Long-term benefits of combining chemical fertilizer and manure applications on crop yields and soil carbon and nitrogen stocks in North China Plain

Application of manure is reported to effectively improve agricultural soil fertility. However, the potential of annual manure applications to increase stocks of soil organic carbon (SOC) and total nitrogen (TN) needs to be better understood. In this study, we evaluated the long-term (1991-2012) effects of various fertilization regimes on crop yield, and SOC and TN in the topsoil (0-20 cm) of a fluvo-aquic soil in a wheat (Triticum aestivumL.)-maize (Zea mays L.) cropping system in the North China Plain. The treatments included no N fertilizer (PK), chemical N, P, K fertilizers (NPK), NPK plus 22.5 t ha−1 swine manure (NPKM), and NPK plus 33.75 t ha−1 swine manure (NPKM+). Compared with NPK and PK, both NPKM and NPKM+ significantly increased crop yields, with no significant difference between NPKM and NPKM+. NPKM and NPKM+ treatments had a lower N use efficiency (NUE) than NPK through the whole period and the average NUE were 30.1%, 14.8% and 12.3% in NPK, NPKM and NPKM+ treatments, respectively. Compared with the initial value of 17.2-17.4 Mg C ha−1 and 2.01-2.07 Mg N ha−1, SOC and TN stocks in NPKM and NPKM+ increased by 94.7-114.2% and 83.0-98.0% over the 22 experimental years, respectively, without any indication of C or N saturation in the soil. Furthermore, TN stock was positively correlated to SOC stock (P <  0.01) added with manure, indicating the significance of more organic C input from manure to build up TN pool. In conclusion, long-term application of organic manure in combination with conventional chemical fertilization could increase crop yield and soil C and N stocks in the wheat-maize cropping system of the North China Plain, but it should be noted that large amounts of manure N could also lead to elevated risk of environmental N losses.