Wheat grain yield and grain-nitrogen relationships as affected by N, P, and K fertilization: A synthesis of long-term experiments

Nutrient management can reduce crop yield gaps, but available literature is mostly restricted to studies limited in time, geography, or in the number of nutrients evaluated. Our objective was to synthesize long-term experiments evaluating wheat (Triticum aestivum L.) yield and grain-N concentration (GNC) response to N, P, and K fertilizer rates and their interactions. We used data from three long-term (1966-2016) experiments conducted in Oklahoma (USA) comprising 155 site-years for yield (n = 8035) and 90 site-years for GNC (n = 4580). The last year of the experiments was the baseline to de-trend yield and GNC data. We first explored relationships between grain yield and GNC, grain N removal, apparent recovery of applied N in the grain (N recovery), and N-use efficiency (NUE) as affected by the presence and rate of N, P, and K across the entire dataset. Then, we subdivided the dataset into yield-environments based on the different data quartiles, and analyzed it using descriptive statistics, multi-level modeling, differences from the control, and conditional inference trees. Our main findings were: i) wheat yield was negatively related to GNC, but positively associated with N removal, N recovery, and NUE. ii) The co-application of P and, to a lesser extent, K, increased N removal and NUE but decreased GNC. iii) The proportion of variability in yield and GNC explained by fertilizer management increased with an increase in yield-environment. iv) Wheat yield response to N and to P were typically quadratic, although response to P was restricted to high yielding environments. v) Wheat GNC increased linearly with increases in N rate, but decreased with increases in P and K rate. vi) Conditional inference trees suggested that the co-application of P and K improved yields but decreased GNC. The co-application of P and K can increased wheat yield, N removal, and NUE, but the increases in yield were greater than those in N removal, thus decreasing GNC.