1.0 Introduction
Agriculture farming, previously dominated by production, currently has diverse objectives. The study of the environmental impacts of crops, the reduced costs of production and the balanced use of fertilization are among the main objectives of modern agriculture 1. Rice-upland rotations are important agricultural production systems in South Asian countries 1, covering an estimated 26.7 million hectares 2. The rice in rotation is followed by next crop called upland crop.
This type of rotation has many different sequences, where numerous grain and industrial crops could be rotated with paddy rice. E.g. rice-rapeseed, rice-wheat, rice-potato, rice-Chinese milk vetch etc. In China and other Asian countries, continuous rice planting has had a negative impact on soil properties, such as reduced soil nitrogen supply and organic carbon content 3. Paddy-rice-upland crop rotations have been recommended and used to improve soil quality and reduce input 3.
In China, these rice-based rotations contribute to 72% of the total cereal production and cover an area of approximately 13 million hectares. Rice (Oryza sativa L.) is an important global food crop that ensures food security for many countries. In China, rice is one of the most vital staple food crops, accounting for ~28% of the total grain-sown area and 43% of total grain production 4. With a constantly growing population, Asian irrigated rice production must increase by 43% over the next 30 years 1.
However, further expansion of the rice planted area is a challenge because a majority of the arable land is already utilized for rice production or has been converted into urban infrastructure 5. Food security advances must be achieved by constantly improving grain yield per unit area. Oilseed rape (Brassica napus L.) is the second most significant source of edible oil globally, with a high nutritional value and a favorable composition of fatty acids for both food and feed 7. Consequently, its demand is increasing considerably all over the world 8. China is the leading producer of oilseed rape, containing 23.3% of the cultivated area and contributing 22.2% of global oil-seed rape production 1. Therefore, rice-oil-seed rape rotation is of great significance to the economy and food security of China.
The Yangtze River basin in China is a major planting area for the rice-oilseed rape rotation system, contributing 70% of the total rice and 91% of the total oil-seed rape production. Oil-seed rape in this area is usually cultivated under the single rice-oilseed rape system or the double rice-oil-seed rape system, which limits the crop duration and the soil nutrient supply accessible for increasing yield. Conversely, the yields of rice-upland rotations faced significant decline or yield stagnation with deterioration in soil fertility.
Concerns are also increasing about the loss of soil organic matter and the reduced nutrient-supplying capacity of soils under rice-upland rotations, which may be due to the increasing cropping intensity. Farmers have resorted to the use of imbalanced fertilization (i.e., one fertilizer application; for example, in north-central China and the middle and lower reaches of the Yangtze River, only N fertilizer but no P and K fertilizers were applied by farmers, while in northeast China, N and P were applied but not K) or excessive fertilization to maintain yield levelness,15,16.
This over-fertilization by farmers, driven by the desire for higher yields, does not always contribute to high yield but it has recently become a common practice for farmers in China nonetheless. Unfortunately, over-fertilization decreases the efficiency of nutrient use and causes a series of economic and environmental problem. However, balanced mineral fertilizer inputs have played an important role in increasing the rice and oil-seed rape yield.
Managing agricultural nutrients to provide a safe food supply and secure the environment remains one of the immense challenges of the 21st century. Crop nutrient uptake and crop yields are the principal factors that determine optimal fertilization practice. Therefore, it is very important to apply fertilizers in an efficient way to minimize loss and to improve the nutrient use efficiency.
To the best of our knowledge, previous studies on fertilization responses were rarely conducted on rice and oilseed rape crops individually, and no one has reported fertilization responses in the rice-oilseed rape rotation, particularly in China. In this study, on-farm experiments were conducted at three sites across the Hubei province in central China from May 2011 to May 2013 to study the influence of different mineral fertilizer applications on rice and oilseed rape yield and on the nutrient-supplying capacity of soil during a rice-oilseed rape rotation. These estimates will be helpful for improving fertilizer recommendations and for achieving sustainable production in rice-oilseed rape rotations in central China.
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