Effects of combined application of nitrogen and zinc on winter wheat yield and soil enzyme activities related to nitrogen transformation

Objectives Zinc (Zn) is critical for nitrogen (N) absorption and utilization in winter wheat. This paper investigated the effect of combined application of N and Zn on the activities of soil enzymes related to N transformation, which would provide theoretical evidence for increasing N effeminacy through combined application of N and Zn.

Methods A pot experiment was conducted using winter wheat as tested crop. Four treatments were included, as CK (without applying N and Zn), Zn (only applying Zn 10 mg/kg), N (only applying N 0.2 g/kg) and N+Zn (applying N 0.2 g/kg and Zn 10 mg/kg). Plant and soil samples were collected at four key growing stages of winter wheat. The plant N and Zn concentrations in four growing stages of winter wheat, the concentrations of soil nitrate (NO₃^–-N) and ammonium (NH₄⁺-N), and the activities of soil nitrate reductase, nitrite reductase, urease and protease were measured. At harvest, the yield and yield components of winter wheat were investigated.

Results Compared with CK, all the treatments of Zn, N and N+Zn significantly increased the spike number per pot, grain number per spike and grain yields, increased the N and Zn concentrations in roots, leaves and stems, spikes or grains of winter wheat, and the increase of N+Zn was significantly higher than that of Zn and N treatments. Along winter wheat grew, the soil NO₃^–-N and NH₄⁺-N concentrations were reduced, the nitrite reductase and the urease activities were increased, and the protease activity was decreased gradually. N and N+Zn treatments significantly increased the soil NO₃^–-N concentration, and the increase of N+Zn was significantly higher than that of N treatment at the late growth stage. All the Zn, N and N+ Zn treatments significantly increased soil NH₄⁺-N concentration at the late growth stage, and the increase degree of N+Zn was significantly higher than that of Zn treatments. Zn application significantly decreased the activity of soil nitrate reductase after the jointing stage, N and N+Zn decreased that of nitrate reductase at the late growth stages, and the decrease degree of N+Zn was significantly higher than that of N treatment. All the Zn, N and N+Zn treatments decreased the nitrite reductase activity in soil. Zn and N treatments significantly decreased the soil urease activity, but Zn, N and N+Zn application significantly increased the protease activity in soil.

Conclusions Combined application of N with Zn increases grain yields and N absorption by winter wheat, because it can increase urease and protease activities, promote transformation of N from organic N to ammonium and nitrate, and decrease nitrate reductase and nitrite reductase activities at the late growth stage, which inhibit the denitrification of nitrate, thus keep relatively high concentration of nitrate and ammonium in soil, and benefit the nitrogen absorption by winter wheat.