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Rice cultivar is the most influential factor affecting methane emissions from double rice fields. A two-year field experiment was conducted at Huizhou, Guangdong province, South China, to identify from among nine cultivars those cultivars with high-yield potential and lower yield-scaled methane emissions (YSMEs). Methane emissions were measured using the static chamber – gas chromatograph method. Results indicate that the cultivars Qihuazhan (QH), Yexianzhan 8 (YX8) and Yue’erzhan (YE) provide higher rice grain yield (8.69%) with lower YSME (30.27%) compared to the other six cultivars (Yexianzhan 6, Yuejingsimiao, Hefengzhan, Huangsizhan, Huangruanzhan and Huangxiuzhan) (p < 0.05). In particular, QH has the highest yield potential (6777 kg ha−1) and lowest methane emission intensity (0.36 kg kg−1yield) capacity. Methane emissions from the double rice field was found to be significantly (p < 0.05) and positively correlated with tiller number, culm biomass and soil organic matter, dissolved soil organic carbon and total carbon content, but negatively correlated (p < 0.05) with rice harvest index (HI), and root and panicle biomass, suggesting that organic source strength provides the substrate of methane production while the oxidation potential in the rhizosphere and the methane transport capacity of rice roots and culm dominate the emissions of methane from soil to the atmosphere. Multivariate decision regression tree (DRT) analysis showed a significant class difference between QH, YX8 and YE with the other six cultivars. These three cultivars are suitable for promotion of low carbon agriculture in South China. DRT analysis also successfully illustrated a potential way to identify rice varieties for low YSME by decisive parameters of tiller number (<15), HI (>0.43) and nitrogen assimilation of leaves (<40). These findings suggest that optimization of rice cultivars may represent an effective way to address both food demand and climate change concerns by improving rice yields while simultaneously minimizing the impact of climate change per unit yield. [ABSTRACT FROM AUTHOR]

The resource utilization of organic materials in China cropland is a great challenge we are facing now. To study the ecological effects of biochar utilization, and explore the response of carbon emission intensity (GHGI) to biochar amendment for the long term in a double rice field in South China, two-year field measurements of greenhouse gases based on a static chamber-gas chromatography method were conducted. Six experimental treatments were set up, including 1) CK (famer traditional operation), 2) BC1 (biochar amendment at 5 t/hm2), 3) BC2 (biochar amendment at 10 t/hm2), 4) BC3 (biochar amendment at 20 t/hm2), 5) RS (raw rice straw residue return) and 6) RI (composted rice straw residue return). As a result, compared to CK, all of the biochar treatments inhibited the emission peaks of greenhouse gases from the double rice field. During the four rice growing seasons, the three biochar treatments decreased the 100 yr-scaled carbon dioxide emission equivalents by 27.53%, 58.65%, and 63.43%, respectively, compared with CK, RS, and RI. The biochar treatments significantly reduced methane emissions (P<0.05) at an average reduction rate of 50.84%. Among the three biochar and two rice straw residue treatments, BC3 had the lowest methane emission potential (2.75 mg/(m2 h)), while RS had the highest (10.05 mg/(m2·h)). For the nitrous oxide emissions, the average value of the three biochar treatments was lower than CK but higher than the two rice straw residue return treatments (P>0.05). Furthermore, there was no statistical significant yield increase effect found in the biochar treatments (P>0.05), compared with CK, RS, and RI, as the four-season average rice yields of the three biochar treatments were increased by 3.21%, 5.11%, and 2.29%, respectively. A significant higher yield in BC3 was observed than that in RS (P<0.05). Additionally, on a 100-year horizon, the GHGI of the three biochar treatments (average at 0.33 kg/kg) were 30.57%, 61.00%, and 64.82% lower than CK (0.48 kg/kg), RS (0.86 kg/kg), and RI (0.48 kg/kg), respectively. By the correlation matrix and principal component analysis, the biochar amendment controlled the distribution of rice yield, harvest index, soil organic matter, total carbon content, and chlorophyll content, while the rice straw residue return treatments dominated the distribution of the methane flux and GHGI. In summary, based on the present study, the application of the regression tree analysis offered a quantitative alternative to decision-making in analyzing the effects of biochar amendment on ecological environment. The results of this study indicated that the biochar amendment in soils has the potential to enhance soil carbon sequestration, increase rice yield, and mitigate the carbon emission intensity and the impact of climate change by optimization of the field managements and biochar return in an appropriate amount. This research could be used as a successful case study of scientific utilization of crop straw residue in China's agriculture. [ABSTRACT FROM AUTHOR]

