Silicon Distribution And Phytolith Morphology In The Karst Critical Zone Of Southwestern China

The terrestrial silicon(Si)biogeochemical cycle and phytolith morphology have received increasing interest in studying global environmental change,ecosystem stability,and vegetation evolution.The ecological environment in the karst region is fragile,but the plant species are diverse.The study of the Si cycle and phytolith morphology in this region is not only important for ecological restoration in the region,but also helpful for better understanding terrestrial Si biogeochemical cycle.This study focuses on the plant-soil system in the karst area of southwestern China,and aims to explore the effects of land use and slope position on the distribution of different Si fractions and the morphological characteristics of phytoliths in plant-soil system of karst area.The main findings are listed as follows:(1)In Chenqi small catchment,the contents and distribution characteristics of available Si(ASi)and biogenic Si(BSi)were affected by land use types and slope position.The ASi content in topsoils under different land use decreased as follows:arable land(0.55±0.04 g/kg)> abandoned farmland(0.46±0.09 g/kg)> abandoned orchard(0.46±0.02 g/kg)> secondary forest(0.40±0.01 g/kg)> shrubland(0.34±0.08g/kg)> grassland(0.28±0.01 g/kg).The BSi contents decreased as follows: arable land(3.88±0.48 g/kg)> abandoned farmland(3.49±0.33 g/kg)> abandoned orchard(3.18±0.39 g/kg)> grassland(2.41±0.06 g/kg)> secondary forest(2.38±0.28 g/kg)>shrubland(2.17±0.91 g/kg).The ASi contents in topsoils at different slope position decreased as follows: lower slope position(0.48±0.08 g/kg)> middle slope position(0.41±0.06 g/kg)> upper slope position(0.33±0.10 g/kg).BSi contents also decreased as follows: lower slope position(3.40±0.25 g/kg)> middle slope position(2.66±0.72g/kg)> upper slope position(2.06±0.84 g/kg).(2)The morphological characteristics of phytoliths were diverse and the contents of phytoliths were abundant in herbaceous plants.The dumbbell phytoliths(33.8?63.5%)accounted for a relatively high proportion in herbaceous plants,which could be regarded as representative identification type of vegetation phytoliths in southwestern karst regions.The tracheid phytoliths(4?45.5%)accounted for a relatively high proportion in woody plant phytoliths.Therefore,it could be regarded as the representative woody plant phytolith in southwestern karst area.(3)Based on the analysis of the sources of phytoliths in secondary forest,shrubland,abandoned orchard and abandoned farmland,the morphology and proportion of phytoliths in the topsoil mainly were derived from the accumulation of plant phytoliths over the years.To some extent,they were also affected by soil erosion,migration and accumulation.The morphological composition characteristics of the phytolith in surface soils under different land use were different.A large number of phytoliths of herbaceous plants appeared in secondary forest and shrubland mainly covered with herbs in the early period,because the phytoliths from woody plants had not largely deposited in the topsoil.The large number of woody plant phytoliths in abandoned orchard and abandoned farmland indicated that a large number of woody plants had recently grown in this area.(4)Studying the distribution characteristics and proportion of phytolith assemblages at different soil depths in the soil profile can deduce the succession process of above-ground vegetations.The proportion of phytoliths of woody plants was not changed largely in secondary forest,indicating that this area should always be covered by woody plants for a long time.In shrubland,the proportion of phytoliths from Poaceae and Ferns were dominant in the early period,while the proportion of phytoliths from woody plants increased in the later period.This means that the bushes have recently grown,while it may be grasses in the early period.In the process of vegetation succession in abandoned farmland,the proportion of phytoliths from woody plants decreased and that from Poaceae increased,indicating that the abandoned farmland might experience the process of transformation from woody plants to Poaceae plants.