Challenge to Resume Production of Mushroom Bed Logs by Potassium Fertilizer Application

Abstract

In order to resume production of mushroom bed logs in a forested area where radiocesium was deposited, it is important to reduce its absorption from the soil. In this study, we found that aggregated transfer factors of radiocesium in Konara oak (Quercus serrata Murray) shoots were low in forests with high amounts of exchangeable potassium in the soil. Moreover, application of potassium fertilizer to the soil surface increased the amount of exchangeable potassium in the soil, thereby suppressing absorption of radiocesium by newly planted Konara oak trees.

This chapter is a translation, with modifications, of Section III-2 of a Japanese publication, “Resumption of Use and Restoration of Shiitake Mushroom Log Forests in Radioactively Contaminated Areas” by the Forestry and Forest Products Research Institute, 2018, ISBN:978-4-905304-92-0.

17.1 Potential Effect of Forest Management to Reduce Radiocesium Content in Hardwood Trees

Radiocesium that adhered to the surfaces of tree trunks and branches immediately following the nuclear power plant accident has gradually diminished as bark peels off the trees. However, some radiocesium still remained on trunk and branch surfaces even several years after the accident (Fig. 17.1). Cutting these trees to induce regeneration of new shoots from stumps (coppicing) might be one way to reduce radiocesium contamination, since the radiocesium concentration in new trunks is assumed to be lower than that of directly contaminated trunks (unpublished). Another option would be to plant new seedlings, instead of coppicing. Although new seedlings would take years longer to achieve harvestable size compared to regenerating forests by coppicing, producing trees with lower concentrations of radiocesium should be prioritized. The source of contamination for both coppiced shoots and newly planted seedlings is radiocesium remaining in the soil that can be absorbed by roots. Thus, in order to produce bed logs for shiitake mushrooms in forests where radiocesium has been deposited, it is important to minimize absorption of radiocesium as much as possible.

Fig. 17.1

Autoradiographs of cross sections of oak trunks grown in a contaminated area. The upper left portion of each section, indicated by dashed lines, was partially stripped of its bark. Strongly radioactive spots can be seen in sections of the trunk where radioactive materials were directly deposited at the time of the accident (top two cross sections). Radioactivity was also found in the bark of trunks that sprouted from stumps after the accident (bottom two cross sections)

17.2 Negative Correlation Between Exchangeable Potassium in Soil and the Radiocesium Concentration in New Shoots

It has been reported that radiocesium content in current-year shoots correlates well with trunk radiocesium content (Kanasashi et al. 2020). Accordingly, we analyzed how much radiocesium was transferred to current-year shoots in hardwood stands in various regions of Fukushima. We found that there was a large variation in the amount of radiocesium transferred to new shoots in hardwood forests, even if the degree of soil contamination was about the same. Forests with larger contents of exchangeable potassium in the soil commonly have lower transfer factors of radiocesium to the shoots (Fig. 17.2). In many crops, radiocesium absorption by roots is suppressed when potassium is abundant around the roots (Yamaguchi et al. 2016). The results of this study indicate that potassium has a similar effect in woody plants such as Konara oak.

Fig. 17.2

The relationship between exchangeable potassium in soil and aggregated transfer factors to current year branches. Higher potassium content in the soil resulted in lower amounts of radiocesium transferred from the soil through roots to branches of the current year. Aggregated transfer factors were calculated by adding data from former crop fields to current-year branch data (Kanasashi et al. 2020)

In forest stands with abundant exchangeable potassium in the soil, such as a former crop field that had been continuously fertilized, it should be possible to grow trees that have absorbed little radiocesium.

17.3 Application of Potassium Fertilizer to Increase Exchangeable Potassium in the Soil

We applied potassium chloride fertilizer to the soil surface to see if we could increase the amount of exchangeable potassium. After 1 year, the amount of exchangeable potassium near the surface, where root density is generally high, increased in most forests, although some forests showed no increase (Fig. 17.3). We presumed this was due to differences in soil type. A comparison of radiocesium concentrations absorbed by newly planted seedlings over the course of a year showed a trend toward lower concentrations in forests where potassium fertilizer had been applied (Fig. 17.4). Potassium fertilizer likely reduces absorption of radiocesium by newly planted seedlings.

Fig. 17.3

Increase in potassium content in surface soil 1 year after application of potassium fertilizer to the surface. At most study sites, potassium chloride fertilizer application to the soil surface increased the exchangeable potassium content in the surface soil after 1 year, compared to before the application, but the potassium content returned to the original level after 2 years if the amount of potassium fertilizer applied was small

Fig. 17.4

Reduction of radioactivity in newly planted seedlings by application of potassium fertilizer to the soil surface. When potassium chloride fertilizer was applied to the soil surface (◇), exchangeable potassium in the soil increased and less radiocesium was transferred to branches of newly planted seedlings than in an adjacent plot, where no fertilizer was applied (○). However, no reduction effect was observed at sites that originally had high potassium levels (the lowest point)

However, 2 years after application of potassium fertilizer of several tens of grams per square meter (= several hundred kg/ha), the content of exchangeable potassium began to return to the original level (Fig. 17.3). To sustain the potassium fertilizer effect until harvest, it may be necessary to apply larger amounts or to repeat the application of potassium fertilizer.

Investigations at these experimental fertilization sites are ongoing and time-course results will be reported in the future.