The mycelia of mycorrhizal fungi in the soil make contact with plant roots, extend the reach of the root system of plants, solubilise phosphorous and improve nutrient availability to plants. In return, they take carbohydrates from plants. In fact, these farmer-friendly fungi help crops tolerate several biotic and abiotic stresses.
But salinity and arsenic contamination in soil reduce the population of these friendly fungi.
And flooded paddy fields are also not conducive to these fungi.
Rice cultivation in the lowlands of Bangladesh and Bengal face these problems.
Last fortnight, researchers from the University of Rajshahi, Bangladesh in collaboration with researchers from Belgium reported the status of mycorrhizal fungi in such lowland rice fields. They selected 45 rice fields across eight different geographical sites in Bangladesh, with wide variations in salinity and arsenic content. All fields grew the kalijira variety of rice, known for its aroma, taste and grain quality. The variety is grown in flooded paddy fields that dry up towards harvesting time.
At the time of harvesting, the researchers collected soil samples and fine roots of five random plants.
They measured the chemical properties – pH, moisture, organic carbon, phosphorus, sodium and arsenic – of the soil samples. The concentration of arsenic ranged from 0.32 to more than 8 milligrams per kilogram of soil and the sodium concentration from 103 to 1601 milligrams per kilogram in the samples.
The researchers isolated genomic DNA from the root samples and amplified the ribosomal RNA genes specific to the fungi. The amplified genes from the samples were then sequenced to identify the mycorrhizal genera. The genus Glomeraceae was present in more than fifty percent of the samples. Claroideoglomeraceae, Acaulosporaceae, Gigasporaceae, etc. were also identified but in small percentages. The team found a total of eight mycorrhizal families in their samples.
The researchers attributed the survival of these fungi in the flooded fields to the presence of aerenchyma (tissue with air spaces) in the roots of rice. Perhaps the close association with the plant allows the fungi to survive the anoxic conditions in the soils of flooded paddy fields.
The mycorrhizal diversity reduced in proportion to the amount of arsenic and salt in the soil, as expected. But the researchers also found correlations between soil pH and soil organic carbon. The greater the soil organic carbon, the higher the population and diversity of mycorrhiza. The higher the pH, the lower the diversity of the fungi.
So it appears that controlling the pH and improving soil organic carbon are steps that farmers can take – to help increase mycorrhizal communities and thus to improve rice crop production in Bengal and Bangladesh.
Sci. Tot. Environ., 686: 546-554 (2019);