4.1. Influence of Sub-Materials on Radiocesium Levels and Chemical Properties of Bamboo and Woody Composts
The level of Cs content in each of the materials did not change throughout the composting process. However, the concentration of radioactive Cs in the source rice bran and wheat meal was very low. Thus, the observed decrease in radioactive Cs concentration was achieved by mixing highly contaminated raw materials (WC, BL or BP) with low contaminated sub-materials (rice bran or wheat meal). The higher moisture content of the final compost compared to the raw materials would also influence Cs concentration. In addition, the greater 137
Cs concentration may be related to the longer half-life of 137
Cs compared to 134
Cs. Our results suggest that radioactive Cs contamination can be successfully reduced through the addition of low-contaminated sub-materials, even for highly contaminated raw composting materials. Thus, our study shows that composting can be used as a viable strategy to remediate and reclaim sites contaminated with radionuclides. However, the authors suggested avoiding the use of organic materials highly contaminated with radioactive Cs in soils to prevent its accumulation in crops. To the extent of our knowledge, there are no studies describing the variation in Cs concentration before and after composting. Some previous research has assessed the transfer of radioactive Cs from soil to crops following soil fertilization with contaminated organic materials [26
]. Alexakhin (1993) [27
] also reported that agricultural practices could effectively mitigate the radiological consequences of the Chernobyl accident in pasture grasslands. Entry et al. (2001) [28
] reported that adding organic matter to the soil can initiate remediation and reclamation of contaminated soils.
The results of this study suggested that the addition of sub-materials effectively changed the pH of WC and BP from slightly acidic to neutral, thereby improving the quality of the final compost. According to previous research, the optimal pH value for green waste compost is neutral [29
], and it is closely related to microbial activity during composting [30
]. A neutral pH enhances the water solubility of nutrients available for plant uptake, in particular that of P and many micronutrients [31
]. In addition, previous research has reported an optimal pH of finished compost in the range of 7.5–8 [32
]. According to Mulligan (2005) [33
], an increase in microbial activity would also increase the decomposition rate of readily degradable organic nitrogen, increase the amount of ammonia released and lead to a sharp increase in pH. The later phase of the secondary fermentation is associated with a reduction of readily degradable organic matter and a decline in the decomposition rate; at this point, the pH values decreased in all treatments and subsequently remained stable [30
]. This decrease in pH could be related to an increase in nitrification experienced toward the end of the composting process, which acidifies the environment [34
In our study, we prevented mineral salt leaching and precipitation by frequently mixing the compost and using plastic trays as experimental plots. Thus, our findings suggest that mixing the materials during composting enhances the degradation process by supplying sufficient nitrogen to enrich the composting process. Our results regarding EC followed those reported by Li et al. (2012) [35
], who composted pig manure using bentonite. Liu and Price (2011) [36
] also reported an increase in EC by 107% in compost derived from spent coffee ground compared to the raw material. As described by Larney et al. (2008) [10
], the EC of WC is 10-times lower than that of straw EC. As described by Villar et al. (1993) [37
], the mineralization of organic matter and the subsequent release of soluble salts, such as phosphates and potassium, during the composting process would be the principal reason behind the observed increase in EC. In general, EC could be higher in the final product than in the initial mixture because of the release of soluble salts; however, it could also decrease because of ammonia volatilization and mineral salt leaching and precipitation [38
Adding rice bran and wheat meal to the compost increased the charge density per unit surface area by enhancing organic matter oxidation. Our results showed that adding sub-materials to the compost increased TN content and decreased the C/N ratio, enhancing the decomposition rate. According to Harada and Inoko (1980) [40
], compost maturity is characterized by a CEC > 60.0 cmol·kg−1
, suggesting that all of our final composts had reached maturity. A similar reduction in TOC content, associated with an increase in TN content, has been previously reported by Zhang and Sun (2014) [41
]. These trends have been previously reported in many previous composting studies and are related to a rapid decomposition of organic matter [39
]. These results follow those by Fang et al. (1999) [42
], who reported that, by the end of the composting period, the C/N ratio was below 20 for mature composts. Furthermore, Rodriguez-Kabana et al. (1987) [43
] reported that the C/N ratio of organic amendments should remain within the range of 12–25 to avoid phytotoxicity. Thus, our results suggest that our composts were suitable as a soil amendment.
