Effects of N fertilizer rate and planting density on short-season cotton yield, N agronomic efficiency and soil N using 15N tracing technique

Straw returning has been widely used as a nitrogen (N) reduction measure in the North China Plain in recent years. However, little is known of the optimal planting density and N application rate of cotton under straw returning. In order to study the effect of the planting density and N application rate on cotton yield, nitrogen agronomic efficiency (NAE), fertilizer N recovery efficiency (FNRE), N balance and soil N under straw returning, a split-plot design in randomized complete blocks was used in the experiment. The main plots were assigned to plant densities (4.50 x 104, 6.75 x 104 and 9.00 x 104 plants hm-2), and the subplots to N application rates (0, 180, 225 and 270 kg N hm-2). The results showed that the biological yield increased as the planting density and the N application rate increase, but the harvest index decreased. Therefore, the highest biological yield was found under D9.00N270, but the highest cotton yield was found under D6.75N225. Excessive N (N270) application rate was not conducive to the improvement of NAE and FNRE. The amounts of plant N derived from fertilizer (Ndff) was reduced with the increase of planting density. Therefore, soil N would be consumed because the N balance was a negative value when N application rate was decreased at N180, especially at high density. The N balance was closest to zero at D6.75N225. In summary, with the yield, NAE, FNRE, N balance and the stability of soil N taken into consideration, the optimal combination of planting density and N application rate in the North China Plain was D6.75N225, because this treatment had the highest cotton yield, and most importantly, it maintained sustainable cotton production in the field.

Automated summary

Straw returning is a widely used measure to reduce nitrogen in the North China Plain. This study found that the optimal combination for cotton yield and sustainable production in the field was D6.75N225.