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To investigate the effect of paddy-upland (PU) rotation system on greenhouse gas emissions, methane (CH4) and nitrous oxide (N2O) emissions were monitored for three years for a PU rotation field (four cultivations (wheat-soybean-rice-rice) over three years) and continuous paddy (CP) field on alluvial soil in western Japan. Soil carbon storage was also calculated using an improved Rothamsted Carbon (RothC) model. The net greenhouse gas balance was finally evaluated as the sum of CO2eq of the CH4, N2O and changes in soil carbon storage. The average CH4 emissions were significantly lower and the average N2O emissions were significantly higher in the PU field than those in the CP field (p < 0.01). On CO2 equivalent basis, CH4 emissions were much higher than N2O emission. In total, the average CO2eq emissions of CH4 plus N2O in the PU field (1.81 Mg CO2 ha−1 year−1) were significantly lower than those in the CP field (7.42 Mg CO2 ha−1 year−1) (p < 0.01). The RothC model revealed that the changes in soil carbon storage corresponded to CO2eq emissions of 0.57 and 0.09 Mg CO2 ha−1 year−1 in the both fields, respectively. Consequently, the net greenhouse gas balance in the PU and CP fields were estimated to be 2.38 and 7.51 Mg CO2 ha−1 year−1, respectively, suggesting a 68% reduction in the PU system. In conclusion, PU rotation system can be regarded as one type of the climate-smart soil management.

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Major factors that decrease soybean yields in Japan include excessive soil moisture and drought stress. In the present study, a WI (Wet Index) was developed using soil moisture estimates based on the FAO56 model to evaluate both risks of soil dryness and wetness. In addition, to assess the risk of wet losses, the days when ratio of WI was equal to one (RWI), which implied that soil moisture content reached a maximum, was adopted for every growth stage. When the relationship between WI or RWI and yield was evaluated using a partial correlation coefficient, WI had a positive correlation with emergence period approximately 30 -40 days after emergence, seed filling phase, and maturity phase, and RWI had a negative correlation with emergence period approximately 20 -30 days after emergence and approximately one week before flowering period. We evaluated moisture and drought stress risks, which present challenges in farmers' fields, and revealed the growing stages at which moisture and drought pose the highest risks. The results could offer critical insights for the estimation of dryness and wetness losses in the field.

Potassium (K) is an essential element for plants and plays a very important role in stomatal movement, water retention and enhancement of enzyme activity (Marschner, 2012). Therefore, its deficiencies cause inhibition of plant growth and a decrease of crop yield (Usherwood, 1985). It is the seventh most abundant element in the earth's crust, and it is also very abundant in soil, ranging from 0.4 to 30 g K kg −1 with a median