Soil microbial community responses to Bt transgenic rice residue decomposition in a paddy field

Purpose

Genetic modifications (GM) of commercial crops offer many benefits. However, microbial-mediated decomposition might be affected by GM crop residues in agricultural ecosystems. The objective of this study was to assess the possible impacts of cry1Ab gene transformation of rice on soil microbial community composition associated with residue decomposition in the paddy field under intensive rice cultivation.

Materials and methods

A 276-day field trial was set up as a completely randomized design for two types of rice residues, KMD (Bt) and Xiushui 11 (non-Bt parental variety) in triplicate by conventional intensive rice cropping system. The litterbag method was used in the rice residue decomposition and a total of 120 straw and root litterbags were either placed on the soil surface or buried at 10 cm depth in the field on Dec. 24, 2005. The litterbags were sampled periodically and their soil bacterial and fungal communities were determined by terminal restriction fragment length polymorphism (T-RFLP). The additive main effects with multiplicative interaction (AMMI) model were performed for the analysis of T-RFLP on binary variables of peak presence (presence/absence). The analysis of variance and linear regressions were performed for analysis of AMMI data.

Results and discussion

Total AMMI model analysis revealed that microbial community composition in the litterbags was affected by temporal and spatial factors. Compared with the non-Bt rice residue treatment, Bt rice straw had no significant effects on the soil bacterial and fungal community composition during the study period, regardless of the litterbags being placed on the surface or buried in the soil. There were no significant differences in the bacterial community composition profiles in root decomposition between Bt transgenic and non-Bt varieties. However, significant differences in soil fungal community composition between the buried Bt and non-Bt rice roots were observed in soils sampled on days 31, 68, and 137, indicating that Bt roots incorporated into paddy soil may affect soil fungal community during the initial stage of their decomposition.

Conclusions

There were some significant differences in fungal community composition between Bt rice root and non-Bt root treatments at the early stage of root decomposition in the paddy field. It is important that, before Bt rice is released for commercial production, more research should be conducted to evaluate the ecological effects of the Bt rice residues returned to paddy field upon grain harvesting.     

Litterbag decomposition of residues from Bacillus thuringiensis (Bt) rice hybrids and the parental lines under multiple field conditions

Purpose

The cultivation of genetically modified (GM) crops has raised environmental concerns, since large amounts of plant materials remain in the field after harvesting. Specific proteins of GM crops might negatively impact soil ecosystem by changing residue decomposition dynamics. Particularly, the residue decomposition of crop-wild hybrids, which were formed through transgene escape to wild population, remains unexplored.

Materials and methods

We used litter bags to assess residue (leaves, stems and roots) decomposition dynamics of two stacked genes from Bacillus thuringiensis (Bt) Cry1Ac and the sck (a modified CpTI gene encoding a cowpea trypsin-inhibitor) (Bt/CpTI) rice lines (Kefeng-6 and Kefeng-8), a non-transgenic rice near isoline (Minghui86), wild rice (Oryza rufipogon) and Bt wild rice at three sites. The enzyme-linked immunosorbent assay (ELISA) was used to monitor the changes of the Cry1Ac protein in Bt rice residues.

Results and discussion

Mass remaining, total N and total C concentrations of rice residues declined over time and varied among plant tissues, with significant differences among cultivar, crop-wild hybrids and wild rice, but no differences between Bt and non-Bt rice cultivars. The initial concentration of Cry1Ac was higher in leaves and stems than in roots and was different between rice types. The degradation dynamics of Cry1Ac fitted best to a first-order kinetics model and correlated with the level of total nitrogen in residues but did not correlate with the mass decomposition rate. The predicted DT50 (50 % degradation time) of the protein ranged from 10.7 to 63.6 days, depending on plant types, parts and burial sites. By the end of the study (~170 days), the protein was present in low concentration in the remaining residues.

Conclusions

Our results suggest that the impacts of the stacked Bt/CpTI gene inserts on the decomposition dynamics of rice residues are insignificant.     

