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 (15)N-labelled urea with 20.8% of atom were used and uniformly applied into the micro-plots of the treatments with N, NPK, M(1)NPK, M(2)NPK 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 M(2)NPK demonstrated the highest rate of (15)N 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 (15)N recovery from residue N in the second maize season plant despite of showing a lower (15)N recovery compared with the first soybean season. The recovery rates of (15)N in soils were ranged from 38.2 to 49.7% by the end of the second cropping season, and the residuals of (15)N distribution in soil layers revealed significant differences. The M(2)NPK treatment demonstrated the highest residual amounts of (15)N, and a total of 50% residual (15)N 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.