Masao Higo

Impact of a 5-year winter cover crop rotational system on the molecular diversity of arbuscular mycorrhizal fungi colonizing roots of subsequent soybean

The impact of winter cover crops, specifically wheat (Triticum aestivum L.), red clover (Trifolium pratense L.), and rapeseed (Brassica napus L.) or winter fallow, on community composition of arbuscular mycorrhizal fungi (AMF) in subsequent soybean roots was investigated in a 5-year field trial on andosolic soils in Japan. Soybean roots were sampled at full-flowering and analyzed for AMF communities using a partial LSU rDNA region. Phylogenetic analysis detected 22 AMF phylotypes, including eight Glomus, three Gigaspora, two Scutellospora, three Acaulospora, two Rhizophagus, and one of Funneliformis, Diversispora, Paraglomus, and an unknown glomeromycete in the roots. The 5-year rotation of different winter cover crops or winter fallow did not impact the molecular diversity of AMF communities colonizing the roots of subsequent soybean. In all of the rotations, Glomus and Gigaspora phylotypes were common to soybean roots over the 5-year period. Redundancy analysis (RDA) demonstrated that AMF communities in the roots of subsequent soybean were not significantly different among winter cover crop rotations or fallow. However, AMF communities in soybean roots were clearly influenced by rotation year suggesting that climate or other environmental factors were more important than winter cover cropping system management.

Inoculation with Arbuscular Mycorrhizal Fungi or Crop Rotation with Mycorrhizal Plants Improves the Growth of Maize in Limed Acid Sulfate Soil

Abstract Arbuscular mycorrhizal fungi (AMF) improve the uptake of immobile mineral nutrients such as phosphate, thereby improving plant growth. In acid sulfate soil (ASS), AMF spore density is generally low which impacts root colonization and phosphate uptake. Thus, inoculation may help increase AMF colonization of crops grown in ASS. AMF spore density decreases after cultivation of a non-host crop or bare fallow. In addition, preceding crops affect the growth and yield of subsequent crops. The production of AMF inocula requires AMF-compatible plants. The objective of the present study is to elucidate the effect of preceding crops on the persistence of inoculated AMF and growth of succeeding maize under an ASS condition with lime application. Spore density of AMF after cultivation of preceding crops (soybean or job’s tears) was maintained in comparison to fallow leading to higher AMF colonization of maize and improved plant growth. Thus, maintenance of AMF spore density, either through selection of preceding crops or application of AMF inoculum, may be a viable strategy to improve maize growth in limed ASS of Thailand.

Molecular diversity and spore density of indigenous arbuscular mycorrhizal fungi in acid sulfate soil in Thailand

Acid sulfate soil (ASS) has an extremely low pH (3.0) and a high capacity to fix phosphate; symptoms of phosphate deficiency are commonly observed in many crop plants. Arbuscular mycorrhizal (AM) fungi form mutualistic relationships with plant roots, and improve uptake of phosphate from soil. However, there is little information on the actual situation of AM fungi in ASS in Thailand. The purpose of the present study is to determine the indigenous AM fungal density and species in ASS in Thailand. AM fungal spores were retrieved and identified by molecular approaches from ASS field at the central plain of Thailand. This study showed that AM fungal spore density in ASS was 0.232 spores per g dry soil. Among the plant species growing in the natural ASS, there was no AM fungal colonization in the roots of four plant species, i.e. Digitaria sp., Fimbristylis sp., Mimosa pudica L., and Sesbania sp.; however, AM colonization was found in Wedelia roots. Using phylogenetic analysis, four operational taxonomic units (OTUs), i.e., one Glomus, one Entrophospora, one Paraglomus and one unknown species were identified from the AM fungal spores. Five OTUs, i.e., two Glomus, one Acaulospora, one Entrophospora and one unknown Glomeromycota were indentified from Wedelia roots. To our knowledge, this is the first report of the actual situation of AM fungi in ASS in Thailand determined by using molecular approaches.

