Silicon Cycle And Phytolith-occluded Carbon In Soil-plant System Of China’s Paddy Fields

The roles of silicon（Si）biogeochemical cycle and phytolith carbon sequestration of rice-paddy ecosystem should not be ignored in regulating global climate change.In this study,we combined national field investigation and regional condition control experiment（including early and late rice cultivation,straw return,and water table management）to discuss the spatial distribution characteristic of plant-available Si（PASi）and its influencing factors in paddy soils of China,explore the patterns of carbon（C）occlusion during the phytolith formation in rice straw,elucidate the effects of agricultural production practices on soil-plant Si cycle and phytolith C sequestration in paddy fields,and assess the phytolith production,return and its C sequestration potential in China’s rice-paddy ecosystem.The main findings are shown as follows:（1）The spatial variation of PASi content in soils of China’s paddy fields was controlled by natural（i.e.,soil physiochemical properties and climate conditions）and anthropogenic（i.e.,rice cultivation and straw remove）factors.Among all investigated natural factors,soil p H and mean annual temperature were the most important influencing factors.On a national scale,the regions of PASi deficiency were mainly located in southern China,with the levels of Si deficiency lowering as the paddy fields at located further north.Currently,more than 65%of paddy fields in China were deficient in Si,which was an increase in area of around 15%compared with data from the1990s.（2）The large variation of C content in phytoliths(1.70～77.5 g kg^–1)was mainly attributed to the existence of different carbon trap patterns within phytoliths.Generally,the C content in phytolith first decreased with increasing digestion strength and then kept constant when the digestion strength reached up to a certain degree（in this case,digestion at 130℃for 0.5 h+digestion at 135℃for 2.5 h）.Additionally,theδ¹³C values of phytoliths extracted from different digestion strengths exhibited a wide variation（–30.9‰～–29.2‰）,indicating that the organic compositions of PhytOC（phytolith occluded carbon）were different among each digestion strength.（3）In the process of extracting phytoliths from rice straw,when the digestion reached a stable state,although phytolith content in the early rice straw(56.0～77.0 g kg^–1)was much lower comparing with late rice straw(71.5～111 g kg^–1),the C content in phytoliths of early rice straw(2.13～4.23 g kg^–1)was significantly higher than that of late rice straw(1.36～2.83 g kg^–1).In all studied early and late rice straw,the efficiency of carbon encapsulation by small size phytoliths（<20μm）was higher than that by large size phytoliths（>20μm）,and the heavy C(¹³C)fraction tended to be sequestered in silica matrix of small size phytoliths,while the light C(¹²C)fraction was prone to be sequestered in silica matrix of large size phytoliths.Furthermore,under different digestion strengths,the estimated phytolith and PhytOC production fluxes(270～308 kg ha^–1 year^–1 and 2.76～49.6 kg CO₂ ha^–1 year^–1,respectively)of early rice straw were significantly lower than those of late rice straw(618～670 kg ha^–1 year^–1and 4.67～81.8 kg CO₂ ha^–1 year^–1,respectively),suggesting that the potential of phytolith carbon sequestration in paddy fields of China could be significantly influenced by the cultivation of early rice and late rice.（4）In the experiment field with long-term straw return,the contents of Na₂CO₃-Si（Si extracted by 1%Na₂CO₃ solution）and H₂O₂-Si（Si extracted by hot 30%H₂O₂solution）increased significantly with increasing dose of straw return,explaining the increases in total labile Si content.Under the same groundwater table,the Si content and phytolith content in the planted rice straw increased significantly with increasing straw application.For the plots with the same dose of straw return,the Ca Cl₂-Si content under the 80 cm groundwater table was greater than that under the 20 cm groundwater table.Correspondingly,the Si content and phytolith content in planted rice straw under the 80 cm groundwater table were significantly higher than those under the 20 cm groundwater table.In addition,the densities of phytoliths and PhytOC within the topsoil（20 cm）of each plot generally exhibited an increase trend with increasing straw application under two groundwater tables,respectively.The above findings suggest that long-term straw return and the maintenance of the groundwater table management significantly influence the soil-plant Si cycle and further increase soil Si bioavailability and phytolith carbon sequestration by promoting the accumulations of soil organic matter and soil phytoliths.（5）According to the estimation of this study,the phytolith production rate of China’s paddy fields was approximately 16.9～19.0×10⁶t year^–1.Those phytoliths could approximately sequester（3.16±0.89）×10⁶ t of CO₂ from atmosphere annually;the contribution of Middle-Lower Yangtze River rice cropping region,South Yangtze River rice cropping region,and Northeast China rice cropping region was 25.3%,26.5%,and 20.0%,respectively.Approximately 50%of rice straw phytolith would be stably stored in paddy soils after rice straw return.On a national scale,the storage of PhytOC within topsoil（0–20 cm）of China’s paddy fields was（23.3±15.3）×10⁶ t-e-CO₂;the contribution of Middle-Lower Yangtze River rice cropping region,South Yangtze River rice cropping region,and Northeast China rice cropping region was 28.3%,28.0%,and14.4%,respectively.Based on the current production rate of PhytOC in rice straw,assuming all rice straw was returned into paddy soils with scientific and reasonable ways,the total amount of atmospheric CO₂ sequestered by phytoliths in topsoil of China’s paddy fields could reach up to（65.4±27.1）×10⁶ t after 20 years later.In conclusion,the deficiency of plant-available Si in soils of China’s paddy fields was very serious,while the potential of phytolith C sequestration was great in China’s rice-paddy ecosystem.Therefore,in the future rice production practices,optimizing the planting of early and late rice cultivation,choosing scientific and reasonable ways to increase the amount of rice straw return,and combining with field groundwater table management and suitable Si fertilization are not only beneficial to maintaining the sustainable rice production,but also helpful to enhancing the phytolith C sequestration potential in rice-paddy ecosystem of China.