The Evolvement Of Quartzs And Phytoliths In Basalt-Derived Chronosequence Soils In Tropics Area Of China

Soil system is the most important interface in terrestrial ecosystem on Earth. During the process of soil development, the physical-chemical and biological processes could influence thetransfer transformation and redistribution of different silicon-containing minerals, which could finally be stored as as different fractions or forms in soil-plant system.This study, we collected chronosequence red soils, which derived from basalts erupted in different time periods in Leizhou Peninsula and northern Hainan Island of China.The content of soil organic carbon (SOC) and silicon oxide, soil pH and the content and morphotype of phyoliths and quartz were studied. The results obtained were as follows:(1) The content of organic carbon was relatively higher in the surface horizon of soil profiles, but decreased rapidly with depth, which had the trend of accumulation in surface soils. Soils showed acid, the pH value in surface horizon was lower in the some profile, the oldest soil had higher pH value. The content of total silicon was lower in the surface horizon in the younger soil profile, but with the increasing weathering of intensity and soil age, the content of total silicon was higher in the surface horizon, which showed the trend of silicon accumulation. The content of soil active Silicon was lower in the surface horizon of soil profiles, was and increased slowly with depth, which had significant correlations with soil age.(2) The content of phytoliths in the soils, ranging from 1.54 to 21.95 g·kg-1, was high in the surface horizon, but decreased with the depth in profile; From entisols to ferrosols, the content of phytoliths droped rapidly, but less significantly decreased from ferrosols to ferralsols. After entering ferrosols, the trend leveled off; As is know to all, the phytolith was mainly originated from poaceae and palmae, which ranged from 5 to 130μm in size and appeared in 12 shapes. for instance, spherical echinate, dumbbell, fan, rectangular, square, saddle, hat, rod, tapered, crenate, round. The warmth index of phytolith samples were all higher than 0.5, which indicated that this area had basically been a state of a warm climate since 6.12 million years (Ma). In tropical regions, phytoliths may have effects on in hindering the process of clay minerals weathering from 2:1 to 1:1 at the stage from primosols to ferralsol. and also played an important role in maintaining soil silicon "stable" or "balance" at the stage of ferrosols.(3)The contents of quartz in soils were widely variable from 5.09 to 28.83 g·kg-1. In the surface horizon, the contents of quartz were relatively higher, but decreased with the depth in profile. The contents had no good connections with soil age. and could not adequately express the red soil weathering degree. Whereas in the same profile, the weathering degree of quartz grains was stronger in the surface horizon. During the development precess of soil, the weathering degree increased gradually from Primosols to Ferralsols (0.01～1.33Ma). and the mechanically-formed features were totally obliterated. In Ferrallitic soil (6.12Ma). decomposition phenomena was occurred in quartz grains. This study confirmed the validity of the weathering score method of Darmody in the tropical area of China, and also showed that the degree of surface weathering in quartz grains from soil which developed from the same parent material and under the same environmental conditions.could indicate the relative degree of soil development.