Decomposition processes under Bt (Bacillus thuringiensis) maize: Results of a multi-site experiment

The effects of maize expressing the Bacillus thuringiensis Cry1Ab protein (Bt maize) on decomposition processes under three different European climatic conditions were assessed in the field. Farming practices using Bt maize were compared with conventional farming practices using near-isogenic non-Bt maize lines under realistic agricultural practices. The litter-bag method was used to study litter decomposition and nitrogen mineralization dynamics of wheat straw. After 4 months incubation in the field, decomposition and mineralization were mainly influenced by climatic conditions with no negative effect of the Bt toxin on decomposition processes.     

室内恒温条件下稻田土壤中菌渣的分解过程及CO2释放特征

菌渣是栽培食用菌后的下脚料,可作为有机肥再利用。本文通过实验室条件下培养不同比例的菌渣和稻田土壤混合物[不施用菌渣(TS),土壤与菌渣质量比为10∶1(SM1)、5∶1(SM2)和2∶1(SM3),全部菌渣(TM)],研究不同处理有机碳和全氮的变化,探讨菌渣在稻田土壤中的分解过程,并分析CO_2释放特征,为菌渣合理利用提供参考。结果表明,在相同培养时间,添加不同比例菌渣处理有机碳和氮含量均比TS处理高,其中TM处理的有机碳和全氮分别比TS处理提高了10.7倍和11.0倍。有机碳、氮含量的提高量主要依赖于菌渣的添加量。总体来说,各处理随培养时间的延长,由于碳氮的分解,有机碳、氮均有下降趋势;在35 d后TM处理有机碳氮下降较快。添加菌渣越多,有机碳残留率也越大。在培养63 d后,菌渣有机碳(YC)和氮(YN)的分解残留率与菌渣添加量(X)的关系式分别为:YC=71.26X-0.607 5,r2=1.000 0**和YN=74.039X-0.413 3,r2=0.999 9**。各处理土壤CO_2释放速率均表现出先增后降然后趋于稳定趋势。菌渣用量越高,CO_2释放速率越高,各处理在不同培养时间CO_2释放速率均表现为TMSM3SM2SM1TS。在第7 d时各处理CO_2释放速率最高,在第14 d时渐渐处于平稳下降状态,培养35 d后,各处理土壤有机碳矿化强度很小,大部分有机碳被固定在土壤中,其中TM处理有机碳矿化强度最小。总之,还田菌渣越多,土壤中被固定的碳越多。     

Exposure and effects assessments of Bt-maize on non-target organisms (gastropods, microarthropods, mycorrhizal fungi) in microcosms

Potential differences between Bt-maize (MEB307 expressing the insecticidal Cry1Ab protein) and a near-isogenic non-Bt variety (Monumental) in their influence on the garden snail (Helix aspersa), soil microarthropods (Collembola, Actinedida, Acaridida, Gamasida and Oribatida) and mycorrhizal fungi were studied. Growing snails were caged in microcosms allowing the development of Bt or non-Bt-maize (Zea mays L.) on a sandy loam soil. After 3 months exposure, survival and growth of snails were similar in both treatments. Cry1Ab protein was detected in the Bt-maize leaves (22–42.2 μg Bt protein g⁻¹ dry wt), in the snail tissues (0.04–0.11 μg Bt-protein g⁻¹ dry wt) and in their faeces (0.034–5 μg Bt-protein g⁻¹ dry wt). Total soil microarthropod abundance and diversity were similar between control (non-Bt-maize) and the genetically modified (GM) Bt-maize microcosms. The mycorrhizal colonization of roots did not differ between Bt and non-Bt-maize (frequency of mycorrhizal roots was 88.7% and 83.3% respectively). The mycorrhizal infectivity of soils, expressed as MI₅₀ (minimum soil dry weight required to colonize 50% of plants) was measured using red clover. MI₅₀ was similar for soils where Bt or non-Bt-maize was cultivated for 4 months. The detection of Cry1Ab protein in the viscera and faeces of H. aspersa exposed to Bt-maize indicates that snails contribute to the transfer of the Bt-protein from plant to soil or snail predators. This may constitute an alternative route of exposure for Bt-protein in soil, but this was without a negative influence on mycorrhizal fungi or microarthropods. Results showed that Bt-maize was not toxic for the selected non-target species exposed for 3 or 4 months. The microcosms and analyses used in this study represent new methods for assessing effects of chronic exposure to GM plants of several diverse, yet ecologically and temporally associated species. As the soil organisms we studied can also be used in standardized ecotoxicological tests (XP X31-205-2 for mycorrhizal fungi, ISO 11267 for Collembola and ISO 15952 for snails), microcosm exposures represent a way to link laboratory and field methods for the ecotoxicological evaluation of GM plants.     

