Environmental impacts, human health, and energy consumption of nitrogen management for maize production in subtropical region

Over-application of fertilizers could not improve crop yield and agronomic efficiency, but result in increasing nitrogen (N) surplus and adverse effects on the ecosystem sustainability. Although some previous studies have addressed one or a few environmental aspects in crop production, an integrated assessment for the effects of N fertilizer on multiple environmental impacts, and the optional steps of normalization and weighting is required. A consecutive 2-year plot-based field experiment was conducted with five N fertilizer levels (0, 90, 180, 270, and 360 kg N ha−1) in maize production at three sites in Southwest China, to evaluate the environmental performance and sustainability through joint use of life cycle assessment (LCA) and energy consumption analysis. Results demonstrated that the optimal N rate (180 kg N ha−1) showed greater potential for maintaining high yield (achieved 86% of the yield potential) and reducing the global warming (− 31%), acidification (− 47%), eutrophication (− 44%) compared to farmers’ practice, and energy depletion potentials, by reducing pollutants emission during the production and transportation of N fertilizer and Nr losses at farm stage. Optimal N treatment indirectly reduced the land use, life-cycle human toxicity, aquatic eco-toxicity, and terrestrial eco-toxicity potentials by improving grain yield and agronomic efficiency. In addition, the optimal N treatment reduced the energy consumption by enhancing the energy use efficiency (EUE) (+ 74%) and reducing non-renewable energy form (− 45%) than the farmer’s practice. This study will provide comprehensive information for both scientists and farmers involved in maize production and N management in subtropical region.


Introduction
There are increasing concerns about the effects of agricultural practices on crop production, environmental pollution, and ecosystem sustainability (Chen et al. 2014;Daniel et al. 2019). Excessive fertilizers could not improve crop yield and agronomic efficiency, but result in increasing N surplus and environmental risks, such as greenhouse gases emissions (Eric and Céline 2020), soil acidification (Hao et al. 2020), and eutrophication of groundwater (Huang et al. 2017). There is an urgent need to optimize N fertilizer management, including N rate, to improve N use efficiency (NUE), reduce environmental risks, and thus meet the dual challenges of crop productivity and ecosystem sustainability.
Over the last decade, N management and NUE studies have been undertaken for the major cropping production systems. Previous studies showed that the optimal N rates were still high in maize production regions of China (Guo et al. 2020;Zheng et al. 2017). Several studies Zheng et al. 2017) indicated a great potential to increased grain yield and NUE, and reduce environmental impacts through reducing reactive nitrogen (Nr) losses (Huang et al. 2021). However, previous studies are limited in scope as they focused on only a single aspect of the environmental risks, such as the global warming potential (GWP), eutrophication potential (EP), and acidification potential (AP) (Cui et al. 2018;Chen et al. 2014). Therefore, due to the increasing public awareness and government concern of environmental damage, integrated measurement and assessment of the environmental impacts should be analyzed by considering the various impact indicators related to ecosystem quality and human health (Ahmed et al. 2017). Some previous studies have indicated that lower environmental impacts and higher eco-efficiency could be achieved by optimizing N fertilizer rates (increasing agronomic efficiency) for maize and wheat production in USA (Kim et al. 2014), Europe (Król-Badziak et al. 2020), Mediterranean environments (Todorović et al. 2018), and North China Plain (Yang et al. 2019). The information on how N fertilizer management affects the integrated potential environmental impacts and energy consumption is still unclear in subtropical regions.
Several studies have used life cycle assessment (LCA) to investigate the environmental impacts of the application of pesticide (Willkommen et al. 2021) and fertilizer (Hasler et al. 2015) in crop production systems. Normalization and weighting of LCA can facilitate the comparison of category indicator results (Blumetto et al. 2019;Esnouf et al. 2019). Only a few LCA studies in China integrated measurement and assessment of environmental impact categories, ecosystem quality, and human health, by using the weighting steps and normalization to evaluate the different environmental indicator results (Wang et al. 2015;Liang et al. 2018).
Southwest China is located in the subtropical region and accounts for 11% of maize in China, and 2.4% of the global maize production (FAO 2019; NBSC 2019). The over-application of N fertilizer in this area has caused serious environmental problems due to high precipitation, high temperature, and low soil pH. A consecutive 2-year plotbased field experiment was conducted with five N fertilizer levels (0, 90, 180, 270, and 360 kg N ha −1 ) in maize production at three sites in Southwest China to provide an integrated assessment of agronomic efficiency and environmental performances in maize production systems. The specific objective was to quantify (i) the agronomic impact (grain yield and NUE), (ii) the environmental impacts, and (iii) the energy consumption, and thus to evaluate the food security and overall environmental sustainability for maize production in Southwest China.

