Spatio-temporal evolutions and driving factors of wheat and maize yields in China

Explore the spatio-temporal characteristics and key driving factors of wheat and maize yields, and provide a theoretical basis for food security strategic decision-making in the whole country. Based on the national farmland monitoring data of the Ministry of Agriculture and Rural Affairs in the past 30 years (825 points, 1988 to 2019), we analyzed the spatio-temporal variation characteristics of wheat and maize yield. Then we explored the effects of fertilizer application, climate, and soil properties on the yield change by the stepwise regression and random forest model. Finally, partial least squares method was used to explore the main path of each index influencing yield change. In the past 30 years, the average yields wheat and maize under single cropping and rotation system (wheat and maize) were 5.05, 9.05, 6.01, and 7.08 t/hm2, respectively (the latter two were rotation, the same below), with the corresponding coefficient of variations of 36.6%, 26.2%, 32.1%, and 28.0%, and the corresponding increase rates with time were 100, 159, 46, and 98 kg/(hm2·a), respectively (P<0.05). The yield, coefficient of variation and rate of change of wheat in rotation with other crops were 5.29 t/hm2, 28.6%, and 67 kg/(hm2·a) (P<0.001), respectively. The high-yielding areas for maize were mainly distributed in Northwest China (single cropping) and North China (rotation cropping). The high-yielding areas for wheat are mainly in the Lower of Yangtze River and North China (rotation with maize). The lowest yields of wheat and maize were distributed in Southwest China. The relationship between yield and monitoring duration in all regions was generally consistent with the national scale. However, the increased rates of wheat yield under wheat-maize rotation were negative in the Southwest China and Middle of Yangtze River. The results of stepwise regression showed that the nitrogen fertilizer input had a highly significant effect on wheat and maize yield (P<0.01), while the pH not have. For the different regions, the results of random forest showed that in addition to the importance of climate and nitrogen fertilizer, organic matter played a relatively strong role in regulating yield in most regions (especially in Southwest China), while potassium fertilizer played a relatively strong role in regulating maize yield in North and Northwest China. Partial least squares analysis showed that the total effects of soil properties, fertilizer input and climate on crop yield were 47.6%, 29.4%, and 23.0% respectively. Climate mainly regulated crop yield by affecting soil properties and fertilizer input. Wheat and maize yields have been increasing overall in China from 1988 to 2019, while the wheat yield under rotation cropping system in Southwest China and the lower reaches of the Yangtze River has been decreasing with time. The yield of wheat and maize is more regulated by soil properties than by fertilization and climate. Nitrogen fertilizer is the key factor to ensure the high yield in each region. In addition, the potassium fertilizer reasonable application should be paid more attention at maize season to ensure high and stable yields in North China, Northeast and Northwest China. The improvement of soil organic matter and available phosphorous should be as the main measures to achieve high and stable yields in Southwest China.