Soil fertility is one of the most important influencing factors on the rice yield. Among them, the relationship between soil physical and chemical properties and rice yield can greatly contribute to rice production and fertilizer cultivation in different regions. This study aims to analyze the absolute rice yield, relative yield, and sustainable yield index on the county scale. The research area was also selected as Fangzheng County in northeast China, Ningxiang City in the middle reaches of the Yangtze River, and Jinxian County in the middle and lower reaches of the Yangtze River. The key factors of soil fertility were then determined to affect the rice yield in different regions. The results show that the two-season rice yield (9414.89 kg/hm2) in Ningxiang City was significantly higher than that in Fangzheng County (8224.31 kg/hm2) and Jinxian County (5691.38 kg/hm2) in the late season, due to different cropping patterns. But the average relative yield in Fangzheng County (89.88%) was significantly higher than that in Ningxiang City (73.84%) and Jinxian County (65.67%). The sustainable yield index of Fangzheng County was also higher (0.86), indicating high yield and stability. Furthermore, the low, medium, and high yield levels were calculated using relative yields. Therefore, the soil fertility index and integrated fertility index of Fangzheng County, Ningxiang City, and Jinxian County were higher under the high yield level. Among them, the soil organic matter index, available nitrogen index, and integrated fertility index of Fangzheng County at the high yield level increased by 25.50%, 31.17%, and 25.45%, respectively, compared with the low yield level. The soil available phosphorus increased by 35.41% in Ningxiang City, compared with the low yield level. The soil organic matter, available nitrogen, available phosphorus, available potassium, and integrated fertility index of Jinxian County showed that the high yield level was significantly higher than the middle and low yield levels, which increased 13.28%-27.28%, 21.45%-34.82%, 62.42%-111.90%, 29.51%-59.51%, and 34.21%-75.86%, respectively. There was no significant difference in the soil pH value among different yield levels. There was a variation in the fertility influencing factors on the relative yield in different counties. The soil's available phosphorus, nitrogen, and organic matter content were the key fertility factors for the sustainable yield index of rice in Fangzheng County, Ningxiang City, and Jinxian County, respectively. Some suggestions were provided for soil fertilizer cultivation in the high and stable rice yield at the county scale. In Fangzheng County, the soil available phosphorus shared the largest relative contribution to the rice sustainable yield index (8.68%), followed by organic matter and available potassium. In Ningxiang, the relative contribution of soil available nitrogen to rice sustainable yield index was 12.92%, followed by pH. In Jinxian County, the soil organic matter shared the greatest impact on the rice sustainable yield index (15.37%), followed by available nitrogen and available phosphorus. As such, there was the highest influencing level in the sustainable production of rice available phosphorus in the soil index, followed by available potassium and available nitrogen. Therefore, Fangzheng County should be focused on the soil available phosphorus in future rice production. While Ningxiang and Jinxian County should increase the soil available nitrogen and organic matter contents.
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The analysis of rice yield, nitrogen use efficiency and the quantitative relationship between apparent nitrogen balance and soil alkali-hydrolyzed nitrogen under different nitrogen fertilizer management conditions could provide a more comprehensive understanding of the effects of long-term fertilization on soil fertility, so as to provide the theoretical guidance for efficient production and scientific nitrogen management of red soil paddy fields.
Based on the red soil double cropping rice long-term fertilization positioning experiment (started in 1981, located in Jinxian County, Jiangxi Province), five treatments were selected: no fertilizer (CK), nitrogen and phosphorus fertilizer (NP), nitrogen and potassium fertilizer (NK), nitrogen, phosphorus and potassium fertilizer (NPK), nitrogen, phosphorus and organic fertilizer (NPKM), and then the grain and straw yield and nitrogen uptake of rice in each season were investigated and analyzed, and the soil alkali-hydrolyzed nitrogen content was analyzed after late rice. The nitrogen uptake, nitrogen utilization rate, nitrogen apparent balance and the changes of soil alkali-hydrolyzed nitrogen were calculated and analyzed on a 10-year basis.
During the 40 years of experiment (1981-2020), the rice yield and nitrogen uptake under NPKM treatment were the highest, increased by 65.9%-108.4% and 85.1%-132.5% compared with CK, respectively, and increased by 19.3%-92.1% and 19.4%-99.8% compared with fertilizer treatments (NPK, NK and NP), respectively, showing significant differences. With the increase of the experimental period, the nitrogen use efficiency of fertilizer treatment gradually decreased, and the NPKM treatment also showed a decreasing trend in the first 30 years (1981-2010), but the rate was slower than that of fertilizer treatment, and increased in the recent 10 years (2011-2020), and from the lowest in the first 10 years (1981-1990) to the highest in the recent 10 years, increased by 25.3%-271.2% compared with fertilizer treatment. The nitrogen surplus was the highest under NPKM treatment during the 40 years of experiment, with an increase of 137.1%-577.2% compared with fertilizer treatment, but in the last 30 years (1991-2020), the nitrogen surplus gradually decreased with the increase of the experimental period. The soil alkaline hydrolyzed nitrogen content was the highest under NPKM treatment during the 40 years of experiment, increased by 7.1%-24.4% compared with CK, but the difference was not significant in the first 10 years, and increased by 11.0%-35.2%compared with fertilizer treatment, while there was no significant difference between fertilizer treatment and CK. Correlation analysis showed that the nitrogen surplus was significantly positively correlated with the soil alkaline hydrolyzed nitrogen content in the last 20 years (2001-2020).
In the red soil double-cropping rice system, with the increase of fertilization years, the combined application of organic and inorganic fertilizers had better effects on rice yield, nitrogen uptake, nitrogen uptake and utilization, and soil alkali-hydrolyzed nitrogen content. Meanwhile, the increase of nitrogen surplus caused by long-term fertilization also further increased the soil alkali-hydrolyzed nitrogen content in the topsoil. The contribution capacity of nitrogen surplus to soil alkali-hydrolyzed nitrogen increased gradually.
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