4.1 Environmental cost of pepper production at different altitudes
Both economic and physiological factors drive the high environmental cost of vegetable production (Cai et al., 2018). Our results demonstrated that high-altitude vegetable production resulted in higher Nr and GHG emissions than low altitude production. On an area basis, due to the higher resource input, Nr (84.4 kg N ha−1) and GHG (4,240 kg CO2-eq ha−1) emissions were higher than reported in previous studies of other low-altitude vegetable production systems (Nr emission: 43 kg N ha−1, GHG emission: 4,638–4,854 kg CO2-eq ha−1; Zarei et al., 2019). However, the emissions were lower than reported for greenhouse vegetable production (GHG emission: 7,061–19,820 kg CO2-eq ha−1; Wang et al., 2018b; He et al., 2016), due to the additional structural materials (metal, plastic, irrigation facilities, etc.) and resources (electricity, water) required in greenhouses compared to open-field production systems. Greenhouse system emissions are much higher than in grain production systems (GHG emission: 2,210–3,629 kg CO2-eq ha−1; Bernesson et al., 2006; Zhao et al., 2016) due to the higher resource inputs. On a yield basis, the N and C footprints at the HAL were also higher than for other vegetable systems, mainly due to the lower yield. For example, the average N footprint (7.6 kg N t−1) at the HAL was 348% higher than at the LAL (2.1 kg N t−1) due to the higher yield at the former level (11.1 t ha−1). This was still far below the “ox horn” pepper yield reported in the eastern plains area of China (41 t ha−1) (Wang et al., 2018b).
There was a large difference in the N and C footprints between the high and low altitudes in this study. Compared to the LAL, the N and C footprints at the HAL were 23.0% and 24.0% higher, respectively. There were several reasons for this. First, fertilizer was the main source of Nr and GHG emissions (Liang et al., 2018). Compared to the LAL, the total N and P fertilizer input at the HAL was 4.2% and 19.3% higher, respectively, which could be attributed to the higher runoff loss associated with the steeper slope (13°, Table 1) in the arable area at the HAL (Preltl et al., 2017). Second, the type of fertilizer (organic or inorganic fertilizer) had an important effect on Nr and GHG emissions during vegetable production. Compared to N from organic fertilizers, inorganic N fertilizer contributed 18.7–22.2% of the runoff per unit N input (Shan et al., 2015;). This resulted in the Nr and GHG emissions on an area basis being 4.6% and 5.3% greater at the HAL than LAL, respectively. Third, yield also exerted an important influence on the Nr and C footprints (Cui et al., 2014; Davies et al., 2020), with the yield at the HAL being 16.2% lower than at the LAL. This large variation in yield was the main driver of the higher N and C footprints at the HAL. The pepper yield was higher at the LAL than HAL. There are several explanations for this. First, climate differs by altitude (Nie et al., 2011; Evan et al., 2018). The optimal temperature range for pepper cultivation is 25–31°C (Zhao et al., 2016; O'Sullivan and Bouw, 1984; Coons et al., 1989). The temperature during the growth stage at the LAL (22.7℃) in this study was more suitable than at the HAL (18.6℃). Soil properties also play an important role in pepper production (Vincent, 2012; Zhao et al., 2016). The optimum soil pH range for pepper production is 6.2–8.5 (Chen, 2010), so the pH (5.3 ± 0.96) at the HAL may have had a more adverse impact on pepper yield than the higher pH at the LAL (6.1 ± 1.3). Third, the production conditions affected the yields at the two altitudes. The HAL had a poor road system and uneven vegetable fields, both of which are very important with respect to the accessibility of yield-improving technology and agricultural machinery. For example, the poor road system at the HAL limited the application of agricultural machinery and organic fertilizer. Compared to the LAL, the diesel consumed by agricultural machinery and organic C applied at the HAL were 66.6% and 48.1% lower, respectively (Table2.).
