Effects of plant density and nitrogen rate on cotton yield and nitrogen use in cotton stubble retaining fields

Increasing nitrogen(N) rate could accelerate the decomposition of crop residues, and then improve crop yield by increasing N availability of soil and N uptake of crops. However, it is not clear whether N rate and plant density should be modified after a long period of cotton stubble return with high N rate. This study seeks to assess the effects of N rate and plant density on cotton yield, N use efficiency, leaf senescence, soil inorganic N, and apparent N balance in cotton stubble return fields in Liaocheng, China, in 2016 and 2017. Three plant densities 5.25(D_(5.25)), 6.75(D_(6.75)) and 8.25(D_(8.25)) plants m~(-2) and five N rates 0(N_0), 105(N_(105)), 210(N_(210)), 315(N_(315)), and 420(N_(420)) kg ha~(-1) were investigated. Compared to the combination used by local farmers(D_(5.25) N_(315)), a 33.3% N reduction and a 28.6% increase in plant density(D_(6.75) N_(210)) can maintain high cotton yield, while a 66.7% N reduction at 6.75 plants m~(-2)(D_(6.75) N_(105)) can only achieve high yield in the first year. Biological yield increased with the increase of N rate and plant density, and the highest yield was obtained under 420 kg N ha~(-1) at 8.25 plants m~(-2)(D_(8.25) N_(420)) across the two years under investigation. Compared to D_(5.25) N_(315), N agronomic efficiency(NAE) and N recovery efficiency(NRE) in D_(6.75) N_(210) increased by 30.2 and 54.1%, respectively, and NAE and NRE in D_(6.75) N_(105) increased by 104.8 and 88.1%, respectively. Soil inorganic N decreased sharply under 105 kg N ha~(-1), but no change was found under 210 kg N ha~(-1) at 6.75 plants m~(-2). N deficit occurred under 105 kg N ha~(-1), but it did not occurr under 210 kg N ha~(-1) at 6.75 plants m~(-2). Net photosynthetic rate and N concentration of leaves under N rate ranging from 210 to 420 kg ha~(-1) were higher than those under N rate of 0 or 105 kg N ha~(-1) at all three densities. The findings suggest that D_(6.75) N_(210) is a superior combination in cotton stubble retaining fields in the Yellow River Valley and other areas with similar ecologies.