農業気象 Vol. 73(2017) No. 2

　Genotypes with high potential yield would improve crop production. Trials that examined potential yield have been conducted around the world, producing databases that can be mined to reveal “hidden” high-yield cultivars. However, yield data from different times and locations are not comparable because yield integrates the effects of cultivar-specific potential with the effects of weather and management practices. Here, we hypothesized that cultivar-specific yield can be expressed as a function of the climatic potential yield, which is calculated using a model based on daily solar radiation, temperature, and phenology data. To test this hypothesis, we used a rice (Oryza sativa L.) yield database from Japan, including only data from years with normal climatic conditions and trials with optimal N fertilization. For cv. ‘Sasanishiki’, which is widely grown in northern Japan, data from four prefectures and 20 years showed yield. The yield variations could be expressed by a single unique statistically significant regression across prefectures and years as a function of the climatic potential yield. This method demonstrated that ‘Koshihikari’ produced 10% less and ‘Fukuhibiki’ produced 19% more than ‘Sasanishiki’ for a given climatic potential yield (1000 g m^-2). We confirmed this ranking by direct comparisons of the cultivars in identical years and at the same locations. Our method can be used for data mining to identify high-yield cultivars through data from previous yield research. We discuss the limitations and advantages of this method, its potential for other crop species, and its potential for determining responses to abiotic and biotic stresses.

　Leaf litter decomposition strongly affects the global carbon cycle through carbon dioxide (CO₂) emissions to the atmosphere. The litter bag method (LB) and chamber method with litter addition and removal treatments (C-LART) have been used to quantify the litter decomposition rate and its resultant CO₂ flux. The C-LART method measures soil CO₂ fluxes in control, litter addition, and litter removal plots, and thereby decomposition rates are calculated from differences of the fluxes. However, no report has described the applicability of C-LART in comparison with LB. This study measured the litter decomposition rate and its resultant CO₂ flux using C-LART and LB in a temperate evergreen forest in central Japan to assess the applicability of the two methods. Annual soil respiration in the control plot was 1572 gC m^-2 yr^-1, which was approximately twice as high as the mean of temperate evergreen forests in the world. The litter decomposition rate was 0.42 g g^-1 yr^-1 in mass loss or 0.49 gC gC^-1 yr^-1 in carbon loss by LB, which are compatible with those reported from other temperature forests. In contrast, the decomposition rate of litter carbon ascertained using C-LART was greater than 1 (1.96 3.76 gC gC^-1 yr^-1), meaning that carbon emissions increased more than applied, and that the carbon emissions were decreased more than those removed by litter treatments. The incredibly high decomposition was attributed to the enhanced or restricted microbial activities in the underlying mineral soil. Changes in microbial activity are probably caused by the alteration of material supply from the leaf litter layer to the soil by litter treatment (the priming effect). In conclusion, C-LART is not applicable to evaluate CO₂ emissions through litter decomposition. Another approach must be used to compensate the priming effect for application of the chamber method.

　To clarify the effect of organic matter application on soil carbon sequestration in orchards, long-term field experiments (>10 years) were conducted at three sites (in Tsukuba, Yamanashi, and Omura) characterized by different fruit crop species, soil types, and climate. Three treatments were established in plots at all sites: (i) clean cultivation (CC, the control), in which chemical fertilizer was applied and the ground was kept bare; (ii) sod culture (SC), in which chemical fertilizer was applied and the ground was covered by grass or weeds; and (iii) organic amendment (OA), in which chemical fertilizer and cattle manure (OA_cat) or bark compost (OA_brk) were applied and the ground was kept bare. At Tsukuba, annual changes in soil organic carbon concentration (a_soc) were lowest in CC and highest in OA_cat and OA_brk plots. At Yamanashi, CC plots lost soil carbon, and a_soc increased the highest in OA_cat plots. At Omura, a_soc was negative in CC and SC plots and was positive in OA_brk plots. Within treatments, annual changes in soil organic carbon were highest in OA plots and lowest in CC plots at all sites; positive differences between control and treatment plots indicated that application of organic matter increased soil carbon sequestration.

　Methods have been developed and evaluated to collect ¹³⁷Cs in stemflow to investigate the possibility of secondary radiocaesium contamination via stemflow in Japanese persimmon (Diospyros kaki Thumb.) orchards. Collection pads were made by encapsulating sphagnum or absorbent cotton in a tea pack (polyester-polyethylene composite fiber). In addition, stemflow was collected directly by a plastic zipper bag (collection bag). A significant linear relationship between the amount of ¹³⁷Cs per 1 g sphagnum collected and the accumulated precipitation over the measurement period was evident, indicating a close relationship between outflow of ¹³⁷Cs from the bark and precipitation. There was a significant difference between the percentages of dissolved ¹³⁷Cs to the total ¹³⁷Cs in stemflow collected by the sphagnum pads and the collection bag (Kruskal-Wallis test, P < 0.01), suggesting that organic mediums consisting mainly of cellulose should not be used for the investigation of dissolved ¹³⁷Cs. The weight of stemflow, the amount of ¹³⁷Cs in particulate form and the total ¹³⁷Cs in the stemflow collected with sphagnum were significantly higher than those in the water collected with cotton. Accordingly, it is concluded that collection pads using sphagnum are more appropriate as stemflow collectors to quantify secondary deposition because of enabling to fix anywhere.