Effect of planting density on nitrogen uptake, utilization anddistribution in winter wheat

Intensive wheat cultivation requires large inputs of nitrogen (N) fertilizer. However, only a limited proportion of the applied N is taken up by winter wheat, meaning that large amounts of N are lost to the environment. This loss of N fertilizer represents not only a considerable cost to the grower, but may also to the environment. Therefore, there has been increasing global emphasis on optimized management strategies for high nitrogen use efficiency (NUE; i.e., grain yield/available N) to help reduce excessive N fertilizer inputs while maintaining acceptable grain yields. The amount and spatial distribution of plant roots in soil profile reflect potential nutrient up-take. Also plant density can influence root distribution in soil profile, increasing seminal roots per unit area and reducing nodal roots per culm. Various planting densities were therefore considered in relation to high NUE. The aim of the study was to find suitable wheat planting density that simultaneously produced high grain yields and NUE. To that end, two winter wheat cultivars (large-spike "Tainong 18" and medium-spike "Shannong 15") were selected and experimented under field conditions at various planting densities. The planting densities were 1.35 million·hm^-2, 2.70 million·hm^-2, 4.05 million·hm^-2 and 5.40 million·hm^-2 for "Tainong 18"; and 1.725 million·hm^-2, 3.45 million·hm^-2, 5.175 million·hm^-2 and 6.90 million·hm^-2 for "Shannong 15". The tow wheat cultivars were analyzed for the uptake, distribution and utilization of N, and for grain yield and inorganic N accumulation in soil profile. The results showed that N accumulation at maturity, grain yield, N uptake efficiency and N partial factor productivity were highest under the planting densities of 2.70 million·hm^-2 for "Tainong 18" and 3.45 million·hm^-2 for "Shannong 15". Grain N accumulation at maturity, gain N after anthesis and N translocation to grain after anthesis reduced with increasing planting density. Translocated N amount, proportion and contribution to grain from stored N in vegetative organs before anthesis increased with increasing planting density. N harvest index, grain N concentration, and NO₃^-N, NH4₊^-N and inorganic N accumulation at different depths of the soil profile decreased at wheat maturity with increasing planting density. In conclusion, grain yield and NUE were highest under planting densities of 2.70 million·hm^-2 for "Tainong 18" and 3.45 million·hm^-2 for "Shannong 15". The recommended planting densities were therefore 2.70 million·hm^-2 for "Tainong 18" and 3.45 million·hm^-2 for "Shannong 15".