Carbon flux from decomposing 13C-labeled transgenic and nontransgenic parental rice straw in paddy soil

Purpose

Genetic modification of Bt rice may affect straw decomposition and soil carbon pool under flood conditions. This study aims to assess the effects of cry gene transformation in rice on the residue decomposition and fate of C from residues under flooded conditions.

Materials and methods

A decomposition experiment was set up using ¹³C-enriched rice straws from transgenic and nontransgenic Bt rice to evaluate the soil C dynamics and CH₄ or CO₂ emission rates in the root and non-root zones. The concentrations and stable carbon isotope compositions of the soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), CH₄, and CO₂ of the root and non-root zones were determined from 7 to 110 days after rice straw incorporation.

Results and discussion

Rice straw incorporation into soil significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates. The percentage of ¹³C-SOC remaining in the root zone was significantly lower than that in the non-root zone with rice straw decomposition. The DOC and MBC concentrations significantly increased in both the root and non-root zones between 0 and 80 days after rice straw incorporation. However, no significant differences were found after Bts (Bt rice straw added into soil) and Cks (nontransgenic Bt rice straw added into soil) incorporation in the root and non-root zones. This result may be attributed to the priming effects of sufficient oxygen and nutrients on straw degradation in the root zone.

Conclusions

Bt gene insertion did not affect the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates in both the root and non-root zones. However, rice straw incorporation and root exudation significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates.     

Dynamics of soil amino sugar pools during decomposition processes of corn residues as affected by inorganic N addition

Purpose  

Identifying the impact of inorganic-nitrogen (N) availability on soil amino sugar dynamics during corn (Zea mays L.) residue decomposition may advance our knowledge of microbial carbon (C) and N transformations and the factors controlling these processes in soils. Amino sugars are routinely used as microbial biomarkers to investigate C and N sequestration in microbial residues, and they are also involved in microbial-mediated soil organic matter (SOM) turnover. We conducted a 38-week incubation study using a Mollisol which was amended with corn residues and four levels of inorganic N (i.e., 0, 60.3, 167.2, and 701.9 mg N kg⁻¹ soil). The objective of this study was to examine the effects of inorganic-N availability on fungal and bacterial formation and stabilization of heterogeneous amino sugars during the corn residue decomposition in soil.     

Adsorption and Desorption of Cry1Ab Proteins on Differently Textured Paddy Soils

In recent years, selected cry genes from Bacillus thuringiensis (Bt) encoding the production of Cry proteins (Bt toxins) have been engineered into crop plants (Bt-crops). Through the cultivation of Bt crops and the application of Bt pesticides, Cry proteins could be introduced into arable soils. The interaction between the proteins and soils was analyzed in this study to investigate the affinity of Cry proteins in paddy soil ecosystems. Four Paddy soils were selected to represent different soil textures. Cry proteins were spiked in soils, and the amount of protein adsorbed was measured over 24 h. Desorption of Cry1Ab proteins from paddy soils was performed by washing with sterile Milli-Q water (H₂O_MQ), and subsequently extracted with an extraction buffer. The paddy soils had a strong affinity for Cry1Ab proteins. Most of the Cry1Ab proteins added (> 98%) were rapidly adsorbed on the paddy soils tested. More Cry1Ab proteins were adsorbed on non-sterile soils than on sterile soils. Less than 2% of the adsorbed Cry1Ab proteins were desorbed using H₂O_MQ, while a considerable proportion of the adsorbed proteins could be desorbed with the buffer, ranging from 20% to 40%. The amount of proteins desorbed increased with the increases in the initial amount of Cry1Ab proteins added to the paddy soils. The concentration of Cry1Ab proteins desorbed from the paddy soils was higher for sterile soils than non-sterile ones. Our results indicate that Bt toxins released via the cultivation of Bt crops, the application of Bt pesticides can be adsorbed on paddy soils, and soil texture could impose an impact on the adsorption capability.     