Site description
The field experiment was conducted from 2018-2019 with the same design at three experimental sites in Ya'an of Sichuan Province, Qianxi of Guizhou Province, and Fuling in Chongqing in China, respectively (Fig. 1). All three sites were located in the subtropical rain-fed maize production area in Southwest China and have a subtropical humid monsoon climate. The mean air temperature and total precipitation over the maize season were showed in Fig. 2. The selected initial soil chemical and physical properties of the 0 to 30 cm soil depth at each experimental site were provided in Table S1.

Experimental design and management
This study was designed with five N rate treatments as N0 (control, CK), N90, N180, N270, N360 (farmer's practice rate), N rate were 0, 90, 180, 270, and 360 kg N ha −1 respectively. The field experiment was a randomized block design, with four replications. The size of each plot was 10 m long × 10 m wide. For the treatments that received N fertilizer, urea was spitapplied with 30% of N at pre-plant, 30% at six-leaf, and 40% at tasseling stage, respectively. Before sowing, calcium superphosphate (75 kg P 2 O 5 ha −1 ) and potassium chloride (75 kg K 2 O ha −1 ) as basal fertilizers. Details of fertilizer and pesticides application during maize growing seasons are showed in Table S2 and S3. The pre-plant basal fertilizers were band-applied at approximately 10 cm next to the plant row and 10 − 15 cm below the soil surface. The in-season application of urea was conducted by furrow.
The maize cultivars used in this study were Zhongyu 3 (Ya'an), Zhongdan 808 (Chongqing), and Hongdan 6 (Qianxi), which have adapted to the experimental stations' climate and topography conditions in Southwest China. Seeding occurred in April with a planting density of 67,500 plants ha −1 (length × width, 60 × 25 cm). Harvest occurred in late August or early September, depending on the site and year.

Sample analysis
At the harvest stage, maize in a 12-m 2 area were handharvested and threshed in each plot and grain were oven-dried at 75 °C to constant weight. At both the tasseling and harvest stages, four uniform plants were collected within each plot by clipping plants at ground level. The samples were separated into grain, leaf, and straw (stem) and then oven-dried at 75 °C to constant weight to determine the above-ground biomass. All samples were subsequently grounded to pass sieve (0.25 mm), digested using a mixture acid of H 2 SO 4 and H 2 O 2 , and then determined for N concentration using the Kjeldahl procedure (Horowitz 1995). The aboveground N uptake of maize and agronomic efficiency (AE) of N use were calculated for the maize seasons in 2018 and 2019 using the following equations (Fang et al. 2006): where N uptake is the maize plant N content at harvest, N concentration is the N concentration of the above-ground biomass (grain, leaf, and straw), and W biomass is the weight of the above-ground biomass. Y N and Y CK are maize grain yield (kg ha −1 ) in the N fertilization treatments and N0 (control, CK), respectively. (1)

Goal and scope
In this study, the objective of the LCA was to comprehensive assessment environmental impacts of N fertilization in maize production in Southwest China. One Mg of maize grains was selected as the basis (functional unit) for the analysis. The system boundary was defined as from the cradle to the farm gate, which included two subsystems as (1) the agricultural materials system (MS) and (2) the farming system (FS). The MS referred to the raw material extraction, processing and production of fertilizers and pesticides, as well as the transportation to farm in this study. The FS included soil preparation, sowing, and applications of fertilizer and pesticides. The production processes of machines and irrigation were not considered in this study due to the rare machine use and the rain-fed production system in Southwest China. The life cycle inventory per Mg maize production under different N treatments is shown in Table 1.

