Biogeochemical Characteristics Of Si In Degraded Grasslands And Their Impacts On C,N And P Cycles

Grassland ecosystems play an important role in global terrestrial silicon?Si?cycle.However,about half of global grasslands have been degraded to different degrees because of the drying climate and intense human disturbance,and the impacts of grassland degradation on the biogeochemical cycle of Si are largely unknown.In this study,in order to assess the impacts of grassland degradation on Si cycle,we analyzed the Si distribution and bioavailability of different soil profiles?0–10 cm,10–20 cm and 20–40 cm?and the Si distribution of plants in the grasslands with different degradation degrees in the agro-pastoral ecotone of northern China.Furthermore,in order to examine the impacts of plant Si on carbon?C?,nitrogen?N?and phosphorus?P?biogeochemical cycles in grassland at landscape scale,we examined the correlations of Si with stoichiometry and accumulation of C,N and P in grasses.Major results and conclusions are as follows:?1?The Si_avail contents of soil profiles significantly decreased with grassland degradation,being 518.26±79.18 mg kg^-1,440.47±53.12 mg kg^-1,232.65±22.21mg kg^-1 and 249.79±23.89 mg kg^-1 in NDG,LDG,MDG and SDG,respectively.The corresponding contents and storages of noncrystalline Si and the relative bioavailability of soil Si also decreased with grassland degradation.The possible reasons for these processes include the changes of vegetation community structure and soil physical and chemical properties because of grassland degradation.?2?The impacts of grassland degradation on average Si content in above-ground parts of plants were not obvious,while the Si production flux significantly decreased with degradation degrees,being 0.81±0.12 g m^-2 a^-1?NDG?,0.60±0.42 g m^-2 a^-1?LDG?,0.36±0.29 g m^-2 a^-1?NDG?and 0.36±0.22 g m^-2 a^-1?SDG?,respectively.The possible reasons for these processes include the change of vegetation community structure and the decrease of Si bioavailability in soils due to grassland degradation.?3?The storage of soil noncrystalline Si and Si production flux of above-ground vegetation potentially decreased by 50%as results of grassland degradation.These results suggest that the grassland degradation could lead to the decreases of Si biogeochemical cycle flux and potential of bio-sequestered C in phytoliths.?4?C concentrations in above-ground parts of grasses?ranging from 45.49%to41.71%?and?¹³C?-24.68‰–-28.03‰?significantly decreased with Si concentrations?0.23%to 2.27%?,but P concentrations increased significantly,from lower than0.10%to higher than 0.20%,while the correlation between Si and N was not obvious.Additionally,the C:P and N:P ratios were negatively relative with Si concentrations.These results suggest that Si incorporation in grasses may affect organic C synthesis and adjust nutrient utilization strategy.Therefore,Si may be a significant factor in regulating C,N and P biogeochemical cycles in grasslands and other ecosystems dominated by Si-accumulating plants.