was placed in a resealable, polyethylene bag which was kept on ice for transport to the laboratory (10 min) and immediate processing. The plant residue remaining in each bag was dried (60°C, 48 h) in a forced air oven and then combusted (450°C, 12 h) in a muffle furnace to obtain an ash-free final dry weight. The percent residue remaining was calculated from the difference between the initial (known) and final dry masses, corrected for the organic matter content of the soil entrained in each bag (ash weight x % soil organic matter, SOM) (Harmon et al., 1999).

Formula

Soil Properties.
At the time of planting, four soil samples (randomly spaced across the entire area being cultivated) were collected by soil probe (3.2 cm diam.) from the 5 to 25 cm depth interval and analyzed for total C and N by dry combustion, nitrate on KCl extracts, pH (1:1 with deionized water), and electrical conductivity using standard methods (Rhoades, 1996). At each time interval when litterbags were retrieved, four soil samples were collected by soil probe (5–25 cm depth) for percent SOM determination using weight loss on ignition (450°C, 12 h) (Nelson and Sommers, 1996).

After the last set of litterbags had been removed, an additional set of soil samples was collected for analysis. Ten soil samples (3.2 cm diam.; 40 cm depth) were collected at regularly-spaced intervals from the interrows cultivated with CL344 and from the interrows cultivated with CL344CRW/Bt. These two sets of 10 soil samples were split into depth intervals of 5 to 25 cm and 25 to 40 cm and analyzed for nitrate N, P, K, pH, conductivity, and total N at the South Dakota State Soil Testing Laboratory, Brookings, SD (Gelderman et al., 1995). Extractable P (Olsen P) was determined using the NaHCO3 method (Olsen et al., 1954). Exchangeable K was determined using the NH4Ac method (Brown and Warncke, 1988). In situ soil moisture measurements were made across the entire plot by installing 28 sets of 30-cm probes in a regular grid pattern and measuring moisture by time domain reflectometry (TDR; Trase model 6050X1, SoilMoisture Equipment Corp., Golata, CA).

Statistics.
Differences in the amount of residue remaining at each time point due to seed manufacturer and Bt trait and the interaction between these factors were tested for significance at the 0.05 alpha level by two-way ANOVA for each of the two burial zones (Bt and non-Bt corn plots.) Residue compositional elements (lignin, hemicellulose, cellulose, carbon, N, and C/N ratio) were tested for significance differences using a two-way ANOVA with seed manufacturer and Bt trait as the factors. Mechanical strength (flexural strength and Modulus of Elasticity) data were analyzed for differences among the hybrids and for differences between the internodes by two-way ANOVA (hybrid type x internode), using a significance level of 0.05.

Greenhouse Study.
Twenty-four 33-L fiber pots were filled with a 2:1:1 soil/perlite/peat mixture and one seed of either CL344 or 344CRW/Bt, using a completely randomized design. The corn plants received water and fertilizer (20–20–20) on an as needed basis and were grown under a 14/10 photoperiod and 24°C day/13°C night temperature regime in the greenhouse. Soil temperature and growing degree-days were monitored at 10 cm depth within a single pot using a biophenometer (BIO-51, Wescor, Inc., Logan, UT).

At plant emergence, six dried and preweighed cellulose filter disks (5.5 cm, <0.01% ash; Fisherbrand Q8, Fisher Scientific, Pittsburgh, PA), each encased in nylon mesh (1 mm) heat sealed envelopes, were buried in each pot 15 cm from the plant stem and with the bottom of the envelope at 10 cm depth. Sets of two filters were removed from each pot at 29, 51, and 79 d after burial and ash free-dry weights were determined to calculate the fraction of filter paper decomposed. The corn plants were grown until nearly mature (R4 stage) and then killed by withholding water for 2 wk. At that time, two more sets of filters were buried to measure decomposition during senescence of belowground biomass. Watering of the pots resumed at a reduced level (twice per week) to keep the soils moist. These two sets of filters were removed after 49 and 67 d burial, respectively. Differences in filter decomposition between the CL344 and CL344CRW/Bt at each sampling intervals were tested with two-tailed, two-sample Student's t test.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Residue Composition.
Carbon/nitrogen ratios for freshly collected plant residue were between 47 and 51, with no significant main effect or treatment interaction between seed manufacturer or Bt trait (Table 1 ). Lignin contents (g kg–1, dry weight basis, db) for freshly collected plant residue ranged between 53 and 59 with no significant main effect or treatment interaction between seed manufacturer or Bt trait. The equivalent residue compositions of the Bt hybrids and their non-Bt near isolines are similar to findings of two other studies that examined cry1Ab hybrids only (Jung and Sheaffer, 2004; Mungai et al., 2005). While Folmer et al. (2002) reported a slightly higher lignin content for Cry1Ab-containing residue, other researchers have found that several cry1Ab hybrids contained significantly higher lignin contents (Poerschmann et al., 2005; Saxena and Stotzky, 2001; Stotzky, 2004). At present, there is no consensus explanation for the disparate findings with respect to Bt and non-Bt corn residues, although differences in analytical methods and fertility management during cultivation were some of the possibilities discussed by Jung and Sheaffer (2004) and Poerschmann et al. (2005). In addition to plant genetics, environmental conditions during a given growing season may influence plant compositional properties.


