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[AGE30-P06] Initial aggregate formation of masa soil with repeated wet/dry cycles
Keywords:soil aggregation, wet/dry cycles, organic matter
Soil has both permeability and water retention properties, and these properties support plant growth on the earth. In ancient times, the Earth's surface was composed of only primary weathered materials, and single-grained structures are thought to have prevailed. With the emergence of plants, organic matter was added to the surface, and aggregate structure developed. The mechanism of the formation of soil aggregates in ancient times is still unclear, but if it can be clarified, it may lead to the improvement of today's degraded soils. In this study, unweathered soils were mixed with different proportions of organic matter, and the particle size distribution and water holding capacity of the soils were evaluated to determine whether or not initial soil aggregates could be formed in a short period of time.
Considering that rainfall and subsequent drying, i.e., repeated wetting and drying, are necessary for the formation of soil aggregates, we conducted an experiment in which wet incubation and hot drying were alternated. Air-dried masa soil sieved through a 2 mm sieve was prepared as unweathered soil, and starch was prepared as organic matter. 0, 1, and 5 w/w% starch was dissolved in water and mixed with the masa soil. This was filled into a 50 ml centrifuge with 1.0 mm diameter holes at the top and bottom for air exchange and free drainage. The volumetric water content was kept at 30 %. Three replicates were made for each amount of organic matter, for a total of nine samples. After measuring the weight, they were placed in a thermostatic bath at 25 °C for 12 hours and then in a drying bath at 40 °C for 12 hours. Twenty-four hours after the initial mass was measured, and the wetting and drying cycle was repeated in the same way, adding water to compensate the decrease from the initial mass. After 5 days, the samples were air-dried and the particle size distribution of each sample was examined using a laser diffraction particle size analyzer. The water holding capacity of each sample was also examined.
The flocculation ratio, which is the difference in particle size distribution with and without organic matter decomposition and dispersion treatment, was highest for 5 % organic matter and lowest for 0 % organic matter. The aggregation sizes with particle size accumulation exceeding 1 % for the first time were 0 %: 40 μm, 1 %: 300 μm, and 5 %: 20 μm, respectively.
The average water retention curves for each organic content showed that the difference in volumetric water content was small when the water absorption was small, but when the water absorption was large, the volumetric water content of 5 % organic matter was relatively high, indicating that water retention was promoted by increasing the organic matter content. At 5% organic matter, small soil particles formed a kind of aggregate, suggesting that water retention was enhanced under high suction.
In conclusion, it was confirmed that the addition of 0, 1, and 5 mass% organic matter and alternating wet and dry environments resulted in the formation of initial aggregates even in unweathered soils such as masa soil within a short incubation period of 5 days, and the addition of 5% also improved the water holding capacity.
Considering that rainfall and subsequent drying, i.e., repeated wetting and drying, are necessary for the formation of soil aggregates, we conducted an experiment in which wet incubation and hot drying were alternated. Air-dried masa soil sieved through a 2 mm sieve was prepared as unweathered soil, and starch was prepared as organic matter. 0, 1, and 5 w/w% starch was dissolved in water and mixed with the masa soil. This was filled into a 50 ml centrifuge with 1.0 mm diameter holes at the top and bottom for air exchange and free drainage. The volumetric water content was kept at 30 %. Three replicates were made for each amount of organic matter, for a total of nine samples. After measuring the weight, they were placed in a thermostatic bath at 25 °C for 12 hours and then in a drying bath at 40 °C for 12 hours. Twenty-four hours after the initial mass was measured, and the wetting and drying cycle was repeated in the same way, adding water to compensate the decrease from the initial mass. After 5 days, the samples were air-dried and the particle size distribution of each sample was examined using a laser diffraction particle size analyzer. The water holding capacity of each sample was also examined.
The flocculation ratio, which is the difference in particle size distribution with and without organic matter decomposition and dispersion treatment, was highest for 5 % organic matter and lowest for 0 % organic matter. The aggregation sizes with particle size accumulation exceeding 1 % for the first time were 0 %: 40 μm, 1 %: 300 μm, and 5 %: 20 μm, respectively.
The average water retention curves for each organic content showed that the difference in volumetric water content was small when the water absorption was small, but when the water absorption was large, the volumetric water content of 5 % organic matter was relatively high, indicating that water retention was promoted by increasing the organic matter content. At 5% organic matter, small soil particles formed a kind of aggregate, suggesting that water retention was enhanced under high suction.
In conclusion, it was confirmed that the addition of 0, 1, and 5 mass% organic matter and alternating wet and dry environments resulted in the formation of initial aggregates even in unweathered soils such as masa soil within a short incubation period of 5 days, and the addition of 5% also improved the water holding capacity.