Bacterial community incorporating carbon derived from plant residue in an anoxic non-rhizosphere soil estimated by DNA-SIP analysis

Purpose

Plant residues are one of the main sources of soil organic matter in paddy fields, and elucidation of the bacterial communities decomposing plant residues was important to understand their function and roles, as the microbial decomposition of plant residues is linked to soil fertility. We conducted a DNA stable isotope probing (SIP) experiment to elucidate the bacterial community assimilating 13-carbon (¹³C) derived from plant residue under an anoxic soil condition. In addition, we compared the bacterial community with that under the oxic soil condition, which was elucidated in our previous study (Lee et al. in Soil Biol Biochem 43:814–822, 2011).

Materials and methods

We used the ¹³C-labeled dried rice callus cells as a model of rice plant residue. A paddy field soil was incubated with unlabeled and ¹³C-labeled callus cells. DNA extracted from the soils was subjected to buoyant density gradient centrifugation to fractionate ¹³C-enriched DNA. Then, polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) analysis of bacterial 16S rDNA band patterns and band sequencing method were used to evaluate bacterial community.

Results and discussion

DGGE analysis showed that the band patterns in the ¹³C-enriched fractions were distinctly changed over time, while the changes in the community structure before fractionation were minor. Sequencing of the ¹³C-labeled DGGE bands revealed that Clostridia were a major group in the bacterial communities incorporating the callus-derived carbon although Gram-negative bacteria, and Actinobacteria also participated in the carbon flow from the callus under the anoxic condition. The proportion of Gram-negative bacteria and Actinobacteria increased on 14 days after the onset of incubation, suggesting that the callus was decomposed by diverse bacterial members on this phase. When the bacterial groups incorporating the ¹³C were compared between under anoxic and oxic soil conditions, the composition was largely different under the two opposite conditions. However, some members of Gram-negative bacteria were commonly found under the anoxic and oxic soil conditions.

Conclusions

The majority of bacterial members assimilating the callus carbon was Clostridia in the soil under anoxic conditions. However, several Gram-negative bacterial members, such as Acidobacteria, Bacteroidetes, and Proteobacteria, also participated in the decomposition of callus under anoxic soil conditions. Our study showed that carbon flow into the diverse bacterial members during the callus decomposition and the distinctiveness of the bacterial communities was formed under the anoxic and oxic soil conditions.

     

Microbial response of soils with organic and conventional management history to the cultivation of Bacillus thuringiensis (Bt)-maize under climate chamber conditions

An experiment was carried out in a climate chamber to analyse if Bt-maize may cause particular changes in soils with different levels of microbial biomass and activity due to long-term management history. Among the soils selected, the ones managed organically for 30 years exhibited twice the microbial biomass and 2.6 times the dehydrogenase activity (DHA) of the soil from a field with long-term conventional maize monoculture. Soils were cultivated twice in a row with Bt-maize, its near-isogenic line and a conventional breeding line. We tested the hypotheses that (a) soil microbial biomass and activity are affected by the cultivation of Bt-maize and that (b) the influence of Bt-maize depends on the level of soil microbial biomass and activity. Shoot and root yield and shoot C-content of Bt-maize were higher than the ones of the near-isogenic line. DHA under Bt-maize was 6 % higher, and the metabolic quotient for CO₂ (qCO₂) was 9 % lower than under its near-isogenic line, giving some support to hypothesis (a). No significant interactions of the soils and the varieties used were found in this study, thus hypothesis (b) was not confirmed, and soils with different microbial biomass and activity appear to react in a similar way to the cultivation of Bt-maize.     

