4.1 Effects of different fertilization strategies on GNrEs
The N fertilizer used in the field could increase the initial levels of NH4+ and NO3− during nitrification and denitrification processes (Cardoso et al. 2017), which in turn regulates the N2O emissions (Liu et al. 2012). In our experiment, CHF2 treatment exhibited significantly higher cumulative N2O emissions than CF treatment, which probably due to higher moisture content in the former (since the water content of human feces slurry as high as 95%). This suggests that an increase in soil moisture content may serve as a key driver of N2O emissions (Uchida et al. 2011; Hu et al. 2017; Feng et al. 2018), and such phenomenon has been observed in previous researches (Pezzolla et al. 2012; Cardenas et al. 2016), particularly when the soil water-filled pores exceeded 60%. Besides, NH4+ nitrification has been recognized as an important pathway for N2O emissions in N fertilizer-amended soils (Skiba et al., 1993). Indeed, the human feces slurry used in this study contained large amounts of ammoniacal nitrogen, which can provide a considerable amount of nitrification substrate. However, urea needs to be mineralized to HCO3− and NH4+ in soils before denitrification can take place. This may explain why the increase in N2O losses induced by human feces slurry application may occur more rapidly than that observed for urea fertilizers.
N fertilization is one of the key points of NO emissions (Sanchez et al. 2010). Our results showed that the types of fertilizer used exhibited no remarkable effect on NO emissions. Liu et al (2016) presumed that ammonium nitrate was the most effective for increasing soil NO emissions among different types of inorganic N fertilizers. However, those findings did not provide information about the pathway of NO production. In recent years, numerous investigations have regarded nitrification as the most predominant process for producing NO (Fan et al. 2020). In the present study, NO emission was the lowest among the three gaseous forms of Nr. NO emission could occur in upland crop systems, but was undetectable in anaerobic soils, which might be attributed to the slow diffusion or fast reduction of NO (Russow et al. 2009; Liu et al. 2016).
NH3 emission is primarily influenced by various abiotic factors, including NH4+ concentration, pH and soil texture (Tasistro et al. 2007; Schraml et al. 2016). In our study, treatment with a relatively high proportion of human feces (CHF2) demonstrated a significantly lower NH3 loss than CF, probably due to (i) higher pH of human feces slurry and (ii) increased infiltration of liquid slurry into soil. Sha and co-workers (2020) have suggested that a deep placement of fertilizers (e.g., ammonium-based fertilizer, liquid manure, etc.) can decrease NH3 emissions in alkaline soils at high air temperature. An interesting point in the study presented here was that the amounts of N2O + NO emissions in the three N-supplied treatment groups decreased in the following order: CHF2 → CHF1 → CF, whereas the trend of NH3 emissions was reversed as follows: CF → CHF1 → CHF2. Similarly, Fan et al. (2017b) found a significant positive relationship between decreased N2O + NO emissions and increased NH3 emissions in an incubation experiment of vegetable soils, given the constant amount of the substrates.
Generally, greenhouse gas intensity and global warming potential (GWP) are used to integrate the effects of greenhouse gases. In our research, GNrEs and GNrI were employed to investigate the overall impact of gaseous reactive nitrogen on the environment. Table 3 showed that N2O and NH3 were the two dominant components of GNrEs, accounting for 39.7–53.1 % and 30.1–42.8 %, respectively. These ratios are in accordance with Chen et al. (2020) who reported on one-year winter wheat/summer maize rotation system in the Northern Central Region of China. In addition, the high amounts of human feces slurry significantly increased the values of GNrEs and GNrI, indicating that the use of human feces neither decreases the emission of gaseous reactive nitrogen nor increases the yields of vegetables. This poses a substantial obstacle to the future application of human feces slurry.
4.2 Effects of human feces replacing part of nitrogen fertilizer on NUE, vegetable growth and net ecosystem economic benefits
We estimated the NUE in all the N-supplied treatments, and found that the NUE ranged from 16.8–20.3 %, which were higher than the values reported by Li et al (2017). However, the two treatments incorporating human feces slurry decreased the NUE in relation to CF treatment (Fig. 2), indicating that replacing the inorganic fertilizer with human feces slurry could not effectively increase the NUE during intensive vegetable production. This could be attributed to the higher Nr emissions in CHF1 and CHF2 groups than those in CF group. Generally, substituting compound fertilizers (usually by organic matter) markedly elevated plant N uptake and reduced gaseous N losses (Zhou et al. 2016; Zhuang et al. 2019), however, the composition of human feces slurry was different from that of standard organic fertilizer, despite containing a certain amount of organic matter, and human feces usually considering as quick-acting fertilizer rather than organic fertilizer.
In this study, human feces replacing part of nitrogen fertilizer exerted no pronounced effects on the vegetable yield in two years (four successive vegetable seasons), possibly due to the fact that the total N application rates were relatively similar in the three N-supplied treatment groups (Table 4). It has been shown that with NH4+ as the dominant nitrogen form, both grain yields and N uptake improved than the conventional fertilizer strategy (Deppe et al. 2016). However, in this study, the vegetable yields were similar between CHF1 and CHF2 treatment groups, but did not differed greatly from CF treatment group. This was probably due to the fact that we applied human feces slurry as base fertilizer before planting/seeding, suggesting that the high NH4+ concentration immediately upon application could not induce toxicity (cf. Müller et al., 2006). Indeed, the results of crop yields were in good agreement with the enhanced uptake of nutrients during the growing season (Cardenas et al. 2016).
Emissions of NOx, N2O and NH3 can threaten human health and cause severe diseases (e.g., cataract, skin cancer, etc.), mainly via particle pollution, ground-level ozone pollution, and stratospheric ozone depletion. In China, the total health damage cost related to atmospheric Nr emissions was estimated at US$19 − 62 billion in 2008, which is much larger than the damage costs in the United States and Europe. According to Gu et al. (2012), agricultural Nr emission is one of the largest sources of health-related damage, accounting for about 50% of total expenditures in China, which are consistent with the results of this study (Fig. 3). However, other reports identified NOx emissions as the largest source of health-related damage in the United States and Europe (Birch et al. 2015), which is contrary to our findings (Fig. 3), possibly because of the high N inputs during intensive vegetable production (Table 2). Moreover, leaching + runoff was an important contribution to the ecosystem N input in this study, which emphasized the importance of reducing leaching + runoff during intensive vegetable production. In addition, we used the coefficient estimated by Ti et al (2015) related to the total N applied, which might have underestimated the leaching + runoff losses in the two treatment groups with human feces slurry due to its rapid infiltration.
Profitability is often regarded as the main driver for farmers to improve their agricultural practices. Taking the yield income, input costs and environmental cost into consideration, we also quantified the net ecosystem economic benefits among the three N-supplied treatment groups (Table 4). Notably, CHF2 had the highest values of NEI and NEEI, representing a sufficient incentive for farmers to alter their N management strategies, if such information were available for them.