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--Improving the sustainability of agricultural product production to meet the needs of society requires the use of fertilizers. To attain optimal production in various agricultural products, the application of chemical fertilizers is essential. To achieve sustainable production through the maintenance and enhancement of soil fertility, reliance solely on chemical fertilizers is insufficient. Consequently, an integrated soil fertility and plant nutrition management model, particularly with respect to the combined application of organic, chemical, and biological fertilizers, has been proposed. To assess the effectiveness of the combined application of fertilizers in enhancing productivity and sustainability of crop production across various crop rotations, a study was conducted in permanent plots at five research stations: the Soil and Water Research Institute station in Meshkin Dasht (Alborz Province), Khosrowshah station in Tabriz (East Azerbaijan Province), Baye-Kola station in Sari (Mazandaran Province), Bakhtajerd station in Darab (Fars Province), and Golestan station in Ahvaz (Huzestan Province). Various management patterns for the combined application of organic, chemical, and biological fertilizers were implemented in a randomized complete block design over a four-year period, commencing in October 2017. Twelve treatments were applied as follows. T1) Control (without fertilizer application), T2) nitrogen, phosphorus and potassium chemical fertilizers based on soil test, T3) 20 tons of cow manure every two years + 75% of the recommended nitrogen and 50% of the recommended phosphorus and potassium, T4) 20 tons of waste compost every two years + 75% of the recommended nitrogen and 50% of the recommended phosphorus and potassium, T5) 20 tons of cow manure annually + 75% of the recommended nitrogen, T6) 20 tons of waste compost anuually + 75% of the recommended nitrogen, T7) 20 tons of cow manure anuually (10 tons in the first crop and 10 tons in the second crop in rotation), T8) 20 tons of waste compost anuually (10 tons in the first crop and 10 tons in the second crop in rotation), T9) T2 + Flavit Biofertilizer, T10) T3+ Flavit Biofertilizer, T11) T4+ Flavit Biofertilizer, T12) T2+20 ton cow manure biennial+ Flavit biofertilizer. At each research station, a regional-scale wheat base crop rotation was implemented. At the Soil and Water Research Institute station, wheat, fodder corn, and rapeseed were grown; at the Khosrowshah station, wheat, fodder rapeseed, and sorghum were grown; at the Golestan station, wheat, sorghum, and mung beans; at the Bakhtajerd station, wheat, rapeseed, cotton, and grain corn; and at the Baye-Kola station, wheat, fodder corn, soybeans, and rapeseed were grown. Municipal waste compost in Alborz and East Azerbaijan, plant residue compost in Fars, wood and paper compost in Mazandaran, and sugarcane bagasse compost in Khuzestan was applied. The yield and yield components of each crop were measured, as well as the concentration of nutrients in plant organs including leaves, straw, and seeds. The results showed that the use of chemical fertilizers in all stations except Khosrowshah had a significant effect (P<0.05) on increasing the production of various crops. Useing chemical fertilizers at recommended rate (T2), enhanced the yield of various crops by 73.9% (17-159%) at the Soil and Water Research Institute station, by 102.6% (25-165%) in Golestan station, by 0.46% (15-66%) in Baya-Kola station and by 0.63% (28-5.96%) in Bakhtajerd station, averagely. In Khosrowshah, chemical fertilizers had no significant effect on crop yield. In 27 crops of the 31 crops grown in all research stations (87% of crops), crop yields with the application of 20 tons of cow manure every two years along with reduced rate of chemical fertilizer (T3) were equal to or greater than the recommended chemical fertilizer rate done (T2). The maximum increase in crop yield with treatment (T3) compared to (T2) was observed at Karaj and Bakhtajerd research stations, equal to 41.4 and 41.9, respectively. Applying 20 tons of compost every two years along with reduced chemical fertilizer (T4) was able to achieve yield equal to or greater than recommended chemical fertilizer in 24 of 31 crops (77.4% of crops). The effect of applying 20 tons of cow manure annually along with 75% nitrogen fertilizer (T5) on crop production in 70.9% of the crops grown in the experimental fields was equivalent to the use of chemical fertilizer and in 12.9% of cases it was more than crop production by (T2). When using various types of compost instead of cow manure (T6), crop yield was equivalent to chemical fertilizer in 64.5% of crops and higher than chemical fertilizer in 12.9% of crops. The application of Flavit biofertilizer (T10) significantly increased the effect of combined use of cow manure and chemical fertilizer (T3) in 16 out of 31 crops (51.6% of crops). This positive effect was observed in 35.5% of the crops when compost was used instead of cow manure (comparing T11 and T4). The relative increase in crop yield with treatment T10 at Bakhtajerd, Baye-Kola, Golestan, and Khosrowshah stations was between 47.8%-0.19%, 0.32%-0.13%, 0.51%-0.24%, and 4.55%-3.24%, respectively. The highest productivity was achieved by adding biofertilizer to the combined use of organic and chemical fertilizers. The highest productivity was achieved in 38.7% of the crops from treatment (T10), in 22.6% of the crops from treatment (T11), and in 16.1% of the crops from treatment (T12). In most crops, fertilizer application increased nitrogen concentration in plant tissue (leaves, straw, and seeds). Combined application of chemical and organic fertilizers had a greater effect on nitrogen concentration in plant tissue than the use of chemical fertilizers alone. In most cases, adding biofertilizer to the fertilization program was associated with an increase in nitrogen concentration. Phosphorus and potassium concentrations also increased with fertilizer use. However, the effect of treatments on increasing nitrogen concentration was greater than their effect on phosphorus and potassium. Combined application of fertilizers had a greater effect on phosphorus and potassium concentrations than chemical fertilizer alone. Overall, it can be concluded that in a 4-year crop rotation period, the combined use of organic and chemical fertilizers not only leads to increased crop production but also reduces the use of chemical fertilizers. Under these conditions, a reduction in nitrogen fertilizers of at least 25% and phosphorus and potassium fertilizers of at least 50% can be achieved. However, sustainable crop production is also very reliable when organic fertilizer is used in combination with nitrogen fertilizer and without the use of phosphorus and potassium fertilizers. Adding biofertilizer to the integrated management of organic and chemical fertilizers can lead to maximizing crop productivity. Product quality is also higher with the combined use of fertilizers than using chemical fertilizers alone.
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