3.2. Nitrogen use efficiency of wheat
Fig. 2 shows the annual wheat NUEs over the period of 1991–2005 in response to fertilizer treatments at the different study sties. The NUE of wheat was clearly in an increasing trend and higher in the NP and NPK treatments than in N and NK treatments. While decreasing in the N and NK treatments, the NUE of wheat in the NP treatment showed increase at Yangling while unchanged at Changping and Zhengzhou, and declined at Qiyang (Table 4). The
NUE of wheat increased at rates of 1.8–4.1%yr−1 at Changping and
Yangling, and did not change over time at Zhengzhou in the balanced nutrient treatments (NPK, NPKm, 1.5NPKm and NPKs). At Qiyang, the NUEs of wheat declined significantly (˛ = 0.01) for all