Degradation of Bioplastics in Soil and Their Degradation Effects on Environmental Microorganisms

Degradation of three kinds of bioplastics and their effects on microbial biomass and microbial diversity in soil environment were analyzed. The degradation rate of bioplastic in soil was closely related to the main components in the bioplastics. Poly (butylene succinate)-starch (PBS-starch) and poly (butylene succinate) (PBS) were degraded by 1% to 7% after 28 days in a soil with an initial bacterial biomass of 1.4 × 109 cells/g-soil, however poly lactic acid (PLA) was not degraded in the soil after 28 days. When the powdered-bioplastics were examined for the degradation in the soil, PBS-starch also showed the highest degradability (24.4% degradation after 28 days), and the similar results were obtained in the case of long-term degradation experiment (2 years). To investigate the effect of bacterial biomass in soil on biodegradability of bioplastics, PBS-starch was buried in three kinds of soils differing in bacterial biomass (7.5 × 106, 7.5 × 107, and 7.5 × 108 cells/g-soil). The rate of bioplastic degradation was enhanced accompanied with an increase of the bacterial biomass in soil. 16S rDNA PCR-DGGE analysis indicated that the bacterial diversity in the soil was not affected by the degradation of bioplastics. Moreover, the degradation of bioplastic did not affect the nitrogen circulation activity in the soil.

Characterization of Orchard Fields Based on Soil Fertility Index (SOFIX)

Soil samples from 139 agricultural orchard fields (apple, grape, tea, and others) were analyzed using the soil fertility index. From these samples, an orchard field database was constructed and the soil properties between orchard, upland, and paddy fields were compared. The average value of bacterial biomass in the orchard fields was 7.4 × 108 cells/g-soil, ranging from not detected (lower than 6.6 × 106 cells/g-soil) to 7.7 × 109 cells/g-soil. The average values of total carbon (TC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK), were 24,000 mg/kg (2670 to 128,100 mg/kg), 1460 mg/kg (133 to 6400 mg/kg), 1030 mg/kg (142 to 5362 mg/kg), and 5370 mg/kg (1214 to 18,155 mg/kg), respectively. The C/N and C/P ratios were 19 (3 to 85) and 27 (2 to 101), respectively. Soil properties of the orchard fields were compared with those of the upland and the paddy fields. The average value of bacterial biomass in the orchard fields was almost the same as that in the upland fields (8.0 × 108 cells/g-soil), but the number was lower than that in the paddy fields (12.9 × 108 cells/g-soil). The average values of TC and TN in the orchard fields fell between those in the upland fields (TC: 33,120 mg/kg, TN: 2010 mg/kg) and the paddy fields (TC: 15,420 mg/kg, TN: 1080 mg/kg). The relationship between the bacterial biomass and TC in the orchard fields resembled that in the upland fields. A suitable soil condition for the orchard fields was determined as TC: ≥25,000 mg/kg, TN: ≥1500 mg/kg, TP: ≥900 mg/kg and TK: 2500 - 10,000 mg/kg. These recommended values will be effective for the improvement of the soil quality in the orchard fields by enhancing the number and activities of microorganisms.

Utilization of Wood Biomass for Organic Soil Based on the Soil Fertility Index (SOFIX)

Possibility of wood biomass for preparing organic soil was examined to construct reproducible and stable organic standard soil. Seven organic soils were constructed from base soils and additive materials based on the recommended values of the soil fertility index (SOFIX) (total carbon ≥ 25,000 mg/kg, total nitrogen ≥ 1500 mg/kg, total phosphorus ≥ 1100, and total potassium of 2500 to 10,000 mg/kg). Base soils were prepared from two types of wood biomass (big- and small-sized wood chips) at 50%, 60%, and 70% (v/v) and other organic materials such as peat moss, black soil, and mountain soil. Additive materials (soybean meal, oil cake, cow manure, and bone meal) were amended into all organic soils at the same amount. Incubation experiment showed that bacterial biomass in all organic soil was greater than 6 × 108 cells/g-soil after addition of 30% of water content for 1 week. In addition, polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) analysis resulted in a stable bacterial diversity of the organic soil prepared from the small size wood chip at 70%. Chemical properties of all organic soils were within the recommended values of SOFIX. The plant cultivation experiment showed that fresh Brassica rapa var. peruviridis weights in the organic soils with 50%, 60%, and 70% of small-sized wood chip were 5%, 16%, and 27% higher than that of the chemical fertilizer-amended soil. The organic soil with 70% of small wood chip was the best in the seven organic soils in this study.

Suitable Soil Conditions for Tomato Cultivation under an Organic Farming System

This study was conducted to determine the suitable soil conditions for tomato cultivation under an organic farming system. Tomatoes were cultivated in chemically and organically fertilized experimental fields from 2013 to 2015 in Moriyama City, Shiga prefecture, Japan. Organically and chemically fertilized soils had different total carbon (TC) and total nitrogen (TN) contents, and different carbon-to-nitrogen ratios (C/N ratios). The tomato yields varied from 1290 to 5960 kg/0.1ha in the organically fertilized fields. The organic soil conditions for the highest tomato yield showed a TC content of ~33,000 mg/kg, TN content of ~1600 mg/kg, and a C/N ratio of ~21. The yield was reproducible in the organic fields under similar values of TC, TN, and C/N ratio in the soil. Significantly higher nitrogen and phosphorus circulation activities were observed in the high-yielding fields. Appropriate control of TC, TN, and C/N ratio is necessary for the enhancement of both microbial activity and tomato yield. Values of the important tomato quality parameters (lycopene, glutamic acid, and acid content) were also increased in the high- yielding tomato fields. We therefore suggest that a suitable soil condition for improving both the yield and quality of tomatoes in an organic farming system is TC of 30,000 - 36,000 mg/kg, TN of 1600 - 1900 mg/kg, and a C/N ratio of 18 - 21.

Analysis of the Alkane Hydroxylase Gene and Long-Chain Cyclic Alkane Degradation in <i>Rhodococcus</i>

To characterize the long-chain cyclic alkane (c-alkane) degradation of bacteria in Rhodococcus, we analyzed the relationship between the alkane hydroxylase gene (alkB) and long-chain c-alkane degradation in 19 species. Eleven strains which were isolated from nature using long-chain c-alkane as a substrate were identified as R. erythropolisc, and all were shown to carry the alkB [alkB R2 type]. This gene type was also carried by two other species, R. rhodochrous and R. baikonurensis. In total, 17 species of the genus Rhodococcus carried alkB, but the gene types differed from each other. The two species R. rhodnii and R. coprophilus did not carry alkB, and their long-chain c-alkane degradation levels were low.