Manure application increased crop yields by promoting nitrogen use efficiency in the soils of 40-year soybean-maize rotation

It is great of importance to better understand the effects of the long-term fertilization on crop yields, soil properties and nitrogen (N) use efficiency in a rotation cropping cultivation system under the conditions of frequent soil disturbance. Therefore, a long-term field experiment of 40 years under soybean-maize rotation was performed in a brown soil to investigate the effects of inorganic and organic fertilizers on crop yields, soil properties and nitrogen use efficiency. Equal amounts of ¹⁵N-labelled urea with 20.8% of atom were used and uniformly applied into the micro-plots of the treatments with N, NPK, M₁NPK, M₂NPK before soybean sowing, respectively. Analyses showed that a total of 18.3-32.5% of applied N fertilizer was taken up by crops in the first soybean growing season, and that the application of manure combining with chemical fertilizer M₂NPK demonstrated the highest rate of ¹⁵N recovery and increased soil organic matter (SOM) and Olsen phosphorus (Olsen P), thereby sustaining a higher crop yield and alleviating soil acidification. Data also showed that no significant difference was observed in the ¹⁵N recovery from residue N in the second maize season plant despite of showing a lower ¹⁵N recovery compared with the first soybean season. The recovery rates of ¹⁵N in soils were ranged from 38.2 to 49.7% by the end of the second cropping season, and the residuals of ¹⁵N distribution in soil layers revealed significant differences. The M₂NPK treatment demonstrated the highest residual amounts of ¹⁵N, and a total of 50% residual ¹⁵N were distributed in a soil layer of 0-20 cm. Our results showed that long-term application of organic fertilizers could effectively promote N use efficiency by increasing SOM and improving soil fertility, and thus leading to an increase in crop yields. This study will provide a scientific reference and guidance for improving soil sustainable productivity by manure application.