Evaluation of soil quality parameters in a tropical paddy soil amended with rice residues and tree litters

Laboratory and greenhouse experiments were conducted to study the effects of applications of rice residue and Pongamia pinnata and Azadirachta indica leaf litters on biochemical properties (extraction yield of humus, composition of humus, microbial biomass carbon, activities of urease and acid phosphatase) of a lowland rice soil under flooded conditions. Bulk soil sample collected from the Mandya paddy fields was used for the green house trials and the laboratory incubation studies. The organic materials were added at three rates – zero, 25.0 g carbon kg⁻¹ (2.5% C) and 50.0 g carbon kg⁻¹ dry soil (5.0% C). Results showed that tree leaf litter and rice residue at 5.0% C rate decreased instantaneous decay constant (k), there by retarded the rate of C mineralization. Carbon contents of HA increased with the rate of C added. Study of delta–log K values and C contents of humic acids revealed that greatest molecular weight of HA was in the pongamia litter treatment, followed by neem litter and rice residue. Grain and straw yields of rice crop in the pot culture study were statistically correlated to the soil quality parameters. Neem and pongamia tree litter incorporation at 2.5% C could be considered for improving soil health and crop yields of rice under flooded conditions; however, application at higher rates significantly (P ≤ 0.05) lowered total dry matter production in rice, despite favorable soil health parameters such as humic yields, microbial biomass – C content and acid phosphatase and urease activity. Among different soil health parameters, microbial quotient was found to be more sensitive indicator of decline in soil quality.     

Molybdenum status and critical limit in the soil for green gram (Vigna radiata) growing in Madurai and Sivagangai districts of Tamil Nadu,India

Abstract

The decay of rice residue was investigated after incubation periods of from 1 to 24 months at 30°C under both flooded and upland soil conditions. Tops and roots of rice plants were cut into about 10-mm length, and separately incorporated in soil which had been passed through a 0.5-mm sieve. Plant debris were fractionated physically according to their sizes and divided into five groups (>4 mm, 4-2 mm, 2-1 mm, 1-0.5 mm, and 0.5-0.25 mm).

Carbon loss from the soils amended with rice residues and decrease in the weight of total plant debris proceeded at a rapid speed in the early periods (around 4 months) and then at a slow speed in the subsequent periods under both flooded and uplana soil conditions. The distribution of the plant debris in the decomposition processes differed under flooded and upland conditions. Under flooded conditions, 2–4 mm-sized plant debris were retained for a long period with slow transformation into the smaller fractions. In contrast, under upland conditions, change of plant debris from large to small size fractions proceeded gradually. This continuous change could be attributed to the high decomposing activities of fungi under upland conditions.     

Impact of coleopteran targeting toxin (Cry3Bb1) of Bt corn on microbially mediated decomposition

Genetically engineered corn expressing crystalline proteins for insect control and encoded by genes derived from soil bacterium Bacillus thuringiensis (Bt) are widely adopted in the United States. Among the seven different events of Bt corn available commercially, YieldGard^® Rootworm (MON863) expresses a variant of the cry3Bb1 protein in the root tissue to control corn rootworm larvae. Although numerous laboratory and field studies show no unexpected ecological risks at the insect community-level above-ground, few studies have addressed the possible impact of cry proteins released from living or decaying roots of Bt corn on soil microbial communities. Here, we test the hypothesis that coleopteran-active Bt corn does not affect nontarget ecological processes, such as decomposition or the function of the associated saprophytic microbial community. Experimental treatments were: (1) a Bt hybrid; (2) a non-Bt, isogenic hybrid treated with a conventional soil insecticide; and (3) a non-Bt, isogenic hybrid without insecticide. Soil and root samples were collected at various times throughout 2 years from experimental plots to estimate microbial community function by quantifying activity of extracellular enzymes on 10 substrates. Decomposition was measured as mass loss by root decay in litter bags. Bt corn (MON863) exuding the cry3Bb1 toxin does not appear to have adverse effects on saprophytic microbial communities of soil and decaying roots or on decomposition. The addition of the soil insecticide had greater effects on microbial function in soil and decaying roots than Bt corn. Our results are similar to those found previously for the cry3Bb1 protein that showed no adverse effects on microbial community composition in controlled and natural environments. This field study is one of the first to report the use of extracellular enzyme assays to examine the effect of transgenic crops on the functional activity of microbes in soil and decaying roots.     

