4.1. Difference in response to fertilization treatment between wheat and corn
The results from this research have shown that manure and P are the most important fertilization treatments to improve the yield and NUE for both wheat and corn (detailed data for corn are reported in Duan et al., 2011). There are significant differences in the crop response to the fertilization treatments. Prior to examination of these differences, it should be noted that the computation of PC was based on NK treatment (low yield and low NUE) and NPK treatment (Eq. (5)); and the MC was based on NPK treatment (higher yield and higher NUE than NK) and NPKM treatment (Eq. (7)). The NPK treatment had substantially higher yield than the NK treatment that resulted in much higher PC values than the MC values; but these values do not necessarily indicate more importance of P than manure. Positive MC values indicate further improvement upon sufficient or high P supply. The crop response to P and manure treatments is discussed below based on the relative differences between the two crops.
The P fertilizer effect on wheat yield and NUE improvement was larger than that observed for corn (Table 7). The contributions of P fertilizer to both grain yield and NUE were higher for wheat, about 2.3–3.4 times that for corn, average of the four sites. Manure, however, did not further improve the yield and NUE of wheat in alkaline soils at Changping, Zhengzhou and Yangling (ave. MC −13% to NUE and 3.4% to yield) but showed significant improvement in the acid soil at Qiyang (33–41%). For corn, manure application increased NUE not only at Qiyang, but also at most other study sites (ave. MC 17% to yield and 14% to NUE) with most MC values relatively higher than that for wheat, though it was not significant in statistical because of the large variability among four sites.
In a similar rotation system, results from a 36-year experiment also showed that wheat was the crop most sensitive and corn was moderate to the absence of P fertilization (Colomb et al., 2007). There were three possible reasons for the different response of wheat and corn to P fertilizer and manure. (1) During the early growth stages of wheat, the mineralization of organic materials and release of P in soil was slow because of low soil temperature (i.e. from early October to early June). This would