View this table:
[in this window]
[in a new window]

  		Table 1. Composition of residue used in field decomposition study.{dagger}

 
Residue Mechanical Strength.
Even without differences in lignin concentrations, it is possible that some genetically-modified plants may possess a different cellular structure (an unintended effect) (Poerschmann et al., 2005) that confers increased resistance to physical disruption. Differences in cellular structure may or may not be manifested in differential decomposition of the residue used in the current study. We used several different internode sections to evaluate the mechanical strength of the stalks. The effect of hybrid on stalk flexural strength (the maximum stress allowable before material failure occurs) was not statistically significant (P = 0.052) (Fig. 1a). Since the main effects of hybrid on the flexural strength were close to alpha = 0.05, separate internode comparisons among the four hybrids were made by least significant difference. Any potential differences among the hybrids in the strength of specific internodes were not related to the presence of the Bt gene. No significant differences (P = 0.673) existed between the hybrids in the stiffness of the stalks (Modulus of Elasticity) for any of the internode sections (Fig. 1b). In our previous study using a testing method modified for shredded materials, we also did not find significant differences between the mechanical strength of Bt and non-Bt corn residues (Lehman et al., 2008). While it was not the objective of the current study to compare mechanical properties of internodes with a given hybrid, we did find significant differences among the internodes for both flexural strength (P < 0.001) and Modulus of Elasticity (P < 0.001), for all hybrids studied (Fig. 1a and 1b ). Internode 5 consistently had the lowest strength and stiffness, whereas internode 7 had the highest. The importance of the compositional and physical nature of corn stalks have been noted in silage and forage research (Johnson et al., 1999; Roberge et al., 1998) and the strength difference in internode segments may be relevant to the production of cellulosic-based fuels and products.


Figure 1
View larger version (43K):
[in this window]
[in a new window]

  		Fig. 1. Mechanical strength of internode sections (3, 5, 7) from intact stalks of each of the four corn hybrids: (i) CL344 (Conventional, base genetics), (ii) CL344CRW/Bt (containing genes for both Cry1Ab and Cry3Bb1 proteins), (iii) DKC46–25 (YieldGuard Plus containing genes for both Cry1Ab and Cry3Bb1 proteins), and (iv) DKC46–26 (Conventional, base genetics); different letters denote values that are significantly different (P < 0.05, least-significant difference test). (a) Flexural strength (MPa); (b) Modulus of Elasticity.

 
Residue Decomposition.
Soils collected from the field plots at experimental initiation had an average (one standard deviation of n = 4 in parentheses) of 19.45 (0.48) g total C, 1.86 (0.06) g total N, and 11.44 (1.08) mg nitrate N per kilogram of dry soil. Soil samples collected over the course of the experiment averaged (one standard deviation of n = 24 in parentheses) 46 (3) g of organic matter per kilogram of dried soil. The first set of litter bags was retrieved from the field plots on 21 Aug. 2006, 73 d after burial and roughly 34 to 40% of the residue had decomposed or was lost from the bags during this period of high temperatures and maximal growth of the surrounding corn plants (Fig. 2 ). There was a significant effect of the Bt trait on residue loss at 73 d (P = 0.031) for the residues buried within the CL344 plot and significant interactions (P = 0.0466 and 0.0464, respectively) between GM trait and seed manufacturer for residues buried in both CL344 and 344CRW/Bt plots. However, there were no significant differences due to the Bt trait, the seed manufacturer, or the interaction term at the 160 d and 384 d sampling intervals. Unexpectedly, all the residues buried in the presence of the CL344CRW/Bt decomposed faster than the same residues buried in the presence of the CL344.