Our results indicated that the addition of sub-materials increased the rate of mineralization of essential nutrients in the compost, as nutrient content was overall enhanced in WCC, BLC and BPC compared to that of WC, BL and BP. Since these nutrients are non-volatile, they tend to remain in the compost; therefore, their increase in concentration is a reflection of the rate of decomposition of organic matter during composting. The results of this composting study are in agreement with Mondonca Costa et al. (2015) [44
], who observed a significant increment in nutrient content in the final compost compared to the raw materials. The same observation was found by Nishanth and Biwas (2008) [45
] in a study on the effect of adding rock phosphate to treated compost. Liu and Price (2011) [36
] also observed an increase in macro- and micro-nutrient concentrations in the final compost compared to the raw materials.
In general, the aeration rate and turning frequency influence the nitrogen dynamics through volatilization and leaching. However, the low NH4+
contents and high NO3−
contents measured in the final composts in our study indicated that the addition of sub-materials created a favorable microenvironment for nitrifying bacteria, which convert ammonia to nitrate, helping N retention in the compost. Similar results have been reported in composting studies using sewage sludge, cattle manure and pig manure [42
]. According to Das et al. (2011) [48
], the NH4+
ratio is a good indicator of compost maturity. In our experiment, final NH4+
ratios were <0.5 (Figure 2
b), suggesting that all final composts had reached maturity.
4.2. Changes in Soil Properties through Compost Amendment
Our results showed that compost amendment reduced soil radioactive Cs concentration compared to amendments with raw materials. In addition, the influence of the compost on the soil Cs concentration was dependent on the type and the level of input. However, amendment with raw materials resulted in significantly high soil Cs concentrations because of their high contamination with radioactive Cs. Harada et al. (2014) [26
] showed that radioactive Cs concentration significantly increases after soil amendment with contaminated cattle farmyard manure. Thus, adding sub-materials, such as rice bran and wheat meal, emerged as an effective strategy to control soil radioactive Cs concentration following the FDNPP accident. In our study, final composts exhibited significantly lower radioactive Cs concentration than the raw materials; therefore, the application of final compost could be a viable strategy to reduce soil radioactive Cs concentrations following the FDNPP accident.
Comparatively lower organic C content in final composts compared to the raw materials supply evidences regarding the high nutrient availability in the compost amendment compared to the amending with raw materials. Thus, these results suggested that compost amendments can increase the concentration of soil nutrients available for plant growth, compared to amendments with raw materials. The results of our study follow those reported by Chaoui et al. (2003) [49
], who observed an increase in soil inorganic nitrogen after compost amendment. Mylavarapu and Zinati (2009) [50
] also reported that amending soil with compost significantly increased soil nitrogen concentrations. The lower C/N ratio of the final composts versus the raw materials indicates that the compost would be an effective source of N through rapid N mineralization reactions. Overall, our results indicated that compost amendment can increase soil inorganic nitrogen content compared to amending with raw materials. These results follow those reported by Courtney and Mullen (2008) [51
], who showed an increase of available K, Ca, Mg and Na in soils following compost amendments.
4.3. Compost Amendment Influence in Crop Growth
In this study, plant biomass increased with the compost input level, mainly due to an increment in nutrient availability with the increase in the amount of compost applied. The addition of sub-materials also enhanced compost nutrient content. The reduction in plant biomass in soils amended with raw materials was mainly due to low nutrient availability in these initial materials and the low concentration of decomposed products. Harada et al. (2014) [26
] demonstrated that the effects of using contaminated farm yard manure on the concentration of radioactive Cs found in crops depend not only on the contamination level, but also on factors, such as soil exchangeable K2
O content. Furthermore, Hoshino et al. (2015) [52
] reported that the ecological behavior of radioactive Cs is influenced by the amount of clay minerals present in the soil. Thus, the low radioactive Cs concentrations found in Komatsuna plants in this study could be related to these two effects. In addition, composts may directly increase the quantity of nutrient available to plants or indirectly through its effects on the CEC [53
]. The results of this study follow those reported by Courtney and Mullen (2008) [51
], who found that barley grain yield increased when the amendment rates of spent mushroom compost were increased. Cherif et al. (2009) [55
] also reported that compost amendments significantly enhanced wheat grain yield and that plots amended with higher compost nutrient contents also showed the highest wheat yields.
The higher levels of N accumulation linked to compost amendments may be related to the direct input of N to the soil through the compost. The addition of sub-materials also effectively enhanced the level of N input through compost compared to WC, BL and BP alone. Our results show a strong significant correlation between the plant’s N and soil inorganic N content (r2
= 0.634, p
< 0.05) in compost treatments. Thus, compost amendments can considerably influence mineral N dynamics in soil. According to Keeling et al. (2003) [56
], N is the key nutrient for plant growth, and yields are usually strongly related to N supply.