Long-term effects of combined application of chemical nitrogen with organic materials on crop yields, soil organic carbon and total nitrogen in fluvo-aquic soil

• Crop yields increase or maintain a higher level under organic materials fertilizer.
• The SOC and TN content with all treatments showed an increasing trend with time.
• TN contents were significantly correlated with SOC at each treatment.
• Soil C/N ratios were generally around 9–14 during cultivate time.
• Organic manure along with N is the best fertilization practice in this area.

Integrating fertilizer nitrogen with organic materials was an important management strategy for sustainable agriculture production systems in most soils low in organic matter. A 33-year-old experiment with various fertilizations in a double cropping system rotated with winter wheat (Triticum aestivum L.) and maize (Zea mays L.) on a fluvo-aquic soil in Tianjin was evaluated. The six treatments used were control, N, NPK, NM, NS and NGM, representing various combinations of N, P, K, organic manure (M), straw (S) and green manure (GM) fertilizer applications. The specific objective of this study was to evaluate the long-term effects of combined use of organic materials and chemical fertilizer nitrogen on crop yields, soil organic carbon (SOC) and soil total nitrogen (TN).As a result, wheat and maize yields in the plot under the N treatment decreased with time, whereas the yields increased in the plot under NM treatment for both crops. The yields in NS and NGM treatments maintained a stable and higher level. Generally, both wheat and maize yields were significantly higher in NM and NPK than those in other treatments. The SOC and TN contents with all treatments showed an increasing trend with time. Compared with the N treatment, the average SOC and TN contents were 38.0 and 17.3%, 14.2 and 6.7%, and 12.9 and 6.1% larger, respectively, for NM, NPK, and NS. In addition, the SOC contents with the five treatments (N, NGM, NS, NPK and NM) increased by 25.5, 33.1, 42.1, 69.7 and 145.6%, respectively, by 2012; for TN they increased by 6.6, 17.8, 23.2, 35.5 and 57.5.5%, respectively, above the values obtained in 1979. TN contents were significantly correlated with SOC at each treatment (P < 0.01). Soil C/N ratios were generally around 9 to 14 during cultivate time. The average soil C/N ratio in NM was significantly higher than those in other treatments, and the soil C/N ratios among the other four treatments were not significantly different.Overall, the results suggest that organic manure along with chemical N must be used to sustain the productivity and promote C and N sequestration of wheat–maize system in the fluvo-aquic soils of the Tianjin areas.