Research On Phytolith Accumulation And Noncrystalline Silicon Distribution Of Forest Soils In East China

Phytolith-occluded organic carbon?PhytOC?,one of the most promising biogeochemical carbon?C?sequestration mechanisms and long-term terrestrial C sinks,plays a crucial role in global biogeochemistry C cycle and mitigation of atmospheric CO₂.This study selected the different forest ecosystems in East China.Then we used sequential chemical extraction,potassium dichromate oxidation heating method,lithium metaborate fusion-dilute nitric acid dissolution,ultraviolet spectrophotometric method,and heavy liquid suspension method to study the soil phytolith accumulation of different forest ecosystems in the subtropics and tropics,and silicon fraction distribution of granite weathering crust in different climate zones.The purpose of this study is to offer some scientific references for long-term biogeochemical carbon sequestration of phytolith in forest ecosystem.We selected three typical forests in subtropics: bamboo,fir,and chestnut.According to the values of phytolith storages and PhytOC storages in different forest soils,the study results indicated that phytolith and PhytOC contents in the soils showed a similar decreasing trend with depth in the soil profiles.PhytOC storage in 100 cm of soil depth of the bamboo forest,within the upper?3.91 tha-1?,was much higher than that in the chestnut forest?2.67 tha-1?and the fir forest?1.18 tha-1?.Generally,PhytOC storage in the studied soil profiles increased significantly from 0 cm to 40 cm depth and then decreased gradually from 40 cm to 100 cm.The PhytOC/SOC ratio in the bamboo forest quickly increased with depth and was much higher than those in the other two forests.These findings suggest that forest type could significantly influence the storage of phytoliths and PhytOC in soil.We selected soil profiles developed form granite and basalt in both the subtropics and tropics.The results indicated that the average PhytOC contents in the soil profiles derived from granite and basalt in subtropics were 0.31 ± 0.02 mg g-1 and 0.13 ± 0.03 mg g-1,respectively,while in tropics were 0.12 ± 0.02 mg g-1 and 0.04 ± 0.01 mg g-1,respectively.At the same time,there was a positive correlation between the phytolith contents and PhytOC contents in the soil profiles?R2=0.477,p<0.01 for basalt;R2=0.603,p<0.01 for granite?.All these findings suggest that both climate and lithology have a distinct impact on soil PhytOC accumulation.The average values of PhytOC storages in the soil profiles were 2.75 ± 0.23 tha-1?granite,subtropics?,1.98 ± 0.37 tha-1?basalt,subtropics?,1.77 ± 0.22 tha-1?granite,tropics?and 0.57 ± 0.08 tha-1?basalt,tropics?,respectively.At the same time,we estimated that the soil phytolith turnover time in the soil profiles developed on the granite and basalt parent materials in the subtropics were 1018 years,960 years,respectively,and in the tropics were 584 years and 433 years,respectively.The turnover times in the subtropics were significantly longer than those in the tropics and there was no distinct difference between the soil profiles developed on different lithologies in the same climatic zone.These results suggest that climatic conditions,rather than lithology,control the soil phytolith turnover times.The bio-available Si contents in the subtropics and tropics soil profiles derived from granite and basalt varied from 0.036 mg g-1 to 0.545 mg g-1,and showed an increasing trend with depth.It is worth noting that the values of bio-available Si contents in the soil profiles derived from basalt were significant higher than those developed on granite.There was a positive correlation between the soil phytolith content and soil bio-available silicon content in the soil profiles developed on basalt parent material?R2=0.208,p<0.05?and a negative correlation in the soil profiles developed on granite parent material?R2=0.241,p<0.05?.All these findings suggest that external silicon amendment?e.g.,basalt powder amendment?in soils derived from granite is a promising way to enhance biogeochemical carbon sequestration.We selected the granite weathering crusts of different climate zones in East China,and determined the noncrystalline Si contents in the soil profiles to research the impact of climate on noncrystalline silicon distribution.The results indicated that the contents of acid Na-acetate-Si,H₂O₂-Si,NH₂-OH HCl-Si,NaOH-Si in the soil profiles have a large variation.There was a distinct difference in the noncrystalline Si contents among the different climate zones.All these findings suggest that the climate have a significant impact on the soil noncrystalline Si distribution.When plant die or decay,phytoliths formed in the plant can be released into the soils or sediments,and took part in the Si bio-geochemical cycle in the terrestrial ecosystem.This study indicated that the phytolith return flux in the subtropical evergreen and deciduous broad-leaf forest and tropical monsoon forest was significant large than those in the coniferous and broad-leaf mixed forest and deciduous broad-or small-leaf forest,which led to the amorphous Si content in the south distinct higher than that in the north.All these findings suggest that phytoliths has a large impact on the soil noncrystalline Si content.