Life cycle inventory (LCI) analysis
For this study, the life cycle inventory compiled all consumed resources and their corresponding emissions related to the functional unit (ISO 2006a). In the MS, the LCI (Table S5) concerning the production of fertilizers, pesticide, fuel, and electricity were taken from Yue (2013), Wang et al. (2018), and Liang et al. (2009), which were widely used for local products. In the FS, the emission factors for Nr loss were derived from the peer-reviewed publications, which were restricted to studies from Southwest China, using the meta-analysis method. The N losses of direct N 2 O emissions, NH 3 volatilization, and N leaching from urea were calculated using the emission factors (Table S4). The indirect N 2 O emission was considered 1.00% of the volatilized NH3-N and 0.75% of the leached NO 3 -N (IPCC 2006). The NO x emission was calculated 10% of the total N 2 O Brentrup et al. 2004), and the phosphorus loss was considered as 1% of total phosphorus fertilizer inputs (Gaynor and Findlay 1995). The pesticide emissions to soil, water, and air were considered as 43%, 1%, and 10% of the applied pesticide mass, respectively (Van Calker et al. 2004). Using heavy metals' input-output balance calculated the losses of them (Zn, As, Cd, Cu, Pb) . The inputs of heavy metals were mainly from seeds and fertilizers, and the related data were derived from Liang et al. (2009).

Life cycle impact assessment
The impact assessment can further process and interpret the LCI data (Table S5), which involves characterization, weighting, and normalization. For this study, eight category indicators were considered: (1) renewable land resource use (LU), (2) energy depletion potential (ED), (3) global warming potential (GWP), (4) acidification potential (AP), (5) eutrophication potential (EP), (6) human toxicity potential (HTP), (7) aquatic ecotoxicity potential (AEP), and (8) terrestrial ecotoxicity potential (TEP). Characterization results were analyzed by using the factors relevant to the use and emission of each type of resource (Liang et al. 2018). The various potential environmental impacts (PEI) were calculated according to the ISO standard (ISO 2006a(ISO , 2006b) using the following equation: where PEI (j) represents the potentials for j environmental impact category; E F(j)i represents the characterization factor of per unit i resource (fertilizers and pesticides, etc.) use or emission potential in relevant to j environmental impact category; Rate (j)i represents the rate of i resource (fertilizers and pesticides, etc.) use or emission potential.
Normalization and weighting analysis can provide further interpretation of potential environmental impacts. Normalization was considered by dividing each category indicator based on the reference values (Brentrup et al. 2004;Liang et al. 2018). Normalization values of percapita environmental impact in China (including LU, ED, GWP, AP, and EP) for 2010, and those of the per-capita eco-toxicity impact in the world (including HTP, AEP, and TEP) were used due to a lack of eco-toxicity references in China, as shown in Table S6. All impact categories were aggregated to one composite environmental indicator (EI) by weighting analysis, which was determined by using the following equation: where W j represents the weighting factors of j impact category (Wang et al. 2007); and Rj represents the normalization value of j impact category.

Interpretation of results
Interpretation of results was conducted to evaluate how and to what extent optimizing N rate could affect the impact category indicators, and to identify the agricultural material inputs and management process that substantially contributed to each impact category indicator. We also assessed the impact of N fertilization on human health using the analysis of disability adjusted life years (DALYs), which were defined as the potential cumulative number of years loss connected to grain production. They were calculated using the equation where DALY i represents the DALY derived from i pollution emission; D Fi represents the endpoint human health damage factors from ReCiPe 2008 (Huijbregts et al. 2017;Goedkoop et al. 2009); Rate i represents the rate of i pollution. (3)

Energy use efficiency of maize production
The energy budget of maize production system includes the input energy consumed in various operation processes for agricultural inputs, and the output energy produced in terms of grain yield and other above-ground straw biomass (Lal et al. 2004). For this study, the input energy consumption was calculated by multiplying the input amounts of fertilizers, pesticide, seeds, human labor, and field operations (Table S2 and S3) with their respective energy conversion coefficients (Table S7). Indirect energy included energy embodied in pesticides, fertilizers, and seeds, while direct energy referred to human labor in the maize production. Renewable energy consists of seeds and human labor, while non-renewable energy included the fertilizers, pesticides. The net energy (NE), energy use efficiency (EUE), and energy productivity of the system were calculated using the equation (Chaudhary et al. 2017):

Statistical analysis
All the study data were processed using Excel 2019. A one-way ANOVA was used to test the interactive and main effects of sub-regions or years on grain yield, fertilizers rate, and environmental indexs using SAS 9.3 statistical software.
Where treatment effects were significant, means were compared by least significant difference (LSD) tests at p < 0.05.

