Symbiotic efficiency and compatibility of native rhizobia in northern Thailand with different soybean cultivars

Symbiotic efficiency and compatibility of 81 isolates of native bradyrhizobia from irrigated areas in northern Thailand with four soybean cultivars and one cowpea cultivar were evaluated under laboratory conditions. Effectiveness and / or compatibility of the tested isolates were compared with those of a standard strain (Bradyrhizobium japonicum CB 1809) by using plants grown on plastic seed bags. Effectiveness of the isolates was also estimated using uninoculated control plants grown in a nitrogen-free solution. Nodulation of a wide range of host plants by the majority of the tested isolates was observed, which agreed well with the results of our previous field experiment (Shutsrirung et al. 2002: Soil Sci. Plant Nutr., 48, 491–499). Up to 75% of the tested isolates induced a higher growth efficiency than that of the uninoculated control in association with one of the tested cultivars, Black soybean. Comparision with uninoculated control plants, enable to estimate the proportion of the tested isolates leading to effective growth promotion (E + e) of each cultivar, namely, Black soybean (local Thai cultivar), 75%; Cowpea, 82%; SJ5 (commercial Thai cultivar), 33%; Bragg (US cultivar), 33%; and Improved Pelican (US cultivar), 9%. These results indicated that although isolates with a high infectiveness with both “Asian-type” and “US-type” soybeans could be found, a high frequency of isolates leading to inefficient nodules was observed in the US cultivar, suggesting the presence of genetic differences in the soybean cultivars that express high-preference (efficient nodules) or low-preference (inefficient nodules) for a certain group of tested isolates. Based on the results of this laboratory experiment together with our previous field experiment, native rhizobial populations in the irrigated area of northern Thailand could be separated into three groups; Group 1: rhizobium strains showing a high effectiveness with only Asian cultivars, Group 2: strains showing a high effectiveness with only US origin cultivars, and Group 3: strains showing a high effectiveness with both Asian and US origin cultivars. The majority of the native rhizobial populations belonged to Group 1. The isolates in Group 3 may display a high potential for manipulating useful rhizobial inoculant.     

Nutrient leaching from the plow layer by water percolation and accumulation in the subsoil in an irrigated paddy field

To estimate the impact of water percolation on the nutrient status in paddy fields, the seasonal variations of the concentrations of cations, anions, inorganic carbon (IC), and of dissolved organic carbon (DOC) in percolating water that was collected from just below the plow layer (PW-13) and from drainage pipes at the 40 em depth (PW-40), as well as in irrigation water were measured in an irrigated paddy field. Total amounts of Ca, Mg, K, Fe, and Mn leached from PW-13 during the period of rice cultivation were estimated to range from about 390 to 770, 65 to 130, 33 to 66, 340 to 680, and 44 to 87 kg ha^-1, respectively. Amounts of losses that were estimated from the differences between the input by irrigation water and the output by percolation water from the plow layer corresponded to 11 to 26, 22 to 47,5.9 to 12, and 13 to 26% of exchangeable Ca and Mg, amorphous Fe, and easily reducible Mn in the plow layer, respectively. The concentrations of Ca, Mg, K, Fe, and Mn in PW13 were higher than those in PW-40. The amounts of these nutrients that were retained in the subsoil between the 13 em and 40 em soil depth corresponded to 83, 86, 61, 99, and 89% of the amounts that percolated from the plow layer, respectively. Total amounts of IC and DOC that percolated from the plow layer ranged from 750 to 1,500 and 85 to 170 kg-C ha^-1, which corresponded to 5.0 to 10.0% and 0.6 to 1.1% of the total carbon content in the plow layer, respectively. Eighty eight % of IC in the percolating water from the plow layer was also retained in the subsoil.     

Effect of fungicide thiram on microbial communities of rice seed surface estimated by culture-dependent and culture-independent (PCR-RFLP) methods

Coating of rice seeds with fungicide Thiram improved the seed germination capability over a long period of time (11 weeks) under low temperature conditions (4 and 8°C), which simulated the sowing of rice seeds in the winter season (the farmer's slack season). To analyze the effect of Thiram on the community structure of microorganisms on the rice seed surface, culture-dependent and culture-independent (PCR-RFLP) methods were applied. PCR-RFLP patterns of 16S rDNA showed that the bacterial communities on the rice seed surface were different between coated and uncoated treatments under 8°C conditions, but that they were very similar under 4°C conditions. PCR-RFLP patterns of 18S rDNA revealed the remarkable effect of Thiram on the fungal community structure under both 4 and 8°C conditions. Although the fungal communities were quite different between coated and uncoated seeds at the beginning of incubation, the fungal communities on the coated seed surface became similar to those of uncoated seeds along with the duration of the incubation period. As the dominance percentage of Fusarium spp. among the isolates increased with the duration of the incubation period for both coated and uncoated seeds, Fusarium was considered to be a responsible for the poor germination of rice seeds that were sown in the winter season.     

