4.1. Non-renewable energy consumption, GWP and pre-farm management
In this study, agricultural material provision consumed far more energy and resulted in higher greenhouse gas emissions than the on-field management. Electricity generation, chemical fertilizers, in particular, N, and pesticide production were the main energy consumers in the pre-farm stage for Wuwei, and electricity, N and steel wire production were the top 3 consumers for both Zhangye and Jiayuguan. Overall, higher inputs lead to heavier environmental emissions in the case of Zhangye. was the biggest energy consumer.
About 90% of fuel for electric power generation comes from coal in China and power sector is the biggest energy consumer as well as a heavy greenhouse gas emitter. About 70% of N production depends on coal as the main fuel. Coal burning contributes 43% of total emissions (Mozell and Thach 2014). The production of 1 t of urea takes 590kg of ammonia, 900kg of coal and 193 kWh of power, which are far higher than the international level which takes Liquefied Natural Gas (LNG) as raw material (Zhao et al. 2019). The total emissions of eight types of industries, including coal-fired power, cement, steel-iron, oil refining, ethylene, synthetic ammonia, among others, were 29.63*108t, of which coal-fired power, cement and steel-iron together accounted for 91.7% in 2004 (Bai et al. 2006). Therefore, these sectors are the focus of most attention for the last decades (Yang et al. 2002).
The GWP impact shows a trend that the material provision phase emits much more than the field operation stage, as do apple production and summer corn-winter wheat rotation systems in north China (Cai et al. 2017; Peng et al. 2015). N production and electric power generation were the main polluters. And both are among the heaviest polluters in China.
Technological innovation, improving laws and regulations, etc. are the top priorities to cut down pollution. In order to reduce the pre-farm environmental impact at the farm level, changes to vineyard infrastructure have to be implemented, for instance, replacing abiotic with biotic materials in trellis establishment.
4.2. Fertilization And Irrigation
N2O, NH3 and \({\text{N}\text{O}}_{3}^{-}\) emissions are the main contributors to GWP, AP and EP impacts respectively in the on-farm stage in the 3 cases (Table 6). They are mainly caused by N application, which is identified as an environmental hotspot for GWP, AP and EP in the pre-farm stage.
Like the rest of China, Gansu has seen a significant rise in chemical fertilizer use in the past decades, leading to severe environmental degradation and inefficient agricultural production as a result (Deng 2014). With a view to reforming production mode and improving the competitive edge of the wine industry, a number of measures have been devised including controlling chemical fertilizer use and limiting yield per unit area in China (Wang et al. 2017). Although N is required in larger quantities than any other inorganic soil nutrient, grapevine’s needs are considerably less than most other crops. Annual use is estimated to range from approximately 40 to 70 kg N ha− 1 (Jackson 2008). Based on yield and pruning weight, application of no more than 56 kg N/ha is recommended to White Riesling grapevines growing in soils low in N (Spayd et al. 1993). Zhang et al. (2006) report that the fertilization rates of N, P, and potash (K) for Cabernet Sauvignon, the main cultivar of red wine grape in China, are 120–180,100–150,150-220kg ha− 1 respectively with a yield of 15 to 22.5t ha− 1, depending on varieties, soil and yield levels. Grapes are seldom deficient in P, and its over-application can interfere with potassium uptake (Jackson 2016). Obviously, Gansu’s fertilizer application rates, in particular N, are all above the international and national standards.
Most vineyards producing high-quality red wines receive little nitrogen fertilization (van Leeuwen 2022). Excessive nitrogen tends to enhance susceptibility to several fungal infections, notably bunch rot. Moderate nitrogen deficiency has been correlated with improved fruit quality in red wine making. The desired level of nitrogen supply is higher in white wine production compared with red wine production (van Leeuwen et al. 2018).
N application rates can be reduced by 30%-60% in China (Ju et al. 2009; Tian et al. 2012). Fertilization should be conducted on the basis of soil and plant testing as well as various wine production demands. Ways to improve fertilization include precise and efficient fertilization, such as plant-soil testing based and green fertilizers; policy tools, for instance, incentivizing green production, taxation and environmental labelling. In addition, integrating eco-farming and waste recycling is necessary in terms of green production. For example, biogas pit facility has proved to be an effective way to improve agricultural cycling in China (Wang et al. 2010). It can solve wastes from both viticulture and vinification processes. Growing green manure crops in the inter-rows is advised, it will benefit both the soil and biodiversity.
According to the evapotranspiration (ET) measurement of wine grape vineyards (Zhang et al. 2011), the irrigation norms of Zhangye, Wuwei, and Jiayuguan are 66%, 45% and 22% higher than the ET respectively. Irrigation consumes almost a half of the total non-renewable energy input. Excessive irrigation can increase yield but reduce fruit quality. Judicious irrigation can regulate vine vigor thus promote fruit quality (Jackson 2008). A number of novel irrigation techniques, such as regulated deficit irrigation and alternate drip irrigation have proved to be valuable in water saving as well as improving fruit quality (Reynolds et al. 2007; Du et al. 2006).
4.3. Product Carbon Footprint
This study reveals that the carbon footprint (CF) for producing 1 t of wine grapes is between 577.2 to 1120.7kg CO2e in Gansu (Tables 5,6). In contrast, the carbon footprint for producing 1 t of wine grapes ranges between 87 to 548 kg CO2e across 240 production scenarios in California (Steenwerth et al. 2015), and it is 0.846 kg CO2e for a Mediterranean table grape variety, 0.556 and 0.283kg CO2e per kg of grapes for wine grape varieties Cabernet Sauvignon and Xynisteri respectively in Cyprus. Fertilizer production and associated N2O emissions are the main contributor (Litskas et al. 2017). Vasquez-Rowe et al. (2013) reported the product carbon footprint for wine grapes ranged between 0.113 to 1.613 kgCO2e kg− 1 in Luxembourg, Italy and Spain respectively and diesel fuel (field energy use) and fertilizer use were the main contributors. The carbon footprint for 1 bottle of wine (750ml) is about 0.8 kg CO2e in Canada with fertilizer and fuel use as the main contributors (Point et al. 2012). Clearly, the low-end carbon footprint of wine grapes in Gansu is higher than that for California, Cyprus and the 3 European countries respectively. The carbon footprint of wine grapes is highly associated with N production and application, and electric power generation in Gansu.
4.4. Grape Yield And Environmental Impact
Zhangye had the greatest N and P application rates and the highest non-renewable energy consumption with the lowest grape yield and the heaviest environmental impacts. On the contrary, Wuwei had the highest yield with the greatest K usage and the lowest environmental impacts (Tables 2,5,6). Unlike other crops, whose efficiency is mainly determined by yield on the whole, the main criterion for wine grape production is the quality of the crop instead (Irimia et al. 2014). In general, the wine grape yield varies between 6–12 t yr− 1 ha− 1 depending on the grape variety, climate, soil and management (Notarnicola et al.2003). The recommended yield for Cabernet Sauvignon is usually 6.75 t ha− 1 in China (Zhang et al. 2019). Wine grape yields are often decreased intentionally to affect aroma and flavor attributes in line with wine production demands (Chapman et al.2004). While increasing yield may seem like an obvious recommendation for to mitigate the environmental impact per mass of produced grapes, it is not necessarily an option for growers seeking for wine grape flavor, aroma, and price points. In any case, attempts to achieve high yields should be consistent with optimal fruit quality as well as in conjunction with maintaining environmental health.