Grain yield, above-ground N uptake, and agronomic efficiency
In both years, grain yield at all sites significantly increased with N rates up to 180 kg N ha −1 (N180 treatment). Increasing N rates over the N180 failed to further increase grain yield (Fig. 3). Grain yield was generally not different between N 180 and N 360 at all site-years, except at Ya'an site in 2018, where N360 resulted in greater grain yield than N180. The mean above-ground N uptake at all site-years was from on average of 42.1 kg ha −1 in (6) NE = Output energy − input energy (7) EUE = Output energy∕input energy (8) Energy productivity = Maize economic yield∕input energy N0 to 166 kg ha −1 in N360. The above-ground N uptake increased with N rates from N0 to the N180 treatment. Increasing N rates over the N180 did not increase aboveground N uptake further (Fig. 3). The AE decreased with increasing N rate, from an average value of 23 kg kg −1 in N90 to 13 kg kg −1 in N360. Across all site-years, the average AE at all site-years was 21.3 and 23.5 kg kg −1 for the N90 and N180 treatment, respectively, which were much higher than that for the N270 and N360 treatment.

Environmental impacts and non-renewable resource consumption
N fertilizer application directly or indirectly affected environmental impacts and non-renewable resource consumption. In 2019, as shown in Fig. 4, the GWP, AP, EP, and ED at all sites increased with the N rate from N0 to N180 treatment, and then increased substantially from N180 to N360 treatment. In 2018, the N180 treatment showed the lowest GWP (Fig. 4a) and ED (Fig. 4g) in Ya'an, and the N0 treatment showed the highest values. AP and EP in N0 treatment has low values at all siteyears, except at Ya'an site in 2018. AP and EP in N90 treatment were higher than N180 treatment at Ya'an site in 2018. In both years, the GWP significantly increased with increasing N rate, from an average value of 292 kg CO 2 -eq Mg −1 in N0 treatment to 593 kg CO 2 -eq Mg −1 in N360 treatment (Fig. 4), except for Ya'an site. The GWP in N180 treatment was 20% and 31% lower than that in N270 treatment and N360 treatment, respectively. The AP significantly increased with N rates, with values being 32% and 47% lower in N 180 than in N270 treatment and N360 treatment, respectively. The N0 treatment had the lowest EP (0.15 kg PO 4 -eq Mg −1 ), whereas the N270 and N360 (1.7 and 2.1 kg PO 4 -eq Mg −1 ) treatments had the highest ones (Fig. 4). The ED increased with N rates, with values being 29% and 43% lower in N180 than in N270 and N360, respectively.
The HTP and AEP were dominated by pesticide emissions to air and water. In contrast, the TEP was dominated by both the pesticides inputs to soil and heavy metal contaminants derived from fertilizers input. In both years, LU, HTP, AEP, and TEP were influenced indirectly by N fertilizer and decreased significantly with the increasing N rate from N0 treatment to N180 treatment, then stabilized at N180 treatment and higher N rate treatments (Fig. 5). All impact category indicators decreased with the increasing grain yield. All impact category indicators were not significantly different between the N180, N270, and N360 treatments. Overall, the N180 treatment showed the highest potential to reduce environmental costs and had the most desired environmental performances (Figs. 4 and 5).

Human health impacts
In both years, 15 environmental pollutants were compiled and expressed as the potential cumulative number of years loss based on per Mg grain production, to evaluate N fertilizer application's effects on human health ( Table 2). The N360 treatment showed the highest impact on human health, being 4.86 × 10 −1 DALY.Mg −1 . The N180 treatment showed the lowest human health burden (3.87 × 10 −1 DALY.Mg −1 ), 12% and 20% lower than the N270 and N360 treatments, respectively (Table 2). Among the environmental pollutants, N 2 O emission had the most significant contribution (62 − -73%) to human health damage, which was mainly associated with the N fertilizer production and electricity Maize grain yield was reported by applying a moisture factor of 15.5%. Means followed by the same lowercase letter are not significantly different among N treatments at p < 0.05 according to LSD. Vertical bars represent ± S.E. of the mean consumption. The NO x and SO 2 showed a comparable level of damage to human health, with a contribution of 14% and 11%, respectively. Cadmium was the most significant heavy metal with a contribution of 4% to human health damage. These four environmental pollutants contributed over 96% to the total human health burden. Among the 5 N rates, the N180 treatment had the lowest human health burden ( Table 2).

