## 4 Discussion
### Effects of Chronic Nitrogen Enrichment on Soil Biochemical Properties  
Nitrogen enrichment improved the soil biochemical properties, which is in line with previous studies [46; 47]. Soil microbial biomass (MBC and MBN) increased with an increasing N rate, which concurs with [PERSON] [48]. The high-N and the medium-N treatments had the highest microbial biomass, probably due to their high available N concentration (Table 3), which enhanced the degradation of SOM, thereby stimulating microbial biomass concentration in the soil. However, our result contradicts previous studies that reported that N enrichment reduces microbial biomass in soils [45; 49; 50; 51; 52]. The discrepancy between our study and previous studies may be attributed to variations in soil pH [53], experimental duration and biome type [49], and nutrient availability [54]. The previous studies were conducted in forest and grassland ecosystems, while the present study was conducted in a cropland ecosystem; nitrogen enrichment decreases microbial biomass in forest and grassland ecosystems, while it increases microbial biomass in cropland and desert ecosystems [49]. Cropland and desert ecosystems are generally limited by nitrogen; therefore, nitrogen enrichment increases soil N availability and promotes microbial growth [49]. Soil DOC plays a key role in the formation of SOM and the movement of nutrients within ecosystems [55]. Nitrogen enrichment decreases soil DOC concentration due to the chemical stabilization of organic carbon [56] and the stimulation of the microbial mineralization rate, which increases DOC consumption [57]. Results of the current study, however, revealed that nitrogen enrichment stimulated soil DOC concentration, which is consistent with [PERSON] et al. [58] and [PERSON] et al. [52]; this could be attributed to the nutrient status of the soil [59; 60]. [PERSON] et al. [60] pointed out that soil DOC significantly correlated with SOC and total N. Soil DOC increased with an increasing N rate (Figure 4b), probably due to aboveground plant biomass, which was returned to the soil annually. Increasing N rate significantly increased aboveground plant biomass (Figure 5a), which in turn stimulated DOC concentration in the soil. This finding corresponds with previous studies that reported that soil DOC increases as N rate increase due to litter production [61; 62]. Nitrogen enrichment increased soil DON, which agrees with [PERSON] et al. [63]. The soil DON content increased with an increasing N rate, probably due to the effect of nitrogen enrichment on soil pH. [PERSON] and [PERSON] [59] documented that soil pH negatively correlated with soil DON concentration. The MBC/MBN and DOC/DON ratios decreased with an increasing N rate (Figures 3d and 4d), since the increase in DON and MBN along the N gradient was higher than that of DOC and MBC contents, respectively. The nitrate concentration in the soil was higher than that of ammonium throughout the experimental period, as nitrate is the main form of available N in soils [46; 64]. High-N treatment had the highest nitrate (NO\({}_{3}\)\({}^{-}\)-N) concentration, partly due to its high rate exceeding the N requirement of the plants, which is consistent with previous studies [46; 65; 66]. However, the linear accumulation of residual soil NO\({}_{3}\)\({}^{-}\)-N content by the high-N treatment could pollute underground water, and thereby affect human health [66; 67; 68]. Nitrogen enrichment decreased soil pH over time and could be attributed to the process of nitrification, which resulted in the production and release of hydrogen ions into the soil solution [53]. [PERSON] et al. [69] pointed out that N enrichment increases the leaching of basic cations such as Ca\({}^{2+}\) and Mg\({}^{2+}\) in the charge balance of the soil solution, thereby decreasing soil pH. An increase in N rate decreased soil pH, which agrees with previous studies [47; 49; 52; 66; 70].