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【Background and Objective】 Hetao irrigation district is one of the three mega-irrigation districts relying on Yellow river water for irrigation in China. However, the reduced water supply for the district to take for the river in the past decade has promoted the region to develop water-saving irrigation. Different agronomic and engineering methods have been developed, and the aim of this paper is to elucidate how to improve groundwater usage by crops by manipulating irrigation amount in regions with different groundwater depths. 【Method】 The experiment was conducted over the district by irrigating the crops three time during their growth season, with the irrigation amount at each growth stage varying from 90 to 135 mm. Added to this is a further irrigation in autumn with an irrigation amount of 300mm. The average annual groundwater depth in the experimental regions varied from 1.8 m to 2.0 m. In each treatment, we measured the change in soil water content and groundwater table before and after the irrigation. 【Result】 Average water content in the 0~60 cm soil changed significantly following irrigation, regardless of the irrigation amount. The first two irrigations in the crop growth season replenished the water in the tillage layer, while in the third irrigation 25% of the irrigation water leached to the subsoils. Overall, the larger the irrigation amount was, the more water would leach into the groundwater. The proportion of irrigation water leached to the groundwater increased with irrigation amount when the average annual groundwater depth was the same, and decreased with the groundwater depth when the irrigation amount was the same. 【Conclusion】 For areas with annual groundwater depth of 1.8 m, irrigating 90 mm in each of the three irrigations during the crop growth season, together with a 300 mm autumn irrigation, can significantly reduce irrigation water leaching and increase groundwater usage by the crops.

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]