Aggregative environmental performance
The China per-capita environmental impact values in the year 2010, and the global per-capita eco-toxicity impact values in 2000 were used as references to analyze the aggregative environmental performance of N fertilization in maize production (Fig. 6). The N180 treatment showed the lowest or relatively lower impact values and the N0 treatments showed the highest values of LU, AEP, TEP, and HTP (Fig. 6a). The GWP, AP, EP, and ED increased slightly with N rate from N0 to N180 treatment, and substantially with N rate from N180 to N360 treatment. These results indicated that the LU and AEP were the most important indicators for environmental impacts, followed by the EP, AP, and AP. On the contrary, the values for ED, GWP, HTP, and TEP were quite low, with all of them being less than 0.4 person equivalent Mg −1 grain. Compared to the N360, the N180 treatment reduced the AP impact by 46%, from 0.43 to 0.23 person equivalent Mg -1 grain; and reduced the EP by 45%, from 0.55 to 0.30 person equivalent Mg -1 grain; and also reduced the ED by 42% and GWP by 31%. Overall, the normalization results in response to N rate showed the same trend as the characterization results (Figs. 4, 5, and 6).
The results indicated that the environmental indicator decreased firstly then increased with the increasing N rate (Fig. 6b). Across the five treatments, the aggregated environmental indicator was dominated by AP (11%), EP (13%), and AEP (67%) (Fig. 6b). The higher the N rate, the greater the AP, EP, and AEP contributed to the aggregated environment indicator. Among the five N treatments, the N180 had the lowest environment indicator (0.26 EcoX Mg −1 ), which was 34% and 16% less than the N0 (0.40 EcoX Mg −1 ) and N360 treatment (0.32 EcoX Mg −1 ), respectively. For the N0 treatment, the aggregated environment indicator was dominated by AEP (87%) and TEP (8.8%) (Fig. 6b). As for N360 treatment, the highest share for the environmental impact was also dominated by AEP (52%), followed by EP (21%), and AP (19%).

Energy use efficiency
In this study, we calculated the energy budget of maize production by assessing the energy flow, accounting for the input energy consumed in various operation processes for agricultural inputs, and the output energy produced in terms of grain yield and other above-ground straw biomass. The output energy and net energy (NE) significantly increased with increasing N rates until the N180 treatment. However, increasing N rates over the N180 treatment failed to further increase NE (Table 3; Table S8). Among the five N treatments, the N180 had the highest NE (106 GJ ha −1 ), which was 78.5% and 14.8% higher than the N0 (59.4 GJ ha −1 ) and N360 treatments (92.3 GJ ha −1 ), respectively. As shown in Table 3 and Table S8, EUE and energy production efficiency decreased slightly with the increment in N rates over 90 kg N ha −1 , while decreased substantially from N0 to N90 treatment. The same trend was found for indirect and nonrenewable energy (Table 4, Table S9).

Effects of N rate on maize production
It is widely acknowledged that the N fertilizer plays a vital role in increasing crop yields, while yield response decreased with increasing N rates. In this study, excessive use of N fertilizer (> 180 kg N ha −1 ) did not show any benefits on grain yield, above-ground N uptake or AE. Similarly, previous studies have showed a linear-plateau relationship between the application of N fertilizer and maize grain yield (Cui et al. 2010). The mean yield with 180-360 kg N ha −1 achieved 86-90% of the yield potential of maize yield in Southwest China . The N180 treatment can maintain the same high grain yield as that with high N fertilizer (≥ 270 kg N ha −1 ), which was attributed to more N uptake by maize plant and accumulation after the post-silking stage ( Figure S1) (Srivastava et al. 2018). Thus, the N application rates currently used by the farmers' practice in maize production (> 280 kg N ha −1 ; PDNDRCC 2019) could be reduced by approximately 36% to 180 kg N ha −1 . The optimal N rate of 180 kg N ha −1 in this study was 13-21% lower than that in some previous studies (206-227 kg N ha −1 ) in the studied region (Li et al. 2020). Higher N surplus and N losses were found in N270 treatment and N360 treatment ( Figure S3) and would lead to high accumulation of soil residual N , and subsequent loss via NH 3 volatilization, N leaching, and  potential (a, b), acidification potential (c, d), eutrophication potential (e, f) and non-renewable energy depletion potential (g, h) per Mg grain production as affected by N rate treatments in year 2018-2019. N0 (control, CK), N90 (90 kg N ha −1 ), N180 (180 kg N ha −1 ), N270 (270 kg N ha −1 ), and N360 (360 kg N ha −1 , farmer's practice rate). Means followed by the same lowercase letter are not significantly different among N treatments at p < 0.05 according to LSD. Vertical bars represent ± S.E. of the mean ◂ greenhouse-gas emissions such as N 2 O (Wang et al. 2020;Sebilo et al. 2013).