Greenhouse gas intensity (GHGI) (methane and nitrous oxide) of double rice paddy was controlled by various field managements, tillage, and rice straw return to the field. On one hand, the choice of the tillage mode affects the gas permeability of soil during the crop-growing stage. On the other, the incorporation of rice straw can be a critical way to amend the loss of soil carbon. Both of the two practices have their advantages and disadvantages on rice production and environmental effects. In fact, no tillage causes less methane emissions from double rice fields than the field with conventional tillage. Nevertheless, rice straw return to the field increases tremendous methane production and emission of organic carbon. In spite of its promotion on rice productivity and sequestration of soil organic carbon pool, this practice is still under discussion .Therefore, multi-management mode should be considered simultaneously when regional or national policy is made to reduce GHGI from agriculture. To date, the study of the interaction effect of tillage and the rice residue return on GHGI from double rice fields is lacking. Most of the previous studies focused only on one field management and its influence mechanism. Consequently, we conducted a measurement for 4 rice growing seasons on GHGI of a double rice field in Hunan, China to investigate the interaction effect of tillage and rice straw return on both the social-economical and environmental benefits. The improved static chamber-GC (gas chromatography) method was used to monitor the greenhouse gas emissions from the rice paddy. Major improvements made on the chamber include the adjustable sampling tube and the pores in the base. The former was designed to change with the growth of the rice plant and the latter favored the communication of soil water between the inside and outside of the base. Both of them enhanced the accuracy of the field measurement. Four field practices were included in this research, viz. CWS (Conventional tillage, without straw residue return), NWS (No till, without straw residue return), HN (High stubble residue retain, no till), and HC (High stubble residue retain, conventional tillage). Results showed that there is a significant interaction effect of tillage and rice residue return on methane emissions (P<0.05) instead of nitrous oxide emissions from the double rice field. The interaction effect has pronounced seasonal and inter-annual variation. By the seasonal average value, the sequence of methane flux of each treatment is the following: HC>HN>CWS>NWS (HC significantly greater than HN while no significant difference between HN and CWS). Similarly, the amount of rice grain yield of each treatment of the 4 seasons is the following: CWS>HN>HC>NWS (no significant difference between HN and CWS). For the GHGI, the following order is disclosed: HC>CWS>HN>NWS (HN less than HC and CWS significantly, P<0.05). This indicated that compared to CWS, "no-till plus high stubble residue (HN)" offset the stimulations of conventional tillage on methane production and emission while simultaneously promoting the rice grain yield. Thus the multimanagement mode mitigated the GHGI of the double rice field significantly. Under the requirement of organic tillage and the sequestration of agricultural soil carbon pool, HN mode should be considered of great importance. This study can provide scientific support on the measurement choice of greenhouse gases effect reduction from the major rice production region of China. [ABSTRACT FROM AUTHOR]

• Three isotopic methodologies were used to quantify the biogenic CH 4 production pathway. • AM processes predominant the CH 4 production in the agricultural river network. • Long-term measurements provide the new understanding of atmospheric CH 4. Methane (CH 4) emitted from river networks provides a globally significant, yet poorly constrained barrier for the refinement of national greenhouse gas (GHG) inventories and the global CH 4 budget. This study employed three isotopic methodologies, i.e., a simple mixing model, apparent fractionation factor, and dual isotopes (δ13C−CH 4 and δD−CH 4) to identify the biogenic CH 4 production pathway (BMPP) in a typical agricultural river network in subtropical China. The results indicated that the contribution of acetoclastic methanogenesis (AM) to BMPP in the Tuojia River network (TRN) ranged from 85 to 99%, with the first reach (S1) supporting the most biogenic CH 4 production. Furthermore, the α C value ranged from 1.00 to 1.039 and increased spatially over the four reaches (S1 to S4), while the α D value varied from 1.393 to 1.394, with S1 having the lowest value. The dual isotopes of δ13C and δD reflected a mixed BMPP. Clearly, the isotopic composition of CH 4 corroborated the predominance of AM in the BMPP in the TRN. Additionally, there was a remarkable spatiotemporal variation in δ13C−CH 4 and δD−CH 4 , as well as α C and α D. In combination with the correlation between the two paired isotopic signatures (antipathetic and positive, p < 0.05, respectively), this suggested a change in the BMPP in the TRN. The findings of our study implied that there was great impact of agricultural production and residential activities on environmental behavior of stable isotopes. Consequently, we recommend that multiple isotopic measures, based on long-term in situ isotopologue monitoring, should be applied when the uncertainties of CH 4 emissions from river networks are addressed. [ABSTRACT FROM AUTHOR]

River networks in subtropical agricultural hilly region become an inconvenient greenhouse gas (GHG, methane and nitrous oxide) source because of the influence of human activities, which has caused large uncertainties for refinement of national GHG inventories and their global budget. Based on field monitoring experiments at high temporal resolution, we employed regression tree and importance analysis to identify quantitatively factors that influence the diffusive flux of GHGs to provide a scientific basis for reducing GHG emissions and controlling regional carbon and nitrogen losses. The results indicate that significant spatiotemporal variation of methane (CH 4) nitrous oxide (N 2 O) diffusion occurs in all the four reaches (W1, W2, W3 and W4) of Tuojia river networks. Among them, W1 contributed lowest CH 4 (22.55 μg C m−2 h−1) and N 2 O (5.00 μg N m−2 h−1) diffusive flux than the other three (P < 0.05), while W4 offered highest CH 4 (166.15 μg C m−2 h−1) and N 2 O (30.47 μg N m−2 h−1) diffusive flux but with no statistically significant difference between W2 and W3 due to homogeneous extraneous nutrition loading into the two reaches. W4 also contributed largest cumulative flux of CH 4 (14.55 kg C ha−1 yr−1) and N 2 O (2.69 kg N ha−1 yr−1) in Tuojia River networks (P < 0.05). Furthermore, the regression tree and importance analysis indicate that, in the anaerobic environment, dissolved oxygen saturation controlled the production and diffusion for both CH 4 and N 2 O. The findings of this investigation highlighted that decision support tools provide an effective pathway to enhance the GHG mitigation technology research in agroecosystems and simultaneously shed light on the global campaign on refinement of national GHG inventories as well as regional nutrient management. Unlabelled Image • Higher reach of Tuojia River contributes more GHG diffusive flux. • There is significant impact of farming and living activities on behavior of GHGs transfer. • RTA revealed saturation of dissolved oxygen as most influential factor of GHG diffusion. [ABSTRACT FROM AUTHOR]