Flooding effects on soil phenol oxidase activity and phenol release during rice straw decomposition

Phenol oxidase (Pox) plays a key role in soil C cycle and its presence may affect soil C mineralization during crop residue decomposition. To examine soil dynamics and relationships between Pox, phenols, Fe²⁺, and C mineralization, we designed a 53‐d laboratory experiment conducted with and without rice straw addition and under non‐flooded and flooded conditions. The results demonstrate that rice straw can indeed decompose faster under flooded conditions. The addition of rice straw significantly increased soil Pox activity (up to 15‐fold), but only under flooded conditions. Rice straw application increased alkali extractable phenol (AEP) concentration by 129% at day 4. However, flooded conditions reduced soil AEP by 61% and 49% at day 53 with and without rice straw application, respectively. Phenol oxidase activity was positively correlated with dissolved organic C and Fe²⁺, while negatively related to AEP, which itself was positively correlated with C mineralization (i.e., CO₂ emission rates). Also, all relationships between soil Pox, AEP, Fe²⁺, and C were stronger under flooded conditions. We therefore conclude that flooded conditions in paddy soil may promote straw decomposition as a result of the stimulation of Pox activity and phenol decomposition.     

Decomposition of rice residue in tropical soils

Nitrogen mineralization and immobilization of rice residue in Maahas clay soil under lowland and upland conditions were investigated by using ¹⁵N-labelled rice straw. The mineralization of residue-nitrogen was taking place even though the net mineralization was depressed by incorporation of rice residue.

There were some significant differences in the pattern of nitrogen transformation between lowland and upland soil conditions. The nitrogen transformation measured by mineralization of soil nitrogen and rice-residue nitrogen and the nitrogen immobilization into rice residue were more active under lowland conditions than under upland conditions, during the earlier period of residue decomposition.     

Enhancing maize grain yield under salt-affected field conditions using salt-resistant maize hybrids and higher planting density

Background

In arid and semiarid countries, grain yield of maize is increasingly impaired by soil salinity. Beside soil amelioration, the development of salt-resistant cultivars is a possibility to enhance crop yield on salt-affected soils.

Aims

This study aimed at testing yield performance in the field of salt-resistant maize hybrids on a salt-affected soil. In addition, planting density was optimized under the saline conditions.

Methods

Four salt-resistant maize hybrids (Zea mays L. SR-05, SR-12, SR-15, and SR-16) were grown under control (EC = 2.0–2.5 dS m⁻¹) and saline (EC = 10.0–12.0 dS m⁻¹) field conditions and compared to the salt-sensitive maize cv. Pioneer-3906. Planting density (5, 8, or 11 plants m⁻²) was optimized for saline soil conditions for SR-12 and the local hybrid EV-78.

Results

Yield of Pioneer-3906 was significantly reduced under salinity because of inhibited kernel setting, whereas the SR hybrids showed no decrease in grain yield. Based on grain yield, the optimum planting density was 8 plants m⁻² with no further increase with 11 plants m⁻². In contrast to SR-12, for cv. EV-78 no increase of harvest index with 8 relative to 5 plants m⁻² was observed.

Conclusions

Vegetative growth of Pioneer-3906 and the SR hybrids was decreased due to Phase-I effects but neither due to water deficiency nor ion toxicity. The experiment corroborated the salt resistance of the SR hybrids under field conditions. Under saline conditions, optimum planting density of salt-resistant cultivars may be higher than under nonsaline conditions when sufficient water supply by artificial irrigation is guaranteed.     

