The composition of soil nitrogen pool is an important indicator reflecting the sustainable productivity of soil. By quantifying the response relationship between changes in soil nitrogen pool components and the application rate of organic and chemical fertilizers, the cultivation characteristics of different nitrogen components in soil by organic and chemical fertilizers were clarified, so as to provide a theoretical basis for soil fertilization and optimizing nitrogen management in farmland soil.

This study used a long-term location monitoring experiment with different dosages of organic fertilizer and chemical fertilizer (starting in 2006) as a platform. Both organic fertilizer and chemical fertilizer were set at 10 nitrogen fertilizer levels (calculated as N, with crop nitrogen application rates of 0, 60, 120, 180, 240, 300, 360, 420, 500, and 600 kg·hm^-2 per season). The planting system was a double cropping system of winter wheat and summer maize per year. Plant samples were taken during the maturity period of maize in the summer of 2024 to determine nitrogen content, biomass, and maize grain yield. After the summer maize harvest, soil samples were collected in 0-20 cm depth to determine the total nitrogen and inorganic nitrogen content of the soil. The Bremner method was used to determine the organic nitrogen component content of the soil.

(1) The total nitrogen content of the soil in both fertilization treatments showed a linear increase with the increase of nitrogen application rate. Compared with the no fertilization treatment, the increase in soil total nitrogen content in organic fertilizer and chemical fertilizer treatments was 19.1%-273.5% and 8.8%-25.0%, respectively. For soil inorganic nitrogen, with the increase of nitrogen application rate, the soil nitrate nitrogen content significantly increased under both fertilization treatments. Under the same nitrogen content, the nitrate nitrogen content in soil treated with organic fertilizer was significantly higher than that of chemical fertilizer, ranging from 1.44 to 5.17 times, but the difference in ammonium nitrogen was not significant. (2) For soil organic nitrogen components, the content of total acid hydrolyzable nitrogen (THN) and non-acid hydrolyzable nitrogen (NHN) in organic fertilizer treatment increased with the increase of nitrogen application rate. The THN content of fertilizer treatment also increased with the increase of nitrogen application rate, but there was no significant difference between the treatments. In the components of acid hydrolyzed nitrogen, the content of acid hydrolyzed ammonium nitrogen (AN), amino acid nitrogen (AAN), and amino sugar nitrogen (ASN) increased with the increase of nitrogen application rate in both organic and chemical fertilizer treatments. However, the trend of changes in the content of acid hydrolyzed unknown nitrogen (HUN) was opposite, with organic fertilizer treatment increasing with increasing nitrogen application, while chemical fertilizer treatment showed a decreasing trend. The linear fitting results indicated that there was no significant correlation between NHN and nitrogen application rate in fertilizer treatment. Under both fertilization treatments, the content of various organic nitrogen components showed a significant linear positive correlation with nitrogen application rate. Among them, the HUN of fertilizer treatment showed a significant linear negative correlation with nitrogen application rate. Long term application of organic and chemical fertilizers has changed the proportion of various acid hydrolyzed nitrogen components to total soil nitrogen. The distribution ratio of chemical fertilizer treatment was AN (28.3%-32.5%)>AAN (25.1%-29.5%)>HUN (5.9%-10.0%)>ASN (2.6%-3.5%); the allocation ratio for organic fertilizer treatment was AAN (23.7%-31.4%)>AN (19.6%-27.6%)>HUN (7.9%-12.6%)>ASN (2.3%-3.2%). (3) Correlation analysis showed that AN, AAN, and ASN were significantly positively correlated with nitrogen uptake in the aboveground parts of summer maize (P<0.01).

Under the winter wheat-summer maize rotation system in the North China Plain, the long-term application of chemical fertilizer and long-term application of organic fertilizer could significantly increase the soil nitrogen pool capacity. In acid hydrolyzed organic nitrogen, compared with the application of chemical fertilizers, organic fertilizer treatment significantly increased the content of AN, AAN, ASN, and HUN, thereby improving soil nitrogen supply capacity. AN, AAN, and ASN exhibited a highly significant positive correlation with the nitrogen absorption in the aboveground parts of summer maize, making them potential effective nitrogen sources for the absorption and utilization by summer maize.