Year-Round Production of Cotton and Wheat or Rapeseed Regulated by Different Nitrogen Rates with Crop Straw Returning

Abstract

Double direct seeding of cotton (with wheat or rapeseed) is a new method for cotton-growing regions in the Yangtze River Basin to adapt to the development of mechanization. It would help to reduce manual labor, optimize the amount of nitrogen fertilizer to be used, reduce the physical and chemical production costs, and improve the benefits of cotton fields. We selected five counties from the major cotton-producing areas of Hubei Province for three consecutive seasons, from winter 2020 to spring 2022. The experimental sites used no tillage with straw returning to the field, double direct seeding, late sowing at high density, and one-time fertilization to study the effects of different nitrogen fertilizer rates on the yield characteristics of cotton, wheat, and rape and calculate the economic benefits of the two cultivation modes under different nitrogen fertilizer input levels through parameters such as land-use efficiency, production efficiency, and profitability. In both cotton–wheat and cotton–rapeseed cropping systems, the number of bolls per plant in cotton was the lowest in the N165 (90 cotton + 75 wheat/rape kg ha−1) treatment. The cotton yield was the highest at N247.5 (135 cotton + 112.5 wheat/rape kg ha−1) in the cotton after the wheat system and N412.5 (225 cotton + 187.5 wheat/rape kg ha−1) in the cotton after the rape system. The yield of wheat and rape increased with the increase in the levels of nitrogen fertilizer, with the N165 treatment showing the lowest values. With an increase in nitrogen fertilizer, the harvest index of wheat first maximized and then started decreasing. The harvest index in wheat was the highest at N247.5 (135 cotton + 112.5 wheat/rape kg ha−1) and N330 (180 cotton + 150 wheat/rape kg ha−1), whereas, in rape, it increased with nitrogen fertilizer application, with the highest value at N495 (270 cotton + 225 wheat/rape kg ha−1). Economically, the expenses and income of both cotton–wheat and cotton–rape systems increased as nitrogen fertilizer increased. The net profit and benefit ratio first increased and then decreased with increasing nitrogen fertilizer, with N247.5 (135 cotton + 112.5 wheat/rape kg ha−1) scoring the maximum values for both of these parameters. The land-use efficiency and production efficiency increased with the increase in nitrogen fertilizer, and the production efficiency of the N165 (90 cotton + 75 wheat/rape) treatment was significantly lower than that of the other four treatments. The profitability increased first and then decreased with the increase in nitrogen fertilizer, with the N247.5 (135 cotton + 112.5 wheat/rape) treatment showing the highest profit. The production cycle of cotton–rape was slightly shorter than that of cotton–wheat, and the system productivity was also lower. The expenses and land-use and production efficiency of the rapeseed system were lower than those of wheat, while the gross income, net profit, and productivity of the cotton–rape system were higher than those of cotton–wheat. The application of nitrogen fertilizer in the cotton–wheat double-cropping system under straw return can achieve the maximum net profit, production ratio, and yield at the low nitrogen level of N247.5, (135 cotton + 112.5 wheat/rape kg ha−1). Due to the price advantage of rape, the net profit, production ratio, and income of the cotton–rape production system are higher than those of the cotton–wheat production system.