Influence of surface printing materials on the degradability of biodegradable plastic films in soil

Effect of surface printing on the biodegradability of plastic films was studied. Biodegradable films (polybutylene-succinate (PBS)) printed with four kinds of gravure inks were placed in soil for 1 year. The inks consisted of carbon black-pigment with four kinds of resins: poly-(ε-caprolactone) (PCL), nitrocellulose-polyamide blended resin (NT), polyvinyl chloride-vinyl acetate copolymer (V), and nitrocellulose (NC). Degradation of film specimens printed on one side and both sides as well as the control film without printing was monitored every 3 months by collecting sample specimens for the measurement of weight loss. No appreciable degradation was observed until 6 months after placement in soil for the control specimens and until 9 months for the printed specimens. And the degradation of the PCL- and NC-printed specimens with one-side printing and V-printed specimens with both-side printing was significantly slower than that of the control specimens without printing after 9-month placement at p < 0.05. Only after 12 months of placement, was the degradation significantly faster for the specimens printed on one side than for those printed on both sides except of the specimens printed with NC. There was no difference in biodegradability among PCL, NT, NC, and V resins. Specimens printed on both sides did not show any appreciable weight loss after 1 year in soil (percentage of maintenance of weight exceeding 98%). Microscopic observation indicated that the degradation mainly proceeded from the non-printed side to the printed side cross-sectionally.     

Microbial mineralization of organic nitrogen into nitrate to allow the use of organic fertilizer in hydroponics

Hydroponics is an excellent technique for the cultivation of vegetable crops and other plants, but organic fertilizers cannot be used in conventional hydroponic systems, which generally use only inorganic fertilizers, because organic compounds in the hydroponic solutions generally have phytotoxic effects that lead to poor plant growth. Few microorganisms are present in hydroponic solutions to mineralize the organic compounds into inorganic nutrients. In this article a novel and practical hydroponic culture method that uses microorganisms to degrade organic fertilizer in the hydroponic solution has been developed. Soil microorganisms were cultured by regulating the amounts of organic fertilizer and inoculum, with moderate aeration. The microorganisms mineralized organic nitrogen via ammonification and nitrification into nitrate at an efficiency of 97.6%. The culture solution containing the microorganisms was usable as a hydroponic solution, and organic fertilizer could be directly added to it during vegetable cultivation. Vegetables grew well in the organic hydroponic system. Organic hydroponics based on this method is therefore a practical tool for the utilization of organic sources of fertilizer.     

Application of N2/Ar ratio method for N2 fixation studies in submerged soil

Abstract

Recently there has been developments in the measurement of N₂ fixation due mainly to the C₂H₂ reduction method (1). This method, however, has several disadvantages, especially for submerged soil, and the estimated amount of fixed N₂ on the basis of the C₂H₂ reduction activity is not very reliable. The tracer ¹⁵N₂ technique which gives a reliable estimation of the fixed N₂ is too expensive for common use. Development of an alternative method suitable for submerged soil would therefore be desirable. The present authors expected that the measurement of the ratio N₂/Ar in the soil solution might provide advantages for the estimation of the fixed N₂ in submerged soil.     

Methane production and its fate in paddy fields

Abstract

Oxidation of methane and total water soluble organic carbon (TOC) in the subsoil, which percolated from the plow layer, was investigated in a column experiment. The amounts of both methane and TOC in the leachate decreased by percolation in the subsoil.

Fe²⁺ percolated from the plow layer was nearly completely retained in the subsoil. The decomposition of methane and TOC in the subsoil was considered to result in the coupling with the formation of Fe²⁺. Methane was estimated to contribute ca. 19–21% to the total amount of Fe²⁺ formed in the subsoil by the organic materials in the leachate.     

Solubilization of Insoluble Inorganic Phosphate by Hyphal Exudates of Arbuscular Mycorrhizal Fungi

Increased phosphate (P) uptake in plants by arbuscular mycorrhizal (AM) fungi is thought to depend mainly on the extension of external hyphae into soil. On the other hand, it is known that the hyphae of some kinds of ectomycorrhizal fungi release organic acids into soil and that they dissolve the insoluble inorganic P. This study collected hyphal exudates of AM fungi within compartmentalized pot culture and clarified their ability to solubilize insoluble inorganic P. Sterilized Andisol was packed in pots that were separated into root and hyphal compartments with a nylon net of 30 μm pore size. Seedlings of Allium cepa inoculated with AM fungi, Gigaspora margarita, or Glomus etunicatum were grown. Control pots were not inoculated. Mullite ceramic tubes were buried in the soil of each compartment and soil solution was collected. The anionic fraction of the soil solution was incubated with iron phosphate (4 mg FePO₄ in 1 mL of 0.4 acetate buffer). Solubilized P was measured. The AM colonization of plants inoculated with G. margarita and G. etunicatum was 86% and 54%, respectively. Adhesion of external hyphae was observed on the surface of the mullite ceramic tubes buried in soil of the hyphal compartment. Colonization of both fungi increased shoot P uptake and growth. Soil solution collected from the hyphal compartments of both fungi solubilized more P than did that from uninoculated plants. It is suggested that hyphal exudates can contribute to increased P uptake of colonized plants.     

Studies on the boron deficiency in field crops (Part I) boron deficiency symptoms in rape plants

In several areas of Japan, rape plants (Brassica napus) show a boron deficient smyptom, resulting in a seed-maturation obstacle. And in these areas, early supply of boron before transplanting were required⁵,⁶.