Over N fertilization is a common problem for the winter wheat (Triticum aestivum L.)–summer maize (Zea mays L.) rotation system in the North China Plain. A field experiment which included control (no N), conventional N (Con. N) fertilization, and optimized N (Nmin) fertilization treatments, was conducted from 1999 to 2003 near Beijing, China. Soil nitrate (NO3) dynamics were measured and N balance was calculated for the period of the eight successive cropping seasons. Soil NO3–N in the 0‐ to 90‐cm profile for the Con. N treatment ranged from 157 to 700 kg ha−1 during the eight successive cropping seasons, much greater than those in the no N and optimized N treatments. Large amounts of soil NO3–N were detected in the 90‐ to 200‐cm layer under the conventional N fertilization treatment, especially in the summer maize season. For the Nmin treatment, the total amount of N applied was 511 kg N ha−1 in the eight successive crops as compared with 2400 kg N ha−1 of the Con. N treatment. Grain yields were not different between the fertilized treatments except for maize in 2003. Soil NO3–N in the root zone under conditions of optimized N fertilization was maintained at a relatively low level as compared with the Con. N treatment, therefore dramatically decreasing NO3–N movement to deeper soil profile. This study indicates that soil NO3 movement out of the effective crop root zone is an important pathway of N losses in this winter wheat–summer maize rotation system in the North China Plain and the optimized N fertilization by an improved Nmin method shows high potential of reducing N‐leaching losses.
A unique physical feature of paddy rice elds is that rice is grown on ooded soil. During the period of ooding and rice transplanting, there is a large proportion of surface water in a land surface consisting of water, vegetation and soils. The VEGETATION (VGT) sensor has four spectral bands that are equivalent to spectral bands of Landsat TM, and its mid-infrared spectral band is very sensitive to soil moisture and plant canopy water content. In this study we evaluated a VGT-derived normalized diVerence water index (NDWI VGT =(B3 MIR)/ (B3+MIR)) for describing temporal and spatial dynamics of surface moisture. Twenty-seven 10-day composites ( VGT-S10) from 1 March to 30 November 1999 were acquired and analysed for a study area (175 km by 165 km) in eastern Jiangsu Province, China, where a winter wheat and paddy rice double cropping system dominates the landscape. We compared the temporal dynamics and spatial patterns of normalized diVerence vegetation index (NDVI VGT ) and NDWI VGT . The NDWIVGT temporal dynamics were sensitive enough to capture the substantial increases of surface water due to ooding and rice transplanting at paddy rice elds. A land use thematic map for the timing and location of ooding and rice transplanting was generated for the study area. Our results indicate that NDWI and NDVI temporal anomalies may provide a simple and eVective tool for detection of ooding and rice transplanting across the landscape.
Metrics & MoreArticle Recommendations CONSPECTUS: Organic photovoltaics (OPVs), in which blend films of organic or polymer electron donor and electron acceptor are used as the active layer, are a promising photovoltaic technology with the great advantages of solution processing, low cost, and flexibility. The development of small molecular or polymer electron acceptors has boosted power conversion efficiency (PCE) of OPVs from 10% to 18%. Among them, polymer acceptors have the merits of superior morphology stability and excellent mechanical properties. However, owing to the key requirement of very low-lying LUMO/HOMO energy levels for polymer acceptors, very few conjugated polymers can work as polymer acceptors in OPVs. The majority of polymer electron acceptors are based on strong electron-withdrawing imide units or cyano substituents. Since 2015, conjugated polymers containing the boron−nitrogen coordination bond (B←N) have emerged as a new kind of polymer electron acceptor with excellent photovoltaic performance in various kinds of organic photovoltaic devices. In this Account, we summarize our research progress on polymer acceptors containing B←N units.At first, we introduce the principle of B←N to greatly down shift LUMO/HOMO energy levels, which enables B←N to be used to design polymer acceptors. Then we describe the two molecular design strategies for polymer acceptors containing B←N units. For high-efficiency OPVs, polymer acceptors should have wide absorption spectra, proper LUMO/HOMO energy levels, high electron mobility, and good donor/acceptor blend morphology. We discuss how to use molecular design to finely tune the absorption spectra, energy levels, and electron mobility of the B←N-containing polymer acceptors. We also discuss how to improve the phase separation morphology of the blends of these polymer acceptors with small molecular donors or polymer donors. These improvements lead to excellent performance of the polymer acceptors containing B←N units in three kinds of organic photovoltaic devices. The small molecular donor/polymer acceptor type organic solar cells show excellent thermal stability and PCE of 8.0%, which is the highest value reported so far. The all-polymer solar cells exhibit PCE of 10.1%. The all-polymer indoor photovoltaics show PCE as high as 27.4% under fluorescent lamp illumination at 2000 lx. This PCE is fairly comparable to those of the best organic or inorganic indoor photovoltaics. These results provide a solid foundation for future advances. Finally, we propose that great attention should be paid to further PCE enhancement of OPVs and indoor photovoltaic applications of this new emerging kind of polymer acceptor.
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