Interactive influence of nitrogen and phosphorus application rate on wheat yield in North Henan, China

A pot experiment was conducted to evaluate the interactive effects of nitrogen and phosphorus fertilizer application rates on wheat yield via measuring agronomic and physiologic indicators such as root activity (RA), acid phosphatase activity (APA) and nitrate reductase activity (NRA). Six treatments 200 kg(N)·hm^-2+100 kg(P₂O₅)·hm^-2, 200 kg(N)·hm^-2+160 kg(P₂O₅)·hm^-2, 220 kg(N)·hm^-2+100 kg(P₂O₅)·hm^-2, 220 kg(N)·hm^-2+160 kg(P₂O₅)·hm^-2, 240 kg(N)·hm^-2+100 kg(P₂O₅)·hm^-2 and 240 kg(N)·hm^-2+160 kg(P₂O₅)·hm^-2 were used in the study. The highest wheat yield occurs under the combination application of 220 kg(N)·hm^-2, 160 kg(P₂O₅)·hm^-2. Wheat yield is significantly correlated with RA, APA and NRA, and is in turn strongly related with wheat cumulative N and P. RA, APA and NRA increase with increasing application rate of phosphorus and nitrogen, but continue to increase until the application rate of nitrogen reaches 220 kg·hm^-2 before declining. This indicates that over-application of nitrogen fertilizer might reduce RA, APA and NRA, which in turn reduce the absorption rate of N and P and thereby decreasing wheat yield.