Optimizing nitrogen application rate and plant density for improving cotton yield and nitrogen use efficiency in the North China Plain

Plant population density (PPD) and nitrogen (N) application rate (NAR) are two controllable factors in cotton production. We conducted field experiments to investigate the effects of PPD, NAR and their interaction (PPD × NAR) on yield, N uptake and N use efficiency (NUE) of cotton using a split-plot design in the North China Plain during 2013 and 2014. The main plots were PPDs (plants m−2) of 3.00 (low), 5.25 (medium) and 7.50 (high) and the subplots were NARs of 0 (N-free), 112.5 (low), 225.0 (moderate) and 337.5 (high). During both 2013 and 2014, biological yield and N uptake of cotton increased significantly, but harvesting index decreased significantly with NAR and PPD increasing. With NAR increasing, internal nitrogen use efficiency(NUE) decreased significantly under three PPDs and agronomical NUE, physiologilal NUE, nitrogen recovery efficiency(NRE) and partial factor productivity from applied nitrogen (PFPN) also decreased significantly under high PPD between two years. Lint yield increment varied during different PPDs and years, but NAR enhancement showed less function under higher PPD than lower PPD in general. Taken together, moderate NAR under medium PPD combined higher lint yield with higher agronomic NUE, physiological NUE, and NRE, while low NAR with high PPD would achieve a comparable yield with superior NRE and PFPN and high NAR under high PPD and medium PPD produced higher biological yield but lower harvest index, lint yield and NUE compared to moderate NAR with medium PPD. Our overall results indicated that, in this region, increasing PPD and decreasing NAR properly would enhance both lint yield and NUE of cotton.

棉花生产中，植株种植密度（Plant Population Density, PPD）与氮肥施用量（Nitrogen Application Rate, NAR）是两个可控栽培因素。本研究于2013年与2014年在华北平原采用裂区设计开展田间试验，探究PPD、NAR及其交互作用（PPD × NAR）对棉花产量、氮素吸收量及氮肥利用效率（Nitrogen Use Efficiency, NUE）的影响。试验主区设置3个PPD水平：3.00株·m⁻²（低密度）、5.25株·m⁻²（中密度）、7.50株·m⁻²（高密度）；副区设置4个NAR水平：0（无氮）、112.5（低施量）、225.0（中施量）与337.5（高施量）。

2013年与2014年的试验结果均表明，随氮肥施用量与植株种植密度的提升，棉花生物产量与氮素吸收量显著增加，而收获指数则显著降低。在三种种植密度下，随氮肥施用量升高，体内氮肥利用效率（Internal Nitrogen Use Efficiency）均显著下降；且在两年试验中，高密度种植条件下，农学氮肥利用效率、生理氮肥利用效率、氮素回收效率（Nitrogen Recovery Efficiency, NRE）与施氮偏生产力（Partial Factor Productivity from Applied Nitrogen, PFPN）同样显著降低。皮棉产量的增幅因种植密度与试验年份的不同而存在差异，但总体来看，相较于低密度处理，高密度处理下提升氮肥施用量对皮棉产量的促进作用更为有限。

综合来看，中密度种植配合中水平氮肥施用量，可同时实现较高的皮棉产量与农学氮肥利用效率、生理氮肥利用效率及氮素回收效率；高密度种植配合低水平氮肥施用量，可获得相当的皮棉产量，并具备更优的氮素回收效率与施氮偏生产力；相较于中密度配施中水平氮肥，高密度配施高水平氮肥可获得更高的生物产量，但收获指数、皮棉产量与氮肥利用效率均更低。本研究整体结果表明，在该区域，适当提升植株种植密度并降低氮肥施用量，可同时提升棉花的皮棉产量与氮肥利用效率。