施肥与灌溉对甘肃省苜蓿碳足迹的影响

Effects of Fertilization and Irrigation on the Carbon Footprint of Alfalfa in Gansu Province

【目的】评估甘肃省苜蓿不同种植模式的碳足迹,探明温室气体的主要排放环节。分析施肥与灌溉对甘肃省苜蓿碳足迹的影响,评估可能的减排潜力,为甘肃省苜蓿生产的可持续发展提供理论依据。【方法】应用生命周期评价理论和IPCC(2006)田间温室气体计算方法,建立苜蓿碳足迹评估方法。通过调研甘肃省苜蓿主产区陇东、陇中和河西地区10个县区的苜蓿种植农户和农场,收集苜蓿的产量、化肥、灌溉等生产数据。根据施肥和灌溉水平及灌溉水源将甘肃省苜蓿种植模式分为4种,使用建立的苜蓿碳足迹评估方法和甘肃省苜蓿生产投入/产出数据,分析4种种植模式的碳足迹构成特点、施氮水平与灌溉对苜蓿干物质产量和碳足迹的影响特征。使用情景分析方法揭示氮肥减施、改进氮肥生产工艺、无机有机肥混施、滴灌和喷灌技术降低苜蓿种植过程温室气体排放的潜力。【结果】甘肃省苜蓿4种种植模式的碳足迹(以CO_2 eq计)从小到大依次为0.02(不施肥不灌溉模式,NFNI)、0.19(施肥不灌溉模式,SFNI)、0.22(施肥+河水灌溉模式,SFRI)、0.64(施肥+井灌模式,SFWI)kg CO2 eq·kg^(-1)苜蓿干物质(DM)。SFNI模式除与SFRI模式之间碳足迹差异不显著外,与其他种植模式之间的差异均显著。不同种植模式之间的碳足迹构成环节和各环节对碳足迹的贡献率存在差异。NFNI模式的碳足迹主要由苜蓿残茬和农机使用两部分的排放组成;SFNI模式和SFRI模式碳足迹的主要产生环节是化肥生产和氮肥田间施用的排放,其次是农机使用;SFWI模式碳足迹的最大来源是灌溉耗电,其次为化肥生产和氮肥田间施用的排放。通过氮肥减施、无机有机肥混施、降低氮肥生产过程中排放可使甘肃省SFNI、SFRI和SFWI模式的苜蓿碳足迹分别降低10.0%—18.0%、-3.0%—8.0%和1.8%—5.8%。如果不考虑节水管材生产的额外温室气体排放,节水灌溉(喷灌和滴灌)可减少SFWI模式苜蓿碳足迹的12.7%—38.5%。【结论】甘肃省4种苜蓿种植模式的产量和碳足迹均存在差异,高投入高产出的SFWI模式的苜蓿产量最高,但碳足迹也显著高于其他模式。除NFNI模式外,其他3种模式均存在过量施肥现象。减施氮肥和降低氮肥生产过程中的温室气体排放均可降低甘肃省苜蓿生产的碳足迹;无机有机肥混施可以降低SFNI和SFRI模式的碳足迹,但同等施氮水平下,无机有机肥混施短期内会降低产量。因此,在保证一定产量同时降低温室气体排放量的目标下,有机肥和化肥混施的最佳比例及实际减排潜力仍需通过田间的长期试验进一步验证;节水灌溉是SFWI模式的主要减排措施,但节水灌溉所能带来的综合减排潜力仍需针对具体区域的田间节水试验和额外耗材带来的温室气体排放进一步评估。

【Objective】The objective of the study is to evaluate the carbon footprint（CF） of alfalfa ,production in different cropping patterns in Gansu Province, clarify the main process emitting greenhouse gases（GHG）, analyze the effects of fertilization and irrigation on the CF, and investigate the potential measures for decreasing CF of alfalfa production in this province. 【Method】Life cycle assessment methodology combined with IPCC（2006） guide for calculating greenhouse gases emission in the field were used to quantify the CF of alfalfa production. Data were collected from farm survey in major regions of alfalfa cultivation in Gansu Province which covered 10 counties in the districts of Longdong, Longzhong and Hexi. Cropping pattern of alfalfa production in Gansu Province was classified into four types according to the level of nitrogen application and irrigation, and the irrigation water source. The CF composition characteristics of four cropping patterns and the effect of nitrogen application rate and irrigation on productivity and CF of alfalfa were analyzed based on CF calculation model and input and output data of alfalfa production. Scenario analysis was used to estimate the GHG abatement potential through strategies such as decreasing chemical fertilizer application, improving fertilizer production technique, combining chemical fertilizer and organic fertilizer, and utilizing sprinkle and dripping irrigation. 【Result】CF of alfalfa production was 0.02, 0.19, 0.22 and 0.64 kg CO2 eq·kg-1 DM under four cropping patterns including NFNI（non-fertilization and non-irrigation）, SFNI（spreading fertilization and non-irrigation）, SFRI（spreading fertilization and river irrigation） and SFWI（spreading fertilization and well irrigation）, respectively. The CF of alfalfa production in SFNI cropping pattern was significantly different from other cropping patterns except for SFRI. The main contributors of CF and their proportion to the total GHG emissions varied among different cropping patterns. For NFNI pattern, CF was mainly composed of GHG emissions from crop residues and the use of agricultural machinery. For SFNI and SFRI patterns, the major contributors of CF were chemical fertilizer production and nitrogen fertilizer application, followed by the use of agricultural machinery. The greatest GHG emissions source for SFWI pattern was electricity for irrigation, followed by chemical fertilizer production and nitrogen fertilizer application. Reducing the amount of applied nitrogen, combining chemical fertilizer with manure, and decreasing the GHG emission from nitrogen fertilizer production would decrease CF by 10.0%-18.0%,-3.0%-8.0% and 1.8%-5.8% in SFNI, SFRI and SFWI patterns, respectively. The water-saving irrigation（sprinkler irrigation and drip irrigation） would decrease the CF by 12.7%-38.5%.【Conclusion】 The alfalfa production and CF of four cropping patterns in Gansu Province are different. The SFWI pattern got the highest production with high input, while its CF was significantly higher than that of other patterns. With the exception of NFNI pattern, excessive fertilizer was used in other three patterns. Reducing the amount of applied N fertilizer and the GHG emission from N fertilizer production will decrease CF of alfalfa production in Gansu Province. The combination of chemical fertilizer and manure will decrease CF in SFNI and SFRI patterns, but also the yield of alfalfa hay. So, the optimal ratio of chemical fertilizer to manure should be investigated further. The water-saving irrigation will be the major mitigation measure in SFWI pattern, while the integrated abatement potential of GHG emission should be evaluated including emissions from the production of pipes used in sprinkler and drip irrigation.