Effects Of Different Topsoil Depths Of A Mollisols On Nutrient Uptake And Root Distribution In Crops

In recent years,soil erosion area is expanding,and the erosion intensity is gradually increasing with great soil and water losses in Black soil(Mollisols)region,which significantly degrades the Black soil,causing gradual thinning of topsoil layer,the declines in soil organic matter content and soil fertility,and farmland productivity,but rise in production cost.A simulation experiment of the reconstruction of different topsoil depths was conducted in the growing seasons of 2016-2017.This simulation experiment was under a maize-soybean rotation system with no-tillage.Depths of simulated topsoil were: 10 cm,20 cm and 30 cm.Normal rates of chemical fertilizers were applied for each crop.The effects of different topsoil depths of a Mollisols on nutrient uptake,root morphology and yield in soybean and maize were investigated.The results were as follows:(1)Within the simulated topsoil depths of 10-30 cm,the highest soybean yield was obtained at 20 cm topsoil depth,while highest maize yield was obtained at 30 cm topsoil depth in 2016,which was15.5% and 24.1% more than that of the 10 cm topsoil depth respectively.Though topsoil depths had no significant effects on both soybean and maize yield in 2017,yield in 30 cm top soil depth was highest,which was 3.9%and 8.2% more than that of the 10 cm topsoil depth respectively.Maize yield was more susceptible to topsoil depth than that of soybean.(2)Topsoil depths could significantly increase soybean root weight density at different soil layers with the greatest effect in the 0-10 cm soil layer.However,topsoil depths reduced root length density and the ratio of root length/root weight,and all values of these two items in the lower topsoil depth,10 cm topsoil depth,were higher than those of 30 cm topsoil depth.The increased soybean yield by topsoil depths might be due to its influences on root weight density and root length spatial distribution during the reproductive stages.Higher root weight per unit root length in the later growth stages,and robust root system is a guarantee for soybean yield.Root weight contributes more than root length to yield formation from the aspect of root system growth in soybean.(3)Topsoil depths also significantly increased maize root weight density at 0-10 cm soil layer,and had similar effects at 10-20 cm soil layer in the earlier growth stages.However,topsoil depths had no effects on root weight density in the later stages and at 20-30 cm soil layer.Topsoil depths influenced the ratio of root length/root weight.The thicker the top soil depths,the lower the ratio at 0-10 cm soil layers.Significant seasonal variations were found in root weight density,root length density and the ratio of root length/root weight.(4)Topsoil depths had no substantial influences on nitrogen content in both root and stem of soybeans.It also had no greater influences on phosphorus content in aboveground parts and root ofsoybean.However,relatively thicker topsoil depths significantly increased nitrogen content in leaf at R1 and R6 stages,promoted potassium uptake and utilization,and significantly increased potassium content in root,stem,and pod shell.The higher leaf nitrogen content and higher potassium content in other parts of the plant are possible reasons for the higher yield of soybean from thicker topsoil depths.(5)Relatively thicker topsoil depths can significantly increase the nitrogen and potassium content in root,stem and leaf of maize.However,thicker topsoil depths had no influences on phosphorus content in different parts of the plant,and it also had no effect on seed nitrogen content.Topsoil depth regulate corn yield probably through increasing nitrogen and potassium content in root,stem and leaf.