Influence of soil type on forage corn and soil
The soil type (SS and TS) influenced the aboveground dry weight, radioactive Cs concentration, deposit density of forage corn, soil Cs concentration at harvest, and TF significantly (Table 3). All these values in TS, except TF, were higher than those values in SS, respectively (Table 4).
The aboveground dry weight of forage corn at harvest was higher in TS than that in SS in each year, and a significant difference was observed in 2014 (Table 4). Generally, in the subsoil, the bulk density is higher and the organic matter and mineral contents are lower than those in the TS (Saigusa 1989). Therefore, soil fertility tends to be higher in top soil than subsoil. In this experiment, the soil fertility might also have been higher in the TS layer than in the SS layer, leading to higher aboveground dry weight of corn in TS than that in SS.
The radioactive Cs concentration in forage corn at harvest was significantly higher in TS than in SS each year, and was lower than that of the provisional regulation value for radioactive Cs in cattle roughage of 100 Bq kg-1 fresh matter (MAFF, 2012a). The 134Cs + 137Cs content in the TS layer (3862 Bq kg-1) collected from the field prior to initiating this study was higher than that in the SS layer (101 Bq kg-1 dry soil, Table 1). The Cs concentration in corn was higher and the TF was lower in TS than that in SS (Tables 4 and 5).
Influence of cattle FYMC application on forage corn and soil
The compost applications (NC, LC, and HC) did not influence the aboveground dry weight, radioactive Cs concentration, deposit density of forage corn, and soil Cs concentration at harvest significantly, but influenced TF (Table 3). TF was the highest in NC, followed by LC and HC (Table 4).
In each year, there were no significant differences among the NC, LC, and HC treatments, and the combined effect of the soil type and compost application on the corn dry weight was not observed (Tables 3 and 4). However, the aboveground dry weight of corn was higher in the compost application treatments (LC and HC) than in treatments with no compost application in 2014 and 2015. Uenosono et al. (2008) reported that the residual 15N in the soil (0–25 cm) derived from 15N-labelled composted cattle manure after the second cropping ranged from 52% in the low-humic Andosol to 74% of the applied 15N in the Cummulic Andosol. In this experiment, we considered that the high rate of residual N in soil derived from FYMC applied in 2013 might cause increased contents of soil nutrients, such as N in 2014 and 2015. This in turn resulted in higher dry weight of corn in compost application treatments than in the treatment without compost application in 2014 and 2015. These results suggested that the continuous compost application to soil, especially SS after stripping off the TS was important to improve the crop productivity, as mentioned Nishiwaki et al. (2016).
The compost application did not increase the radioactive Cs concentration in corn and soil significantly (Tables 3). However, the continuous application of HC increased the soil Cs concentration before starting experiment each year, compared to the application of NC and LC (Table 1). Moreover, the 137Cs concentration of the corn and soil was similar in NC and LC, but the 137Cs concentration in HC was higher than that in NC and LC (Table 4, 5). These results suggested that the application of compost with high Cs concentration (range of 134 + 137Cs concentration in compost: 995–2316 Bq kg-1 fresh matter) can increase the Cs concentration of crops. However, during the study period of three years, the radioactive Cs concentration in the forage corn did not exceed the provisional regulation value for radioactive Cs in cattle roughage, (100 Bq kg-1 fresh matter) (MAFF, 2012a).
FYMC application to soil may increase potassium (K) in the soil and reduce the Cs concentration in crop (Zhu and Shaw 2000; Ogura et al. 2014). The soil exchangeable K2O content effective for reducing the radioactive Cs concentration in crop was approximately 0.35 g kg-1 dry soil (Harada 2014). In the 2014 experiment, the soil exchangeable K2O was higher in HC (0.43–0.63 g kg-1 dry soil) than that in LC and NC (0.15–0.31 g kg-1 dry soil, Fig. 1). In HC treatment, the exchangeable K2O concentration in soil was more than 0.4 g possibly because of the application of compost with high K content in HC than in LC (Table 2). Application of this HC with high exchangeable K2O concentration may have reduced the Cs concentration in corn in 2014.
Influence of the continuous application of cattle FYMC to contaminated soil on forage corn and soil
The continuous application of compost (LC and HC) influenced the aboveground dry weight and 137Cs concentration in forage corn at harvest, and soil 134Cs concentration and TF of 134Cs and 137Cs significantly (Table 3).
Fig. 2 shows average of the aboveground dry matter weight, Cs concentration in corn and soil, and TF included data in compost application treatments. The aboveground dry weight was same in 2013 and 2015, and significantly higher than that in 2014. It is considered that damage by pests is one of the factors that reduced the dry weight of corn in 2014.
In SS, the 137Cs concentration in soil increased every year with no significant differences (Fig. 2). The 137Cs concentration in corn did not increase every year, and it was significantly higher in 2013 than in 2014 and similar to the value in 2015. Consequently, the TF value of 137Cs in SS did not increase every year. Moreover, the 137Cs concentration in TS did not increase every year and showed no significant differences, but the 137Cs concentration in corn and TF increased every year, with a significantly higher value in 2015 than in 2013 and 2014. The 137Cs concentration of corn in LC/137Cs of corn in NC and values in HC/ NC were 0.63 and 0.77 in 2013, 1.02 and 1.32 in 2014, and 0.96 and 1.11 in 2015, respectively (Table 4). These relative values of LC and HC to NC did not increase over the years, although the Cs concentration in corn, and the TF value of 137Cs was higher in HC than that in LC and NC. The reasons for the increase in the Cs concentration in HC, as well as LC and NC every year were not clear. However, it was considered that the effect of compost application on the increase in Cs concentration and TF was not large.
These findings indicate that the continuous application of compost from 2013 to 2015 may have a potential to increase the crop productivity; additionally, it does not significantly change the radioactive Cs content in corn, as reported by Harada et al. (2014).