Effect of Winter Crop Species on Arbuscular Mycorrhizal Fungal Colonization and Subsequent Soybean Yields

Abstract We evaluated how the cultivation of arbuscular mycorrhizal fungi (AMF) host (wheat) and non-host (rapeseed) crops affects the subsequent soybean crop by assessing AMF spore density and AMF colonization, phosphorus (P) uptake by soybean and yields of soybean over a 4-year period. Every year wheat or rapeseed was cultivated from autumn to spring and soybean from spring to autumn. From the first to fourth year, AMF spore density was higher in the plot after the cultivation of wheat (wheat plot) than in the rapeseed plot. From the second to fourth year, the AMF colonization ratio was higher in the wheat plot than in the rapeseed plot. In the first year, there was no difference in the AMF colonization ratio, growth, and P uptake by soybean plants between the rapeseed plot and wheat plot. However, from the second to fourth year, AMF colonization ratio, plant growth, and P uptake by soybean in the wheat plot were higher than those in the rapeseed plot. The soybean yields in both plots gradually decreased from the first to fourth year, but, in the second and the fourth year, soybean yields were higher in the wheat plot than in the rapeseed plot. Soybean yield was significantly correlated with the AMF colonization ratio, but not with AMF spore density. Therefore, we concluded that AMF colonization is not determined by AMF spore density alone, and other factors influence the AMF colonization in subsequent soybean plants. It is important to increase the AMF colonization ratio to increase soybean yield.

Molecular diversity and distribution of indigenous arbuscular mycorrhizal communities colonizing roots of two different winter cover crops in response to their root proliferation

A clear understanding of how crop root proliferation affects the distribution of the spore abundance of arbuscular mycorrhizal fungi (AMF) and the composition of AMF communities in agricultural fields is imperative to identify the potential roles of AMF in winter cover crop rotational systems. Toward this goal, we conducted a field trial using wheat (Triticum aestivum L.) or red clover (Trifolium pratense L.) grown during the winter season. We conducted a molecular analysis to compare the diversity and distribution of AMF communities in roots and spore abundance in soil cropped with wheat and red clover. The AMF spore abundance, AMF root colonization, and abundance of root length were investigated at three different distances from winter crops (0 cm, 7.5 cm, and 15 cm), and differences in these variables were found between the two crops. The distribution of specific AMF communities and variables responded to the two winter cover crops. The majority of Glomerales phylotypes were common to the roots of both winter cover crops, but Gigaspora phylotypes in Gigasporales were found only in red clover roots. These results also demonstrated that the diversity of the AMF colonizing the roots did not significantly change with the three distances from the crop within each rotation but was strongly influenced by the host crop identity. The distribution of specific AMF phylotypes responded to the presence of wheat and red clover roots, indicating that the host crop identity was much more important than the proliferation of crop roots in determining the diversity of the AMF communities.

How are arbuscular mycorrhizal associations related to maize growth performance during short-term cover crop rotation?

BACKGROUND Better cover crop management options aiming to maximize the benefits of arbuscular mycorrhizal fungi (AMF) to subsequent crops are largely unknown. We investigated the impact of cover crop management methods on maize growth performance and assemblages of AMF colonizing maize roots in a field trial. The cover crop treatments comprised Italian ryegrass, wheat, brown mustard and fallow in rotation with maize. RESULTS The diversity of AMF communities among cover crops used for maize management was significantly influenced by the cover crop and time course. Cover crops did not affect grain yield and aboveground biomass of subsequent maize but affected early growth. A structural equation model indicated that the root colonization, AMF diversity and maize phosphorus uptake had direct strong positive effects on yield performance. CONCLUSION AMF variables and maize performance were related directly or indirectly to maize grain yield, whereas root colonization had a positive effect on maize performance. AMF may be an essential factor that determines the success of cover crop rotational systems. Encouraging AMF associations can potentially benefit cover cropping systems. Therefore, it is imperative to consider AMF associations and crop phenology when making management decisions.

