Distribution Of Silicon And Its Impact On Nutrient Stoichiometry,and Carbon And Nitrogen Isotope Compositions In Grasslands Of China

Carbon（C）biogeochemical cycle in terrestrial ecosystem plays an important role in regulating global climate.Mineral elements such as silicon（Si）,nitrogen（N）,phosphorus（P）and sulfur（S）play indispensable roles in the growth of terrestrial plants,evolution of ecosystems and C biogeochemical cycles.Grassland ecosystems,as an important part of terrestrial ecosystems,are very sensitive to global climate change.However,the distribution of silicon in different grassland ecosystems and its effects on N,P and S ecological stoichiometry and carbon isotopes(δ¹³C)and nitrogen isotopes(δ¹⁵N)are still unclear.Therefore,we selected different grassland ecosystems in China,investigated the distribution of plant Si and soil available Si in grassland ecosystems,and their effects on ecological stoichiometry patterns of N,P and S,and discussed the patterns ofδ¹³C andδ¹⁵N composition in different grassland ecosystems and their relationships with Si distribution.We also quantified the effects of climate change and soil physical and chemical properties on the ecological stoichiometry andδ¹³C andδ¹⁵N composition of grassland ecosystems,providing scientific references for predicting the C and N cycle and nutrient biogeochemical cycle under future climate change scenarios.The main results of this study are as follows:（1）The range content of soil available Si in grasslands of China was 41.52–51.28mg kg^-1,and the average content of soil available Si in Inner Mongolia plateau was higher than that in Qinghai-Tibet Plateau.Generally,the average content of soil available Si decreased with soil profile depth,while the average content of soil available Si in temperate desert steppe increased significantly in deep soil,indicating that the vegetation of this grassland type had deeper roots.Soil available Si content was mainly affected by soil bulk density（BD）and soil texture.Soil available Si content increased with the increase of soil organic carbon（SOC）content.The average plant Si content in grassland of China was about 12.14±1.02 mg kg^-1,and the average plant Si content in Tibetan plateau grassland was significantly higher than that in Inner Mongolia Plateau grassland.Plant Si content was mainly affected by mean annual temperature（MAT）and altitude.Plant Si content decreased with the increase of plant C content,indicating that plant Si has a substitution effect on organic structural carbon components.（2）The average contents of soil total nitrogen（TN）,total phosphorus（TP）and total sulfur（TS）in grasslands of China were 1.00±0.05 mg g^-1,0.42±0.01 mg g^-1 and0.18±0.01 mg g^-1,respectively.The average contents of plant N,P and S were 20.67±0.69 mg g^-1,1.50±0.06 mg g^-1 and 1.85±0.09 mg g^-1,respectively.The soil TN contents of temperate meadow steppe and alpine meadow were higher than that of other grassland ecosystems.The content of TS in surface soil of alpine meadow within them were significantly higher than that of other grassland ecosystems.The plant N content in temperate desert steppe was higher than that in other grassland ecosystems.According to the N:P ratio of plants,the growth of plants in temperate meadow steppe was limited by N,while the growth of plants in other grassland ecosystems were limited by both N and P.Soil TN was mainly affected by soil p H,BD,soil texture,MAT and mean annual precipitation（MAP）,soil TP was mainly affected by soil BD,soil texture and MAT,and soil TS was mainly affected by soil BD and soil texture.The plant N,P and S contents in grassland increased first and then decreased with the increase of MAT.Plant P,S contents were negatively correlated with MAP in the range of 400 mm.The increase of soil available Si could significantly increase soil TN and TP contents.There was a significantly positive relationship between plant Si content and plant N content.By increasing Si absorption,plants can promote P absorption,regulate nutrient balance and alleviate P restriction in grasslands.（3）The plant and soilδ¹³C values of grasslands in China decreased with the increase of precipitation gradient.Under the influence of human activities,the variation of soilδ¹³C in temperate typical steppe was not well explained by various environmental factors.Soilδ¹³C values were positively correlated with MAT,BD and p H,but negatively correlated with MAP,SOC and TN.Plantδ¹³C values were positively correlated with BD and S content,but negatively correlated with MAP,topsoil organic carbon and plant C:N ratio.The structural equation model（SEM）based analyses showed that MAT,SOC and plantδ¹³C values had the greatest influence on soilδ¹³C values,and all factors accounted for 54%of the total surface soilδ¹³C values.Support vector regression（SVM）simulations showed that the simulated soilδ¹³C values could better represent the soilδ¹³C values of different grassland types,and soilδ¹³C values were significantly affected by microorganisms in the 10-20 cm soil layer.In temperate desert steppe,there was a positive correlation between plant Si content and plantδ¹³C value,while in temperate meadow steppe,there was a significantly negative correlation between plant Si content and plantδ¹³C value.The main reason is that the increase of plant Si can promote the water use of plants in arid environment,and thus increase plantδ¹³C value.（4）The soilδ¹⁵N values of grassland in Inner Mongolia Plateau decreased with the decrease of precipitation gradient,while the soilδ¹⁵N values of grassland in Tibetan Plateau increased.The plantδ¹⁵N values of different grasslands decreased with the decrease of precipitation gradient.Soilδ¹⁵N values were positively correlated with soil TS and soil EC,but negatively correlated with MAT and MAP.Plantδ¹⁵N values were positively correlated with plant S content,but negatively correlated with MAT,MAP and plant C content.MAP had a greater effect on plant and soilδ¹⁵N values than MAT,according to the rain-heat effect.The SEM based analyses showed that MAT,MAP and plantδ¹³C values had the greatest influence on soilδ¹⁵N values in grasslands of China,and all the factors accounted for 64%of the total surface soilδ¹⁵N values.The results of SVM simulation showed that the simulated soilδ¹⁵N values could better represent the soilδ¹⁵N values of different grassland types,and increased with the increase of soil depth,which had a good fidelity.There was no significant correlation between plant Si and plantδ¹⁵N and between soil available Si and soilδ¹⁵N.In conclusion,Si plays an important role in the biogeochemical cycle of grassland in China.Soil available Si and plant Si could regulate the nutrient balance and alleviate the restriction of P deficiency in grasslands.In arid areas,plant Si can also affect plant water use efficiency by regulating plant transpiration,thereby affecting plantδ¹³C composition and participating in grassland C biogeochemical cycle.Therefore,in future grassland management or restoration,silicon fertilizer can be applied rationally,which is not only conducive to the sustainable development of grasslands,but also can improve the carbon sequestration potential of grasslands.