The NrNEEB and GHGNEEB at the LAL were 32.6% and 33.8% lower than at the HAL, mainly due to the lower yield and net revenue at the HAL. At the HAL, farmers applied excessive resource inputs to overcome the N and P runoff losses and obtain high pepper yields. Their lack of nutrient management expertise resulted in large resource input costs. The higher cost combined with lower total revenue due to the lower yield resulted in a lower net revenue at the HAL.
Overall, pepper production at high altitudes resulted in higher Nr and GHG emissions, larger N and C footprints, and lower yields, net profit, and NEEB (Figure. 5). This clearly showed that the expansion of pepper cultivation to higher altitudes was associated with greater environmental costs and relatively smaller economic benefits.
4.2 Potential for mitigating environmental costs
The N and C footprints of group 1 were 7.9%, 24.9%, 48.5%, and 16.9%, and 9.2%, 23.7%, 48.3% and 18.3%, lower than in groups 2–4 and the average at the HAL, respectively (Figure. 3). Closure of yield gaps offers great potential for mitigating the environmental cost of agricultural systems (Cui et al., 2014). Although the Nr and GHG emissions at the HAL were largest in group 1 in this study, mainly due to the higher fertilizer input than in the other groups, the increase in the rate of N fertilizer application was 17% lower than the rate of yield increase in group 1. This resulted in a smaller N and C footprint at the HAL in group 1 compared to the other groups and the average for all farmers. Many studies (e.g., Chen et al., 2014; Wang et al., 2020) have indicated that the yield gap can be closed by improving crop and nutrient management by adopting best practice farming methods. However, the factors responsible for the yield gap differ among regions and crops, due to large variations in climate, soil conditions, and management practices. In this study, the higher yield in group 1 at the HAL and LAL could be explained by several factors. The top-dress fertilizer rate, especially the N and K fertilizer rate, was the major factor influencing vegetable yield. Our results revealed positive relationships between the N fertilizer top-dressing rate and yield. An increase in the N and K fertilizer application rates promoted rapid crop growth and fruit formation at the later growth stage (Dhillon et al., 2020; Zhong et al., 2006). There were no significant relationships between the total N, P, and K fertilizer application rates and yield (Figure. 4). Second regarding organic management, organic fertilizer application can also improve soil quality, reduce soil Nr loss, and promote crop growth (Zhang et al., 2016). Due to long-term and excessive inorganic fertilizer inputs, several soil problems (e.g., acidification, soil-borne disease, and decreased quantity and stability of structural aggregates) are very common in the survey area. Our results revealed a positive relationship between the organic C input rate and yield. Additional organic C inputs improve soil structure, stimulate soil microbial activity, and improve the yield of vegetable crops. Third, regarding management, an appropriate planting density will be able to fully exploit the light conditions and soil nutrients. Our results indicated that an excessive planting density would have a negative influence on pepper yield (Figure. 4). This might result in undue competition between plants for limited light and heat, thereby reducing carbohydrate production (Yan et al., 2015). In addition, there was room for further optimization of group 1. The N and P fertilizer input was much higher than the vegetable demand and recommended fertilization rate (147–200 kg N ha−1, 75–90 kg P2O5 ha−1) (Chongqing Agro-Tech Extension Station, 2020), which could be optimized with in-season root-zone nutrient management to match the nutrient supply (Ren et al., 2010). Furthermore, the traditional N fertilizer (e.g., urea, ammonium carbonate) applied in the survey area could be substituted for an N fertilizer with a nitrification inhibitor (Abalos et al., 2014).
In the future, it will be important to consider the environmental cost of resource inputs, and their mitigation potential in the context of vegetable production at high altitudes. For the first time, this study systemically compared the environmental cost of resource inputs for pepper production between high and low altitudes. Our results indicated that pepper production at high altitudes was more resource-intensive and led to larger N and C footprints than production at low altitudes. The results provide a reference for studies of the environmental cost of crop production in other high-altitude regions of the world. We quantified the environmental cost of pepper production at high altitudes in southwest China and identified potential mitigation measures. The results indicated that the pepper yield could be improved, while the environmental cost was greatly diminished by optimizing crop and nutrient management. These results have important implications for sustainable vegetable production in high-altitude regions.