## 3 Results
### Effects of Different Organic Materials on Soil ATN Content  
The overall ATN changes followed the order SM \(>\) MS \(>\) FG \(>\) CK from 2018 to 2019, which was the same as that of the TN and PON content (Figure 5). Compared with October 2018, the content of ATN in April 2019 showed different trends. SM and MS increased by 8.89% and 18.89%, respectively, while FG and CK decreased by 4.08% and 33.46%, respectively. Compared with October 2019, all the treatments increased the content of ATN, which were 13.32%, 21.44%, 32.12%, and 19.79%, respectively. Compared with CK treatment, the ATN content under organic material treatments increased by 32.48\(-\)77.28% from 2018 to 2019. Among the organic material treatments, SM had the highest ATN content.  
Figure 4: Application effects of different organic materials on soil particulate organic nitrogen (PON) content at different sampling dates. Lowercase and different letters indicate statistically significant differences at \(\
u<0.05\) between treatments.  
Effects of Different Organic Materials on Soil NH\({}_{4}\)+-N, NO\({}_{3}\)\({}^{-}\)-N, and N\({}_{min}\) Content  
Compared with the original starting point (April 2018), the returning of organic materials to the field reduced the NH\({}_{4}\)+-N content in the soil, and the highest NH\({}_{4}\)+-N content was recorded under the CK treatment (Figure 6). In 2018 and 2019, the overall NH\({}_{4}\)+-N content followed the order CK \(>\) FG \(>\) SM \(>\) MS, and all the treatments followed a similar trend of firstly rising and then falling. Compared with 2018, the content of NH\({}_{4}\)+-N increased by 43.29\(\sim\)75.09% in 2019. From 2018 to 2019, the content of NH\({}_{4}\)+-N in the CK treatment was higher than SM, FG, and MS by 2.2\(-\)45.56%, 1.59\(-\)40.49%, and 26.92\(-\)57.05%, respectively.  
Compared with the original starting point (April 2018), the returning of organic materials to the field increased the content of NO\({}_{3}\)\({}^{-}\)-N in the soil, and the highest was recorded under MS treatment (Figure 7). From 2018 to 2019, the overall NO\({}_{3}\)\({}^{-}\)-N content followed the order MS \(>\) SM \(>\) FG \(>\) CK, and all the treatments followed a similar trend of firstly rising and then falling. Compared with October 2018, the overall NO\({}_{3}\)\({}^{-}\)-N content for April 2019 and October 2019 increased by 8.36\(-\)25.51% and 3.88\(-\)10.34%, respectively. Compared with the CK treatment, the NO\({}_{3}\)\({}^{-}\)-N content of the organic material treatments increased significantly by 13.05\(-\)34.29% from 2018 to 2019. In these two years, MS application resulted in the highest NO\({}_{3}\)\({}^{-}\)-N content compared with other treatments.  
Figure 5: Application effects of different organic materials on soil active organic nitrogen (ATN) content at different sampling dates. Lowercase and different letters indicate statistically significant differences at \(p<0.05\) between treatments.  
Figure 6: Application effects of different organic materials on soil NH\({}_{4}\)\({}^{+}\)-N content at different sampling dates. Lowercase and different letters indicate statistically significant differences at \(p<0.05\) between treatments.  
Generally, N\({}_{\rm min}\) content reflects the change in the total amount of inorganic nitrogen. The effects of different treatments on the overall N\({}_{\rm min}\) content from 2018 to 2019 showed a trend of firstly rising and then falling (Figure 8). In April and October 2019, the N\({}_{\rm min}\) content for the organic material treatments was 22.39-42.99% higher than that in October 2018. Compared with CK treatment, the N\({}_{\rm min}\) content in the organic material treatments increased by 1.7-20.4% from 2018 to 2019. Unlike the NH\({}_{4}\)+-N and NO\({}_{3}\)\({}^{-}\)-N content, the N\({}_{\rm min}\) content followed a different trend and also varied in different periods.