Effects of Bt-maize material on the life cycle of the land snail Cantareus aspersus

Insect resistant Bt-maize (MON 810) expresses active Cry1Ab endotoxin derived from Bacillus thuringiensis (Bt). Snails constitute non-target soil species potentially exposed to Bt-toxin through consumption of plant material and soil in fields where transgenic plants have been grown. We studied the effect of the Cry1Ab toxin on survival, growth and egg hatchability of the snail Cantareus aspersus. From the age of 4 to 88 weeks, snails were fed either powdered Bt-maize or non-Bt-maize and exposed to soil samples collected after harvesting either the Bt-maize or non-Bt-maize. We applied four treatments: non-Bt soil + non-Bt-maize (MM); Bt soil + Bt-maize (BB), non-Bt soil + Bt-maize (MB), Bt soil + non-Bt-maize (BM). Eggs laid by snails not exposed to Bt-toxin were also exposed to the two types of soils (Bt and non-Bt soil).At the end of growth (47 weeks of exposure), snails exposed to Bt-toxin in food and soil (BB) had a growth coefficient (GC) 25% lower than unexposed snails (MM). After the first period of reproduction (68 weeks) a significant difference remained for body mass GC between the BB and MM treatments. Differences in body mass were not significant at the end of exposure (88 weeks). For snails not previously exposed to Bt material, hatchability of eggs was similar in the soils tested. The outcome of the experiments indicates that, in growing snails, long-term exposure is needed to reveal an effect of Bt-maize. The hazard analysis of Bt-maize which we performed, based on a worst-case scenario, i.e. snails having no food choice, should now be complemented by other simple measurements, e.g. food intake, to understand the underlying mechanisms involved.     

Effects of endophytic Bacillus spp. on accumulation and distribution of iron in the shoots of lowland rice grown under iron toxic conditions

Background

The tolerance of plants against abiotic stresses can be greatly influenced by their interaction with microbes. In lowland rice (Oryza sativa) production, the iron toxicity of the soils constitutes a major constraint. Although there are tolerant cultivars, the mechanisms underlying the tolerance against excess iron are not fully understood. Even less is known about the role of microbes in the response to iron toxicity.

Aim

In the study presented here, the effects of different Bacillus isolates on the accumulation and distribution of iron within the shoots of different rice cultivars grown under iron toxicity were analyzed.

Methods

Three lowland rice cultivars with contrasting tolerance to iron toxicity (IR31785-58-1-2-3-3, Sahel 108, Suakoko 8) were inoculated with three Bacillus isolates (two B. pumilus and one B. megaterium) and, after 1 week, exposed to excess iron (1,000 ppm) for 8 days. Tolerance was evaluated by leaf symptom scoring.

Results

Bacterial inoculation mitigated leaf symptoms in the sensitive cultivar IR31785-58-1-2-3-3 despite no significant differences in shoot iron concentration between inoculated and noninoculated plants. In the tolerant excluder cultivar, Suakoko 8, leaf symptoms were exacerbated when inoculated with B. pumilus Ni9MO12. While the total shoot Fe concentration was not affected in this bacteria × cultivar combination, the distribution of iron within the shoot was clearly disturbed. Tolerance to iron toxicity of the tolerant includer cultivar, Sahel 108, was not affected by Bacillus inoculation.

Conclusion

In conclusion, our results show that Bacillus inoculation can affect the tolerance of lowland rice to iron toxicity and that the effects strongly depend on the bacteria × cultivar combination.     

Decomposition of soil organic carbon influenced by soil temperature and moisture in Andisol and Inceptisol paddy soils in a cold temperate region of Japan

Purpose

Understanding the effects of temperature and moisture on soil organic carbon (SOC) dynamics is crucial to predict the cycling of C in terrestrial ecosystems under a changing climate. For single rice cropping system, there are two contrasting phases of SOC decomposition in rice paddy soils: mineralization under aerobic conditions during the off-rice season and fermentation under anaerobic conditions during the growth season. This study aimed to investigate the effects of soil temperature and moisture on SOC decomposition under the aerobic and subsequently anaerobic conditions.