Environmental consequences and energy consumption
To our knowledge, this study is the first to integrally assess the environmental impacts, human health impacts and sustainability for N fertilization in maize production in Southwest China. Eight impact category indicators were assessed in order to evaluate the multi-indicator environmental impacts of five different N rates by employing the LCA approach, which provided more scientific support for decision-making of N management in maize production (Brentrup et al. 2004). Although comparisons between different LCA studies in different regions are not straightforward (Wang et al. 2015;Leach et al. 2012), there were many interesting results from this study. Compared with maize in other regions of China, the GWP and other environmental impacts (e.g., EP, AP) in Southwest China are much higher (Cui et al. 2018). The environmental impacts (e.g., GWP, EP, and AP) are significantly higher than previous research for maize production systems in India (Mohanty et al. 2017) and USA (Kim et al. 2014). The particular challenges for China's agricultural systems are greater than those in other countries with larger land, water endowments, and smaller populations. The LU for maize production in this study was lower than that in the USA and Denmark (Liang et al. 2018). In contrast, the GWP, AP, and EP of maize production in this study was higher than those in other regions of the world (Grassini and Cassman 2012). Intensive farmland is favorable for the field works by agricultural machineries, low inputs and high yield in developed countries. However, high N inputs and low grain yield from extensive farmland in Southwest China.

