| Global warming is mainly caused by increasing concentrations of greenhouse gases(GHGs) in the atmosphere. In the past few decades, emissions of GHGs such as CH4 and CO2 has increased and highest record has been recorded in the recent years. Agricultural soils are potent source of GHGs emission into atmosphere. Organic matter decomposition and microbial diversity and community structure in soils are have great influence on GHGs emissions. Therefore, we conducted several experiments to investigate CO2 and CH4 emissions under various experimental conditions.Soils were sampled from cultivated paddy ?elds from the cities of Qianjiang and Xianning, Hubei Province. A rapeseed(Brassica napus)-rice crop rotation(QR) and a rice–fallow/?ooded rotation(QF) were sampled from Qianjiang, and the same rotations were sampled(XR and XF) from Xianning. Influences of different inputs of carbon sources and moisture conditions on dissolved organic carbon(DOC), redox potential(Eh), community diversities and composition structures of soil microbe and iron-reducing bacteria(FeRB), CH4 and CO2 emissions were studied. The correlations between the soil DOC, CH4 and CO2 fluxes regulated by Eh, carbon application and dissimilatory iron reduction on soil organic carbon mineralization were also analyzed. Furthermore, molecular techniques were applied to analyze the diversities and community structures of the soil microbe and FeRB to unserstand the differences of iron oxide as well as effects on soil organic carbon mineralization. The results of this research are presented as follows:1. In soil incorporated with rice straw and rice straw + urea under flooded conditions, the cumulative CH4 fluxes from soil XR and QF were 969.53~1692.05 and 1739.36~1855.55 mg C kg-1, respectively, while under non-flooded conditions(Water-filled pore space 80%) CH4 emissions were much lower(125.60~192.48 and 420.80~448.80 μg C kg-1) than the flooded conditions. Cumulative CO2 fluxes of XR and QF were 2709.82~2885.53 and 2036.07~2133.82 mg C kg-1 in rice straw and rice straw + urea, respectively, which were also much higher than the same treatments with non-flooded conditions(1729.16~1676.51 and 1628.80~1675.05 mg C kg-1). A significant negative correlation(p<0.05) was observed between Eh and CH4 emission rate under different moisture conditions.2. Glucose addition significantly(p<0.05) stimulated CH4 and CO2 fluxes in QR and QF soils, but had no significant effect on XR and XF soils. Cumulative CH4 ?uxes in the glucose-treated soils were 5.31, 35.26, 13.92 and 27.58 mg C kg-1 for QR, QF, XR and XF, respectively, while for the same treatments and soils, cumulative CO2 fluxes were 594.33, 620.49, 549.42 and 792.46 mg C kg-1, respectively. Within the first 20 days of the experiment, the ratio of Fe(II) /(Fe(II) + Fe(III)) and CH4 emission rate positively correlated(p<0.05), and the Fe(II) production rate was positively correlated with the CO2 flux during the whole incubation period.3. Anthraquinone-2,6-disulfonate(AQDS) had an important impact on CH4, CO2 emissions and dissimilatory Fe reduction. Addition of AQDS(42 mg C kg-1) to the four soils increased the cumulative CH4 flux by up to 9.1% in QR soil, while decreased by up to 8.9%, 15.3% and 42.3% in QF, XR and XF soils, respectively. Among different application doses of AQDS(0, 100, 200, 400 and 800 mg C kg-1) to QF soil, only 200 mg C kg-1 of AQDS application dose increased cumulative CH4 flux in QF soil, while other doses of AQDS decreased by up to 40.2%~49.4%. However, AQDS had no effect on CO2 fluxes, but significantly increased the dissimilatory Fe reduction of soils.4. The microbial diversity was analyzed by terminal restriction fragment length polymorphism(T-RFLP) technology in soil XR and QF soils. Microbial diversity in XR soil was higher than in QF soil. These two type of soils had microbial communities mainly including anaerobic and facultative anaerobes. Study of physiological and biochemical characteristics of microbial communities are advantageious to understand CH4 and CO2 emissions under flooded and un-flooded conditions. Furthermore, Fe(III) reducing bacteria(FeRB) had large proportion(25.7%~30.8%) of the total microbial communities, which suggest that Fe-carbon and Fe-nitrogen cycles were related to FeRB.5. FeRB diversity of the four soils were analyzed by high-throughput sequencing technology. We obtained 25935~44993 DNA optimized sequences and 554~1056 operational taxonomic units(OTUs) in the four enrichment soil samples. The results showed that the relative diversity of FeRB in the order of XF> QR> QF> XR, and FeRB diversity and richness in XF were highest, and FeRB phylogenetic trees had not very close relation with the other soil samples. The analysis of environmental variables also indicated that the FeRB diversity of the four soils were influenced by soil pH, C/N and the Fe content. For the first time, we identified Phingomonas, Pandoraea and Azospira which were not classified as FeRB in previous studies.In summary, CH4 and CO2 emissions from rice soils derived from different parent materials and crop rotations showed significant differences under different incubation conditions. CH4 and CO2 emissions were influenced by soil moisture, sources of carbon application, which also regulated by parent material and crop rotations of soils. This research provides understandings of organic carbon mineralization processes in paddy soils as well as scientific basis for the implementing of carbon sequestration in paddy fields. Our results can be implemented in related regulatory measures to promote the sustainable development of regional agriculture. |
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