Paddy-upland rotation is an unavoidable cropping system for Asia to meet the increasing demand for food. The reduction in grain yields has increased the research interest on the soil properties of rice-based cropping systems. Paddy-upland rotation fields are unique from other wetland or upland soils, because they are associated with frequent cycling between wetting and drying under anaerobic and aerobic conditions; such rotations affect the soil C and N cycles, make the chemical speciation and biological effectiveness of soil nutrient elements varied with seasons, increase the diversity of soil organisms, and make the soil physical properties more difficult to analyze. Consequently, maintaining or improving soil quality at a desirable level has become a complicated issue. Therefore, fully understanding the soil characteristics of paddy-upland rotation is necessary for the sustainable development of the system. In this paper, we offer helpful insight into the effect of rice-upland combinations on the soil chemical, physical, and biological properties, which could provide guidance for reasonable cultivation management measures and contribute to the improvement of soil quality and crop yield.

The yield-increasing mechanism of an optimized nitrogen fertilizer application (OFA) in rice was reported in this work through a three-year test. Results showed that the number of branches and spikelets increased, panicle length, the diameter and vascular bundle number of panicle-neck internode improved with OFA. Under the condition of OFA, high effective leaf areas, especially for the flag and the second upper leaf areas, increased, the net photosynthetic rate of the upper three leaves promoted, so the photosynthetic productivity went up by a large margin; moreover, the content of soluble protein and chlorophyll of leaf also increased, and the content of soluble sugar and malondialdehyde (MDA) decreased, as a result in slowing down the senescence speed in leaves, and increasing the photosynthetic time. Gene expression level, including MOC1, LAX1, SP1, GS1;1, were up-regulated obviously in different panicle initiation stage under OFA condition, which conduced to the increase in the secondary branches and spikelets. So we concluded that the changes in organ formation and panicle structure, together with the responses in physiological and molecular made the photosynthetic area, rate and time all increased with OFA, which provided the matter basis for the big panicle development, consequently, got a higher yield.

Light is a basic environmental factor required for plant growth and development; however, these are not only affected by light quantity, but also by light quality - light and radiation of different wavelengths and different compositions. In four different rice varieties (Oryza sativaL.), two kinds of shading materials, white cotton yarn (Shading (W)) and black nylon net (Shading (B)) were used to simulate cloudy days. Yield decreased under Shading (W) (15.3–17.7%) and Shading (B) (16.6–20.0%) compared to under sunny day (CK), and different effects on rice quality, which is mainly affected by changes in light quality, were observed. The change in light quality (Blue, Green, Red and R/FR proportions) represented under Shading (W) was significantly different from that under CK and Shading (B) conditions. Red light composition under Shading (W) was closer to that of the cloudy day condition. The proportion of blue light under Shading (W) was significantly lower than that under CK conditions; under Shading (B), it was higher than that under all conditions. The differences in light quality may affect photosynthesis in leaves and final starch synthesis, resulting in increased chalky grain rate, chalkiness, and poor rice quality. White cotton yarn as the shading material for further research used to simulate the influence of the light environment on rice growth under cloudy conditions will be better than black net.

The quantity and quality of tillers determine the yield of rice. In order to explore how optimized nitrogen fertilizer application (OFA) increases rice yield by affecting tiller growth, a pot experiment with three nitrogen treatments was performed on the basis of previous researches to investigate the growth and development of tillers. Results showed that under OFA, the emerging rate of secondary tillers and high leaf position tillers decreased, which increased with the number of primary tillers. The decrease of ineffective tillers increased the accumulation of biomass and nitrogen per tiller, which promoted the development of panicles. Compared with traditional nitrogen fertilizer application (TFA), the differentiated number of spikelets increased by 10.85%–21.70%, which led to the total number of filled spikelets increasing by 9.67%–18.95%, resulting in 9.6% increase in rice yield. Primary tillers, especially at the first, second, fifth, and sixth leaf positions, were the superior tillers in good quality, which made great contribution to rice yield and were significantly affected by nitrogen application. Making full use of the regulation effect of nitrogen on the quality of tillers will help to stabilize rice yield with less nitrogen input or increase rice yield without adding nitrogen input. Graphical abstract

Low light stress increases the chalkiness of rice; however, this effect has not been fully characterized. In this study, we demonstrated that low light resulted in markedly decreased activity of ADP-glucose pyrophosphorylase in the grains and those of sucrose synthase and soluble starch synthase in the early period of grain filling. Furthermore, low light also resulted in decreased activities of granule-bound starch synthase and starch branching enzyme in the late period of grain filling. Therefore, the maximum and mean grain filling rates were reduced but the time to reach the maximum grain filling rates and effective grain filling period were increased by low light. Thus, it significantly decreased the grain weight at the maximum grain filling rate and grain weight and retarded the endosperm growth and development, leading to a loose arrangement of the amyloplasts and an increase in the chalkiness of the rice grains. Compared to the grains at the top panicle part, low light led to a greater decrease in the grain weight at the maximum grain filling rate and time to reach the grain weight at the maximum grain filling rate at the bottom panicle part, which contributed to an increase in chalkiness by increasing the rates of different chalky types at the bottom panicle part. In conclusion, low light disturbed starch synthesis in grains, thereby impeding the grain filling progress and increasing chalkiness, particularly for grains at the bottom panicle part.