Hideki Sugimoto

Comparison of different methods for estimating soil surface evaporation in a bare field

In this paper, three methods for estimating soil evaporation in a bare field were evaluated: evaporation ratio method (k ratio), complementary relationship and bulk equation. Micro-lysimeters were used to measure the actual evaporation for validation of the three methods. For the k ratio method, pan evaporation was used as the reference evaporation instead of the value obtained from the Penman–Monteith equation. This result is important for areas where meteorological data are unavailable. The results showed that, for daytime evaporation, the k ratio and bulk equation produced a good fit with the observation data, while the complementary relationship generated a larger deviation from the measured data. We recommend that the k ratio method and bulk equation could be used to calculate daytime soil evaporation with high accuracy when soil water content and pan evaporation data or meteorological data are available, while the complementary relationship could be used for a rough estimation when pan evaporation is available. All the methods could be applied to calculate cumulative evaporation.

Photosynthetic and Transpiration Rates of Soybean as Affected by Different Irradiances During Growth

The diurnal variation of net photosynthetic (PN) and transpiration (E) rates in soybean [Glycine max (L.) Merr. cv. Fukuyutaka] plants grown under 100, 50, or 25 % of full sun irradiance (I100, I50, I25 plants) were compared. In the morning, activities of the plants were measured at irradiances under which they grew. However, during the afternoon, all the plants were tested under full irradiance. The lower the growth irradiance, the lower PN, E, and mesophyll conductance values were found. Stomatal conductance was considerably lower in I25 plants only. Both the increase in specific leaf area (SLA) and the decrease in nitrogen content per leaf area unit contributed to the PN reduction of soybean plants grown under low irradiances. Though E of the plants grown under different irradiances differed less markedly than PN, the water use efficiency declined from I100 to I25.

Effect of topdressing on individual leaf photosynthesis at different position in direct-sown rice with non-woven fabric mulch system

Direct sowing with non-woven fabric mulch is the new organic rice cultivation system. We studied the effect of topdressing on individual leaf photosynthesis at different position and grain yield in rice plants cultivated by this system. Leaf photosynthetic rate at the different leaf position per plant (PN-LP) of the third and fourth to lower leaves was higher when the topdressing amount was increased. Without topdressing or in no-fertilizers plots, the PN-LP values of lower leaves were very low. The leaf photosynthetic rate per unit leaf area (PN-LA) decreased gradually as the leaf position became lower. Again, the PN-LA values of the top-dressed plots at the lower leaves were higher than that of plots without topdressing or without fertilizers. The lower leaves maintained a higher PN because of a higher rate of nitrogen accumulation due to topdressing. The higher rate of photosynthesis in these leaves resulted in better root activity, which contributed to a better ripening percentage and ultimately higher rice grain yield.