Effects of N in crops are profound, but much understanding of crop growth responses to N is empirical. This review attempts to develop a mechanistic understanding of the effects of N on crop biomass accumulation by elucidating quantitative relationships among leaf N content, CO2 assimilation rate, and crop radiation use efficiency. Three crop species were considered: soybean (Glycine max [L.] Men.), rice (Oryza sativa L.), and maize (Zea mays L.). The correlation between leaf N content and leaf CO2 assimilation rates was high within each species, although the response functions were markedly different among species. A relationship was developed predicting crop radiation use efficiency (biomass accumulated per unit solar radiation intercepted) for each of the crops as a function of both leaf CO2 assimilation rate and leaf N content. Radiation use efficiency within each species was nearly constant at high leaf CO2 assimilation rates, but decreased appreciably at low leaf CO2 assimilation rates. At the leaf CO2 assimilation rates typical of a species, the radiation use efficiency was predicted to be about 1.2 g MJ−1 for soybean, 1.4 g MJ−1 for rice, and 1.7 g MJ−1 for maize. Simple calculations during early crop growth examined the competitive use of N for the construction of either large leaf area or high leaf N content. Maize had the greatest biomass accumulation because it had low leaf N contents that allowed the most crop leaf area growth, and it had high radiation use efficiencies. For each rate of N supply to leaves, an optimum leaf N content existed to maximize crop biomass accumulation.

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The subionospheric VLF/LF propagation is extensively used to investigate the lower ionospheric perturbation in possible association with earthquakes. An extensive period of data over 7 yr from January 2001 to December 2007 and a combination of different propagation paths in and around Japan are used to examine the statistical correlation between the VLF/LF propagation anomaly (average nighttime amplitude, dispersion, and nighttime fluctuation) and earthquakes with magnitude >6.0. It is then found that the propagation anomaly exceeding the 2σ (standard deviation) criterion indicating the presence of ionospheric perturbation is significantly correlated with earthquakes with shallow depth (<40 km). Finally, the mechanism of seismoionospheric perturbations is discussed.

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Abstract.A superimposed epoch analysis has been undertaken, in order to find the correlation of the ionospheric perturbations with seismic activity. We take the wave path from the Japanese LF transmitter (frequency=40 kHz) and an observing station of Kochi (wave path length of 770 km), and a much longer period (of five years) than before, is considered. This subionospheric LF propagation can be called "an integrated measurement" in the sense that any earthquakes in the LF sensitive area just around the great-circle path can influence the observed LF signals, so that we define the "effective magnitude" (Meff) by integrating the total energy from different earthquakes in the sensitive area on a current day and by converting it back into magnitude. A superimposed epoch analysis for the effective magnitude greater than 6.0 has yielded that the ionosphere is definitely disturbed in terms of both amplitude and dispersion, and that these perturbations tend to take place prior to an earthquake. The statistical z-test has also been performed, which has indicated that the amplitude is definitely depleted 2-6 days before the earthquake day and also that the dispersion is very much enhanced during the same period. This statistical study has given strong support to the existence of seismo-ionospheric perturbations for high seismic activity.

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