Figure 2
View larger version (15K):
[in this window]
[in a new window]

  		Fig. 2. Percent residue remaining in litter bags (average of n = 10) after intervals of 73, 160, and 384 d for each of four corn hybrids: (i) CL344 (Conventional, base genetics), squares; (ii) CL344CRW/Bt (containing genes for both Cry1AB and Cry3Bb1 proteins), triangles; (iii) DKC46-26 (Conventional, base genetics), diamonds; and (iv) DKC46-25 (YieldGuard Plus containing genes for both Cry1Ab and Cry3Bb1 proteins), circles. The hybrids of residues buried in the root zones of non-Bt (CL344) corn are indicated with dashed lines and open symbols and those buried in Bt (CL344CRW/Bt) corn are indicated with solid lines and closed symbols. Error bars were removed for clarity of presentation.

 
Our approach using chopped litter in buried litter bags to simulate the effect of chopping and tillage operations on the stover showed that residue from Bt hybrids decomposed at rates similar to that of their non-Bt near isolines when buried under similar conditions. For those plants buried in the root zone of a Bt hybrid, the amount of residue remaining after a year was similar (about 20%) to what we observed with residue from other corn hybrids (Bt and non-Bt) buried in fallow ground (Lehman et al., 2008). For residues buried in the non-Bt root zone, decomposition appear to slow greatly between the 160 and 384 d sampling and for all hybrids there was an average of about 40% residue remaining after 1 yr.

The lack of persistent differences in decomposition rates that we observed in the field between the residue (stems and leaves) from two cry1Ab/cry3Bb1 stacked Bt hybrids and their non-Bt near isolines was consistent with our previous study which compared a different cry1Ab/cry3Bb1 stacked hybrid and single-trait hybrids with either cry1Ab or cry3Bb to their non-Bt near isoline (Lehman et al., 2008). Similarly, Zwahlen et al. (2007) reported no differences in residue decomposition in the field between Cry1Ab-containing leaf residue and its non-Bt counterpart. Laboratory studies using evolved carbon dioxide as an indicator of residue decomposition have found no (Hopkins and Gregorich, 2003), inconsistent (Raubuch et al., 2007), or significant (Flores et al., 2005) differences between the decomposition of Cry1Ab-containing residues and their non-GM counterparts; however, these studies had differences in methodological details, including the use of different hybrids, soil to residue ratios, soil fertility, and residue particle sizes.

When it became apparent that all residues buried in between the rows of CL344CRW/Bt decomposed faster than those buried in between CL344, we examined the study area further to see if there were corresponding trends in soil properties. The CL334 and CL344CRW/Bt hybrids were grown in adjacent four-row subplots that were established within a single larger plot (10 by 30 m) that historically (>15 yr) had been treated in a uniform manner. With two exceptions, we found no significant differences between the areas cultivated in the two corn hybrids in soil nitrate, P, K, pH, conductivity, or total N for either of two depth intervals, 5 to 25 cm and 25 to 40 cm (Table 2 ). The two exceptions were conductivity in the 5- to 25-cm depth interval (P = 0.024) and K in the 25- to 40-cm depth interval (P = 0.003) which cannot be readily related to the decomposition results. We also measured volumetric soil moisture in a grid across the two subplots on several occasions and found no significant differences between the areas cultivated with CL344 (28.9 ± 0.9%, n = 12) and CL344 CRW/Bt (28.3 ± 0.9%, n = 12).


View this table:
[in this window]
[in a new window]

  		Table 2. Soil properties following the experiment.{dagger}

 
Greenhouse Experiment.
Soil temperatures (10 cm) ranged between 17 and 25°C over the 6-mo duration of the greenhouse study. The buried filters were collected from 11 plants for each corn hybrid across a range of plant growth stages (Table 3 ; one plant failed to germinate and one plant was diverted to other uses). There were no significant differences in the decomposition of the cellulose filters between the 344 and 344 CRW/Bt hybrids observed at any sampling dates during the growth or senescence phases (Fig. 3 ). The decomposition dynamics under greenhouse conditions would have been minimally affected by macroinvertebrates compared to the field studies.