Varietal Difference in the Occurrence of Delayed Stem Senescence and Cytokinin Level in the Xylem Exudate in Soybeans

Abstract The objectives of this study were to evaluate the varietal difference in the occurrence of delayed stem senescence (DSS) and cytokinin level in the xylem exudate in soybeans and the relationship between the occurrence of DSS and cytokinin level in the xylem exudate. Pot experiments were carried out in the experiment field at Nihon University in 2010, 2012 and 2013. In this experiment, we used 11 soybean varieties, which were cultivated in the Kanto region, Japan. The degree of DSS (DSS score) was positively correlated with the days from sowing to flowering stage (S-R2), and was higher in the varieties with a longer length of S-R2, though the correlation was not significant. Under some conditions, the DSS score was negatively correlated with S-R2. Moreover, the DSS score was positively or negatively correlated with the main stem diameter, total node number, stem dry weight and seed weight depending on the growth parameter or sowing date. Thus, we concluded that the DSS score was not correlated with the growth parameters. On the other hand, the level of cytokinins such as t-ZR and iPA shown by their amount in xylem exudate from a plant at the seed filling stage was negatively correlated with the DSS score. Thus, we consider that one of the reasons for varietal difference in DSS occurrence may be the difference in cytokinin content of stem and leaves after the seed filling stage.

Can phosphorus application and cover cropping alter arbuscular mycorrhizal fungal communities and soybean performance after a five-year phosphorus-unfertilized crop rotational system?

Background Understanding diversity of arbuscular mycorrhizal fungi (AMF) is important for optimizing their role for phosphorus (P) nutrition of soybeans (Glycine max (L.) Merr.) in P-limited soils. However, it is not clear how soybean growth and P nutrition is related to AMF colonization and diversity of AMF communities in a continuous P-unfertilized cover cropping system. Thus, we investigated the impact of P-application and cover cropping on the interaction among AMF colonization, AMF diversity in soybean roots, soybean growth and P nutrition under a five-year P-unfertilized crop rotation. Methods In this study, we established three cover crop systems (wheat, red clover and oilseed rape) or bare fallow in rotation with soybean. The P-application rates before the seeding of soybeans were 52.5 and 157.5 kg ha−1 in 2014 and 2015, respectively. We measured AMF colonization in soybean roots, soybean growth parameters such as aboveground plant biomass, P uptake at the flowering stage and grain yields at the maturity stage in both years. AMF community structure in soybean roots was characterized by specific amplification of small subunit rDNA. Results The increase in the root colonization at the flowering stage was small as a result of P-application. Cover cropping did not affect the aboveground biomass and P uptake of soybean in both years, but the P-application had positive effects on the soybean performance such as plant P uptake, biomass and grain yield in 2015. AMF communities colonizing soybean roots were also significantly influenced by P-application throughout the two years. Moreover, the diversity of AMF communities in roots was significantly influenced by P-application and cover cropping in both years, and was positively correlated with the soybean biomass, P uptake and grain yield throughout the two years. Discussion Our results indicated that P-application rather than cover cropping may be a key factor for improving soybean growth performance with respect to AMF diversity in P-limited cover cropping systems. Additionally, AMF diversity in roots can potentially contribute to soybean P nutrition even in the P-fertilized cover crop rotational system. Therefore, further investigation into the interaction of AMF diversity, P-application and cover cropping is required for the development of more effective P management practices on soybean growth performance.

Effect of Fertilizer with Low Levels of Potassium on Radiocesium-137 Decontamination

Potassium (K) fertilizer closely affects both above-ground biomass production and radiocesium-137 (137Cs) uptake by plants from Cs-contaminated soil. Here we investigated the effects of low levels of K on aboveground biomass, 137Cs concentration, and 137Cs removal ratio (CR) using Napier grass (Pennisetum purpureum Schum.). Experimental plots fertilized with either 2 g K (20% K) or 6 g K (60% K) per m2 were compared with that with no K fertilization (0% K) under low-, medium-, and high-density planting conditions. As a result, with increasing amounts of K fertilizer, the aboveground biomass of Napier grass tended to increase, and the 137Cs concentration significantly decreased. The greatest CR was observed at the plot fertilized with 20% K and with plants grown under highly dense conditions. CR was positively correlated with biomass in all the experimental plots. Moreover, a positive correlation between CR and 137Cs concentration was found only at low levels of K application (0% and 20% K). In the present study, the application of 20% K produced more aboveground biomass and smaller decreases in 137Cs concentration in Napier grass compared with those with 0% K application; therefore, applying a fertilizer with low levels of K, such as 20% K, is better for 137Cs decontamination than applying 0% or 60% K fertilizer.