Acidification And Phosphorus Enrichment Affect Silicon Availability In Two Farmland Soils

The acidification of soils poses a threat to their functions.Soil pH plays a crucial role in the availability and interaction of phosphorus（P）and silicon（Si）.Although Si fertilizer application has been reported to efficiently increase P availability in soils,the response of Si availability to soil P enrichment remains unclear,especially in areas of intensive agriculture where soil P accumulation is significant.This study investigated the effects of P addition and artificial soil acidification on Si sorption,soil Si fractions and availability,and Si release characteristics in two typical farmland soils with different available Si levels,namely lower available Si level（LASi）and higher available Si level（HASi）.The study employed P and Si competitive adsorption experiment,soil incubation experiment,leaching soil column experiment and rice pot experiment.The study results can provide a theoretical and practical basis for managing soil Si and P nutrients and their efficient utilization in soils.The main study results were as follows:（1）In the pH range of 3.5-8.0,the addition of equimolar of Si and P was found to inhibit the adsorption of Si by soil.The adsorption of Si was decreased by 26%-74%and31%-84%for LASi and HASi,respectively,at each corresponding pH due to P addition.Soil pH significantly affected the competitive sorption of Si and P in the soil.The effect of P addition in reducing soil Si sorption was more significant at high pH conditions.（2）The contents of labile Si,organic Si and Fe/Mn oxide Si in LASi soils increased with decreasing pH and increasing P addition,with labile Si increasing by 8.32%-16.48%and 10.36%-20.29%,respectively,while the contents of adsorbed Si decreased by4.57%-12.29%and 8.61%-15.30%,indicating that acidification and P addition can regulates Si fractions mutual transformation by affected soil Si sorption-desorption.For HASi soil,the contents of labile Si,adsorbed Si,organic Si and Fe/Mn oxide Si was decreased with decreasing pH,among which labile Si decreasing by 25.49%-40.56%,while the contents of amorphous Si increased 34.89%-193.27%,acidification is not conducive to the dissolution of soil amorphous Si,acidification affect the precipitation-dissolution of soil Si to regulate Si fractions mutual transformation.Only the contents of organic Si increased significantly with increasing P addition.The HOAc-Na OAc-Si content of LASi and HASi soils decreased significantly with decreasing pH,while increasing P addition had no significant effect.The CaCl₂-Si（labile Si）content of LASi soils increased significantly with decreasing pH and increasing P addition,while the CaCl₂-Si content of HASi soils decreased significantly with decreasing pH and P addition contributed to its increase.Total rice dry matter and rice Si uptake in LASi soil significantly decreased with acidification,it was reduced by 43.26%,33.77%and 32.65%,21.77%at pH 4.5 and pH 5.5,respectively,compared to pH 6.4.Acidification inhibited rice growth and Si uptake in LASi soils,while P addition promoted rice growth and silicon uptake in LASi and HASi soils.（3）The release of Si from LASi and HASi with acidification by 14.59%-200.57%and40.38%-162.48%.Moreover,the Si release tended to be increased by P addition.Soil acidification and P addition promoted soil Si release and increased Si mobility and the risk of leaching loss.For LASi soils,the sorption-desorption and precipitation-dissolution processes of Si in soil are affected by acidification,resulting in mutual transformation between soil Si fractions,reduced availabile Si（HOAc-Na OAc-Si）and increased labile Si（CaCl₂-Si）content.For HASi soils,acidification mainly affected the precipitation-dissolution processes of Si,with mutual transformation between Si fractions,reduced availabile Si（HOAc-Na OAc-Si）and labile Si（CaCl₂-Si）content.P addition mainly affected the sorption-desorption process of soil Si,and P enrichment contributed to the increase content of labile Si（CaCl₂-Si）.In conclusion,the effect of soil acidification on Si fractions,available Si（HOAc-Na OAc-Si）and labile Si（CaCl₂-Si）is greater than the effect of P enrichment.