Mapping winter rapeseed in South China using Sentinel-2 data based on a novel separability index

Large-scale crop mapping using remote sensing data is of great significance for agricultural production, food security and the sustainable development of human societies. Winter rapeseed is an important oil crop in China that is mainly distributed in the Yangtze River Valley. Traditional winter rapeseed mapping practices are insufficient since they only use the spectral characteristics during the critical phenological period of winter rapeseed, which are usually limited to a small region and cannot meet the needs of large-scale applications. In this study, a novel phenology-based winter rapeseed index(PWRI) was proposed to map winter rapeseed in the Yangtze River Valley. PWRI expands the date window for distinguishing winter rapeseed and winter wheat, and it has good separability throughout the flowering period of winter rapeseed. PWRI also improves the separability of winter rapeseed and winter wheat, which traditionally have been two easily confused winter crops. A PWRI-based method was applied to the Middle Reaches of the Yangtze River Valley to map winter rapeseed on the Google Earth Engine platform. Time series composited Sentinel-2 data were used to map winter rapeseed with 10 m resolution. The mapping achieved a good result with overall accuracy and kappa coefficients exceeding 92% and 0.85, respectively. The PWRI-based method provides a new solution for high spatial resolution winter rapeseed mapping at a large scale.

Analysis of Yield and Disease Resistance Traits of New Winter Rapeseed Varieties over the Past Twenty Years in China

Based on the results of the national regional trail for winter rapeseed in four groups,including the upper reaches,middle reaches and lower reaches of Yangtze River and Huang-Huai region in the past twenty years,new varieties of winter rapeseed showed upward trend in average yield,the yield level in Huang-huai group was higher than other groups.The changes of average effective pods per plant were not significant in any group,but the number of grain per pod and 1 000-grain weight showed increase trend.However,the increment of grains per pod in three Yangtze River groups was higher than that in Huang-Huai River group,while the increment of 1 000-grain weight was just opposite.The incidence rate of Sclerotinia sclerotiorum and viral disease in new varieties of winter rapeseed obviously decreased,and the incidence rate of S.sclerotiorum decreased lower than 5%.Genetic improvement for winter rapeseed should be focus on the number of grain per pod and 1 000-grain weight in the future,and yield level of new varieties in all four groups is expected to increase.

Effects of Nitrogen Fertilizer Management on Biological Traits and Physiological Indexes at Seedling Stage and Yield of Directly-sown Rapeseed(Brassica napus L.)

Field experiments were carried out to study the effects of different nitroge- napplication rates and application times on biological traits and physiological indexes of directly-sown rapeseed （Brassica napus L.） at the seedling stage and investigate the relationship between these biological traits or physiological indexes at the seedling stage and yield, so as to provide scientific theoretical support for high yield and efficient fertilization management in production of winter rapeseed. Field trials were conducted in Chengdu plain of Sichuan Province under rice-rapeseed rotation system during the period of 2011-2012. The nitrogen application rate trial consisted of five nitrogen levels （0, 90, 180, 270 and 360 kg/hm2） and the nitrogen application time trial included NTl（single application as base fertilized）, NT2 （bottom application ＋one time of topdressing at seedling stage） and NT3 （bottom application＋two times of topdressing at seedling stage） under the same nitrogen rate （225 kg/hm2）. The results indicated that compared with no nitrogen application （NO） treatment, the in- crease of nitrogen fertilizer is beneficial to the increase of biological traits including plant height, green leaf number, leaf area index and dry weight of rapeseed at the seedling stage, the improvement of physiological indexes including total nitrogen content, chlorophyll content and soluble protein content of functional leaves, and the reduction of soluble sugar content. Nitrogen rate was linearly correlated with various biological traits at the seedling stage and physiological indexes including total nitro- gen content, chlorophyll content and soluble sugar content in functional leaves over- a/I, but in parabolic correlation with soluble protein content. Under the same nitrogen rate, NT2 treatment exhibited biological traits remarkably or significantly higher than NT1 treatment and NT3 treatment. The nitrogen application times were linearly cor- related with the physiological indexes of functional leaves at the seedling stage. The various biological traits and physiological index of functional leaves at the seedlings stage were in quadratic function parabolic correlation with seed yield, and the corre- lation was significant （P〈0.05）. Therefore, under the rice-rapeseed rotation system in Chengdu plain, the economic rational nitrogen rate is 180-225 kg/hm2, and the mode of bottom application ＋ one time of topdressing （NT2） is suitable.