Materials and methods

Two Japanese paddy soils (Andisol and Inceptisol) were firstly incubated under four temperatures (±5, 5, 15, and 25°C) and two moisture levels (60 and 100% water-filled pore space (WFPS)) under aerobic conditions for 24 weeks. Then, these samples were incubated for 4 weeks at 30°C and under anaerobic conditions. Carbon dioxide (CO₂) and methane (CH₄) productions were measured during the two incubation stages to monitor the SOC decomposition dynamics. The temperature sensitivity of SOC was estimated by calculation of the Q₁₀ parameter.

Results and discussion

The total CO₂ production after the 24-week aerobic incubation was significantly higher in both soils for increasing soil temperature and moisture (P < 0.01). During the subsequent anaerobic incubation, total decomposed C (sum of CO₂ and CH₄ productions) was significantly lower in samples that had been aerobically incubated at higher temperatures (15 and 25°C). Moreover, CH₄ production was extremely low in all soil samples. Total decomposed C after the two incubation stages ranged from 256.8 to 1146.1 mg C kg^?1 in the Andisol and from 301.3 to 668.8 mg C kg^?1 in the Inceptisol. However, the ratios of total decomposed C to SOC ranged from 0.29 to 1.29% in the Andisol and from 2.21 to 4.91% in the Inceptisol.

Conclusions

Both aerobic and anaerobic decompositions of SOC in two paddy soils were significantly affected by soil temperature and moisture. Maintaining optimal soil temperature and medium moisture during the off-rice season might be an appropriate agricultural management to mitigate CH₄ emission in the following rice growth season. Although it is high in SOC content, Andisol has less biodegradable components compared to Inceptisol and this could be a probable reason for the distinct difference in temperature sensitivity of SOC decomposition between two paddy soils.

     

Carbon cycling in rice field ecosystems in the context of input, decomposition and translocation of organic materials and the fates of their end products (CO2 and CH4)

Rice fields are intensively managed, unique agroecosystems, where soil flooding is general performance for rice cultivation. Flooding the field results in reductive soil conditions, under which decomposition of organic materials proceeds during the period of rice cultivation. A large variety of organic materials are incorporated into rice soils according to field management. In this review, the kind and abundance of organic materials entering carbon cycling in the rice field ecosystem are evaluated first. Then, decomposition of plant residues and soil organic matter in rice fields is reviewed quantitatively. Decomposition of plant residues is shown to be the active process in carbon cycling in rice fields. Rice releases photosynthates into the rhizosphere (rhizodeposition), and they follow a different avenue of decomposition in soil from that of plant residues. Incorporation of rhizodeposition into microbial biomass and soil organic matter during the period of rice cultivation, and their fates after harvesting are evaluated quantitatively from ¹³C pulse labeled experiments. Percolating water transports inorganic and organic carbon from the plow layer to the subsoil layer. The amounts of their transport and accumulation in the subsoil layer are evaluated in relation to the amounts of soil organic C in the plow layer. Not only CO₂ but also CH₄ are produced in the decomposition process of organic materials in flooded rice fields. CH₄ evolution from rice fields is of global concern from the viewpoint of global warming. Origins of CH₄ evolved from rice fields are estimated first, followed by the fates of CH₄ in rice field ecosystems. Rhizodeposition is shown to be the main origin of CH₄ evolved from rice fields. Evolution to the atmosphere is not the sole pathway of CH₄ produced in rice fields. The amounts of CH₄ retained in soil, percolated to the subsoil layer and decomposed in soil are evaluated in the context of the amounts of CH₄ efflux. Thus, this review focuses on carbon cycling in the rice field ecosystem from the viewpoints of input, decomposition, and translocation of organic materials and the fates of their end products (CO₂ and CH₄).