Contributing factors to environmental impacts
In this study, nitrogen fertilizer and pesticides were the most important two original factors for the environmental impacts, as seen in previous studies (Grassini et al. 2013). Grain yield was also an important factor for the environmental impacts, as all the environmental impacts were assessed based on per Mg maize production. The lack of farmers' knowledge and information in crop, fertilizer management might be a major contributor to the high N surplus in Southwest China (Chen et al. 2014). As a result, high N inputs and low grain yield from extensive farming in Southwest China. Although the grain yield increased at high N rate (farmers' practice, 360 kg N ha −1 ) due to the greater N uptake, the N surplus also increased. Meanwhile, the low yield of maize production at the low N application (90 kg N ha −1 ) resulted in high environmental impacts per Mg production. Therefore, the "4R" principles to N management should be highly recommended for maize production in this region .
Similar to previous studies, pollutant emissions of fertilizer production, transportation and application were major contributors to various environmental impacts (Grassini et al. 2013). On the one hand, the increment of N fertilizer could increase pollutant emissions (CO 2 emission contributing to GWP, SO 2 , and NO x associated with AP and NH 4 and NH 3 emissions associated with EP) and energy consumption during N fertilizer production and transportation (Liang et al. 2009). On the other hand, at the farming stage, high N surplus ( Figure S3) increased exponentially with increasing N rate, leading to substantial nitrate accumulation in soil (Wang et al. 2020;Sebilo et al. 2013). The on-farm NH 3 volatilization increased linearly, while N 2 O emissions and N leaching increased exponentially with N surplus in southwest China ( Figure S3) (Cui et al. 2018). The N losses to the ecosystem through N 2 O, NH 3 , and NO 3¯ leachates cause environmental problems (e.g., GWP, AP, EP, AEP, and TEP) and the threat to human health (HTP). From the perspective of human health damage expressed as DALY per Mg maize production, the five main pollutants came from the application of N fertilizer at farming stage (NH 3 and N 2 O) and the production process of N fertilizer and electricity at agricultural materials stage (such as SO 2 , CO 2 , and NO x ). Reasonable fertilizer application strategy is therefore needed to reduce the pollutant emissions during N fertilizer production and application processes. In this study, the N 180 treatment reduced 20-31% of GWP, 32-47% of 45% of AP, and 10-20% of the human health damage compared to the N 270-360 treatment. Therefore, reducing Nr losses associated with excessive N fertilizer application is the key measure to mitigate the negative environmental and health Table 3 The 2-year mean value of the output energy, net energy, energy use efficiency and energy production efficiency for maize production system as affected by N rate treatments in the subtropical region of China impacts (Bodirskyet et al. 2014;Willkommen et al. 2021;Drzeżdżon et al. 2018). Environmental factors are important in driving N losses and low yield in tropical/subtropical environments (Figure S2) (Alam et al. 2017). On the one hand, the meteorological factors can influence N losses through its direct impact on N 2 O direct, NH 3 volatilization, and N leaching and indirect impact on maize yield (Han et al. 2020). The high precipitation and temperature in maize season (Fig. 2), and the low growing degree-days (GDD)  in Southwest China, could lead to high N losses to the ecosystem through N 2 O, NH 3 , and NO 3¯ leachates, by reducing nutrient availability and solar radiation ( Fig. 2; Figure S2) (Sebilo et al. 2013). Among these, high precipitation was a key driving factor for N leaching over the whole maize growing season (Yao et al. 2021). Excessive fertilizer N input can result in high accumulation of soil residual nitrate, which can further move to the deeper soil layers due to the nitrate's high mobility and the downward water fluxes with high precipitation (Wang et al. 2020;Sebilo et al. 2013). The experimental soil pH was low (pH, 5.3-5.8) (Table S1), could decrease denitrification but increase the risk of N loss through leaching (Wang et al. 2020). Such risk could likely be even more serious for the experimental area as water holding capacity of soils was low (i.e., Luvic Xerosols, Ail-perudic Cambosols and Typic Purpli-Udic Cambosols) (Table S1).

Potential limitations of this study
In this experimental area, although optimized N application showed great potential to reduce the environmental impacts, there are still some effects of stress on maize production because of regional climate, soil characteristics, and N management practices remain (Snyder et al. 2009;Chien et al. 2009). At the arable farming process, it is hard to advance the accuracy and completeness of the LCA results in Southwest China, due to the complexity and variety of ecological conditions, with high temperature, high and uneven precipitation, and variation in soil properties ( Fig. 2; Table S1). In this study, the environmental impacts were not direct measurements under the actual field experiments, but based on estimations through site-specific equations for Southwest China. The LCA results were highly dependent on the accuracy of the system boundary and LCI. Measurements in factor-controlled field experiments over multiple site-years are needed to better assess environmental pollutants in maize production in Southwest China. We should consider previous studies from Southwest China for important parameters that are missing ). Further studies should analyze the long-term trend of environmental pollutants with optimized N management in maize production system under various ecological conditions in Southwest China. Despite these limitations, this study provides meaningful findings about the agronomic, environmental, and ecosystem sustainability in maize production system.

Conclusion
An assessment of a 2-year field study has integrally evaluated the productivity, agronomic efficiency, and the environmental impacts of N fertilization in maize production system in the subtropical region. The results indicated that, nitrogen fertilizer application at the optimal rate of 180 kg N ha −1 year −1 achieving a high grain yield, and meanwhile reduced the negative environmental impacts through increasing agronomic efficiency and reduction of N losses, compared to farmers' N practice. Low N input in the optimized N management reduces pollutants emission and energy consumption during the production and transportation of N fertilizer, on-field Nr losses at farming stage. There was a decrease in life-cycle pollutants emission that mitigated the overall human health damage. Optimized N achieved the highest net energy with low input energy among all N treatments, reduced the losses of non-renewable resource. High N application and environmental factors are important in driving environmental impacts in subtropical region. The evidence could be used in decision-making for future sustainable agricultural policies in the sub-tropical regions with environmental factors similar to that occur in Southwest China. Further studies may be required to identify maize production and the long-term trend of environmental pollutants at the optimal N rate application under various cropping systems and agro-climatic regions, especially in high temperature or high precipitation regions.

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