Nitrogen management in a rice-wheat system in the Taihu Region: Recommendations based on field experiments and surveys

Excessive use of mineral nitrogen (N) fertilizer has been a common practice in the high-yielding rice-wheat double-cropping system in the Taihu Region of southeastern China. As a consequence of high N balance surpluses and low N use efficiencies (NUEs), nitrogen losses to water bodies and to the atmosphere are high. Field experiments on five replicate farmers' field sites were conducted in southern Jiangsu Province over three consecutive rice-wheat double-crop rotations with three different N fertilization treatments ('conventional' (farmers' practice), 'reduced' (by 23% for rice and 32% for wheat) and zero N application). A parallel survey was carried out involving 43 farmers' households, in order to calculate standard gross margins (SGM) and to conduct an environmental assessment of the rice-wheat farming system in the Taihu Region. The results of the field experiment showed that a distinct reduction in fertilizer N application rates to summer rice and winter wheat crops is possible without significant decrease in mean grain yields. Mean grain yields for the entire double-crop rotation were 14.7 Mg ha(-1) yr(-1) under the conventional N fertilization practice and 14.1 Mg ha(-1) yr(-1) under reduced N fertilization. A significant increase in NUEs could be achieved in most years and crops under reduced N fertilization compared to farmers' practice, and N balance surpluses were significantly decreased from 142 kg N ha(-1) to less than 60 kg N ha(-1) yr(-1) under the reduced N fertilization regime. The residual mineral N contents in the soil profiles after winter wheat harvest showed a decreasing tendency over time with a reduction of N fertilization. Mineral N contents in fertilized fields were significantly higher than on the zero N plots. Concentrations of NH4+-N in soil extracts from the puddled layer during the summer rice in 2010 showed significant differences between the two N fertilization treatments and the zero N plots. Results of the agro-economic survey showed that production of rice was 50% more profitable than that of wheat, and that fertilizer costs made up less than 20% in the rice season, but almost 50% of the total production costs during the wheat season. It can be concluded that N losses to the environment can be efficiently decreased by reducing the overall N fertilization rates without any risk of decline in grain yield and related income for farmers. Based on the field experiments and investigations, a reduction in N fertilizer application rates by 15-25% for summer rice and by 20-25% for winter wheat, compared to present levels is recommended. (C) 2015 Elsevier B.V. All rights reserved.