View this table:
[in this window]
[in a new window]

  		Table 3. Greenhouse study events.

 

Figure 3
View larger version (39K):
[in this window]
[in a new window]

  		Fig. 3. Percent of cellulose filter remaining after 29, 51, and 74 d during the growth (G) phase of the corn hybrids CL344 and CL344CRW/Bt and after 49 and 65 d during the senescent (S) phase of the same. Each bar represents an average of filters collected from n = 11 plants with one standard error.

 
We expected that if there was a significant influence of a growing or senescing corn hybrid on decomposition activities in the surrounding soil, it would be due to the suppression of residue decomposition by endotoxins from the Bt hybrid. However, this was not the case in our greenhouse and field studies or Cortet et al. (2006) who examined the decomposition of wheat straw over a 4-mo period under different Bt (cry1Ab) corn hybrids. Cortet et al. (2006) reported that the decomposition of wheat straw on the soil surface under Bt corn was faster or the same as that under their non-Bt near isolines at three experimental sites. The influence of growing or senescing plants on organic matter decomposition in the rhizosphere and surrounding soils has been reported due to species differences in biomass allocation, exudate quantity and quality, extent of root exploration, etc. (Cheng et al., 2003; Fu and Cheng, 2002). In the current field study, we could not identify similar hybrid-specific characteristics other than the presence/absence of the Bt genes that might vary to the same extent as among different plant species, nor did we find gross differences in soil characteristics where these hybrids had been grown.

Over the last 60 yr, there has been a substantial increase in corn residue in parallel with grain yields (Johnson et al., 2006), yet concerns about corn residue persistence have been elevated since the widespread use of some GM corn hybrids. No consistent differences in silage yield between Bt and non-Bt hybrids were reported by Folmer et al. (2002). Similarly, Mungai et al. (2005) found no differences between Bt and non-Bt hybrids with respect to above- or belowground biomass in a 2-yr study. It remains possible that the apparent persistence of Bt corn residue observed by agricultural producers may be related to the relative sturdiness of the Bt residue compared to non-Bt residue where insect pressure is present. No data yet exists regarding the relative decomposition of intact sections of Bt-protected and non-Bt-protected corn stalks that have been subjected to high insect pressure. Studies using shredded or ground residue may fail to detect changes in residue decomposition that are due to differential insect damage to intact stalks such as that inflicted by the European corn borer [Ostrinia nubilalis (Hübner)].


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
We found no persistent differences in the decomposition of chopped Bt and non-Bt corn residue from hybrids produced by two seed manufacturers when exposed to the same field conditions. We found no differences in the flexural strength of Bt and non-Bt corn stalk sections from hybrids produced by two seed manufacturers. We found no evidence in field and greenhouse studies that growing and senescing Bt hybrids inhibit decomposition activities in the surrounding soils. As yet, insufficient data exists to explain the perceived resistance of some GM corn residues to decay. The relative persistence of corn residue remains important because of potential effects on conservation tillage practices and C budgets.


   ACKNOWLEDGMENTS
 
Technical assistance in the field, greenhouse, or laboratory was provided by USDA-ARS employees Amy Christie, Dave Schneider, Kurt Dagel, Aaron Hoese, Kendra Jensen Kallemyne, Alyssa Bechen, Vanessa Cheesbrough, Sharon Nichols, Meggan Kowalski, Levi Vanoverschelde, and Max Pravacek. The manuscript was improved by the reviews of Dr. Randy Anderson, Dr. Deirdre Prischmann, and three anonymous reviewers.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Lehman, R. M.
Right arrow Articles by Rosentrater, K. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Lehman, R. M.
Right arrow Articles by Rosentrater, K. A.
Agricola
Right arrow Articles by Lehman, R. M.
Right arrow Articles by Rosentrater, K. A.
Related Collections
Right arrow Soil Microbiology
Right arrow Microbial Processes
Right arrow Maize
Right arrow Residue management
Right arrow Ecological Risk Assessment
Right arrow Soil Biology

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome